Diets of fishes of the upper continental slope of eastern Tasmania: content,calorific values,dietary overlap and trophic relationships

Diets of 15 species of demersal and pelagic fishes on the upper continental slope (420 to 550 m) were determined, based on samples taken every two months over 13 mo (April 1984 to April 1985) off eastern Tasmania. The calorific contribution of each prey item to the diets was determined. The fish could be divided into four trophic categories: pelagic piscivores, epibenthic piscivores, epibenthic invertebrate feeders and benthopelagic omnivores. Dietary overlap between the groups was low. The pelagic piscivores Apogonops anomalus, Trachurus declivis, Brama brama, Lepidopus caudatus and Macruronus novaezelandiae primarily consume the shelf-break myctophid Lampanyctodes hectoris; their diet is narrow, with a large overlap between species. The epibenthic piscivores Deania calcea and Genypterus blacodes both take a greater variety of prey, but have little dietary overlap. The fish feeding on epibenthic invertebrates, Coelorinchus sp. 2 and Centriscops humerosus, obtain most of their energy from benthic Crustacea and Ophiuroidea, supplemented with Lampanyctodes hectoris; the diet is broad, with little overlap. Among the benthopelagic omnivores (Cyttus traversi, Coelorinchus sp. 4, Lepidorhynchus denticulatus, Neocyttus rhomboidalis, Helicolenus percoides, Epigonus denticulatus and E. lenimen), most diets are broad and show slight overlap. All but E. denticulatus consume significant quantities of Lampanyctodes hectoris as well as Crustacea, particularly Polychaeta, Euphausiacea and Pyrosoma atlanticum. Seasonal changes in diet occurred in G. blacodes, T. declivis, Lepidopus caudatus, Coelorinchus sp. 4, Lepidorhynchus denticulatus, H. percoides, E. denticulatus and E. lenimen; these were related to changes in abundance of particular prey species, not to alterations in feeding habits. Only three species, Lepidopus caudatus, Coelorinchus sp. 2 and H. percoides, showed significant diel feeding periodicity. Ontogenetic dietary changes were evident in Cyttus traversi, Coelorinchus sp. 2, Lepidorhynchus denticulatus and H. percoides. Cyttus traversi and H. percoides progressively changed from crustaceans to fish as their size increased. The diets of size classes within species showed little overlap, except for Lepidorhynchus denticulatus, which eats chiefly euphausiids and Lampanyctodes hectoris at all sizes. In addition to describing the diets and trophic relationships of 90% of the fish biomass, the results emphasize the importance to the entire fish community of mesopelagic food resources, particularly Lampanyctodes hectoris. Many benthopelagic species undertake extensive vertical migrations in search of prey, thus playing a major role in the transport of energy from midwater regions to the benthos of the continental slope.     

Deep-water decapod crustacean communities in the Northwestern Mediterranean: influence of submarine canyons and season

The specific composition and abundance of bathyal decapods in the Catalan Sea were investigated. A total of 109 bottom trawls were effected at depths ranging from 141 to 730 m on the continental slope in the Catalan Sea (northwestern Mediterranean) during two sampling cruises in spring and autumn 1991. Multivariate analysis of the samples revealed four groups of the decapod crustacean communities: (1) A shelf-slope transition-zone group at depths between 146 and 296 m, primarily characterized by the presence of Plesionika heterocarpus; (2) an upper-slope community between 245 and 485 m, characterized by the presence of the mesopelagic species Pasiphaea sivado and Sergestes arcticus, with Processa nouveli, Solenocera membranacea and Nephrops norvegicus as secondary species; (3) a middle-slope community below 514 m, with Aristeus antennatus and Calocaris macandreae as the most abundant species; (4) a group at 430 to 515 m, comprising all samples collected exclusively within or in the vicinity of submarine canyons. Mesopelagic decapods were predominant on the slope, while benthopelagic fishes (Merluccius merluccius, Micromesistius poutassou, Gadiculus argenteus) replaced mesopelagic decapods on the shelf. There were seasonal variations, with higher densities of mesopelagic species in spring, which were probably related, among other factors, to variations in the photoperiod. Our surveys also revealed higher species richness in the canyons together with seasonal changes in the megafaunal biomass. Generally, the upper and middle-slope communities both displayed seasonal changes in the composition and abundance of megabenthos.     

Observations in captivity of the activity patterns and resources utilization of the spider crab <Emphasis Type="Italic">Inachus phalangium</Emphasis> (Decapoda,Majidae)

The spider crab Inachus phalangium is common in the sublitoral fringe of the Mediterranean Sea and north-eastern Atlantic Ocean, where it can be found in association with the snakelocks sea anemone Anemonia viridis. Studies concerning its activity patterns and the role of the host sea anemone are lacking. Our study aimed at investigating activity rhythms and resources utilization of I. phalangium reared in captivity. The main behavioral traits exhibited by I. phalangium are performed mostly at night. Two experiments were designed, one examined the time budget of various behavioral acts and the degree of association with the sea anemone, the other analyzed the behavioral response to algae and anemones. We showed that algae have a crucial role in the biology of I. phalangium and that crabs are ready to leave the protection of their host to obtain them. Algae represent both the major component of the diet and one of the most utilized sources of masking material of I. phalangium, which provide, together with specialized cryptic behaviors, protection against predators. Although our data suggest that the association with A. viridis is not obligatory, but the role of the snakelocks sea anemone in the life of I. phalangium is still central, both as an anti-predatory defense and as a nutritional source. The association of I. phalangium with algae and the anemone is a facultative biotrophic commensalistic symbiosis.     

A survey of the shrimp resources of Nigeria

Previous planned resources surveys of the Nigerian coastal waters were performed with research vessels, gear and personnel not specialised for shrimp fishing. The results have, therefore, only indicated relative abundance of shrimps in the areas fished, rather than showing the absolute catches which a commercial fishing vessel in the area may expect to make. The present survey used two commercial shrimp trawlers together with their crews, who already had long experience of shrimp trawling both in the Nigerian shrimp grounds and elsewhere. Four shrimp species were caught: Penaeus duorarum, P. kerathurus, Parapeneopsis atlantica and Parapenaeus longirostris. Of these, P. duorarum, P. atlantica and P. longirostris are of commercial importance, P. duorarum being the most important. The distributions and concentrations of these 3 species are shown, together with the size distribution of P. duorarum, and the fish by-catch at each station. With the exception of the transect off the Nun Entrance, all transects sampled showed areas of high concentrations of P. duorarum, that is, 54 to 113 lbs (24 to 51 kg) of head-on shrimps per hour of trawling. The oceanographic conditions prevailing at the time of the survey were normal for the area. Records of previous shrimp landings from the area show a regular seasonal variation and the period of the survey to be one of lowest catches. The catch rates obtained in this survey are, therefore, the minimum which might be expected for the year. From a consideration of the operating costs of the type of shrimp trawler considered adequate for use in the area, it is found that the break-even shrimp catch is about 41 lbs (19 kg) of head-on per hour.     

Size-related trophodynamic changes in three target fish species recovering from intensive trawling

The consequences for food-web structure of protecting marine communities from trawling have been little explored. We investigated the Gulf of Castellammare, northwest Sicily, where a trawl ban initiated in 1989 has led to great increases in the biomass of some target species. Using stable isotopes of carbon and nitrogen, our objectives were to see if mean trophic level and omnivory had increased after the ban in three of the most common fishery-target species, namely, the anglerfish Lophius budegassa, Mediterranean hake Merluccius merluccius, and red mullet Mullus barbatus. We compared size data from before and after the fishery closure, but we also compared recently derived data from outside and inside the closed area. In all three species '¹³C and '¹⁵N were found to differ markedly depending on fish size. '¹³C was found to decrease in L. budegassa and Merluccius merluccius but increase in Mullus barbatus with increasing fish length. '¹⁵N increased in all three species, and this was thought to reflect feeding at increasingly higher trophic levels during the animals' lives. Mean length, and therefore '¹⁵N-derived trophic level, increased after the trawl ban only in the anglerfish L. budegassa. Based on '¹³C and '¹⁵N data, none of the species became more or less omnivorous after the ban. After 9 years of no trawling, increases in numerical abundance were not accompanied by substantial size-related trophodynamic shifts in any of the three species of fish studied.     

Karyotype analyses reveal inter-individual polymorphism and association of nucleolus-organizer-carrying chromosomes in Capros aper (Pisces: Zeiformes)

Three different karyomorphs with 2n=46, 2n=44 and 2n=42 for Capros aper (L., 1758) (Zeiformes) collected from the Gulf of Lion, near Banyuls-sur-Mer, France, in July 1990 were determined. Karyomorphs were characterized by the same arm number [Fundamental number (FN)=50], suggesting that chromosome variations are due to Robertsonian fusions. In somatic metaphase spreads stained with silver nitrate, nucleolus organizer regions (NORs) consistently occupied a terminal position on the short arms of two small submetacentric chromosomes. Ca. 30% of silver-stained metaphases in each specimen showed NOR chromosomes associated in pairs by their nucleolus organizer regions.     

Diets of clupeoid,stomiatoid and salmonoid fish of the Rockall Trough,northeastern Atlantic Ocean

The trophic interactions of species of fish of the continental slopes have not been investigated previously in detail. The present study examines the diets of the clupeoid, stomiatoid and salmonoid species occurring demersally and pelagically in the Rockall Trough to the west of Scotland and Ireland. Pelagic fish were collected between the surface and about 2 500-m depth between 1973 and 1978. Demersal fish were sampled at 250-m intervals of depths between 500- and 2 900-m depth during the years 1975 and 1981. Of the 28 species caught, 18 were strictly pelagic and dominated by the stomiatoids while five were strictly demersal. The remaining five species occurred in both the pelagic and demersal environment. The commonest clupeoid was Alepocephalus bairdii, which is a dominant species within the demersal fish associations at depths of 750 to 1 250m. This species, along with the deeper-living A. agassizi, and 3 other alepocephalid species feed primarily on benthopelagic prey but also exploit the epibenthos. The stomiatoid species, such as Cyclothone microdon, C. braueri, Maurolicus muelleri and Argyropelecus hemigymnus, dominate the associations of pelagic fish in the Trough. They feed on different proportions of ostracods, copepods and amphipods. Of the salmonoids, only Argentina silus and Bathylagus euryops occurred commonly and both feed on benthopelagic fauna, the latter at much greater depths than the former. The principal factor acting to prevent direct competition between species is the modal centres of bathymetric distribution of the species. Other contributing factors are briefly discussed.     

Limited trophodynamics effects of trawling on three Mediterranean fishes

Trawling has a significant effect on the structure of marine communities, yet the ubiquity of trawling impacts makes testing such effects difficult. This study examines trawling impacts on trophodynamics of three fishes among the Gulfs of Castellammare and Termini Imerese (northern Sicily), the first of which has been subject to a trawling ban since 1990 that initially resulted in an eightfold increase in total fish biomass. The Gulf of Termini Imerese remains heavily fished and was treated as a control site. Nitrogen and carbon stable isotope data were used to assess fishing induced changes in trophic levels or source of production supporting three demersal fish species; Mullus barbatus, Merluccius merluccius and Lophius budegassa following a control-impact approach. The exclusion of trawling resulted in only small alteration of δ¹⁵N in two of the three-three species. There were no systematic changes in the δ¹³C of any species sampled. Thus, a large influence of trawling on the trophodynamics of the studied species at sampled size was discounted. Although stable isotopes do not have spatial or temporal resolution to identify detailed shifts in diet composition, their integrative nature highlights that the trophic role these species play is robust to fishing impacts at scales over which the fishery operates. This is despite a significant increase in the abundance of these species and of total fish biomass within the protected area.

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Tropical fishes in a temperate sea: evolution of the wrasse <Emphasis Type="Italic">Thalassoma pavo</Emphasis> and the parrotfish <Emphasis Type="Italic">Sparisoma cretense</Emphasis> in the Mediterranean and the adjacent Macaronesian and Cape Verde Archipelagos

The northeastern Atlantic and the Mediterranean Sea share geological histories and display great faunal affinities. The majority of the Mediterranean species have Atlantic origins, with a few species with tropical affinities. These include the parrotfish Sparisoma cretense and the wrasse Thalassoma pavo that are restricted to the subtropical northeastern Atlantic, the Macaronesian archipelagos (Azores, Madeira, and Canaries) and the southern Mediterranean. The Pleistocene glaciations have been described as having different effects on the fauna of the two regions. During glacial peaks, Mediterranean waters remained warmer than those of the adjacent Atlantic. Within the eastern Atlantic, the effects of Pleistocene glaciations were differentiated. Here, we perform a comparative analysis focusing on T. pavo and S. cretense populations from the northeastern Atlantic and the Mediterranean to assess the effects of Pleistocene glaciations in these two species. Sequences from the mitochondrial control region were obtained and analyzed combining phylogeographic and demographic approaches. Gene flow between Atlantic and Mediterranean populations was shown to be very high. The Mediterranean populations of T. pavo and S. cretense showed high levels of genetic diversity, even in the eastern basin, pointing to an ancient colonization event. This suggests that both species must have been able to persist in the Mediterranean during the cold Pleistocene periods. Historical migration estimates revealed a Mediterranean towards Atlantic trend in the case of T. pavo, which may reflect the re-colonization of areas in the Atlantic by fish that survived the cold phases in relatively warmer Mediterranean refugia. Our data also showed that within the Macaronesian Archipelagos, migrations occurred from Madeira towards the Azores, for both T. pavo and S. cretense, thus supporting a post-glacial colonization of the Azores by fish that persisted in the warmer region of Madeira. Similar geographic distributions, thermal affinities, and means of dispersion for T. pavo and S. cretense resulted in a similar response to the effects of Pleistocene glaciations, as evidenced by identical phylogeographic patterns.     

Food resource partitioning in a Mediterranean demersal fish assemblage: the effect of body size and niche width

We investigated the effects of body size, feeding strategy and depth distribution on the trophic resource partitioning among the 26 dominant fish consumers in a fish assemblage on the central Mediterranean shelf-break. The fish assemblage was structured in two major trophic guilds: epibenthic and zooplanktonic feeders, according to the position of each predator along the benthos–plankton gradient. Within each main guild, the species were segregated along a prey-size or fish-size gradient into five further guilds. Fish size and prey size were strongly correlated, indicating that the prey-size niche can be well explained by predator size. Fish consumers showed a significant negative correlation between the similarity in prey type and the similarity in depth distribution; most species with similar trophic preferences segregated along the depth dimension. The only predators overlapping in both food and depth preferences were those with a more specialist trophic behavior. These results suggest that fish body size and depth preferences are the two main niche dimensions, explaining a large part of the coexistence between the Mediterranean shelf-break fish consumers.     

Small-scale spatial and temporal interactions among benthic crustaceans and one fish species in the Bay of Biscay

In the summer of 2004, a video survey was carried out in the northern part of the central mud bank (Grande Vasière) of the Bay of Biscay to study the small scale relationship between the dominant crustacean megafauna Nephrops norvegicus, Munida rugosa and Goneplax rhomboides and juvenile hake (Merluccius merluccius). Using a towed body, high-resolution videos were recorded in six sampling sites. Statistical modelling using generalised additive models (GAM) revealed variations in activity patterns for two species. More N. norvegicus were observed outside their burrows at dawn and somewhat at dusk (no observations during night) while G. rhomboides was less observed in the morning. In addition, reduced spatial overlap between G. rhomboides and N. norvegicus suggested reduced competition for food but also space as both are burrowing species. The observed temporal and spatial activity patterns may contribute to regulating assemblage structure as competing species may be actively foraging at different times and locations thus reducing direct competition.     

Evaluating day–night changes in shallow Mediterranean rocky reef fish assemblages by visual census

Ecological information on coastal fish distribution patterns and habitat use can be greatly improved by nocturnal samplings and observations. To this purpose, the structure of a Mediterranean fish assemblage inhabiting the shallow rocky littoral of Linosa Island (Sicily Strait, Italy) was examined by using visual census to detect possible diel variations in species composition and abundance. Day–night fish distribution patterns were investigated by multivariate and univariate analyses. Overall, 42 fish taxa belonging to 19 families were recorded: 35 during the day and 24 during the night. Seventeen species were common to both diurnal and nocturnal assemblages. Within the diurnal assemblage, Chromis chromis was the most represented species (37.2%), followed by Thalassoma pavo (23.2%) and Sparisoma cretense (10.8%). Within the nocturnal assemblage, the most abundant taxon was Atherina spp. (33.9%), followed by Apogon imberbis (26.4%) and Boops boops (11.5%). Our results indicated wide variation in the abundance and species composition during the day and during the night. Multi Dimensional Scaling plot showed a clear-cut separation between the two assemblages and analysis of similarities found significant differences as well. SIMPER analysis revealed that ten species individually contributed by more than 2.5% to the dissimilarity between diurnal and nocturnal assemblages, T. pavo, C. chromis and S. cretense being the first three species in order of decreasing percentage. ANOVA performed on species richness and fish abundance detected significant differences between diurnal and nocturnal assemblages, the latter showing far lower average values for both variables.     

Diets and bathymetric distributions of the macrourid fish of the Rockall Trough,northeastern Atlantic Ocean

The Macrouridae are the most common fish caught in demersal trawls on the continental slope and rise of the Rockall Trough. They represented 41% of all fish caught, with Coryphaenoides rupestris amounting to 28% of the catch. No previous study of the trophic interaction of these fish has been made over a wide bathymetric range. Samples were obtained at 250-m intervals of depth between 400 and 2 900 m in the period 1975 to 1981. The stomachs of 5 326 fish belonging to 12 species were examined to define their diets. Eight species are primarily benthopelagic feeders while four are primarily epibenthic feeders. The bathymetric centres of distribution of the populations of the benthopelagic feeding Trachyrhynchus murrayi, Malacocephalus laevis, Coryphaenoides rupestis, C. guentheri, Nematonurus armatus, Chalinura brevibarbis, C. leptolepis and C. mediterranea are different from each other. Similar differences in bathymetric distribution occurred among the epibenthic feeding Nezumia aequalis, Coelorhynchus coelorhynchus, C. occa and Lionurus carapinus. These differences decrease competition among species that exploit similar resources.     

Megabenthic decapod crustacean assemblages on the Galician continental shelf and upper slope (north-west Spain)

The structure of megabenthic decapod crustacean assemblages on the Galician (north-west Spain) continental shelf (100 to 200 depth) and upper slope (200 to 500 m) was analyzed based on surveys carried out in autumn and spring, from 1980 to 1987. Forty species belonging to 19 families were caught. The portunid crab Polybius henslowii, a species with pelagic phases, was the most abundant species, but displayed strong spatial and temporal fluctuations. Other dominant species were the Norway lobster Nephrops norvegicus, the portunid Liocarcinus depurator, the galatheids Munida intermedia and M. sarsi, and the shrimps Solenocera membranacea, Plesionika heterocarpus, Pasiphaea sivado and Dichelopandalus bonnieri. Total abundance and biomass (average values excluding Polybius henslowii = 255 individuals and 2.06 kg/30 min tow) and species richness and diversity, H′ (6.85 species and H′ = 1.45 per tow) displayed a significant positive correlation with depth, and strong interannual fluctuations. The factors determining community organization were depth and, to a lesser extent, spatial structure. There was clear evidence of bathymetric zonation, differentiating between species characteristic of the slope (D. bonnieri and Pasiphaea sivado), shelf-slope edge (Macropipus tuberculatus, Pontophilus spinosus, Munida sarsi, S. membranacea, Processa spp.) and shelf (L. depurator, Macropodia tenuirostris, Paguridae and Chlorotocus crassicornis). The spatial zonation was related to changes in oceanography and sediment along the continental margin. Goneplax rhomboides, N. norvegicus, C. crassicornis and Alpheus glaber are benthic species which generally exhibit burrowing behaviour, and they were found mainly in the southern area where there are fine sediments due to the outwelling from the Rías Baixas. Different benthopelagic shrimps (Pontophilus spinosus, Plesionika heterocarpus,Processa spp. and Pasiphaea sivado) were typical of the zone just north of Fisterra, characterized by a convergence of water masses bringing about an increase in productivity due to upwelling. The benthic anomuran and brachyuran crabs Munida intermedia, M.␣sarsi, L.␣depurator and Macropipus tuberculatus were characteristic of the northwestern zone between Fisterra and Estaca, where the infauna reaches high biomass despite coarser sediments with a lower concentration of organic material than in the southern area. Lastly, both the Paguridae and Macropodia tenuirostris were species typically found in the waters in the northern shelf. Based on interannual changes in assemblage structure, two periods could be distinguished: between 1980 and 1984, when Polybius henslowii, D. bonnieri and Pasiphaea sivado had abundance peaks; and another period from 1985 to 1987 when L. depurator, Munida intermedia, M.␣sarsi and Macropipus tuberculatus increased in abundance. Received: 21 May 1996 / Accepted: 9 August 1996     

Spatial distribution patterns of the gorgonians <Emphasis Type="Italic">Eunicella singularis</Emphasis>, <Emphasis Type="Italic">Paramuricea clavata</Emphasis>, and <Emphasis Type="Italic">Leptogorgia sarmentosa</Emphasis> (Cape of Creus,Northwestern Mediterranean Sea)

Most of the current knowledge on Mediterranean gorgonians is restricted to investigations of those populations found within shallow sublittoral waters, and only limited data are available for populations located below scuba depth. To overcome this lack of information, the occurrence and abundance of the gorgonians Eunicella singularis, Paramuricea clavata, and Leptogorgia sarmentosa were investigated in northwestern Mediterranean benthic communities over a wide geographical (60 km of coastline) and bathymetrical (0–70 m deep) extent using a remotely operated vehicle (ROV). The greatest occurrence and abundances of E. singularis and P. clavata were concentrated in areas that are directly exposed to strong near-bottom currents. E. singularis was the most common and abundant species and displayed great plasticity and amplitude in its environmental preferences. Conversely, P. clavata showed a very patchy distribution that was associated with vertical rocky walls. Only isolated colonies of L. sarmentosa were observed in the study area. Hot spots of abundance of E. singularis and P. clavata were identified below a depth of 40 m, which demonstrates the importance of studying the distribution of benthic species over a wide geographical and bathymetrical extent.     

Unexpected new species of deep-water Hydroidomedusae from Korsfjorden,Norway

Two new species of Hydroidomedusae, Parateclaia norvegica sp.nov. and Foersteria quadrata sp.nov., are described from specimens collected in Korsfjorden, western Norway. Neither genus has previously been observed in the North-Atlantic Ocean. The closest congenerics to the new species occur in the Mediterranean, where family Teclaiidae has been considered endemic. P. norvegica and F. quadrata were only collected at sites 500 m or deeper and are possibly deep-water benthopelagic species like their congenerics elsewhere. Sampling took place for the entire year 2003 at 2- to 4-week intervals. F. quadrata, the more numerous of the new species, was absent from the water column only during the spring months, the implications of which regarding its lifecycle are discussed. Hypotheses on speciation in the Foersteria species group and family Teclaiidae are shortly discussed.     

Elemental composition of otolith growth marks in three geographically separated populations of European hake (Merluccius merluccius)

The composition of the opaque and translucent bands in the otoliths of European hake (Merluccius merluccius) from three different populations (Celtic Sea, Galicia shelf and Cretan Sea) was analysed with wavelength dispersive spectrometry (WDS) and differences in Ca, Sr and Na were compared. The translucent bands (annual marks and checks altogether) were significantly richer in Sr and Ca and significantly poorer in Na than opaque bands. Results support the idea of a more compact arrangement of crystals in translucent bands compared to opaque. Analyses did not help to discriminate between putative annual marks and checks. Clear differences in the range of Sr/Ca variation across the otolith as well as in the composition of the otolith core were observed among the three populations. As a result it was possible to discriminate the natal origin of each fish using the composition of the otolith core in Ca, Sr and Na. Nevertheless, the overall pattern across the otolith was similar among the three populations suggesting the existence of ontogenetic trends. Variation of Na/Ca ratios was largely uninformative. Based on these results, a conceptual model for hake is proposed which integrates the different sources of otolith Sr/Ca variation: ambient Sr/Ca, endogenous cues (ontogeny) and exogenous cues (environment).     

Biological traits of three closely related species of <Emphasis Type="Italic">Pycnoclavella</Emphasis> (Ascidiacea) in the Western Mediterranean

The reproductive cycles and abundance of the sympatric colonial ascidians Pycnoclavella brava, Pycnoclavella aurilucens and Pycnoclavella communis from two Northwestern Mediterranean sites over a period of 2.5 years are reported. The species showed some differences in their biological patterns. Gonad maturation and larval brooding took place during autumn and early winter for P. communis, during spring for P. aurilucens and during late winter and spring for P. brava. Summer was found to be the unfavorable season for all species, and an aestivation period occurred in P. communis and P. brava. Maximum abundance for the three species was observed during winter. Growth rates of the most abundant species, P. communis, were also monitored and found to peak during the initial phases of reactivation after aestivation. Growth rates decreased afterwards, becoming negative as water temperature started to increase after the winter minima. Reproductive activity and growth rates for P. communis displayed a temporal lag that suggested partitioning of resources to either reproduction or growth in this species.     

Trophic implications of cross-shelf copepod distributions in the Southeastern Bering Sea

Spring distributions of some numerically dominant copepods reflect associations with two distinct water masses separated along the 80- to 100-m isobaths. Seaward of this middle shelf front, the oceanic Bering Sea hosts populations of Calanus cristatus, C. plumchrus, and Eucalanus bungii bungii; Metridia pacifica, Oithona similis, and Pseudocalanus spp. are also present. The large oceanic species are much less abundant in waters shallower than 80 m where the community is seasonally dominated by smaller copepods, O. similis, Acartia longiremis, and Pseudocalanus spp. Experimental and field-derived estimates of carbon ingestion indicate that the oceanic/outer shelf copepods can occasionally graze the equivalent of the daily plant production and probably routinely remove 20–30% of the primary productivity. Conversely, stocks of middle shelf copepods rarely ingest more than 5% of the plant carbon productivity. During 45 d between mid April to late May, 1979, approximately three times more organic matter was ingested m^-2 by the outer shelf/oceanic copepod community than by middle shelf species. This imbalance in cross-shelf grazing permits middle shelf phytoplankton stocks to grow rapidly to bloom proportions, and to sink ungrazed to the seabed. Over the outer shelf and particularly along the shelf break, a much closer coupling to phytoplankton supports a large biomass of oceanic grazers. Here, copepod stocks approaching 45 g dry wt m^-2 occur in late spring as a narrow band at the shelf break.Supported by National Science Foundation Grant DPP 76-23340Contribution no. 485, Institute of Marine Science, University of Alaska, Fairbanks