Showing papers on "Pelagic zone published in 1979"

Continuous Plankton Records: Seasonal cycles of phytoplankton and copepods in the North Atlantic ocean and the North Sea

Abstract: Data from the Continuous Plankton Recorder survey of the North Atlantic Ocean and the North Sea are used to study geographical variations in the amplitude, duration and timing of the seasonal cycles of total phytoplankton and total copepods. It is shown that the distribution of overwintering stocks influences the distributions throughout the year. There is a relationship between the timing of the spring increase of phytoplankton and the amplitude of the seasonal variation in sea surface temperature. In the open ocean, the timing of the spring increase of phytoplankton corresponds with the spring warming of the surface waters. In the North Sea the spring increase occurs earlier, associated, perhaps, with transient periods of vertical stability, resulting in a relatively slower rate of increase. It is suggested that in the open ocean the higher rate of increase is under-exploited by copepods due to low overwintering stocks and longer generation times. Exceptionally early spring increases of phytoplankton off the west coast of Greenland and over the Norwegian shelf are probably associated with permanent haloclines. A high and late autumn peak of phytoplankton off the coast of Portugal may be associated with coastal upwelling.

Diel Pattern of Pelagic Distribution and Feeding in Planktivorous Fish

Abstract: The spatial distribution of juvenile roach (Rutilus rutilus), rudd (Scardinius erythrophtalamus), bream (Abramis brama) and bleak (Alburnus alburnus) was registered by echosounding during two years in small Bavarian lakes. The gut contents of the fish were analysed in order to reconstruct their rhythmicity of food intake. Fish were found in the pelagic zone only during the night, feeding before midnight with maximal rate. The vertical position of the fish was dependent on the water transparency following a light intensity in the order of 10-3 Lux. The diel migration between littoral and pelagic zones was a mainly horizontal one with a vertical component. Both the speed of increase of fish density in the open water and of the vertical movement was correlated to the speed of change of light intensity. Shoals broke up into single fish during the offshore migration. The span of presence in the pelagic zone was identical with the duration of the dark period. During winter no diel migration nor periodical feeding took place.

Ecological Significance of Fronts in the Southeastern Bering Sea

Abstract: A series of three fronts divides the continental shelf of the southeastern Bering Sea into two interfrontal zones which contain different food webs. Large stocks of birds, mammals, and pelagic fish, primarily walleye pollock, occur in the outer shelf zone between the 200 meter isobath and the middle front near the 100 meter isobath. Large stocks of benthic infauna, demersal fish, and crabs occur in the middle shelf zone between the middle front and the inner front at the 50 meter isobath. Very low cross-shelf advection and the presence of the middle front which acts as a diffusion barrier restrict large oceanic herbivores to the outer shelf zone. Large diatoms are not grazed by the small coastal herbivores which inhabit the middle shelf zone, resulting in an accumulation of phytoplankton biomass which settles to the benthos.

Fish production and the marine ecosystems of the Scotian Shelf, eastern Canada

Abstract: Fishery production and food webs have been studied on the Scotian Shelf and upper continental slope along a transect running 270 km SE of Halifax, Canada. The area (TCNAF Division 4W) supports a fishery of roughly 0.15x106 metric tons. Overall primary production of the shelf waters is 102 g C m-2 year-1 and of the slope wacers about 128 g C m-2 year-1. Demersal fish production (average 4.1 kcal m-2 year-1) is highest over the shelf and declines in an offshore direction, while pelagic fish production (average 16 kcal m-2 year-1) is highest over the slope and declines in an inshore direction. Hypothetical food webs of these two intergrading ecosystems have been constructed, based on data for primary production, fish catches, and the biomasses of zooplankton and macrobenthos. These lead us to suggest that there are basic differences in food chains and efficiencies between the two ecosystems that account for their differences in production. Although primary production is 17% higher on an average on the Nova Scotian transect than in the North Sea, the apparent zooplankton and macrobenthos production is 31% lower and macrobenthos production may also be lower. Overall fish catch from the Scotian Shelf and slope is about 47% lower per unit area than the catch in the North Sea, despite the fact that the demersal catches are identical. This is accounted for by a much lower overall pelagic catch from the Nova Scotian area, centered in a region that is small compared to the total area. Fish production in different regions cannot be predicted merely on the basis of differences in level of primary production, but must take into account differences in the structure of the ecosystems.

Ethno-Archaeology and the Development of Polynesian Fishing Strategies

Abstract: In Oceania, the "hunting-and-gathering" component of subsistence is focused primarily upon the sea and its varied resources. This is particularly so in inner or "remote" Oceania,1 for as one ventures eastward from the larger continental islands of New Guinea and island Melanesia, the indigenous terrestrial flora and fauna become increasingly less numerous. The impov erished land biotas of the Polynesian islands excepting only the avifauna offered but a restricted range of potential foodstuffs. At the same time, however, the coral reef communities, lagoons, and pelagic waters around the tropical Pacific islands support a large number of edible fish, molluscs, crust cea, echinoderms, seaweeds, and the like. We would emphasise further that "hunting-and-gathering" is as apt a rubric for marine exploitation as it is for the exploitation of a terrestrial biota. Seaweeds and invertebrates are certainly gathered, and anyone who has participated in a Polynesian fishing expedition will realise that fishing is by no means a passive activity; the excitement of the chase is just as fully expressed as among the more often cited land-based hunters. To be sure, there are significant differences between land and sea hunting, these being in large part a reflection of the unique characteristics of the sea buoyancy, turbulence, refraction of light, and the general unsuitedness of man for the marine environment (Hewes 1948). Thus, most of the technological apparatus and behavioural strategies for marine exploitation are unique developments, and not mere transferences of land hunting devices. While terrestrial hunting is almost certainly of a greater antiquity than marine exploitation, the origins of sea hunting-and-gathering clearly extend well into the Pleistocene among people inhabiting the littoral margins of the continents. The ultimate origins of Oceanic fishing strategies doubtless lie in insular South-east Asia, that region of the "shadow" continents Sunda and Sahul, whose coasts and shorelines shifted so remarkably during the Pleistocene (Jones 1977). Sauer's hypotheses (Sauer 1952) on the origins of coastal South-east Asian adaptations of the interactions of the littoral

Vertebrate and invertebrate predation intensity on freshwater zooplankton communities

Abstract: PREDATION is widespread in the pelagic zone of lakes and both vertebrate and invertebrate predators are abundant and diverse in their feeding habits. Vertebrate predators are often size-selective and can virtually eliminate larger prey items1–3. These dramatic effects resulting from fish predation have led several workers to conclude that vertebrates contribute the largest percentage of the total predation intensity on freshwater zooplankton. Particular invertebrate predators, however, have also been shown to crop a substantial proportion of their prey populations4–6. I report here a study showing that invertebrate predation is more intense than vertebrate predation in the pelagic zone of Gull Lake, Michigan, with the first in situ evaluation of both types of predation on a single zooplankton prey assemblage. The significance of identifying major predation pathways in lakes is basic to many contemporary discussions of nutrient and energy flow as well as considerations of community structure and stability.

Stages in the early development of 40 marine fish species with pelagic eggs from the Cape of Good Hope

Abstract: ....................................................................................................................... 1 STAGES IN THE EARLY DEVELOPMENT OF 40 MARINE FISH SPECIES WITH PELAGIC EGGS FROM THE CAPE OF GOOD HOPE1

Methane supersaturation and ATP-zooplankton blooms in near-surface waters of the Western Mediterranean and the subtropical North Atlantic Ocean

Abstract: The concentrations of methane and 333 to 1800-μm ATP-biomass were determined at 15 stations along a transect from Gibraltar to Africa to Puerto Rico. Near-surface waters at one station in the Western Mediterranean and at one station off the coast of North Africa were highly supersaturated with methane. High concentrations of zooplankton encountered at the same locations offer biological evidence that the production of excess methane in the water column may be associated with the pelagic food chain. A possible cause may be the presence of methane bacteria in the digestive tracts of the herbivorous zooplankton.

Relationships of the blue shark, prionace glauca, and its prey species near santa catalina island, california 1

Abstract: Small fishes and cephalopods associated with both pelagic and inshore habitats composed the major prey for the blue shark, Prionace glauca, near Santa Catalina Island, Calif. The northern anchovy, Engraulis mordax, was the predominant prey for sharks in the immediate study area while at least 13 species ofpelagic cephalopods constituted major prey for sharks in more distant oceanic waters. Inshore species taken by sharks included pipefish, Syngnathus californiensis; jack mackerel, Trachurus sym­ metricus; and blacksmith, Chromispunctipinnis. In addition, sharks moved inshore to feed on winter spawning schools ofmarket squid, Loligo opalescens. Digestive rate studies and telemetric monitoring of activity patterns indicate that sharks forage in waters near the surface from around midnight through dawn. Diel activities of prey species were examined and show that most prey dispersed in the upper water column at night and refuged during the day either by schooling (anchovies and jack mackerel) or by retreating to deeper waters (pelagic cephalopods). Field observations ofshark feeding behavior indicate that predatory modes vary in response to prey behavior. The blue shark,Prionaceglauca (Carcharhinidae) (Figure 1), is a pelagic carnivore cosmopolitan in tropical and warm temperate seas. Because of its pelagic habits, the majority of ecological studies on this species have been predicated on data from sharks captured by sport and commercial fisheries. As a result data has been largely qual­ itative, and the shark's role as a predator in the epipelagic habitat has remained unclear. The importance of small fish as prey items for blue sharks has been described by Couch (1862), Lo Bianco (1909), Bigelow and Schroeder (1948), Strasburg (1958), LeBrasseur (1964), Bane (1968), Stevens (1973), and others. These prey generally are schooling species common in productive coast­ al waters. Cephalopods were also reported as major prey but little information is available on specific identifications (see Stevens 1973; Clarke and Stevens 1974). Although blue sharks have been observed feed­ ing on dead or wounded cetaceans (Bigelow and Schroeder 1948; Cousteau and Cousteau 1970) there is little indication that they habitually prey on live, healthy marine mammals. The occurrence

Growth and production of some important species of fish in a Mexican coastal lagoon system

Abstract: In terms of biomass, the three dominant fish species in the Huizache-Caimanero lagoon system during 1975–76 were the mullet Mugil curema, the anchovy, Anchoa panamensis and the catfish, Galeichthys caerulescens. Peaks in the abundance of the secondary predators occurred at the end of the wet season (September-October), while pelagic forms and members of the centropomid-gerreid association (Warburton 1968a) were most common during November-February and December-February respectively. Twelve species, including the ten having the highest overall mean biomass, were selected for growth and production studies. Lagoon-specific differences in the patterns of growth and recruitment of several species were observed. The total estimated annual fish production in Caimanero lagoon was 34·48 g m−2 y−1, with M. curema contributing 9·36 g m−2, A. panamensis 9·24 g m−2 and G. caerulescens 6·15 g m−2 (maximum estimate). Mean turnover ratios (annual production: mean biomass) were 3·25 (demersal species), 8·44 (pelagic species) and 4·48 (all species). Implications for possible fishery and aquacultural expansion are discussed.

Genetic Variability in the Pelagic Environment: A Paradox?

Abstract: The pelagic environment is often regarded as rather homogeneous by comparison with shallow—water benthic environments. Species of krill of the pelagic genus Euphausia (Crustacea) display a trend in genetic variability from low in high latitudes to high in low latitudes, closely similar to trends displayed by shallow—water benthic invertebrates. We interpret the trends as genetic strategies, with few functionally—broad alleles in high latitude species or in species that range widely ecologically, and numerous functionally—narrow alleles in low latitude species that are narrowly restricted ecologically. The functionally—narrow alleles are maintained by forms of balancing selection and permit a high degree of specialization. Therefore the tropical kill species is highly sensitive to spatial heterogeneities within the pelagic zone, which does not appear homogeneous to selection. See full-text article at JSTOR

326Ra in Bering Sea sediment and its application as a geochronometer

Abstract: 232Th, 230Th and 226Ra were determined for three pelagic cores collected in the northern North Pacific and the western and eastern basins of the Bering Sea. The surface sediment collected in the Pacific contains much 232Th and 230Th decreasing in concentration with depth indicating a sedimentation rate of 2.2 × 10-3 cm/yr. The Bering Sea sediments do not show vertical profiles of decreasing concentration with depth. This is due to a fast sedimentation rate in the Bering Sea. The excess radioactivity of 226Ra relative to that of 230Th was observed in the surface sediment collected in the eastern basin of the Bering Sea where the sediment was of siliceous ooze, showing a vertical profile decreasing in concentration with depth. The excess 226Ra is caused by the biogenic silicate material and has been used as a geochronometer. The sedimentation rate in the Bering Sea basin turns out to be 5 × 10-2cm/yr, which shows good agreement with that obtained by the 14C method. Therefore, 226Ra chronology is applicable to pelagic or hemi-pelagic sediment of siliceous ooze.

The swimming of the mandarin fish Synchropus picturatus (Callionyiidae: Teleostei)

Abstract: In order to gain a general understanding of the problems of fish locomotion and appreciate them in an evolutionary context, it is important that we have information available on the kinematics, hydrodymamics and energetics of the swimming of as wide a range of fish as possible. At present, most of our knowledge of fish locomotion is restricted to the swimming of streamlined, pelagic and nektonic teleosts, capable of high speeds and accelerations, that are either neutrally buoyant or have a buoyancy close to neutral (see Lighthill (1969), Webb (1975), Bone (1975) for reviews).

Clostridium botulinum in fish.

Abstract: 1407 fish caught in Scandinavian waters, the North Sea and the North Atlantic have been examined for the presence of Cl. botulinum. The incidence in gut samples expressed as percentage of fish tested was generally highest in fish from Scandinavian coastal waters and the Baltic Sea (4--43%), decreasing in fish from the North Sea (0--8%), and the organism was practically absent in fish from the North Atlantic. When gut samples were examined, the incidence was highest in demersal fish (cod and flatfish) as compared with pelagic fish (herring). The latter fish species were mainly contaminated on outer surfaces and gills. Only type E was detected in this survey, and Cl. botulinum was not detected in any wild fresh water fish. It is suggested that type E spores may originate in the sea bed and that they be spread by fish and water currents.

Benthic resting periods of pelagic cyclopoids in an oligotrophic lake

Abstract: The benthic resting stages of pelagic cyclopoids were studied in the oligotrophic lake Paajarvi (maximum depth 87 m), southern Finland. Stage 5 copepodids of Thermocyclops oithonoides were found in the bottom from September to April, with highest abundances in the littoral at 1–2 m. Dormant Mesocyclops leuckarti (mainly stage 5 copepodids) were found from mid-August so April, with a strong concentration at the depth of 1.5 m, and resting stage 4 copepodids of Cyclops kolensis from mid-summer to late winter at the depths of 1–13 m. Single stage 4 and 5 copepodids of C. strenuus and C. lacustris were also found in the bottom during autumn and winter. Available information on the periodicity of these species conforms well with the hypothesis of a temperature-adjusted photoperiodic control of diapause. In the sediment, the resting stages of pelagic cyclopoids were confined to the uppermost 2–3 cm, C. kolensis penetrating slightly deeper than the smaller Thermocyclops and Mesocyclops species. Losses during the resting period were highest in T. oithonoides, in which the seasonal mortality rates were significantly correlated with temperature. Its overall mortality was slightly lower at 2 m than at 13 or 40 m. C. kolensis had the lowest mortality, with no clear relation to temperature. The mean winter biomass of the benthic resting stages of pelagic cyclopoids exceeded that of the true benthic copepods, and the winter losses of the former were equal to about one fifth of the total annual production of the true benthic copepods.

Marine production mechanisms

Abstract: 1. Introduction M. J. Dunbar 2. Primary production in Frobisher Bay, Arctic Canada E. H. Grainger 3. Primary production in some tropical environments S. Z. Qasinm 4. Biological productivity of some coastal regions of Japan K. Hogetsu 5 Factors determining the productivity of South African coastal waters J. R. Grindley 6 The Strait of Georgia Programme T. R. Parsons 7. Biological production in the Gulf of St Lawrence M. J. Dunbar 8. Patterns of the vertical distribution of phytoplankton in typical biotopes of the open ocean H. J. Semina 9. The Dutch Wadden Sea M. van der Eijk 10. Seaweed utilization in the Philippines G. T. Velasquez 11. Trophic relationships in communities and the functioning of marine ecosystems: I Studies on trophic relationships in pelagic communities of the Southern Seas of the USSR and in the tropical Pacific T. S. Petipa 12. Trophic relationships in communities and the functioning of marine ecosystems: II Some results of investigations on the pelagic ecosystem in tropical regions of the ocean E. A. Shushkina and M. E. Vinogradov 13. Soviet investigation of the benthos of the shelves of the marginal seas A. A. Neynian 14. Studies of trophic relationships in bottom communities in the southern seas of the USSR E. A. Yablonskaya 15. Studies of the pattern of biotic distribution in the upper zones of the shelf in the seas of the USSR A. N. Golikov and O. A. Scarlato.

Tsesis oil spill: its impact on the coastal ecosystem of the Baltic Sea

Abstract: The grounding of the Soviet tanker Tsesis in the archipelago south of Stockholm in October 1977 resulted in a spill of over 1000 tons of medium-grade fuel oil. The environmental impact of the spill was studied by an international research team. Severe but not catastrophic initial effects were found in pelagic, littoral, and benthic ecosystems. The effects proved to be of short duration in the pelagic system, and after about one year the littoral zone had also recovered considerably; whereas, in the soft bottom communities, not even the beginning of a recovery was found. The results from the first year of study presented here illustrate the recovery rates of a coastal ecosystem in a cold brackish sea, following an oil spill.

Birds of the Hebrides: seabirds*

Abstract: Coastal water moving north up the coast of Europe mixes with oceanic water from the Atlantic off the north of Scotland, which leads to a high marine productivity. In summer the mixing tends to be concentrated along fronts which form between the stratified water out at sea and that mixed by coastal currents, and in the sounds between the land masses. Small fish, and the birds which feed on them, frequent these areas, and the birds breed nearby, so that the area is a major nursery for temperate North Atlantic seabirds. Most of the large breeding colonies of pelagic species are not found along the Outer Hebridean island chain, but along the mainland coast and on the outlying islands. There is also a substantial population of birds which feed along the coast scattered throughout the area. When the stratification of the water out at sea breaks down at the end of the summer, many moulting guillemots, razorbills and kittiwakes move inshore with their young for a time. Some birds also tend to move north at this time, presumably following some change in the distribution of the available food. As the weather deteriorates in the winter, many of the seabirds disperse. Some auks move into the more sheltered North and Irish Seas, the shearwaters, small petrels, skuas and terns migrate to the southern hemisphere, and the puffins, kittiwakes and young fulmars scatter throughout the temperate North Atlantic. Variable proportions of many species, including cormorants, gulls, auks and the adult fulmars remain in the area, retiring to more sheltered waters during bad weather. There may have been some long-term changes in populations; thus the southern Manx shearwater declined and the northern puffin increased in the last century, and the process was reversed in this one, possibly as a result of changes in climate. Some species, notably the gulls and fulmar, are now increasing following their exploitation of human wastes for food, and all species are benefiting from the cessation of human exploitation and its replacement by protection over the last century.

Zinc concentrations in juvenile flatfish

Abstract: In most demersal and some pelagic marine fish O- and I-group stages are found on inshore nursery grounds which are often situated in coastal indentations and estuarieswhere flushing rates are low and the build up of persistent pollutants can be correspondingly high. Their movements are restricted, only leaving the nursery areas towards the end of their second year (Lockwood, 1974; Riley, 1973). This, coupled with the fact that juveniles may be more susceptible than adults to pollutants, makes them particularly vulnerable to the potential hazards of inshore pollution.The physico-chemical properties of most trace metals, however, generally preclude their occurrence at high concentrations in sea water itself. Instead, most of the metal is transferred to the particulate phase of the water mass and to sedimentary reservoirs (Renfro, 1973) so that concentrations of metals in sea water very rarely reach levels which are known to be acutely toxic to fish. The main risk from direct effects of metals lies in poisoning resulting from internal accumulation.Little information is available on metal concentrations occurring in young fish. Hardisty et al. (1974) and Hardisty, Kartar & Sainsbury (1974) have examined zinc, cadmium and lead levels in O-group and older flounders from the Bristol Channel. Andersen, Dommasnes & Hesthagen (1973) found high concentrations of zinc in O- and I-group herring and sprat from Oslo Fjord, Cross & Brooks (1973) reported on manganese, iron and zinc concentrations in juvenile estuarine fish. Pentreath (1973 b) has reported on zinc concentrations in the organs of I-group plaice.

Observations on the food and feeding habits of the eel, muraenesox cinereus (forskal) from porto novo

Abstract: Feeding in Muraenesox cinereus was studied from the catches of hooks and line fishing operations from^ the catamarans This fish is a carnivore and largely piscivorous, feeding on the pelagic, demersal and bottom living organisms Cannibalistic tendency is noticed Pelagic fishes like the mackerel and the clupeids are found mostly preferred

Relationships of the blue shark, przonace glauca, and its prey species near santa catalina island, california'

Abstract: Small fishes and cephalopds associated with both pelagic and inshore habitats composed the major prey for the blue shark, Prionace glauca, near Santa Catalina Island, Calif. The northern anchovy, Engraulis mordm, was the predominant prey for sharks in the immediate study area while at least 13 species of pelagic cephalopdsconstituted major prey for sharks in more distant oceanic waters. Inshore species taken by sharks included pipefish, Syngnathus californiensis; jack mackerel, Trachurus symmetricus; and blacksmith, Chromispunctipinnis. In addition, sharks moved inshore to feed on winter spawning schools of market squid, Loligo opalescens. Digestive rate studies and telemetric monitoring of activity patterns indicate that sharks forage in waters near the surface from around midnight through dawn. Die1 activities of prey species were examined and show that most prey dispersed in the upper water column at night and refuged during the day either by schooling (anchovies and jack mackerel) or by retreating to deeper waters (pelagic cephalopods). Field observations of shark feeding behavior indicate that predatory modes vary in response to prey behavior. The blue shark,Prionace glauca (Carcharhinidae) (Figure 11, is a pelagic carnivore cosmopolitan in tropical and warm temperate seas. Because of its pelagic habits, the majority of ecological studies on this species have been predicated on data from sharks captured by sport and commercial fisheries. As a result data has been largely qualitative, and the shark's role as a predator in the epipelagic habitat has remained unclear.

Interrelationships Between Pelagic Fishes and Plankton in the Coastal Fishing Ground of the Southwestern Japan Sea

Abstract: Interrelationships between the occurrences of pelagic fishes and plankton in coastal fishing grounds have been studied, in two phases, in coastal waters close to Esaki, southwestern Japan Sea, from April through December, 1970. The first phase comprised a general investigation of seasonal changes in the occurrences of plankton and pelagic fishes; the second phase, of an examination of the interrelationships between distribution of pelagic fishes and zooplankton. The results reveal complicated interrelations. For pelagic fishes, the distribution of copepods, their major prey, seems to be of paramount importance. The fishes appeared primarily in close vicinity to dense copepod populations. High aggregations of Noctiluca scintillans or cladoceran species exerted unfavorable effects on the fish=, even in the presence af copepods Apparent interrelationships were also established in regard to seasonal abundance changes of plankton and pelagic fishes. The results obtained differ somewhat from known prey-predator osciIlations postulated by theoretical equations. Alternations in water masses are suggested to act as significant modifying influences.

Dynamic ecology of Aldabran seabird communities

Abstract: The characteristics of the present seabird community of Aldabra are described, and compared briefly with others in the tropics. By comparison with Pacific Ocean communities, Aldabra is deficient especially in petrels and shearwaters, which are poorly represented in the western Indian Ocean generally and are absent probably for zoogeographic reasons. Ground-nesting species are also scarce, especially pelagic feeders that form large colonies, and this is attributed partly to their extermination by rats and partly to the proximity of Assumption, and perhaps Cosmoledo, that offer better nesting sites to these species. Inshore-feeding terns are also scarce on Aldabra, probably owing to insufficient areas of shallow water nearby. Relative population sizes are roughly in agreement with those that would be predicted, except for the fairy tern whose population may have been reduced in the past by barn owls. Almost all Aldabran seabirds nest either in mangroves or on small lagoon islets, the habitats in which birds are least vulnerable to introduced rats, which appear to have had a major effect on nesting distribution. The seabird community is made up of two different trophic guilds, the pelagic feeders which bring in nutrients entirely from outside the ecosystem, and the inshore feeders which cycle nutrients between the intertidal and inshore parts of the system and the terrestrial part. Both result in a net input of nutrients, in the form of bird droppings, to the terrestrial ecosystem. It is estimated that about 1680 t of food are removed from the sea per year, most of the 105 t of guano resulting being channelled into the intertidal, rather than the terrestrial, parts of the ecosystem. The seabird communities likely to have occupied Aldabra in the past are reconstructed on the basis of the known changes in the atoll’s size and structure during the Pleistocene. At most times there would have been more species of seabird using Aldabra than now, and more of their guano would have passed into the terrestrial ecosystem. Probably the most significant event in the recent history of the atoll was the arrival of rats, which exterminated probably quite considerable colonies of ground-nesting seabirds and drove the survivors into the only places where the rats could not survive - the tall mangroves and the tiny lagoon islets. In so doing, the rats destroyed a significant source of nutrient input to the terrestrial ecosystem, with major consequences for the ecology of the atoll as a whole.

Food habits as an ecological partitioning mechanism in the nearshore rockfishes (Sebastes) of Carmel Bay, California

Abstract: In the kelp forests of Carmel Bay there are six common rockfishes (Sebastes). Three are pelagic (S. serranoides, S. mystinus, and S. melanops) and two are demersal (S. chrysomelas and S. carnatus). The sixth (S. atrovirens) is generally found a few meters above the sea floor. The pelagic rockfishes which are spatially overlapping have different feeding habits. All rockfishes except S. mystinus utilize juvenile rockfishes as their primary food source during the upwelling season. Throughout the non-upwelling season, most species consume invertebrate prey. The pelagic rockfishes have shorter maxillary bones and longer gill rakers than their demersal congeners, both specializations for taking smaller prey. They also have longer intestines, enabling them to utilize less digestable foods. S. mystinus, which has the longest intestine, may be able to use algae as a food source. Fat reserves are accumulated from July through October, when prey is most abundant. Fat is depleted throughout the rest of the year as food becomes scarce and development of sexual organs takes place. Gonad development occurs from November through February for all species except S. atrovirens.

Aspects of the ecology of Gadoid Fish of the West Coast of Scotland

Abstract: The distribution, abundance, growth, length-weight relationships and reproductive ecology of whiting, Norway pout and poor-cod populations were studied in inshore and offshore areas around Oban from November 1974 to March 1977. Planktonic stages and mature adults were found only at offshore sites, the inshore areas being nursery grounds for all three species. Immigration occurred between June and December of the O-group when the fish were nektonic, with part of the population pelagiC and part demersal. In whiting the young fish aggregated for a short period inshore, but in other species abundance was always greater offshore. Norway pout and poor-cod remained on the nursery. grounds for between one and two years, whereas few whiting remained longer than one year. In all species shallower sampling sites were vacated more rapidly than deeper ones. The migration of fish offshore is considered in relation to growth, diet and maturation. The ecological significance and management implications of the migrations are discussed. The rate of growth in length and weight varied seasonally, being more rapid in summer and autumn in all species. Differences were found between localities in rate of growth and are thought to be due to a preference by older fish for deeper water. Only the poor-cod spawned in the study area, at Tiree Passage and on the south-west coast of Null. Spawning occurred between January and May, with a peak in April. Females matured at a mean.length of 15.6 cm in the 2-group with only a small proportion of 1-group fish maturing. In males the average age of first maturity was lower (14.0 - 14.6 cm) and a greater proportion of 1-group fish matured, which compensated for a slightly lower survival rate in older fish and equalised the sex-ratio of the spawning popUlation. The success of reproduction was highly dependent on a single year-class, since 83.3% of reproducing females belonged to the 2-group. Although ripe and spent whiting and Norway pout were caught at offshore sites no spawning fish were encountered. The possibility of a spawning migration out of the area and the locality of spawning is discussed for each species. Length-weight relationships were calculated for each species and significant differences were found between males and females in all cases. Seasonal variations in condition were studied by calculation of condition factors and by analysis of monthly length-weight relationships.