Food habits and intestinal parasites of deep demersal fishes from the upper continental slope east of Newfoundland,northwest Atlantic Ocean

Stomach contents and intestinal parasite faunas of 471 individuals of demersal fishes in 14 species were examined from the Carson Canyon region (Lat. 45°30N; Long. 48°40W) of the upper continental slope of the Grand Banks off Newfoundland, Canada. Individual species tended to feed either on benthic or on pelagic/benthopelagic organisms, but pelagic prey assumed the greatest importance overall. Data from stomach contents were supported by the parasite information. Prevalence of parasites was higher in benthic feeders (53.1%) than in pelagic feeders (28.9%), and relative abundance by major group was: Digenetic Trematoda 5.8% benthic vs 27.8% pelagic, Nematoda 53.1% vs 72.2%, and Acanthocephala 40.9% vs 0%. Of the dominant fishes, there were more species of benthic feeders (5) than pelagic feeders (3), but pelagic feeders were numerically more abundant (pelagic 70.9%, benthic 20.5%). Benthic feeders were on average larger (=270.6g) than pelagic ones (=130.6g), but pelagic feeders represented a larger proportion of the biomass (pelagic 43.3%, benthic 25.9%). The results of this study combined with those from other areas suggest that feeding from the pelagial by demersal fishes at upper continental slope depths is probably the general rule.     

Diets of the demersal fishes on the shelf off Iwate,northern Japan

Diets of demersal fishes were determined on the shelf (ca. 130 m deep) off Iwate, Japan. Samples were taken in three different types of habitat, an artificial reef (AR) site, a natural reef (NR) site, and sandymud bottom (SB) site, from May 1987 to September 1991, mostly every two months. A total of 67 prey items were recognized in the stomachs of 45 predator fish species. The most important preys were the pelagic fishes Sardinops melanostictus and Engraulis japonicus, which comprised 37% wet wt of the overall stomach contents. The percentage of pelagic fishes was highest at AR site, where fish density was highest. The dominant ten species could be divided into five feeding types. The pelagic fish feeders Physiculus maximowiczi and Gadus macrocephalus fed mainly on S. melanostictus. The dietary breadth of P. maximowiczi was wide, while that of Gadus macrocephalus was narrow. The pelagic crustacean feeder Theragra chalcogramma mostly consumed Themisto japonica and euphausiids and showed the least dietary overlap with other fishes. Benthic fish feeders were Hemitripterus villosus and Liparis tanakai. The benthic crustacean feeders Alcichthys alcicornis and Hexagrammos otakii consumed benthic crustaceans as well as pelagic and benthic fishes and showed the largest dietary breadth. The benthic invertebrate feeders Gymnocanthus intermedius, Dexistes rikuzenius and Tanakius kitaharai fed mainly on polychaetes and benthic crustaceans. But Gymnocanthus intermedius consumed a significant proportion of pelagic fishes. Ontogenetic dietary shift was recognized for these fishes. Pelagic fishes were consumed more intensively by larger individuals, especially true of A. alcicornis, Theragra chalcogramma and Gadus macrocephalus. Predominancy of the two most adundant species, P. maximowiczi and A. alcicornis, may be supported by their wide dietary breadth and the significant proportion of pelagic fish in their diets. Interspecific dietary overlap was low in most cases suggesting that food resources were well partitioned, although some high overlap was observed among the pelagic fish feeders, A. alcicornis, and Gymnocanthus intermedius, and among the benthic invertebrate feeders. Interspecific competition seemed more likely in the benthic invertebrate feeders than in the pelagic fish feeders partly because of superabundance of the pelagic prey S. melanostictus.     

The megabenthic fauna in the deep sea south of New England,USA

Data from 105 benthic trawls made in an area south of New England, USA between 40 and 5000 m show the fauna to be zoned with depth, areas of rapid faunal change separating regions of relative faunal homogeneity. Distinct faunal assemblages with characteristic catch rates, diversity, and dominant species are found on the shelf (40 to 264 m), upper continental slope (283 to 650 m), middle continental slope (653 to 1290 m), lower continental slope (1380 to 1947 m), the transitional region from slope to rise (2116 to 2481 m), the upper continental (2504 to 3113 m), the middle continental rise (3244 to 3470 m), and lower continental rise to abyssal plain (3879 to 4986 m). Catch rates and diversity are greatest on the lower continental slope and transition to the upper rise, and are lowest at the greatest depths. Dominance, particularly by echinoderms, is an important aspect of community structure. The 3 major taxa represented (decapod crustaccean, echinoderms, and fishes) do not always display the same patterns within and between assemblages. Generalities derived from study of a single group need not apply to all segments of the deep-ocean community. Overall patterns in the megafauna are similar to those described in other groups and areas, but species assemblages are not the same everywhere and perhaps too much has been made of the horizontal extent of zones. Trophic level is related to degree of zonation, but where predators are generalists their ranges may be wide rather than restricted. Diversity patterns can be understood in terms of the interrelationships of predation, competition, environmental heterogeneity, and trophic level. Faunal zones are of importance as the geographical units within which evolution, community development, and diversification take place.This research was supported by the National Science Foundation under Grant OCE76-21878, the Office of Naval Research under Contract N00014-74-C0262, and, in part, by the United States Department of Energy under Contract No. EY-76-C-02-0016 and the Canadian Natural Sciences and Engineering Research Cancil under Grant A-7230     

Biodiversity in the ecosystem of the Portuguese continental shelf: distributional ecology and the role of benthic amphipods

The biodiversity and distributional ecology of amphipod crustaceans were analysed from benthic surveys in 1985 and 1986 along the coast of Portugal, from the Tagus canyon to the coast of Minho, in order to determine their role in the ecosystem of the Portuguese continental self. In samples obtained from different types of substrate between 18 and 545 m, we identified 113 species belonging to 51 genera of 24 families. Seventeen of these species were new records for the Portuguese coast. The relative frequencies and number of individuals of all species were analysed. The genus Ampelisca clearly dominated the group of the most frequent and abundant species, which also exhibited a large range of vertical distribution. Most of the 113 species were found only down to 150 m depth, corresponding roughly to the lower limit of the continental shelf. Fifteen species exhibited clear bathyal traits, being consistently found deeper than 100 m, especially in the Tagus canyon and the upper zone of the continental slope. The most frequent and abundant species were found predominatly or in large numbers in medium to fine sand bottoms. This type of substrate also contained the highest number of species. The granulometric structure of the habitat could constitute the most important factor controlling both biodiversity and development of large amphipod populations, since oxygen level, water exchange, number of available shelters, and probably the quantity and quality of nutritional resources all vary as a function of substrate texture. Because of their dominance in both frequency and abundance, several Ampelisca species may constitute an important food source for many secondary consumers of the shelf sediments, especially for demersal fishes.     

Recherches sur la situation trophique d'un groupe d'organismes pélagiques (Euphausiacea). V. Relations avec les thons

The examination of stomach contents of large subsurface tunas caught by long-line fishing, as well as of those of micronektonic fishes which they had eaten, allows us to assess the role played by euphausiids in food webs concerning these fishes. Long-line tunas do not utilize euphausiids as food because, in the tropical Pacific Ocean, their feeding activity is restricted to light hours, i.e., to a time when large euphausiids which could serve as potential prey, occupy deeper (below 400 m) water layers. Euphausiids constitute approximately 10% of the total food ingested by epipelagic micronektonic fishes which, in turn, are preyed upon by tunas; these fishes, measuring usually 30 to 60 mm in total length, are fast swimmers and are rarely collected by pelagic trawls. Among this ichthyofauna, the main predators of euphausiids appear to be Gempylidae (Diplospinus multistriatus), Paralepididae, and Bramidae (Collybus, Taractes, Pteraclis). Euphausiids ingested by these fishes chiefly belong to the genera Stylocheiron (64%) and Nematoscelis (15%). This means that the fishes which serve as prey for tunas are able to utilize for food only organisms which inhabit subsurface (0 to 400 m) layers during daytime and not those (Euphausia, Thysanopoda) which remain at this time in the deeper layers and rise to the subsurface only at night. Therefore, these fishes, like tunas themselves, appear to display their main feeding activity in the 0 to 400 m water layer and during daytime.     

Use of pelagic prey subsidies by demersal predators in rocky reefs: insight from movement patterns of lingcod

Allochthonous subsidies of energy and nutrients can affect community structure in patchy marine habitats, including rocky reefs, and their ecological consequences may depend on the mechanism of energy transfer. Lingcod (Ophiodon elongatus) are demersal predators that trophically link nearshore rocky reefs with offshore pelagic habitats through consumption of pelagic fishes. We quantified lingcod habitat use and movement patterns to make inferences about the temporal and spatial conditions under which lingcod may acquire pelagic prey. Lingcod maintained small home ranges (21,272 ± 13,630 m²) within a rocky reef in the San Juan Archipelago, Washington; eight of nine individuals used rocky habitat exclusively. Depths occupied by lingcod (0–50 m) coincided with pelagic fish distribution on the rocky reef; however, diel patterns in lingcod activity varied inversely with occurrence of pelagic fishes on the reef. Our findings suggest that the pelagic subsidy to lingcod is not strongly mediated through directed off-reef foraging by lingcod.     

Recherches sur la situation trophique d'un groupe d'organismes pélagiques (Euphausiacea). VI. Conclusions sur le rôle des euphausiacés dans les circuits trophiques de l'océan Pacifique intertropical

The role of euphausiids in the food webs of the Intertropical Pacific Ocean is defined through analysis of their nutrition, vertical distributions and migrations, and their utilization by pelagic predators. It is suggested that the abundance of the group, the extensive vertical migrations of many species and the fact that feeding takes place mainly in subsurface layers, result in a leading role of euphausiids in energy transfer between different bathymetric levels. For night-time feeding predators, they represent a noticeable food source only in the 0 to 300 m water layer, as 97% of the euphausiid biomass concentrates in this layer at night. In the daytime, only the smaller specimens (chiefly genus Stylocheiron), accounting for 10 to 15% of the whole biomass of the group, remain available for epipelagic (0 to 400 m) predators, larger individuals dwelling deeper. Euphausiids account for 8 to 10% of the food ingested by micronektonic fishes, but the species are not the same for different categories of fishes. Migrating fishes caught by pelagic trawls, more or less connected with the deep scattering layer, feed on migrating species in subsurface layers at night as well as in deeper layers during the daytime, and on non-migrating species inhabiting shallower and intermediate layers. On the other hand, fishes which comprise the prey of large long-line tunas, which are not caught by trawls because they are fast swimmers, feed almost solely on species which remain above 400 m in the daytime. These results suggest a certain degree of independance between the trophic webs which concern, on the one hand, epipelagic ichthyofauna (including tuna), and, on the other hand, migrating and deep-living faunas. Migrating populations are able to feed at night upon subsurface organisms, a part of this resource being then transmitted during the day to the deep-living fauna; but the epipelagic ichthyofauna, with a feeding activity restricted to light hours, has few possibilities to benefit from the migrating or deepliving biomass. Therefore, energy transfers seem to be intense only from subsurface (0 to 400 m) to deeper layers. From a more general point of view, these investigations suggest that, in the pelagic system, vertical distributions and migrations, and feeding rhythms, are the main factors determining the structure of the food webs.     

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.     

Underwater and above-water search patterns of an Arctic seabird: reduced searching at small spatiotemporal scales

How predators vary search patterns in response to prey predictability is poorly known. For example, marine invertebrates may be predictable but of low energy value, while fish may be of higher energy value but unpredictable at large (pelagic schools) or small (solitary benthics) spatial scales. We investigated the search patterns of the thick-billed murre (Uria lomvia), an Arctic seabird feeding on invertebrates, pelagic fish, or benthic fish. Foraging ranges at the Coats Island colony are generally smaller (<240 min per trip) than at larger colonies, and many birds specialize in foraging tactics and diet. Underwater search times for benthic fish were higher than for pelagic fish or invertebrates while above-water search times for pelagic fish were higher than for benthic fish or invertebrates. There were few stops during trips. Total trip time, flying time, number of flights, and number of dives were intercorrelated and increased with prey energy content, suggesting that longer trips involved fewer prey encounters due to selection of higher-quality, but rarer, prey items. Flight times were not Lévy-distributed and seabirds may have used area-restricted searches. The high degree of specialization, apparent absence of information center effects, and reduced above-water searching times may be linked to the relatively small colony size and the resulting short commuting distances to feeding areas, leading to greater prey predictability. We concluded that prey predictability over various scales affected predator search patterns.     

Restoring piscivorous fish populations in the Laurentian Great Lakes causes seabird dietary change

Ecosystem change often affects the structure of aquatic communities thereby regulating how much and by what pathways energy and critical nutrients flow through food webs. The availability of energy and essential nutrients to top predators such as seabirds that rely on resources near the water's surface will be affected by changes in pelagic prey abundance. Here, we present results from analysis of a 25-year data set documenting dietary change in a predatory seabird from the Laurentian Great Lakes. We reveal significant declines in trophic position and alterations in energy and nutrient flow over time. Temporal changes in seabird diet tracked decreases in pelagic prey fish abundance. As pelagic prey abundance declined, birds consumed less aquatic prey and more terrestrial food. This pattern was consistent across all five large lake ecosystems. Declines in prey fish abundance may have primarily been the result of predation by stocked piscivorous fishes, but other lake-specific factors were likely also important. Natural resource management activities can have unintended consequences for nontarget ecosystem components. Reductions in pelagic prey abundance have reduced the capacity of the Great Lakes to support the energetic requirements of surface-feeding seabirds. In an environment characterized by increasingly limited pelagic fish resources, they are being offered a Hobsonian choice: switch to less nutritious terrestrial prey or go hungry.     

Coordinated mechanics of feeding,swimming, and eye movements in <Emphasis Type="Italic">Tautoga onitis</Emphasis>, and implications for the evolution of trophic strategies in fishes

Fish feeding behavior results from successful coordination of the fins, jaws, and sensory systems, and the organization of this behavior may affect the fish’s foraging abilities and trophic ecology. Using quantitative kinematic methods, movements of the jaws, fins and eyes of Tautoga onitis (Teleostei: Labridae) were analyzed during feeding events. Tautog feeding events consisted of three phases: approach, strike, and recovery, each defined by a combination of kinematic events. The approach was characterized by changes in fin movements and in body position, with the eyes directed forward at the food item. The strike began with the onset of jaw opening and protrusion, then cranial elevation, with the eyes no longer looking at the food item. The end of the strike and the beginning of the recovery involved a braking maneuver with the pectoral fins; the fish turns and swims away from the original food location item after prey capture. The coordination performance variables of tautog were quantitatively compared to published data from closely related cheiline wrasses and parrotfishes, which represent different feeding ecologies within a monophyletic assemblage. Fishes feeding on molluscs and benthic invertebrates (Cheilinus fasciatus and Tautoga onitis) represented an intermediate coordination condition, with herbivores (the parrotfishes, Scarus quoyi and Sparisoma radians) at one extreme, and fishes feeding on elusive prey (Epibulus insidiator and Oxycheilinus digrammus) at the other extreme. The analysis suggests that the biomechanical demands of coordination for feeding on benthic invertebrates may represent a generalized, and perhaps ancestral behavior in the wrasses, whereas more specialized trophic niches have evolved divergent, more specialized demands. Examining the movement and coordination of the jaws, fins, and eyes during fish feeding provides a detailed mechanistic basis for behavior, and comparison of coordination patterns during feeding among different taxa can measure how these trophic strategies differ. Understanding the evolution of feeding ecologies in these demersal fishes may have implications for understanding their role within their shallow water reef community.     

Enzyme activities of fish skeletal muscle and brain as influenced by depth of occurrence and habits of feeding and locomotion

Activities of lactate dehydrogenase (LDH), pyruvate kinase (PK), malate dehydrogenase (MDH) and citrate synthase (CS) were measured in the white skeletal muscle of marine fishes having different depths of occurrence and different feeding and locomotory strategies. There were significant depth-related differences in the two glycolytic enzymes, LDH and PK. LDH activity was most variable, and differed by 3 orders of magnitude between the most active shallow-living species and certain deep-sea fishes likely to have only minimal capacities for active locomotion. Superimposed on the depth-related patterns was a high degree of interspecific variation (up to 20-fold) in enzymic activity among species from any given range of depth of occurrence. Variation of both LDH and PK activities, noted for shallow- and deep-living fishes, seems to be largely accounted for by differences in feeding habits and locomotory performance. Active pelagic swimmers have much higher activities of LDH and PK than, for example, deep-living sit-and-wait predators. Benthopelagic fishes like rattails and the sablefish have the highest activities found among deep-living fishes, suggesting that these species engage in relatively active food-searching behavior compared to most other deep-sea fishes. The activity of CS, an enzyme of the citric acid cycle and an indicator of aerobic metabolism, varied little among species. Thus, the large interspecific variation in glycolytic potential (LDH and PK) among species is not associated with a similar variation in aerobic metabolism of white muscle. The much higher and more variable activity of MDH relative to CS suggests that, in addition to its function in the citric acid cycle, MDH may play an important role in redox balance in fish white muscle. In a comparison of white muscle composition between the shallow- and deep-living species, water content did not differ significantly, but protein content was significantly higher in shallow- than in deep-living fishes (211 and 130 mg g^-1 wet wt of muscle, respectively). The differences in muscle protein content are small relative to the differences between shallow- and deep-living species in LDH, PK and MDH activities. Thus, depthrelated differences in muscle enzymic activity are caused by factors other than enzyme dilution. Enzyme activities (LDH, PK and CS) in brain tissue were relatively constant among species regardless of depth of occurrence or feeding and locomotory habits. Habitat and lifestyle do not seem to influence the demands for overall metabolic function in brain. The utility of muscle enzymic activity data for making predictions about the ecological characteristics of difficult-to-observe, deep-living, fishes is discussed.     

Importance of jumbo squid Dosidicus gigas (Orbigny, 1835) in the pelagic ecosystem of the central Gulf of California

The Humboldt squid is an important predator in the pelagic ecosystem of the central Gulf of California and the commercial catch of this species has increased over the past decade, probable due to a decrease of several top predators (sharks, large pelagic fish and the marine mammals) and the optimal feeding conditions in this area. Its high abundance and important position in the pelagic food web was quantified through two trophic models of the pelagic ecosystem of the central Gulf of California. Models represented conditions in 1980 and 2002, to document the decadal changes in ecosystem structure and function. The models were composed of 18 functional groups, including marine mammals, birds, fish, mollusks, crustaceans, and primary producers. Model results show direct negative effects on principal prey groups such as myctophids and pelagic red crab and positive effects on sharks, marine mammals and specifically sperm whales. It thus appears that the jumbo squid has an important role in the ecosystem and plays a central part in the overall energy flow as main food item for most top predators, and due to its predation of organisms on lower tropic levels.     

Diets of deep-sea brachyuran crabs in the Western Mediterranean Sea

This study examines the feeding habits of Paromola cuvieri (Risso, 1816) and Geryon longipes A. Milne Edwards, 1881, the only two common deep-sea brachyuran crabs inhabiting the bathyal mud assemblages in the Catalan Sea (Western Mediterranean). Samples were obtained by bottom trawls at depths between 360 and 1871 m during 1983 to 1992. Both species had highly diverse diets, but very low feeding activity, as reflected by the high proportion of empty stomachs. Both characteristics may be important factors enabling deepsea crabs to adapt to bathyal zones, where trophic resources are scarce. The most important food items found in P. cuvieri were fish remains (teleost, sharks) and benthic decapods (Monodaeus couchii, Munida tenuimana). Scavenging activity plays an important role in this species. The diet of G. longipes included a broad range of benthic invertebrates. In the upper middle slope, the bivalve Abra longicallus, decapods (Calocaris macandreae and Monodaeus couchii), echinoderms and polychaetes were the dominant prey, with epibenthic peracarids as a secondary resource. On the lower middle slope, the incidence of decapod crustaceans (C. macandreae, Pontiphilus norvegicus) and peracarids in the diet declined. Small macrobenthic prey (glycerids, cumaceans or amphipods) were rare in the diet of both species, in accordance with the large size of the crab specimens studied. The absence of preferred prey items and the lack of food items of an optimum size on the lower slope may contribute to the progressive decline in abundance of P. cuvieri and G. longipes with increasing depth.     

Metabolic enzyme activities in shallow- and deep-water chondrichthyans: implications for metabolic and locomotor capacity

Biochemical indices of white (WM) and red muscle (RM) aerobic and anaerobic metabolic capacity were measured in 14 species of benthic and benthopelagic chondrichthyans from a depth of ~90 to 2,200 m to evaluate the relationship between metabolic capacity and depth of occurrence, phylogeny, and locomotor mode. Maximal activities of the enzymes citrate synthase, malate dehydrogenase (MDH), lactate dehydrogenase (LDH), and pyruvate kinase (PK) were analyzed in muscle tissue at 10 °C. These were combined with previously published elasmobranch data in order to represent a comprehensive range of depths, phylogeny, and locomotor modes (i.e., benthic, benthopelagic, pelagic). Significant decreases in WM PK and LDH activities and a lack of significant trends in RM enzyme activities with increasing median depth of occurrence (MDO) indicate a depth-related reduction in both burst-locomotor and metabolic capacity. These trends are consistent with the “visual-interactions hypothesis.” Phylogeny and locomotor mode had little influence on enzyme activities compared to MDO, and the present study suggests similar activities in co-occurring demersal sharks and rays. Overall, the present study indicates low metabolic capacities in deep-sea chondrichthyans, which is important to consider when managing deep-sea fisheries.     

Terrestrial, benthic, and pelagic resource use in lakes: results from a three-isotope Bayesian mixing model

Fluxes of organic matter across habitat boundaries are common in food webs. These fluxes may strongly influence community dynamics, depending on the extent to which they are used by consumers. Yet understanding of basal resource use by consumers is limited, because describing trophic pathways in complex food webs is difficult. We quantified resource use for zooplankton, zoobenthos, and fishes in four low-productivity lakes, using a Bayesian mixing model and measurements of hydrogen, carbon, and nitrogen stable isotope ratios. Multiple sources of uncertainty were explicitly incorporated into the model. As a result, posterior estimates of resource use were often broad distributions; nevertheless, clear patterns were evident. Zooplankton relied on terrestrial and pelagic primary production, while zoobenthos and fishes relied on terrestrial and benthic primary production. Across all consumer groups terrestrial reliance tended to be higher, and benthic reliance lower, in lakes where light penetration was low due to inputs of terrestrial dissolved organic carbon. These results support and refine an emerging consensus that terrestrial and benthic support of lake food webs can be substantial, and they imply that changes in the relative availability of basal resources drive the strength of cross-habitat trophic connections.     

On predation of pelagic larvae and early juveniles of marine bottom invertebrates by adult benthic invertebrates and their passing alive through their predators

The dynamic quantitative balance between prey and predator invertebrate species inhabiting the same shallow-shelf (sublittoral level bottom) benthic communities was first discussed by Thorson (1953). Thorson considered the exact timing of larval settlement of prey and predator species possessing pelagic development and temporal supression of the adult predators' feeding activities during reproduction at the time of the preys' settlement to constitute the major factors which facilitate survival of the prey species in such communities. However, information obtained demonstrates that Thorson's “mechanism of balance between predator and prey species of benthic communities” is not always effective in securing survival from predation not only of the prey's spat but even sometimes of the predator's spat also. Because of this, the “mechanism” can not be rated as universally effective in all situations. Analysis of the data so far published demonstrates that, in marine benthic communities, especially in shallow-shelf waters, it is not uncommon for gametes, larvae, or early juveniles of different prey species to pass alive through suspension (filter)-feeding and deposit-feeding adult invertebrates preying on them. Sometimes development can even continue after excretion by predators. The hypothesis of Voskresensky (1948) and Goycher (1949) of the importance of this phenomenon for the maintenance and recruitment of the mussel Mytilus edulis and other filter-feeding lamellibranchs of nearshore waters preying on their own and other lamellibranch pelagic larvae must be rejected on the basis of accumulated data on their feeding and general biology and on the adverse influence of the mucous of their faecal pellets and pseudofaeces on the larvae excreted by them alive. The data considered here demonstrate that, although the passing alive of larvae and spat of benthic invertebrates through benthic predators is not uncommon in shallow-shelf bottom-communities, it plays no important role in the processes of maintenance and recruitment of the species and communities involved nor of the marine benthos as a whole. The actual ecological significance of predation on pelagic larvae and bottom spat of benthic invertebrate prey species by all three main trophic groups of marine benthos (suspension or filter-feeders, deposit-feeders, carnivores) and its importance to predator-prey dynamics in marine benthic communities remains open to debate until more reliable quantitative data become available.     

The application of predicted habitat models to investigate the spatial ecology of demersal fish assemblages

Benthic habitats are known to influence the abundance and richness of demersal fish assemblages; however, little is known about how habitat structure and composition influences these distributions at very fine scales. We examined how the benthic environment structures marine fish assemblages using high-resolution bathymetry and accurate predicted benthic habitat maps. Areas characterised by a mosaic of habitat patches supported the highest richness of demersal fishes. A total of 37.4% of the variation in the distribution of the fish assemblage was attributed to 6 significant variables. Depth explained 23.0% of the variation, with the boulders explaining 12.6% and relief 1.4%. The remaining measures (seawhips, light/exposure and solid reef) provided a small (<1.0%) but significant contribution. Identifying components of the benthic environment important in structuring fish assemblages and understanding how they influence the spatial distribution of marine fishes is imperative for better management of demersal fish populations.     

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

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.     

Lake of flies, or lake of fish? A trophic model of Lake Malawi

Ecosystem-focused models have, for the first time, become available for the combined demersal and pelagic components of a large tropical lake ecosystem, Lake Malawi. These provide the opportunity to explore continuing controversies over the production efficiencies and ecological functioning of large tropical lakes. In Lake Malawi these models can provide important insight to the effect of fishing on fish composition, and the potential competition that the lakefly Chaoborus edulis may have with fisheries production. A mass-balanced trophic model developed for the demersal fish community of the southern and western areas of Lake Malawi was integrated with an existing trophic model developed for the open-water pelagic. Input parameters for the demersal model were obtained from a survey of fish distributions, fish food consumption studies, and from additional published quantitative and qualitative information on the various biotic components of the community. The model was constructed using the Ecopath approach and software. The graphically presented demersal food web spanned four trophic levels and was based primarily on consumption of detritus, zooplankton and sedimented diatoms. Zooplankton was imported into the system at trophic levels three and four through fish predation on carnivorous and herbivorous copepods and Chaoborus larvae. It is proposed that the primary consumption of copepods was by fish migrating into the pelagic zone. Chaoborus larvae in the demersal were probably consumed near the lakebed as they conducted a daily migration from the pelagic to seek refuge in the sediments. This evidence for strong benthic-pelagic coupling provided the opportunity for linking the demersal model to the existing model for the pelagic community so producing the first model for the complete ecosystem. Energy fluxes through the resulting combined model demonstrated that the primary import of biomass to the demersal system was detritus of pelagic origin (72.1%) and pelagic zooplankton (10.6%). Only 15.8% of the biomass consumed within the demersal system was of demersal origin. Lakefly production is efficiently utilised by the lake fish community, and any attempt to improve fishery production through introduction of a non-native plantivorous fish species would have a negative impact on the stability and productivity of the lake ecosystem.