Showing papers on "Pelagic zone published in 2002"

Fishes as integrators of benthic and pelagic food webs in lakes

Abstract: Studies of lake ecosystems generally focus on pelagic food chains and processes. Recently, there has been an emerging recognition of the importance of benthic production and processes to whole-lake ecosystems. To examine the extent to which zoobenthos contribute to higher trophic level production in lakes, we synthesized diet data from 470 fish populations (15 species) and stable isotope data from 90 fish populations (11 species), all of which are common inhabitants of north-temperate lakes. Across all species considered, zoobenthos averaged 50% of total prey consumption. Indirect consumption of zoobenthos (i.e., feeding on zoobenthos-supported fishes) contributed another 15%, for a total of 65% reliance on benthic secondary production. Stable isotopes provided estimates of mean zoobenthivory ranging from 43% to 59%. For most fish species, consumption of zoobenthos was highly variable among populations. The overwhelming concern of ecologists with pelagic food chains and processes contrasts sharply with our finding that benthic secondary production plays a central role in supporting higher trophic level production. This extensive zoobenthivory can subsidize fish populations, leading to apparent competition and otherwise altering trophic dynamics and ecosystem processes in the pelagic zone. We argue for a more integrated view of lake ecosystems that recognizes the duality of benthic and pelagic production pathways. Food web models that explicitly consider energy flow from pelagic and benthic sources will provide a more realistic energy flow template for understanding the regulation of lake ecosystem functioning.

Habitat coupling in lake ecosystems

Abstract: Lakes are complex ecosystems composed of distinct habitats coupled by biological, physical and chemical processes. While the ecological and evolutionary characteristics of aquatic organisms reflect habitat coupling in lakes, aquatic ecology has largely studied pelagic, benthic and riparian habitats in isolation from each other. Here, we summarize several ecological and evolutionary patterns that highlight the importance of habitat coupling and discuss their implications for understanding ecosystem processes in lakes. We pay special attention to fishes because they play particularly important roles as habitat couplers as a result of their high mobility and flexible foraging tactics that lead to inter-habitat omnivory. Habitat coupling has important consequences for nutrient cycling, predator-prey interactions, and food web structure and stability. For example, nutrient excretion by benthivorous consumers can account for a substantial fraction of inputs to pelagic nutrient cycles. Benthic resources also subsidize carnivore populations that have important predatory effects on plankton communities. These benthic subsidies stabilize population dynamics of pelagic carnivores and intensify the strength of their interactions with planktonic food webs. Furthermore, anthropogenic disturbances such as eutrophication, habitat modification, and exotic species introductions may severely alter habitat connections and, therefore, the fundamental flows of nutrients and energy in lake ecosystems.

Evidence of self-recruitment in demersal marine populations

Abstract: The majority of shallow-water marine species have a two-phase life cycle in which relatively sedentary, demersal adults produce pelagic larvae. Because these larval stages are potentially subject to dispersal by ocean currents, it has been widely accepted that local populations are open, with recruitment resulting from the arrival of larvae from non-local sources. However, a growing number of studies indicate that larvae are capable of recruiting back to their source population. Here, we review the evidence for selfrecruitment in demersal marine populations, drawing from studies of endemism, introduced species, population genetics, stock-recruitment relationships, larval distributions, populations at the limit of a species’ range, and applications of environmental and chemical markers. These studies indicate that self-recruitment can and does occur across species representative of most life history traits and geographical localities. Thus, the ability of larvae to recruit back to their natal population may be a pervasive phenomenon among marine species. The mounting evidence in support of self-recruitment dynamics indicates a pressing need for a reevaluation of the appropriateness of demographically-open population models and their applicability to the management and conservation of marine ecosystems. Until the early 20th century, marine systems were considered analogous to many terrestrial systems that consist of groups of demographically-closed or self-recruiting populations in which reproduction by local adults gives rise to the next generation. For many pelagic fisheries, it was believed that fluctuations in abundance were caused by adult migration, with poor fishing years a result of few fish returning to the usual fishing grounds. Hjort (1914) marked the beginning of a paradigm shift away from the migration theory to the view that fluctuations in adult abundance might be caused by variable recruitment (Sinclair, 1997). This shift in focus towards the early life history stages highlighted important differences in the life cycles of terrestrial and marine fauna. The majority of shallow-water marine taxa have a bipartite life cycle in which relatively sedentary, demersal adult stages produce larvae that develop in the pelagic environment before recruiting to the benthos. Because many pelagic larval stages are too small to contend with ocean currents, Thorson (1950) proposed that recruitment variability was primarily determined by larval mortality resulting from advection away from suitable settlement sites. If pelagic larvae are subject to transport by ocean currents away from the parental population, local marine populations should be demographically open with local recruitment resulting from the transport of larvae from non-local sources. Since the 1950’s, the demographically open population model has gained wide acceptance (e.g., Roughgarden et al., 1985; Sale, 1991; see Caley et al., 1996 for a review).

Predicting self-recruitment in marine populations: Biophysical correlates and mechanisms

Abstract: Mounting evidence suggests that some populations of benthic marine organisms may be less demographically 'open' than previously thought. The degree to which a population receives recruits from local sources versus other populations has important ecological and management ramifications. For either of these reasons, it is often desirable to estimate the degree to which a population of interest is self-recruiting. Although methods for actual estimation of population self-recruitment are limited and often difficult to employ, the presence of several biological and physical conditions may improve our estimates of self-recruitment for particular populations. Biological traits of benthic adults (relative fecundity, spatial and temporal patterns of spawning and larval release, parental investment), as well as pelagic larvae (stage of development at hatching, pelagic larval duration, vertical migration behavior, horizontal swimming ability, and sensory capabilities) influence where and when larvae are released, where and how they are transported, their ability to move actively in the pelagic realm, and finally, spatial and temporal settlement patterns. Physical variables potentially influencing self-recruitment include site isolation, coastal complexity and flow variability. Within these physical variables we discuss explicit mechanisms by which larvae may be retained in proximity to their natal population. We provide examples from specific locations such as coral reefs, isolated islands and seamounts, and semi-enclosed embayments such as lagoons and estuaries, as well as characteristic oceanographic features such as upwelling systems, fronts, moving convergences, eddies and counter currents. We evaluate direct and indirect evidence to predict the relationship between these biophysical variables and the degree of self-recruitment in benthic marine organisms. We conclude that physical factors that result in a departure from unidirectional, depth-uniform water flow provide the opportunity for retention of larvae, and therefore of self-recruitment. These physical factors are common in the ocean and vary in intensity among locations and times. Some enable retention of passive larvae (physical retention), whereas others lead to retention only with active behavioral input by the larvae (biophysical retention). Larval behavior that can contribute to or result in retention or return to natal sites ranges from simple vertical orientation (within the capabilities of the larvae of most taxa) to complex sensory abilities and strong swimming (known to occur in larvae of a few taxa, particularly decapod crustaceans and fishes). For all taxa, both the pelagic larval duration and the time to behavioral competency will have a strong influence on likelihood of self-recruitment. Understanding the biophysical mechanisms by which larvae are retained near or return to their natal population will be necessary before generalizations can be made. Examples highlight the importance of each variable to processes controlling self-recruitment. For most correlates, further study is clearly warranted. Although certain variables hold promise for predicting self-recruitment, complex, non-linear interactions among these biophysical variables must be considered.

Overfishing drives a trophic cascade in the Black Sea

Abstract: During recent decades, environmental conditions have deteriorated in the Black Sea. Population explosions of phytoplankton and jellyfish have become frequent and several fish stocks have collapsed. In this study, literature sources and long-term data are explored in order to find empirical evidence for ecosystem effects of fishing. Inverse trends of decreasing predators, increas- ing planktivorous fish, decreasing zooplankton and increasing phytoplankton biomass are revealed. Increased phytoplankton biomass provoked decreasing transparency and nutrient content in surface water. A massive development of jellyfish during the 1970s and 1980s had a great impact on con- sumption and consequent decrease in zooplankton. The turning point for these changes occurred in the early 1970s, when industrial fishing started and stocks of pelagic predators (bonito, mackerel, bluefish, dolphins) became severely depleted. A 'trophic cascade' is invoked as a mechanism to explain observed changes. According to this hypothesis, reduction in apex predators decreases con- sumer control and leads to higher abundance of planktivorous fish. The increased consumption by planktivorous fish causes a consequent decline in zooplankton biomass, which reduces grazing pres- sure on phytoplankton and allows its standing crop to increase. The effects of fishing and eutrophi- cation are explored using a dynamic mass-balance model. A balanced model is built using 15 eco- logical groups including bacteria, phytoplankton, zooplankton, protozoa, ctenophores, medusae, chaetognaths, fishes and dolphins. Ecosystem dynamics are simulated over 30 yr, assuming alterna- tive scenarios of increasing fishing pressure and eutrophication. The changes in simulated biomass are similar in direction and magnitude to observed data from long-term monitoring. The cascade pat- tern is explained by the removal of predators and its effect on trophic interactions, while the inclusion of eutrophication effects leads to biomass increase in all groups. The present study demonstrates that the combination of uncontrolled fisheries and eutrophication can cause important alterations in the structure and dynamics of a large marine ecosystem. These findings may provide insights for eco- system management, suggesting that conserving and restoring natural stocks of fish and marine mammals can contribute greatly to sustaining viable marine ecosystems.

The biology, behaviour and ecology of the pelagic, larval stage of coral-reef fishes

Abstract: [Extract] Reef fish biologists are keenly aware that nearly all bony fishes on coral reefs have a pelagic larval phase that is potentially dispersive, and that this has major implications for reef fish populations not only at evolutionary (or biogeographic) scales, but also at ecological (or demographic, including management) scales. The literature is full of statements of how important this type of life history is for reef fishes, and for study and management of them. However, this realization has not been accompanied by a major shift in research effort to studying this pelagic phase, what one might refer to as “prerecruitment” studies. Neither has it led to a widespread view of the pelagic phase as much more than a “black box” that results in open populations and large fluctuations in recruitment. Even attempts to assess the population connectivity that presumably results from larval dispersal typically make simplifying assumptions, either explicitly or implicitly, that portray the larvae as little more than passive tracers of water movement that “go with the flow”, doing nothing much until they bump into a reef by chance and settle at once. Are larvae really as simple and as uninteresting as the assumptions made by this “black box” view of larval biology? We think not. The work reviewed here reveals larvae of coral reef fishes to have remarkably good swimming abilities, good sensory systems that develop early in ontogeny, and sophisticated behavior that is very flexible. Little of this would have been predicted from the much better known larval biology of temperate, non-reef species such as herring, cod, and plaice. We explore some of the reasons for this. The interaction of larval distributions with oceanography is the subject of Chapter 7 in the present volume, and we do not address that subject area. This chapter is not a revision of former work by Leis (1991a), nor does it cover ground already dealt with in reviews of coral reef fish larval biology by Boehlert (1996) and Cowen and Sponaugle (1997). Instead, here the focus is on recent research that examines reef fish larvae as animals interacting with their environment. The emphasis is on a perspective from the pelagic environment toward the demersal reef environment. The larvae have a similar perspective. Other studies take the opposite view, and indirectly examine the pelagic stage from the reef. These utilize information gleaned from otoliths of recruits or from abundance patterns either of settlement stage larvae captured by reef-edge light traps and reef-based nets, or of recruits on the reef (e.g., Dufour and Galzin, 1993; Milicich, 1994; Sponaugle and Cowen, 1994; Thorrold et al., 1994b,c; Robertson et al., 1999). Studies of this sort provide valuable insight, but they are largely beyond the scope of the present review. We review here new information on the pelagic stage, from spawning to settlement, including metamorphosis, but not postsettlement issues.

Increases in jellyfish biomass in the Bering Sea: implications for the ecosystem

Abstract: There has been a dramatic increase in jellyfish biomass over the eastern Bering Sea shelf since the early 1990s, which was previously hypothesized to have been triggered by changing climate and ocean conditions. We examine the hypothesis that the presence of these large carnivores has affected fisheries resources, either through direct predation on larval stages, or through compe- tition for zooplankton prey. In this paper, we explore the impact of this jellyfish increase on zoo- plankton and fish communities based on field data on the composition of the jellyfish community, and the abundance, size, stable isotopic signatures, and feeding habits of the principal scyphomedusae in the region. These data, together with those on zooplankton biomass, are used to estimate the ecosys- tem impacts of this increase. The center of jellyfish biomass has shifted from the SE Middle Shelf Domain in the early 1980s to the NW in the late 1990s. In recent years, the species composition of large medusae caught in trawls was dominated (>80% by number and >95% by weight) by the scyphozoan Chrysaora melanaster. Dense aggregations of this species occupied the water column in daytime between 10 and 40 m. Their food habits consisted mainly of pelagic crustaceans (euphausi- ids, copepods, amphipods), although other jellyfish and juvenile pollock were also consumed. Based on stable isotope ratios, the trophic level of this scyphozoan is equivalent to, or higher than, that of Age 0 pollock. Preliminary estimates showed that medusae have a moderate grazing impact on zoo- plankton in the area around the Pribilof Islands; C. melanaster was estimated on average to consume seasonally about one-third of the standing stock and 4.7% of the annual production of zooplankton in this region. Daily consumption of Age 0 pollock was estimated to be 2.8% of the standing stock around the Pribilof Islands during 1999. A hypothesis for the increase in jellyfishes observed in the eastern Bering Sea, based on release from competition from planktivorous forage fishes, is proposed.

Food webs and the transmission of parasites to marine fish

Abstract: Helminth parasites of fish in marine systems are often considered to be generalists, lacking host specificity for both intermediate and definitive hosts. In addition, many parasites in marine waters possess life cycles consisting of long-lived larval stages residing in intermediate and paratenic hosts. These properties are believed to be adaptations to the long food chains and the low densities of organisms distributed over broad spatial scales that are characteristic of open marine systems. Moreover, such properties are predicted to lead to the homogenization of parasite communities among fish species. Yet, these communities can be relatively distinct among marine fishes. For benthos, the heterogeneous horizontal distribution of invertebrates and fish with respect to sediment quality and water depth contributes to the formation of distinct parasite communities. Similarly, for the pelagic realm, vertical partitioning of animals with depth will lead to the segregation of parasites among fish hosts. Within each habitat, resource partitioning in terms of dietary preferences of fish further contributes to the establishment of distinct parasite assemblages. Parasite distributions are predicted to be superimposed on distributional patterns of free-living animals that participate as hosts in parasite life cycles. The purpose of this review is first, to summarize distribution patterns of invertebrates and fish in the marine environment and relate these patterns to helminth transmission. Second, patterns of transmission in marine systems are interpreted in the context of food web structure. Consideration of the structure and dynamics of food webs permits predictions about the distribution and abundance of parasites. Lastly, parasites that influence food web structure by regulating the abundance of dominant host species are briefly considered in addition to the effects of pollution and exploitation on food webs and parasite transmission.

Are populations of coral reef fish open or closed

Abstract: Dispersal plays a crucial role in several aspects of the biology, management and conservation of many species, including coral reef fish and other demersal marine organisms with pelagic larval stages. To know the origin of propagules that replenish benthic populations is a major challenge, yet, whereas earlier studies emphasized the broadly extensive dispersal of reef fish larvae, recent publications have emphasized the extent to which these larvae succeed in returning to their natal populations. Here, we critically analyse the evidence concerning the dispersal of coral reef fish, and conclude that: (1) at present, the extent to which reef fish populations are open or closed must be regarded as unknown; and (2) further improved research is likely to confirm that larval dispersal structures populations into more or less open populations depending on the particular attributes of species, physical oceanographical systems in which they occur and the scale at which the question is posed.

Long-term trends in the trophic structure of the North Sea fish community: evidence from stable-isotope analysis, size-spectra and community metrics

Abstract: Fishing has wide-ranging impacts on marine ecosystems. One of the most pervasive signs of intensive fishing is "fishing down the food web", with landings increasingly dominated by smaller species from lower trophic levels. Decreases in the trophic level of landings are assumed to reflect those in fish communities, because size-selective mortality causes decreases in the relative abundance of larger species and in mean body size within species. However, existing analyses of fishing impacts on the trophic level of fish communities have focused on the role of changes in species composition rather than size composition. This will provide a biased assessment of the magnitude of fishing impacts, because fishes feed at different trophic levels as they grow. Here, we combine body size versus trophic level relationships for North Sea fishes (trophic level assessed using nitrogen stable-isotope analysis) with species–size–abundance data from two time-series of trawl-survey data (whole North Sea 1982–2000, central and northern North Sea 1925–1996) to predict long-term trends in the trophic structure of the North Sea fish community. Analyses of the 1982–2000 time-series showed that there was a slow but progressive decline in the trophic level of the demersal community, while there was no trend in the trophic level of the combined pelagic and demersal community. Analyses of the longer time-series suggested that there was no trend in the trophic level of the demersal community. We related temporal changes in trophic level to temporal changes in the slopes of normalised biomass size-spectra (which theoretically represent the trophic structure of the community), mean log2 body mass and mean log2 maximum body mass. While the size-based metrics of community structure showed long-term trends that were consistent with the effects of increased fishery exploitation, these trends were only correlated with trophic level for the demersal community. Our analysis suggests that the effects of fishing on the trophic structure of fish communities can be much more complex than previously assumed. This is a consequence of sampled communities not reflecting all the pathways of energy transfer in a marine ecosystem and of the absence of historical data on temporal and spatial changes in the trophic level of individuals. For the North Sea fish community, changes in size structure due to the differential effects of fishing on species and populations with different life histories are a stronger and more universal indicator of fishing effects than changes in mean trophic level.

Mercury accumulation in the fish community of a sub‐Arctic lake in relation to trophic position and carbon sources

Abstract: Summary 1 Stable isotope analysis has improved understanding of trophic relationships among biota. Coupled with contaminant analysis, stable isotope analysis has also been used for tracing the pattern and extent of biomagnification of contaminants in aquatic food webs. 2 Combined analysis of nitrogen (δ 15 N) and carbon (δ 13 C) isotopes from fish species in a sub-Arctic lake were related to tissue mercury (Hg) concentrations to assess whether carbon sources influenced Hg accumulation in fish, in addition to trophic position. 3 Statistical models were used to estimate Hg biomagnification and uptake, to elucidate Hg accumulation dynamics and to appraise the relative importance of Hg exposure routes for the fish species. 4 Species Hg contamination increased as a function of trophic position (δ 15 N) and was inversely related to the δ 13 C signature. Species connected to the benthic food chain had lower Hg concentrations than species connected to the pelagic food chain. Species undergoing ontogenetic dietary shifts with increasing size, e.g. lake trout Salvelinus namaycush , also showed increased Hg concentrations with increasing reliance on pelagic fish as prey. 5 The results indicate that both vertical (trophic) and horizontal (habitat) food web structure influence Hg concentrations in fish tissue. 6 The biomagnification and uptake models indicated that contamination at the base of the food chain in the lake exceeded estimates for more southerly environments, thereby demonstrating the importance of dietary and water column Hg exposure routes in the sub-Arctic for determining Hg concentrations in fish. 7 Overall, the data reported here demonstrate how a combination of ecological concepts (food webs), developing ecological methods (stable isotopes) and environmental geochemistry can combine profitably to indicate the risks of exposure to environmental contaminants. Additional studies of the dynamics of Hg accumulation in the food webs of sub-Arctic lakes are needed, particularly in the light of the estimated high biomagnification rates and the heavy reliance of Inuit communities on subsistence fish harvests.

Pelagic food web configurations at different levels of nutrient richness and their implications for the ratio fish production:primary production

Abstract: Based on existing knowledge about phytoplankton responses to nutrients and food size spectra of herbivorous zooplankton, three different configurations of pelagic food webs are proposed for three different types of marine nutrient regimes: (1) upwelling systems, (2) oligotrophic oceanic systems, (3) eutrophicated coastal systems. Up-welling systems are characterised by high levels of plant nutrients and high ratios of Si to N and R. Phytoplankton consists mainly of diatoms together with a subdominant contribution of flagellates. Most phytoplankton falls into the food spectrum of herbivorous, crustacean zooplankton. Therefore, herbivorous crustaceans occupy trophic level 2 and zooplanktivorous fish occupy trophic level 3. Phytoplankton in oligotrophic, oceanic systems is dominated by picoplankton, which are too small to be ingested by copepods. Most primary production is channelled through the ‘microbial loop’ (picoplankton — heterotrophic nanoflagellates — ciliates). Sporadically, pelagic tunicates also consume a substantial proportion of primary production. Herbivorous crustaceans feed on heterotrophic nanoflagellates and ciliates, thus occupying a food chain position between 3 and 4, which leads to a food chain position between 4 and 5 for zooplanktivorous fish. By cultural eutrophication, N and P availability are elevated while Si remains unaffected or even declines. Diatoms decrease in relative importance while summer blooms of inedible algae (Phaeocystis, toxic dinoflagellates, toxic prymnesiophyceae, etc.) prevail. The spring bloom may still contain a substantial contribution of diatoms. The production of the inedible algae enters the pelagic energy flow via the detritus food chain: DOC release by cell lysis — bacteria — heterotrophic nanoflagellates — ciliates. Accordingly, crustacean zooplankton occupy food chain position 4 to 5 during the non-diatom seasons. Ecological efficiency considerations lead to the conclusion that fish production:primary production ratios should be highest in upwelling systems and substantially lower in oligotrophic and in culturally eutrophicated systems. Further losses of fish production may occur when carnivorous, gelatinous zooplankton (jellyfish) replace fish.

Long‐term changes in the trophic level of the Celtic Sea fish community and fish market price distribution

Abstract: Summary 1. The intensive exploitation of fish communities often leads to substantial reductions in the abundance of target species, with ramifications for the structure and stability of the ecosystem as a whole. 2. We explored changes in the mean trophic level of the Celtic Sea (ICES divisions VII f‐j) fish community using commercial landings, survey data and estimates of trophic level derived from the analysis of nitrogen stable isotopes. 3. Our analyses showed that there has been a significant decline in the mean trophic level of survey catches from 1982 to 2000 and a decline in the trophic level of landings from 1946 to 1998. 4. The decline in mean trophic level through time resulted from a reduction in the abundance of large piscivorous fishes and an increase in smaller pelagic species which feed at a lower trophic level. 5. Similar patterns of decline in the trophic level of both catches and landings imply that there have been substantial changes in the underlying structure of the Celtic Sea fish community and not simply a change in fishery preferences. 6. We suggest that the reported changes in trophic structure result from reductions in the spawning stock biomass of traditional target species associated with intensive fishing, together with long-term climate variability. 7. The relative distribution of fish market prices has changed significantly over the past 22 years, with high trophic level species experiencing greater price rises than lower trophic level species. 8. Although decreased abundance of high trophic level species will ultimately have negative economic consequences, the reduction in mean trophic level of the fish community as a whole may allow the system to sustain higher fishery yields. 9. Management objectives in this fishery will depend on the relative values that society attaches to economic profit and protein production.

Evolution of local recruitment and its consequences for marine populations

Abstract: Advantages of dispersal on the scales that are possible in a long pelagic larval period are not apparent, even for benthic species. An alternative hypothesis is that wide dispersal may be an incidental byproduct of an ontogenetic migration from and then back to the parental habitat. Under this hypothesis, the water column is a better habitat than the bottom for early development. Because the parental area is often an especially favorable habitat for juveniles and adults, selection may even favor larval retention or larval return rather than dispersal. Where larval capabilities and currents permit, a high percentage of recruits may then be produced from local adults. Expected consequences of a high proportion of local recruitment are stronger links between stock and recruitment, greater vulnerability to recruitment overfishing and local modifications of habitat, greater local benefits from fishery reserves, and possibly more localized adaptation within populations. Export of some larvae is consistent with a high proportion of retained or returning larvae, could stabilize populations linked by larval exchange, and provide connectivity between marine reserves. Even a small amount of larval export could account for the greater gene flow, large ranges, and long evolutionary durations seen in species with long pelagic larval stages.

Oceanographic habitats of two sympatric North Pacific albatrosses during the breeding season

Abstract: We characterized the movements and oceanographic habitats of black-footed (Phoebas- tria nigripes) and Laysan (P. immutabilis) albatrosses during the brooding and the rearing periods of the breeding cycle. Analyses of satellite telemetry data in conjunction with remotely sensed sea sur- face temperature and chlorophyll concentrations revealed substantial differences in habitat use between these 2 sympatrically breeding species. During the brooding period, black-footed albatross restricted their foraging to tropical waters (>20°C), while Laysan albatross ventured into the colder waters of the Transition Domain (15 to 12°C) and the Subarctic Frontal Zone (12 to 10°C). This pelagic segregation became more apparent with the expansion of the foraging ranges later in the breeding season. During the chick-rearing period, black-footed albatross commuted to the California Current (15 to 12°C) and Laysan albatross foraged in subarctic (<12°C) and Transition Domain (15 to 12°C) waters. The foraging behavior of albatrosses was scale-dependent. Over macro-mega scales of (1000 to 3000 km) albatross dispersion was influenced by large-scale ocean productivity patterns and water mass distributions. Over smaller coarse-meso scales of (10 to 100 km) albatrosses focused their forag- ing activities along oceanic habitats characterized by elevated ocean productivity and prey aggrega- tion. The foraging birds traveled more slowly in the vicinity of highly productive continental shelves (central California to Washington State, Aleutian Islands), and hydrographic fronts (Transition Domain, North Pacific Transition Zone Chlorophyll Front). Conversely, the satellite tracked alba- trosses commuted rapidly over tropical and subtropical waters between these foraging areas and the breeding colony. These results highlight the significance of macro-mega scale of (1000 to 3000 km) water mass distributions and coarse-meso scale (10 to 100 km) hydrographic features to far-ranging marine predators, and underscore the need to understand how physical-biological processes sustain predictable regions of elevated ocean productivity and prey aggregation in marine systems.

Effects of habitat and food resources on morphology and ontogenetic growth trajectories in perch

Abstract: Studies on resource polymorphism have mainly been considered at the end stage of ontogeny, whereas many species undergo diet changes as they grow We conducted a field survey to analyze the role of adaptive variation during ontogeny in Eurasian perch (Perca fluviatilis) We caught perch from the littoral and pelagic zones of a lake to investigate whether perch differ in morphology and diet between these habitats We also investigated whether there were any differences in morphological trajectories during the ontogeny of perch collected from the two habitats We found that perch caught in the littoral habitat, independently of size, had a deeper body, larger head and mouth and longer fins than perch caught in the pelagic zone Macroinvertebrates and fish dominated the diet of littoral perch, whereas the diet of the pelagic perch consisted mainly of zooplankton and to some extent fish Independently of size, the more streamlined individuals had a larger proportion of zooplankton and a smaller proportion of macroinvertebrates in their diet than the deeper-bodied individuals, indicating a relation between diet and morphology Some morphological characters followed different ontogenetic trajectories in the two habitats; eg the changes to a deeper body and a larger head were faster in the littoral than in the pelagic perch The relationship between the length of perch and the size of the mouth and fins also differed between perch from the two habitats, where the increase in the length of the pelvic fin and the area of the mouth increased faster with size in the littoral perch Our findings show that variation in morphology between habitats differs during ontogeny in a way that corresponds to functional expectations for fish species that occupy these habitats

Attraction of wild fish to sea-cage fish farms in the south-western Mediterranean Sea: spatial and short-term temporal variability

Abstract: Aggregations of wild fish were counted around 9 floating sea-cage fish farms along a 300 km stretch of the Spanish coastline in the south-western Mediterranean Sea. Each fish farm cul- tivated Sparus aurata and Dicentrarchus labrax in 6 to 16 floating sea cages between 10 m and 7.4 km from the coast. During September and October 2001, assemblages of fish were counted on 3 separate days at each of 9 farms. Six 5 min rapid visual counts using SCUBA and covering 11 250 m 3 were per- formed within each farm complex and at open water control sites 200 m distant from farms. Abun- dance (52 to 2837× ), biomass (2.8 to 1126 × ) and number of species (1.6 to 14 × ) were greater in fish farm counts than control counts at all locations. Twenty-seven species were recorded at fish farms, with 2 families, Sparidae (12 species) and Carangidae (4 species), being particularly abundant. Over 85% of farm-associated fish were of adult size. Assemblages of wild fish differed greatly between farms separated by 10s to 100s of km, although there was some evidence to suggest that similar assemblages occur at farms separated by 100s of m to several km. Abundance, biomass and number of species differed among fish farms, with all 3 variables negatively correlated with distance of farms from shore and positively correlated with size of farms. Limited variability of wild fish assemblages and abundance of the dominant taxa at each farm among times sampled indicated some degree of temporal stability on a scale of several weeks. Due to the strong aggregative effect of fish farms, pos- sible residence of fishes for periods of weeks to months and the prohibition of fishing within farm leasehold areas, we suggest that coastal sea-cage fish farms may act as small (up to 160 000 m 2 ), pelagic marine protected areas (MPAs). Furthermore, at farms where wild fish are abundant, ecolog- ical interactions that may influence both wild fish stocks and the impact of farms must be considered.

Parasites as biological tags in population studies of marine organisms: an update

Abstract: This paper reviews the work published over the past decade on the use of parasites as biological tags in population studies of marine fish, mammals and invertebrates. Fish hosts are considered in taxonomic and ecological groups as follows: demersal, anadromous, small pelagic, large pelagic and elasmobranch. Most studies were carried out on demersal fish, particularly on members of the genera Merluccius (hake), Sebastes (rockfish) and on Atlantic cod Gadus morhua L., but Pacific salmonids and small pelagic fish of the genus Trachurus are also well-represented. A current multidisciplinary study of the population biology of horse mackerel Trachurus trachurus in European waters, which includes the use of parasites as tags, is described. Two studies recognize the potential for using parasites as tags for cetaceans but, in spite of the considerable potential for this approach in population studies of elasmobranchs, no original study has been carried out on this group for over ten years. Studies of parasites as tags for marine invertebrates have concentrated on squid. Recent trends in the use of parasites as biological tags for marine hosts are discussed.

Larval growth predicts the recruitment success of a coral reef fish

Abstract: While growth rates of pelagic larvae have been argued to be one of the principal determinants of the recruitment success of temperate marine fishes, it is not known if this is the case in the tropics. Here, we use lar- val growth histories derived from otoliths of a Caribbean reef fish to show that monthly variation in the intensity of settlement and recruitment of pelagic juveniles onto reefs is positively correlated with variation in growth rates 1-2 weeks after larvae begin feeding. Our results suggest that the processes thought to underlie recruitment of marine fishes in temperate regions may also operate in the tropics and contrasts with current research on the causes of recruitment variability in coral reef fishes, which emphasises the role of larval transport.

Bacteria and Foraminifera: key players in a short-term deep-sea benthic response to phytodetritus

Abstract: The deep-sea floor has long been considered a 'food desert' but recent observations suggest that episodic inputs of relatively fresh organic matter (phytodetritus) occur and that benthic processing of this material may be rapid. Although the responses of the total community in terms of oxygen consumption and of some individual benthic groups have been identified, the quantitative role of the different groups in the short-term response remains largely unknown. We examined the short-term response in major benthic compartments in an in situ experiment in the NE Atlantic (2170 m water depth) using 13 C-enriched diatoms as a tracer of labile carbon. Within 35 h, 6 mg C m -2 was processed by the benthos, with the majority of the processed carbon recorded as respiration (45%). Among the fauna retained on a 300 µm sieve, Foraminifera were rapid consumers which, together with Bacteria, accounted for 50% of the processing. Therefore, although Bacteria dominate long-term carbon mineralization (as suggested by their general dominance in the benthic biomass), some faunal components, in this case Foraminifera, may play a central role in the rapid initial pro- cessing of fresh organic carbon in deep-sea sediments.

Blue mussels, Mytilus edulis, at the edge of the range: population structure, growth and biomass along a salinity gradient in the north-eastern Baltic Sea

Abstract: The blue mussel, Mytilus edulis, is the most conspicuous animal species in the northern Baltic rocky sublittoral. In the studied area the species lives at the margin of its salinity tolerance. Although dwarfed by the low-salinity conditions, blue mussels in the northern Baltic are very abundant and have a decisive role in the benthic and pelagic ecosystems. We studied abundance, size distribution, biomass and growth rate of blue mussels along a 270 km salinity gradient in the northern Baltic Sea. Samples (n=317, 1998–1999) from moderately exposed and exposed rocky shores at seven study areas were collected in the southern Archipelago Sea in the west and into the central Gulf of Finland in the east, where the species is becoming increasingly rare. The results show a marked decline in mean mussel size and biomass from the saline west to the less saline east. The growth rate also decreased with lower ambient salinity. However, abundance of small mussels was considerably higher in the central and eastern parts of the study area. Vertically, the highest biomass was recorded at intermediate depths (5 and 8 m), being lower at both shallower (3 m) and deeper bottoms (12 m). It is concluded that salinity is the foremost factor determining size structure and growth rate among populations within the area. The results suggest that predation further influences the population structure of blue mussels living at the edge of their range in the central Gulf of Finland ultimately set by their salinity tolerance.

Spawning on the edge: spawning grounds and nursery areas around the southern African coastline

Abstract: The southern African coastline is dominated by strong currents. Along the eastern seaboard, the warm western boundary Agulhas Current sweeps close inshore along the shelf edge before diverging from the coast on the Agulhas Bank and retroflecting back into the Indian Ocean. On the western seaboard, strong jet currents develop in the southern Benguela, associated with the strong thermal gradients induced by upwelling and Agulhas Current intrusions and eddies. There is, in general, northward drift of surface waters in the Benguela Current with strong offshore losses in the vicinity of an exceptionally active upwelling region off Luderitz. Several potent mechanisms exist for offshore dispersal and loss from the productive shelf waters, such as eddies, filaments, retroflections and offshore Ekman drift, which pose special problems for successful retention of planktonic eggs and larvae from broadcast spawners. Most fish species in southern Africa have evolved highly selective reproductive patterns, which ensure that sufficient progeny are retained or can enter the nursery grounds along the coastline. Four important reproductive habitats, comprising spawning areas, transport mechanisms and nursery grounds, occur between Mocambique and Angola. These are used by a wide variety of pelagic, demersal and inshore-dwelling fish species.

Large-scale species-richness gradients in the Atlantic Ocean.

Abstract: The increase in species richness from the poles to the Equator has been observed in numerous terrestrial and aquatic taxa. A number of different hypotheses have been put forward as explanations for this trend, e.g. area and energy availability. However, whether these hypotheses apply to large spatial scales in marine environments remains unclear. The present study shows a clear latitudinal gradient from high to low latitude (from 80 degrees N to 70 degrees S) in marine species richness for 6643 species (fishes and invertebrates) in 10 different taxa dwelling in benthic and pelagic habitats on both sides of the Atlantic. The patterns in benthic taxa are strongly influenced by coastal hydrographic processes, with marked peaks and troughs, and consequently the gradients are not symmetric along both Atlantic sides. Pelagic taxa show a plateau-shaped distribution and the influence from coastal events on gradients could not be demonstrated. The relationships between species richness and different environmental factors indicate that area size does not explain the latitudinal pattern in benthic species richness on a large spatial scale. Sea-surface temperature (positive relationship) is the best predictor of this pattern for benthic species, and nitrate concentration (negative relationship) is the best predictor for pelagic species. The results call into question the existence of a single primary cause that would explain the pattern in marine species richness on a large spatial scale.

The Role of Sharks and Longline Fisheries in a Pelagic Ecosystem of the Central Pacific

Abstract: The increased exploitation of pelagic sharks by longline fisheries raised questions about changes in the food webs that include sharks as apex predators. We used a version of Ecopath/Ecosim models to evaluate changes in trophic interactions due to shark exploitation in the Central North Pacific. Fisheries targeted on blue sharks tend to produce compensatory responses that favor other shark species and billfishes, but they have only modest effects on the majority of food web components. Modest levels of intraguild predation (adult sharks that eat juvenile sharks) produce strong, nonlinear responses in shark populations. In general, analysis of the Central North Pacific model reveals that sharks are not keystone predators, but that increases in longline fisheries can have profound effects on the food webs that support sharks.

The role and contribution of the seagrass Posidonia oceanica (L.) Delile organic matter for secondary consumers as revealed by carbon and nitrogen stable isotope analysis

Abstract: The δ 13 C and δ 15 N values of primary producers and consumers were studied to obtain information on the trophic role of Posidonia oceanica L. Delile, the dominant primary producer, in a Mediterranean shallow environment (the Stagnone di Marsala, western Sicily). δ 13 C strongly discriminated between pelagic and benthic pathways, with the former based on phytoplankton and the latter on a mixed pool of seagrass detritus, epiphytes and benthic algae as carbon sources. A particularly important trophic role appears to be performed by the vegetal epiphytic community on seagrass leaves (δ 13 C = –14.9 ± 0.1‰), which supports most of the faunal seagrass community (i.e. Amphipoda, Isopoda, Tanaidacea; δ 13 C = –14.9 ± 0.1‰, –12.5 ± 0.1‰ and –14.8 ± 1.0‰, respectively). Although P . oceanica (δ 13 C = –11.3 ± 0.3‰) does not seem to be utilised by consumers via grazing (apart from a few Palaemonidae species with δ 13 C value of –10.8 ± 1.8‰), its trophic role may be via detritus. P . oceanica detritus may be exploited as a carbon source by small detritivore invertebrates, and above all seems to be exploited as a nitrogen reservoir by both bottom and water column consumers determining benthic–pelagic coupling. At least three trophic levels were detected in both the pelagic (mixture of phytoplankton and cyanobacteria, zooplankton, juvenile transient fish) and benthic (sedimentary organic matter and epiphytes, small seagrass-associated invertebrates, larger invertebrates and adult resident fish) pathways.

CHAPTER 7 – Larval Dispersal and Retention and Consequences for Population Connectivity

Abstract: The larval stage of most coral reef fishes is spent in the pelagic environment, away from the reef proper. Survival at this stage is tenuous, being mediated by factors such as food availability, predator abundance, and physical conditions. The complex biological and physical interactions of these factors can result in a seemingly stochastic larval supply that drives temporal and spatial variation in recruitment intensity. Although fish larvae are often considered strict constituents of the zooplankton community, evidence suggests that many species exhibit some form of active behavior during their pelagic stage. Variability in larval transport is determined by the interaction of water masses and the effects of external forces such as winds and tides. Active behavior by larvae may modulate some of this variability, yet a strong change in the direction or intensity of flow of a particular water mass may result in a substantial change in larval supply to a given reef or island, or may carry larvae away from a suitable food environment. The scale over which larval transport or retention occurs varies substantially among species and even among locations within a single species. For some species, larval dispersal is minimal and possibly nonexistent, whereas at the other extreme dispersal can occur over thousands of kilometers. For most coral reef fish species, with larval durations of weeks to months, transport will usually be on the scale of tens to hundreds of kilometers.

Salp/krill interactions in the Southern Ocean:spatial segregation and implications for the carbon flux

Abstract: Available data on the spatial distribution and feeding ecophysiology of Antarctic krill, Euphausia superba, and the tunicate, Salpa thompsoni, in the Southern Ocean are summarized in this study. Antarctic krill and salps generally display pronounced spatial segregation at all spatial scales. This appears to be the result of a clear biotopical separation of these key species in the Antarctic pelagic food web. Krill and salps are found in different water masses or water mass modifications, which are separated by primary or secondary frontal features. On the small-scale (o100 km), Antarctic krill and salps are usually restricted to the specific water parcels, or are well segregated vertically. Krill and salp grazing rates estimated using the in situ gut fluorescence technique are among the highest recorded in the Antarctic pelagic food web. Although krill and salps at times may remove the entire daily primary production, generally their grazing impact is moderate (p50% of primary production). The regional ecological consequences of years of high salp densities may be dramatic. If the warming trend, which is observed around the Antarctic Peninsula and in the Southern Ocean, continues, salps may become a more prominent player in the trophic structure of the Antarctic marine ecosystem. This likely would be coupled with a dramatic decrease in krill productivity, because of a parallel decrease in the spatial extension of the krill biotope. The high Antarctic regions, particularly the Marginal Ice Zone, have, however, effective physiological mechanisms that may provide protection against the salp invasion. r 2002 Elsevier Science Ltd. All rights reserved.

Copepoda-Cladocera-Tunicata : The role of three major mesozooplankton groups in pelagic food webs

Abstract: Copepods, cladocerans and tunicates form major groups of herbivorous mesozooplankton. The former two are found in fresh and marine waters, while the latter are restricted to marine systems. In the present review, we compile existing ecophysiological knowledge about between-group differences in metabolic and reproductive rates, feeding selectivity and elemental composition. From this, we derive predictions about their impact on the lower trophic levels (phytoplankton and microbial food web) and predictions about their prevalence under different ecological conditions (e.g. nutrient richness, Si : N ratio, phytoplankton size structure and top-down control).

Trophic ecology of Atlantic cod Gadus morhua on the northeast US continental shelf

Abstract: Atlantic cod Gadus morhua is an extremely important fish in the northern hemisphere with respect to culture, economics, and ecology. However, the effects of over-fishing, environmental change, fish community dynamics and other factors that have altered the trophic ecology of cod are not well understood. We present an analysis of the trophic patterns of cod in the northeast US shelf ecosystem from a 25 yr time series of food habits data. Additionally, we compared the diet of this spe- cies with the spatio-temporal distribution of its prey species and evaluated prey preference over time. Atlantic cod exhibit an omnivorous diet; we assessed the temporal, spatial, and ontogenetic trends in this diet by examination of >15000 stomachs. Ontogenetic shifts in diet were observed; early juve- niles consumed more pelagic than benthic invertebrates, medium cod consumed benthic inverte- brates and fish, and larger cod consumed larger amounts of fish. Cannibalism also increased with ontogeny. Diet shifted significantly over a period of 3 decades, concurrent with changes in forage species abundance and distribution. Most of the major prey species were eaten in periods when they had high spatio-temporal overlap with cod and were abundant, indicating opportunistic feeding by cod. Similar to many other ecosystems, cod prefer sand lance, Cancer spp. crabs and herring, regard- less of the abundance or spatio-temporal overlap with these prey species. It is unclear whether the observed changes in the trophic dynamics of cod have broad implications for cod populations, yet the evidence does suggest that cod are not likely to influence the abundance and distribution of their prey populations in this ecosystem.