Showing papers on "Pelagic zone published in 2005"

Eutrophication of Chesapeake Bay: historical trends and ecological interactions

Abstract: This review provides an integrated synthesis with timelines and evaluations of ecological responses to eutrophi- cation in Chesapeake Bay, the largest estuary in the USA. Analyses of dated sediment cores reveal initial evidence of organic enrichment in ~200 yr old strata, while signs of increased phytoplankton and decreased water clarity first appeared ~100 yr ago. Severe, recurring deep-water hypoxia and loss of diverse submersed vascular plants were first evident in the 1950s and 1960s, respectively. The degradation of these benthic habitats has contributed to declines in benthic macro- infauna in deep mesohaline regions of the Bay and blue crabs in shallow polyhaline areas. In contrast, copepods, which are heavily consumed in pelagic food chains, are relatively un- affected by nutrient-induced changes in phytoplankton. Intense mortality associated with fisheries and disease have caused a dramatic decline in eastern oyster stocks and associated Bay water filtration, which may have exacerbated eutrophication effects on phytoplankton and water clarity. Extensive tidal marshes, which have served as effective nutrient buffers along the Bay margins, are now being lost with rising sea level. Although the Bay's overall fisheries production has probably not been affected by eutrophication, decreases in the relative contribution of demersal fish and in the efficiency with which primary production is transferred to harvest suggest funda- mental shifts in trophic and habitat structures. Bay ecosystem responses to changes in nutrient loading are complicated by non-linear feedback mechanisms, including particle trapping and binding by benthic plants that increase water clarity, and by oxygen effects on benthic nutrient recycling efficiency. Observations in Bay tributaries undergoing recent reductions in nutrient input indicate relatively rapid recovery of some ecosystem functions but lags in the response of others.

Electronic tagging and population structure of Atlantic bluefin tuna

Abstract: The Atlantic bluefin tuna is at the centre of an international debate in fisheries conservation. Last summer the western Atlantic bluefin tuna fishery collapsed and some conservationists say it will not recover unless the International Commission for Conservation of Atlantic Tunas adopts a tougher regime. Results from an electronic tagging programme on bluefin tuna are presented this week, and they make alarming reading. Western tagged bluefin tuna are shown to migrate freely across the international stock boundary into the eastern Atlantic, where they are vulnerable to European fisheries; and both known spawning grounds, in the Gulf of Mexico and the Mediterranean Sea, are shown to be linked to the endangered western Atlantic fishery. Electronic tags that archive or transmit stored data to satellites have advanced the mapping of habitats used by highly migratory fish in pelagic ecosystems1,2,3,4,5,6. Here we report on the electronic tagging of 772 Atlantic bluefin tuna in the western Atlantic Ocean in an effort to identify population structure. Reporting electronic tags provided accurate location data7,8,9 that show the extensive migrations of individual fish (n = 330). Geoposition data delineate two populations, one using spawning grounds in the Gulf of Mexico and another from the Mediterranean Sea. Transatlantic movements of western-tagged bluefin tuna reveal site fidelity to known spawning areas in the Mediterranean Sea. Bluefin tuna that occupy western spawning grounds move to central and eastern Atlantic foraging grounds. Our results are consistent with two populations of bluefin tuna with distinct spawning areas that overlap on North Atlantic foraging grounds. Electronic tagging locations, when combined with US pelagic longline observer and logbook catch data, identify hot spots for spawning bluefin tuna in the northern slope waters of the Gulf of Mexico. Restrictions on the time and area where longlining occurs would reduce incidental catch mortalities on western spawning grounds.

The relationship between fish species richness, abundance and habitat complexity in a range of shallow tropical marine habitats

Abstract: A simple habitat assessment score (HAS) was designed to assess habitat complexity across several different shallow tropical marine habitats including sandy patches, algal beds, seagrass beds and reefs. It measured rugosity, variety of growth forms, height, refuge size categories, percentage live cover and percentage hard substratum. Multiple regression models using HAS variables as predictors accounted for 71 and 22% of the variation in observed species richness and total fish abundance respectively. The two most important predictors of observed species richness were rugosity and variety of growth forms, while height was the most important predictor of total fish abundance. The HAS method worked consistently across a variety of habitat types and the complexity map closely mirrored the map of observed species richness, reflecting the patchy habitat mosaic of shallow tropical marine areas. Stations at the mouth of an enclosed lagoon, however, had a higher number of species than might have been expected judging from the habitat complexity scores. It is possible that this was linked to the preferential settling of pelagic fish larvae in this area as tidal water exchanges between the bay and the reef were funnelled through one small gap. This study highlights the need for fish biodiversity studies to take habitat complexity into account.

Environmental correlates of population differentiation in Atlantic herring.

Abstract: The marine environment is characterized by few physical barriers, and pelagic fishes commonly show high migratory potential and low, albeit in some cases statistically significant, levels of genetic divergence in neutral genetic marker analyses. However, it is not clear whether low levels of differentiation reflect spatially separated populations experiencing gene flow or shallow population histories coupled with limited random genetic drift in large, demo- graphically isolated populations undergoing independent evolutionary processes. Using information for nine micro- satellite loci in a total of 1951 fish, we analyzed genetic differentiation among Atlantic herring from eleven spawning locations distributed along a longitudinal gradient from the North Sea to the Western Baltic. Overall genetic differ- entiation was low ( u5 0.008) but statistically significant. The area is characterized by a dramatic shift in hydrography from the highly saline and temperature stable North Sea to the brackish Baltic Sea, where temperatures show high annual variation. We used two different methods, a novel computational geometric approach and partial Mantel correlation analysis coupled with detailed environmental information from spawning locations to show that patterns of reproductive isolation covaried with salinity differences among spawning locations, independent of their geograph- ical distance. We show that reproductive isolation can be maintained in marine fish populations exhibiting substantial mixing during larval and adult life stages. Analyses incorporating genetic, spatial, and environmental parameters indicated that isolating mechanisms are associated with the specific salinity conditions on spawning locations.

From wind to whales: trophic links in a coastal upwelling system

Abstract: Blue whales Balaenoptera musculus meet the highest prey demands of any predator that has ever existed by feeding exclusively upon dense but patchy schools of pelagic euphausiids. We examined the role that seasonally high primary production supported by coastal upwelling com- bined with topographic breaks off California play in creating, collecting, and maintaining euphausi- ids at densities sufficient to allow exploitation by whales. We used concurrent ship- and mooring- based oceanographic, hydroacoustic, and net sampling, whale-sighting records, visual surveys, and time-depth recorder deployment to examine temporal and spatial linkages between (1) intensity of upwelling, (2) primary production, (3) development, density and distribution of euphausiids, and (4) the distribution, abundance, and foraging behavior of blue whales in Monterey Bay, California between 1992 and 1996. Blue whales fed exclusively upon adult euphausiids Thysanoessa spinifera and Euphausia pacifica that were larger than those generally available in the Bay. Foraging whales dove repeatedly to dense euphausiid aggregations between 150 and 200 m on the edge of the Mon- terey Bay Submarine Canyon. Euphausiid aggregations where whales were foraging averaged 153 g m -3 , approximately 2 orders of magnitude greater than mean euphausiid densities in the Bay (1.3 g m -3 ). High euphausiid densities are supported by high primary production between April and August (249 mgC m -3 d -1 ) and a submarine canyon that provides deep water down-current from an upwelling region. Peak euphausiid densities occur in late summer/early fall, lagging the seasonal increase in primary production by 3 to 4 mo. This lag results from the temporal development of euphausiids spawned around the spring increase in primary production and the shoreward collapse of productivity due to decreased upwelling in late summer. The migratory movements of the Califor- nia blue whale probably reflect seasonal patterns in productivity in other foraging areas similar to those we describe for Monterey Bay.

Fishing methods to reduce sea turtle mortality associated with pelagic longlines

Abstract: Changes in hook design and bait type were investigated as measures to reduce the bycatch of sea turtles on pelagic longlines in the western North Atlantic Ocean. Specifically, the effectiveness of ...

Phytoplankton community ecology: principles applied in San Francisco Bay

Abstract: In his seminal 1961 paper 'The paradox of the plankton' Am Nat 95:137-147, G E Hutchinson asked why many species of phytoplankton can coexist while competing for a small num- ber of limiting resources in an unstructured habitat Hutchinson anticipated the resolution of his paradox, recognizing that communities are organized by processes beyond resource competition including species interactions, habitat variability and dispersal Since 1961 we have made funda- mental discoveries that have revolutionized our conceptual understanding of pelagic ecology, includ- ing (1) habitat heterogeneity at all scales relevant to plankton population dynamics, (2) community shifts in response to global climate cycles, (3) fast and selective predation as a powerful top-down force to shape phytoplankton communities, (4) turbulent mixing as a physical process that selects species on the basis of their size and form, (5) mixotrophy that allows some algal species to tap organic nutrient pools and function at multiple trophic levels, (6) taxon-specific life cycles including alternating vegetative and resting stages, and (7) the pelagic as an open system where communities are continually reshaped by species immigration Here we synthesize these discoveries to show how they validate and amplify Hutchinson's hypothesis that phytoplankton communities are assembled by many processes Our synthesis is built around observations of phytoplankton species composition from a decade of study in San Francisco Bay, used as a case study to illustrate the contemporary prin- ciples of phytoplankton community ecology We apply these principles to address 2 central questions: (1) What processes assemble phytoplankton communities? (2) How does phytoplankton community composition influence ecosystem functions such as production in pelagic and benthic food webs?

On the origin of Antarctic marine benthic community structure.

Abstract: Environmental conditions fostering marine communities around Antarctica differ fundamentally from those in the rest of the world's oceans, particularly in terms of pronounced climatic fluctuations and extreme cold. Here, we argue that the rarity of pelagic larval stages in Antarctic marine benthic invertebrate species is a consequence of evolutionary temperature adaptation and that this has greatly contributed to the current structure of the Antarctic benthic community. In arguing this position, we challenge the likelihood of previously suggested survival strategies of benthic communities on the Antarctic continental shelf and slope during Cenozoic glacial periods. By integrating evidence from marine geology and geophysics, we suggest that the Antarctic continental shelf and slope were both unfavourable environments for benthic communities during glacial periods and that community survival was only possible in the deep sea or in shelters on the continental shelf as a result of the diachronism in maximum ice extent.

Cascading effects of overfishing marine systems

Abstract: Profound indirect ecosystem effects of overfishing have been shown for coastal systems such as coral reefs and kelp forests. A new study from the ecosystem off the Canadian east coast now reveals that the elimination of large predatory fish can also cause marked cascading effects on the pelagic food web. Overall, the view emerges that, in a range of marine ecosystems, the effects of fisheries extend well beyond the collapse of fish exploited stocks.

Shifts in open-ocean fish communities coinciding with the commencement of commercial fishing

Abstract: We identify changes in the pelagic fish community of the tropical Pacific Ocean by comparing recent data collected by observers on longline fishing vessels with data from a 1950s scientific survey when industrial fishing commenced. A major shift in size composition and indices of species abundance and community biomass accompanied the start of fishing. The largest and most abundant predators, such as sharks and large tunas, suffered the greatest declines in abundance (21% on average). They also showed striking reductions in mean body mass. For example, the mean mass of blue shark (Prionace glauca) was 52 kg in the 1950s compared to 22 kg in the 1990s. The estimated abundance of this species was 13% of that in the 1950s. Overall, the biomass of large predators fell by a factor of 10 between the periods. By contrast, several small and formerly rare species increased in abundance, e.g., pelagic stingray (Dasyatis violacea). However, the increases in small species did not balance the reductions in the biomass of large predators. Of three possible explanations (fishing, environmental variation, and sampling bias), available evidence indicates fishing to be the most likely cause for the observed patterns.

Sustainable exploitation of small pelagic fish stocks challenged by environmental and ecosystem changes : a review

Abstract: Small pelagic fish contribute up to 50% of the total landing of marine species. They are most abundant in upwelling areas and contribute to food security. Exploited stocks of these species are prone to large interannual and interdecadal variation of abundance as well as to collapse. We discuss why small pelagic fish and fisheries are so “special” with regard to their biology, ecology, and behavior. Two adjectives can sum up the characteristics of pelagic species: variability and instability. Analyses of the relationships between small pelagic fish and their physical environment at different time-scales illustrate the complexity of the interplay between exploitation and environmental impacts. How small pelagic fish species are positioned and related within the trophic web suggests that these species play a central role in the functioning and dynamics of upwelling ecosystems. Finally, we discuss the sustainable exploitation of small pelagic fisheries through appropriate management, focusing on the resilience to exploitation, a comparison of different management options and regulatory mechanisms. We recommend that statistical, socio-economical, and political merits of a proposed two-level (shortand long-term) management strategy be undertaken. Despite constant progress in understanding the complex processes involved in the variability of pelagic stock abundance, especially at short and medium time scales, our ability to predict abundance and catches is limited, which in turn limits our capacity to properly manage the fisheries and ensure sustainable exploitation. Substantial progress can be expected from an integrated modeling approach of spatialized models coupling hydrodynamics, biogeochemical, and ecological processes. Although there are many definitions of “small pelagic fish,” this expression most commonly refers to shoaling epipelagic fish characterized by high horizontal and vertical mobility in coastal areas and which, as adults, are usually 10–30 cm in length. The upper limit is often the most debated, since some authors use the term “medium-sized pelagic fish” to designate larger fish ranging from about 20–60 cm (Bas et al., 1995). This distinction between the two size groups simultaneously allows distinctions to be made with regard to the position of the species within the food web: the conventional “small pelagic fish” includes typical forage species like sardine and anchovy preying on phytoplankton and/or micro-mesozooplankton, while the group of “medium-sized pelagic fish” includes mostly species from intermediate trophic levels like horse-mackerel, mackerels, and coastal tunas. When adult, the latter species prey mainly on macro-zooplankton, ichthyoplankton, and small fish or mollusks. In this work, we will focus mainly on the conventional “small pelagic fish,” but will also often refer to, or contrast findings with, medium-sized pelagic fish. When referring to both small and medium-sized pelagic fish, we will use the terms “pelagic fish” or “coastal pelagic fish” to separate them from the large oceanic tunas, although not all medium-sized pelagic fish remain on the continental shelf for the duration of their whole life cycle. BULLETIN OF MARINE SCIENCE, VOL. 76, NO. 2, 2005 386 Small pelagic fish are abundant in all oceans and seas except the Antarctic, where large euphausids (a schooling species of zooplankton) constituting the krill occupy a similar trophic level. According to FAO data (FAO, 2002; Fig. 1), small pelagic fish contributed up to 39 million t or 50% of the total landing of marine species. The ratio of pelagic/demersal catches varied from 1.0–2.1 over the last 51 yrs, with a positive trend at least partly related to overexploitation of the higher trophic levels (Pauly et al., 1998). Although these official landing statistics are likely to be inaccurate for some countries (Watson and Pauly, 2001), the relative values given here provide consistent trends during the last decades. Despite their usually low commercial value, small pelagic fish provide a substantial source of income for many developing countries due to their abundance, especially in the four major upwelling regions located on the eastern border of the Pacific and Atlantic Oceans. In these regions, a few species of small pelagic fish represent a large amount of the total fish biomass. This is typically the case for Peru (Peruvian anchoveta and sardine) and Chile (anchoveta, sardine and also horse-mackerel, a mediumsized pelagic species) in the Humboldt Current ecosystem, Mauritania and Senegal (sardine and sardinella) in the Canary Current ecosystem, Namibia and South Africa (sardine, anchovy and horse-mackerel) in the Benguela Current ecosystem and, to a lesser extent, Mexico (sardine and anchovy) in the California Current ecosystem. In this work we will often refer to these productive areas and others, such as the Kurushio-Oyashio Extension (Japanese sardine), the North Sea and the Norwegian Sea (the latter two being inhabited by several herring stocks) or the Gulf of Guinea (sardinellas). In some of these regions, coastal pelagic fish contribute substantially to food security, either directly (human consumption of fresh, smoked, or frozen fish) or indirectly (income for a component of society). Many examples of small pelagic fishery collapses occurred during the second half of the 20th century (Figs. 2A,B) and, in most cases, these collapses were clearly associated with an abrupt decline in the corresponding fish stock biomass with delayed reFigure 1. World pelagic and demersal marine fish catch from 1950–1999 (data source FAO, FishStat, www.fao.org). FREÓN ET AL.: SUSTAINABLE EXPLOITATION OF SMALL PELAGIC FISH STOCKS 387 covery, if any (Troadec et al., 1980; Beverton, 1990; Hutchings, 2000). Whether these biomass variations were mainly driven by natural variability (due to environmental changes and/or species interactions), exploitation, or both has been, and is still, debated at length (Lluch-Belda et al., 1989; 1992a; Schwartzlose et al., 1999). In this work we will try to shed more light on this issue and propose management strategies aimed at minimizing the risk of stock and fishery collapse. This introduction is followed by four sections and a final section of general conclusions and recommendations. In the first section we explain why small pelagic fish and fisheries are “unique” in terms of their biology, ecology, behavior, and commercial aspects. The second section gives details on the relationships between small pelagic fish and their physical environment at different time-scales. The third section indicates how small pelagic fish are related within the trophic web and presents some modeling approaches in this regard. Finally, the fourth section deals with the sustainable exploitation of small pelagic fisheries through appropriate management, focusing on the resilience to exploitation, different management options, and regulatory mechanisms. Figure 2. Catches of (A) sardine and (B) anchovy in the Pacific Ocean [source: Scientific Committee on Oceanic Researh Working Group (SCOR WP) 101]. BULLETIN OF MARINE SCIENCE, VOL. 76, NO. 2, 2005 388 WHY PELAGIC FISH AND FISHERIES ARE “UNIQUE” HABITAT By definition, pelagic fish live in the pelagic domain, that is, they move freely in the water-column where they spend most of their time. Most of the small pelagic fish species are epipelagic, that is, they live in the uppermost layer of the ocean, usually at depths of 0–200 m, although the limit of the deeper boundary varies according to the species and region. For instance, Sardinella aurita Valenciennes in Cuvier and Valenciennes, 1847 off the Gulf of Guinea migrate in deeper waters (>200 m) out of the upwelling season (Longhurst and Pauly, 1987). Many medium-sized pelagic fish are found in a wider range of depths, typically from the surface to 500 m. The view that pelagic fish are remote from the sea floor is largely incorrect. Firstly, a minority of pelagic species, such as herrings and capelins, are demersal spawners, laying adhesive eggs on the bottom (Blaxter and Hunter, 1982). Secondly, many pelagic species, especially medium-sized ones, are frequently found close to the bottom during daytime and are therefore vulnerable to semi-pelagic fishing gear and even to bottom trawls. Some of these species (e.g., horse-mackerel and mackerel) feed on or near the bottom, but for others, like the gilt sardine S. aurita, only the sediment found in their stomachs testifies to their bottom activity and suggests that they might make use of the bacterial film (Nieland, 1982). Therefore, the reason for large “pole shaped schools” (Petitgas and Levenez, 1996) several tens of meters high and of which only the base is in contact with the bottom for prolonged periods of time remains unclear. MORPHOLOGY Small pelagic fish are characterized by a streamlined body shape (fusiform and laterally compressed) and forked caudal fin. These characteristics make them good and fast swimmers, despite their small size. Another typical phenotypic trait of coastal pelagic fish is their discrete coloration, ranging from dark gray to silver. The flanks are usually highly reflective and the dorsal surface dark in order to render the fish inconspicuous to predators: light is reflected in such a way that it matches the background light against which the fish is viewed (Blaxter and Hunter, 1982). Pelagic fish are also capable of camouflage by adjusting the state of the melanophores on their dorsal surface. The body of most small pelagic fish species is fragile, comprised of soft tissue and a thin skin. Physical protection is provided by small scales that cover nearly the whole body in most species, and by the mucus produced by their skin. Scales and mucus are easily lost by manipulation or contact with fishing gears, leaving the thin skin exposed to wound infection by bacteria, fungal diseases, or external parasites. Small pelagic species are extremely frail

Scales of benthic–pelagic coupling and the intensity of species interactions: From recruitment limitation to top-down control

Abstract: Large and usually unpredictable variation in species interaction strength has been a major roadblock to applying local experimental results to large-scale management and conservation issues. Recent studies explicitly considering benthic-pelagic coupling are starting to shed light on, and find regularities in, the causes of such large-scale variation in coastal ecosystems. Here, we evaluate the effects of variation in wind-driven upwelling on community regulation along 900 km of coastline of the southeastern Pacific, between 29°S and 35°S during 72 months. Variability in the intensity of upwelling occurring over tens of km produced predictable variation in recruitment of intertidal mussels, but not barnacles, and did not affect patterns of community structure. In contrast, sharp discontinuities in upwelling regimes produced abrupt and persistent breaks in the dynamics of benthic and pelagic communities over hundreds of km (regional) scales. Rates of mussel and barnacle recruitment changed sharply at ≈32°-33°S, determining a geographic break in adult abundance of these competitively dominant species. Analysis of satellite images demonstrates that regional-scale discontinuities in oceanographic regimes can couple benthic and pelagic systems, as evidenced by coincident breaks in dynamics and concentration of offshore surface chlorophyll-a. Field experiments showed that the paradigm of top-down control of intertidal benthic communities holds only south of the discontinuity. To the north, populations seem recruitment-limited, and predators have negligible effects, despite attaining similarly high abundances and potential predation effects across the region. Thus, geographically discontinuous oceanographic regimes set bounds to the strength of species interactions and define distinct regions for the design and implementation of sustainable management and conservation policies.

Composition of fish communities in German lakes as related to lake morphology, trophic state, shore structure and human‐use intensity

Abstract: Summary 1. Previous comparative analyses of fish communities in European lakes have mainly focused on the response of community composition to eutrophication. In addition, frequently only one or two lake habitats have been sampled. 2. Here, we present fish community data from 67 lakes in north-east Germany. Fish abundance was estimated in littoral, benthic and pelagic habitats from which a composite parameter indicating lake-wide relative species abundances was derived. This parameter was used in group comparisons and non-metric ordination procedures to explore, among 40 lake habitat descriptors sampled, those most important in structuring community composition. 3. Fish community composition was mainly determined by maximum and mean depth, chlorophyll a content and lake volume. The impact of anthropogenic alterations of shore structure and human-use intensity of lakes were of minor importance. The dominant fish species were vendace Coregonus albula, perch Perca fluviatilis, smelt Osmerus eperlanus and several cyprinids (roach Rutilus rutilus, bream Abramis brama, white bream Abramis bjoerkna and bleak Alburnus alburnus). 4. A response of relative species abundance to lake productivity could be demonstrated for small perch, ruffe and bream. However, when the relationship between lake morphology and productivity was controlled for, differences in species abundances were not longer attributable to differences in productivity, but to maximum lake depth. 5. This suggests that there are two distinct fish community types in Germany, namely the cold-water community with vendace and perch inhabiting deeper lakes, and the warm-water cyprinid community inhabiting more shallow lakes. The previously established conceptual model of a community succession from salmonids through perch to cyprinids with increasing eutrophication is hence not continous, but includes a switch between two lake and fish community types.

Estuarine respiration: an overview of benthic, pelagic, and whole system respiration

Abstract: This chapter reviews rates of benthic, pelagic, and whole system respiration in estuaries. We define estuaries as semi-enclosed coastal bodies of water with some degree of mixing between fresh and salt water. Rates of respiration in these locations are high, reflecting high rates of organic loading from both autochthonous and allochthonous sources. Areal rates of pelagic respiration (58–114 mmol C m−2 d−1) are 2–4 times higher than benthic respiration rates (34 mmol C m−2 d−1), consistent with estimates that only about 24% of total organic inputs to estuaries are respired by the benthos. Estimates of whole system respiration derived from open-water techniques (294 mmol C m−2 d−1) are substantially higher than those obtained by summing component rates (92–148 mmol C m−2d−1), most likely due to the different spatial scales sampled by the two different approaches. The fundamental limit on benthic, pelagic, and whole system respiration appears to be the supply of organic matter, and in many locations allochthonous inputs fuel a major portion of estuarine respiration. Nonetheless, information on the factors that affect benthic respiration is far greater than it is for pelagic respiration, and knowledge of whole system respiration is particularly lacking. This prevents a full understanding of the fate of the vast amount of organic carbon that is imported and produced in estuarine ecosystems.

Measuring marine fish biodiversity: temporal changes in abundance, life history and demography

Abstract: Patterns in marine fish biodiversity can be assessed by quantifying temporal variation in rate of population change, abundance, life history and demography concomitant with long-term reductions in abundance. Based on data for 177 populations (62 species) from four north-temperate oceanic regions (Northeast Atlantic and Pacific, Northwest Atlantic, North mid-Atlantic), 81% of the populations in decline prior to 1992 experienced reductions in their rate of loss thereafter; species whose rate of population decline accelerated after 1992 were predominantly top predators such as Atlantic cod (Gadus morhua), sole (Solea solea) and pelagic sharks. Combining population data across regions and species, marine fish have declined 35% since 1978 and are currently less than 70% of recorded maxima; demersal species are generally at historic lows, pelagic species are generally stable or increasing in abundance. Declines by demersal species have been associated with substantive increases in pelagic species, a pattern consistent with the hypothesis that increases in the latter may be attributable to reduced predation mortality. There is a need to determine the consequences to population growth effected by the reductions in age (21%) and size (13%) at maturity and in mean age (5%) and size (18%) of spawners, concomitant with population decline. We conclude that reductions in the rate of population decline, in the absence of targets for population increase, will be insufficient to effect a recovery of marine fish biodiversity, and that great care must be exercised when interpreting multi-species patterns in abundance. Of fundamental importance is the need to explain the geographical, species-specific and habitat biases that pervade patterns of marine fish recovery and biodiversity.

Modelling diatom and Phaeocystis blooms and nutrient cycles in the Southern Bight of the North Sea: the MIRO model

Abstract: The link between anthropogenic nutrient loads and the magnitude and extent of diatom and Phaeocystis colony blooms in the Southern Bight of the North Sea was explored with the complex ecosystem model MIRO. The model was adapted for resolving the changing nutrient loads, the com- plex biology of the bloom species and the tight coupling between the benthic and pelagic compart- ments that characterise this shallow coastal shelf sea ecosystem. State variables included the main inorganic nutrients (nitrate (NO3), ammonium (NH4), phosphate (PO4) and dissolved silica (DSi)), 3 groups of phytoplankton with different trophic fates (diatoms, nanophytoflagellates and Phaeocystis colonies), 2 zooplankton groups (copepods and microzooplankton), bacteria, and 5 classes of detrital organic matter with different biodegradability. The capability of the MIRO model to properly simu- late the observed SW-NE gradient in nutrient enrichment and the seasonal cycle of inorganic and organic C and nutrients, phytoplankton, bacteria and zooplankton in the eastern English Channel and Southern Bight of the North Sea is demonstrated by running the model for the period from 1989 to 1999. The MIRO code was implemented in a simplified multi-box representation of the hydrody- namic regime. These model runs give the first general view of the seasonal dynamics of Phaeocystis colony blooms and nutrient cycling within the domain. C, N and P budget calculations show that (1) the coastal ecosystem has a low nutrient retention and elimination capacity, (2) trophic efficiency of the planktonic system is low, and (3) both are modulated by meteorological forcing.

Changes in the structure and function of the North Sea fish foodweb, 1973–2000, and the impacts of fishing and climate

Abstract: North Sea environmental and biological data were analysed to examine 30-year changes in production and consumption in the fish food web. The analysis revealed that the demand for secondary production placed on the ecosystem by fish declined from approximately 20 g Cm-2 y-1 in the 1970s to 16 g C m-2 y-1 in the 1990s. Over the same period, the proportion of demand provided by zooplankton production increased from around 70% to 75%. The overall decrease was mainly due to a reduction in piscivorous demersal fish. Average secondary production by omnivorous zooplankton was estimated to be 35 g Cm-2 y-1, and annual fluctuations were positively correlated with the gross production of planktivorous fish. The results suggest a ''bottom-up'' control of the pelagic foodweb. Individual planktivore species have been impacted by fishing, but the populations of other functionally similar species have expanded to fill the vacant niches, thus maintaining the planktivore role in the system. In contrast, the results indicate that benthos production was more "top-down" controlled. Overall, demersal fish species have been depleted by fishing, with no obvious species expansions to fill the vacant niche, releasing the benthos from predation pressure, and leading to an increase in benthic production and fisheries for invertebrates.

Population persistence of california current fishes and benthic crustaceans: a marine drift paradox

Abstract: Because flow in the California Current is unidirectional for months, larvae of coastal species may drift and settle downstream of their parents, potentially causing the parental population to go extinct from upstream to downstream; this is a marine equivalent of the drift paradox in streams. Unidirectional larval drift might, however, be minimized by when, where, and how organisms reproduce. We compiled data on California Current fishes (89 nearshore, 65 shelf/slope) and benthic crustaceans (35 nearshore, 15 shelf/slope) and found three unique sets of life history traits displayed by (1) shelf/slope species, (2) nearshore pelagic-spawning fishes (all from the Southern California Bight [SCB] and Baja California), and (3) all other nearshore species. Pelagic larval durations (PD) of shelf/slope species are long (∼136 d); offspring are pelagic winter through summer and are found at depth below the mixed layer offshore. Offspring experience northward flow in winter and southward flow in spring/summer, perhaps minimizing net alongshore drift. Adults are both long-lived and highly fecund. Nearshore species have short PDs (∼45 d), and offspring are pelagic during spring/summer upwelling and are found nearshore and near the bottom. Nearshore larvae may experience southward flow during upwelling and northward flow during relaxation events, minimizing net alongshore transport. Nearshore species have shorter lives and are less fecund than shelf/slope species. Nearshore pelagic-spawning fishes (all from the SCB and Baja) have short PDs (∼48 d), but in contrast to other nearshore species, their larvae are pelagic from spring into fall and are found far from shore. Adults are long-lived, highly fecund, and have numerous broods per year. These life history traits may have evolved to exploit eddies and countercurrents present in the SCB and off Baja for larval retention. Each set of life history traits appears to improve chances of offspring recruiting to parental populations. The pelagic phase, rather than being dispersive, may be selected to achieve a migration between larval pelagic and adult benthic habitats.

Multi-decadal synthesis of benthic–pelagic coupling in the western arctic: Role of cross-shelf advective processes

Abstract: Using geographic information systems (GIS) software and geostatistical techniques, we utilized three decades of water-column chlorophyll a data to examine the relative importance of autochthonous versus allochthonous sources of reduced carbon to benthic communities that occur from the northern Bering to the eastern Beaufort Sea shelf. Spatial trend analyses revealed areas of high benthic biomass (>300 g m−2) and chlorophyll (>150 mg m−2) on both the southern and northern Chukchi shelf; both areas are known as depositional centers for reduced organic matter that originates on the Bering Sea shelf and is advected northward in Anadyr and Bering shelf water masses. We found a significant correlation between biomass and chlorophyll a in the Chukchi Sea, reflective of the strong benthic–pelagic coupling in a system that is utilized heavily by benthic-feeding marine mammals. In contrast, there was no significant correlation between biomass and chlorophyll in the Beaufort Sea, which by comparison, is considerably less productive (biomass and chlorophyll,

Fish–macrofauna interactions in a kelp ( Laminaria hyperborea ) forest

Abstract: predator–prey interactions between fish and macrofauna in kelp (laminaria hyperboea) forest were examined. fish were caught in gill-nets and fish traps at two sites and throughout the diurnal cycle. their stomach contents were analysed. the most abundant fish species, except pollachius virens, were caught in larger numbers at night than during daytime. the examined fish fed throughout the diurnal cycle and kelp-associated fauna were found to be important prey. pollachius virens was caught in the same numbers throughout the diurnal cycle and contained mainly pelagic prey, in larger numbers during the day than at night. small labrids were specialists, feeding mainly on the two numerically dominant kelp-associated gastropods rissoa parva and lacuna vincta. the larger labrus bergylta, gadus morhua and myoxocephalus scorpius were generalists, feeding on a large range of kelp-associated invertebrates. some fish species showed preference for particular vertical levels when feeding in the kelp forest. the most mobile and/or the largest invertebrates were most heavily preyed upon.

Climate, zooplankton, and pelagic fish growth in the central Baltic Sea

Abstract: Oceanographic conditions in the brackish central Baltic Sea are strongly linked to atmospheric forcing and the unusual period of persistently strong westerlies that, since the late 1980s, have resulted in an increase in average water temperatures and decreasing salinity. These changes in temperature and salinity resulted in a change in the dominance of the mesozooplankton community from Pseudocalanus sp. to Temora longicornis and Acartia spp. Similar to the copepod community, the central Baltic fish community shifted from cod (Gadus morhua), dominant during the 1980s, to sprat (Sprattus sprattus), dominant during the 1990s. Further, the commercially important pelagic fish species herring (Clupea harengus) and sprat exhibited reductions in growth. Using Principal Component and Correlation Analyses we investigated the temporal variability in the importance of the food supply as well as competition on condition of central Baltic pelagic fish species. Our results indicate that herring condition results from a combined effect of changes in the food environment and increased competition with sprat, while sprat condition appeared to be primarily determined by intra-specific competition.

The comparative roles of suspension-feeders in ecosystems

Abstract: Modeling particle selection efficiency of bivalve suspension feeders.- Field measurements on the variability in biodeposition and estimates of grazing pressure of suspension-feeding bivalves in the northern Baltic Sea.- Can bivalve suspension-feeders affect pelagic food web structure?- Motile suspension-feeders in estuarine and marine ecosystems.- Impact of suspension-feeding nekton in freshwater ecosystems: patterns and mechanisms.- Influence of eastern oysters on nitrogen and phosphorous regeneration in Chesapeake Bay, USA.- How does estimation of environmental carrying capacity for bivalve culture depend upon spatial and temporal scales?- Impact of increased mineral particle concentration on Acartia clausi (Copepoda) behaviour, suspension feeding and reproduction.- Suspension-feeders as factors influencing water quality in aquatic ecosystems.- Neoplasia in estuarine bivalves: effect of feeding behaviour and pollution in the Gulf of Gdansk (The Baltic Sea, Poland).- Bivalves as biofilters and valuable by-products in land-based aquaculture systems.- Significance of suspension feeder systems on different spatial and temporal scales.- Invaders in suspension-feeding systems: variations along the regional environmental gradient and similarities between large basins.- Contrasting distribution and impacts of two freshwater exotic suspension-feeders, Dreissena polymorpha and Corbiculafluminea.- Functional changes in benthic freshwater communities after Dreissena polymorpha (Pallas) invasion and consequences for filtration.- Does the introduction of the Pacific oyster Crassostrea gigas lead to species shifts in the Wadden Sea?- One estuary, one invasion, two responses: phytoplankton and benthic community dynamics determine the effect of an estuarine invasive suspension-feeder.- Development of human impact on suspension-feeding bivalves in coastal soft-bottom ecosystems.- Oyster reefs as complex ecological systems.- Synthesis/Conclusions.- Index .

Monitoring pelagic ecosystems using plankton indicators

Abstract: Beaugrand G. 2005. Monitoring pelagic ecosystems using plankton indicators. e ICESJournal of Marine Science, 62: 333e338.Planktonic organisms are an important food resource of pelagic ecosystems, but they alsoserve as an integrator of hydroclimatic forcing. Four types of recently developed planktonindicator, based on the Continuous Plankton Recorder survey, are summarized here:indicators based on individual taxa; indicators based on functional attributes of theecosystem (diversity); species assemblage indicators; and indicators of larval fish survival.All provide information on the state of a pelagic ecosystem, but have different limitations.Therefore, their combined application provides the most accurate diagnosis of ecosystemstate. In most of the examples described, statistical analyses help to identify major spatialand temporal patterns, and may allow future ecosystem changes to be anticipated.

Swimming speeds of larval coral reef fishes: impacts on self-recruitment and dispersal

Abstract: The dispersal of larvae during their time in the pelagic environment is critically important to our understanding of marine populations Recent publications have highlighted the potential importance of larval behaviour in influencing dispersal patterns of larval reef fishes However, it has not been clearly established if their abilities are of a magnitude comparable to the potential effects of oceanic processes and whether larval behaviour is sufficient to facilitate self-recruitment This study presents new data on the swimming speed of late-stage larvae to determine how they can swim relative to oceanic currents The families examined comprised the Acanthuridae, Siganidae, Lutjanidae, Lethrinidae, Pomacentridae, Chaetodontidae, Nemipteridae, Monacanthidae, Psuedochromidae, Pomacanthidae and Apogonidae The late-stage larvae of all reef fish families examined were able to swim at speeds greater than the mean transport speeds reported around reefs in most locations However, even the best-swimming reef fish families could not swim faster than the maximum current speeds reported Based on new and previously published data it appears that the development of swimming ability can be described adequately (80% of variation explained) as a linear increase from zero at hatching to a species-specific maximum at settlement Calculations based on this developmental pattern suggest that most reef fish families could substantially influence their dispersal patterns relative to ocean currents for over 50% of their larval phase For all families examined, swimming behaviour could potentially affect dispersal patterns on a magnitude similar to the dispersing effect of oceanic currents In addition, the swimming capabilities of several reef fish families have the potential to facilitate active self-recruitment in a range of reef systems

Food web structure in the high Arctic Canada Basin: evidence from δ13C and δ15N analysis

Abstract: The food-web structure of the Arctic deep Canada Basin was investigated in summer 2002 using carbon and nitrogen stable isotope tracers. Overall food-web length of the range of organisms sampled occupied four trophic levels, based on 3.8‰ trophic level enrichment (δ15N range: 5.3–17.7‰). It was, thus, 0.5–1 trophic levels longer than food webs in both Arctic shelf and temperate deep-sea systems. The food sources, pelagic particulate organic matter (POM) (δ13C=−25.8‰, δ15N=5.3‰) and ice POM (δ13C=−26.9‰, δ15N=4.1‰), were not significantly different. Organisms of all habitats, ice-associated, pelagic and benthic, covered a large range of δ15N values. In general, ice-associated crustaceans (δ15N range 4.6–12.4‰, mean 6.9‰) and pelagic species (δ15N range 5.9–16.5, mean 11.5‰) were depleted relative to benthic invertebrates (δ15N range 4.6–17.7‰, mean 13.2‰). The predominantly herbivorous and predatory sympagic and pelagic species constitute a shorter food chain that is based on fresh material produced in the water column. Many benthic invertebrates were deposit feeders, relying on largely refractory material. However, sufficient fresh phytodetritus appeared to arrive at the seafloor to support some benthic suspension and surface deposit feeders on a low trophic level (e.g., crinoids, cumaceans). The enriched signatures of benthic deposit feeders and predators may be a consequence of low primary production in the high Arctic and the subsequent high degree of reworking of organic material.

Littoral energy mobilization dominates energy supply for top consumers in subarctic lakes

Abstract: We used stable carbon (d 13 C) and nitrogen (d 15 N) isotopes to assess the energy sources supporting the top consumer (Arctic char, Salvelinus alpinus) in nine subarctic lakes in northern Sweden. The d 13 C of littoral (epipelic algae) and pelagic (bacterioplankton and phytoplankton) energy sources were clearly separated in the lakes, as reflected in habitat-specific consumers (zoobenthos, zooplankton). Char were enriched in 13 C compared with pelagic energy sources and prey and isotopically more similar to littoral energy sources and prey. The contribution of littoral energy sources to char body carbon was estimated to range between 62% and 94% among the lakes. The reliance on littoral energy sources was independent of char size and did not change when char coexisted with a small-sized prey fish (nine-spined stickleback, Pungitus pungitus). The strong reliance of top consumers in subarctic lakes on littoral energy sources may be due to the higher energy mobilization and larger sizes of primary consumers in littoral than in pelagic habitats. The energy flow through the ecosystem, i.e., from the energy mobilizers at the base of the food web to top energy consumers, is of fundamental importance for the function of lake ecosystems. Mobilization and transfer of energy within the food web sets the limit for production at higher trophic levels, and the presence of several diverse energy-flow pathways may affect ecosystem properties such as food-web dynamics and nutrient cycling (Polis et al. 1997). Still, energy flow between benthic and pelagic compartments has received little attention, reflecting both methodological constraints and a scarcity of whole-lake studies (Vadeboncoeur et al. 2002). Research into lake energy flow has largely focused on pelagic food webs. Pelagic energy mobilization derives from photosynthesis and from bacterioplankton growing on allochthonous organic carbon (Tranvik 1989; Jones 1992; Karlsson et al. 2002). The energy mobilization at the base of the food web is consumed, redistributed in the food web, and subsequently transferred to top consumers such as fish. However, recent studies have shown that benthic resources are of great importance for lake food webs, since fish communities may make considerable use of zoobenthos (Schindler and Scheuerell 2002). In the profundal zone, exploitation of benthic resources can be regarded as mainly comprising the re