Prey Production Research Articles

Management of an ornamental pond as a conservation site for a threatened native fish species, crucian carp Carassius carassius

Ornamental ponds are important sites for conserving threatened native fish species (e.g. crucian carp Carassius carassius L.), but pond management plans rarely include considerations of native fishes. We developed and implemented a management plan for a small (0.8 h), ornamental estate pond in Hertfordshire (England) using historical information (aquatic plant and animal surveys) and a 9-year data set on climatic variables and crucian carp body condition. Crucian carp growth was not correlated with climatic variables, but body condition decreased with increasing temperature (in degree-days), which suggests that temperature influences on growth are counter-balanced by environmental factors. Management included the removal of one fish species (to eliminate hybridization with another species) and the introduction of two native species (to re-balance the fish assemblage), a reduction in floating aquatic plants (to reduce shading of the sediments) as well as the use of a chemical agent to compact the pond’s fine sediments and barley straw to enhance invertebrate habitat and thus fish prey production.

Responses of Songbirds to Magpie Reduction in an Urban Habitat

ABSTRACT Bird communities change in response to urbanization, which poses a challenge for conservationists. We examined the consequences of the recent increase in European cities of black‐billed magpie (Pica pica), which has become the main bird nest predator in many urban parks, yet its impact remains disputed. We tested predator role in the limitation of postfledging and adult numbers of 10 common songbird species. We conducted before‐after, control‐treatment experimental magpie removal in the suburbs near Paris, France, during our 3‐year study. We also compared the productivity and the relative densities between urban and rural habitats for 14 songbirds. We found that magpies had very limited effect on songbird productivity, even for species sensitive to predation by corvids. In addition, impact of the magpie on population levels of passerines would also be minimized because we found no relationship between productivity of prey and their densities. Thus, the recent colonization of urban parks by magpies should not threaten persistence of local songbird populations. Yet, there was evidence that a small number of non‐territorial bird species were attracted to places where magpies were removed. Magpie densities may have modified habitat selection for foraging that might be explained by predator avoidance. But overall, characteristics of urban habitat explain variations of productivity and densities of songbirds better than did magpie predation. In France, conservationists widely use the control of predators, aiming at increasing levels of prey populations. Despite very high densities of magpies in urban parks, we gave evidence that removal of this predator was ineffective to preserve populations of common passerines. This would suggest that the risk induced by the presence of magpies is independent of its density, and conservationists must carefully assess its impact. In the future, we recommend management policies include long‐term monitoring of magpie‐prey interactions during breeding season to detect potential changes in songbird responses to magpie presence.

Feeding and production of zooplankton in the Catalan Sea (NW Mediterranean)

Zooplankton are key components of the structure and functioning of marine planktonic food webs. They are the main link of planktonic primary production towards top pelagic consumer levels (fish), and play a relevant role on the nutrient recycling in the water column and on the export of particulate matter out of the photic zone. In this paper, we review the present knowledge on the feeding and production of zooplankton in the Catalan Sea (NW Mediterranean), with special emphasis on copepods. Feeding of zooplankton in the Catalan Sea appears typically food limited, with average daily rations on a yearly basis in the order of 48% body C d −1. Heterotrophic prey constitute a relevant fraction of their diet, as an alternative to the scarce phytoplankton in the area. From a structural point of view, the trophic impact and control of their prey populations are low on standing stocks but, at certain times, zooplankton can exert a meaningful effect on their prey production. Regarding zooplankton production, the available estimates of growth rates in the area are based on the egg production rate of copepods. Egg production rates appear to be limited, especially in summer. Tentative estimates of copepod production in the area are in the order of 20–40 mg C m −2 d −1. In conclusion, this review confirms that the oligotrophic character of the NW Mediterranean constrains the feeding activity and production of zooplankton.

Ecological balance between ciliate plankton and its prey candidates, pico- and nanoplankton, in the East China Sea

Standing stocks of ciliate plankton and its prey candidates, both picoplankton and nanoplankton, were investigated in spring in the East China Sea. The former was 1.36 × 105–1.54 × 108 μm3 l−1 in biovolume, and the latter was 5.33 × 106–1.11 × 108 μm3 l−1. The biovolume ratio of ciliate plankton to prey candidates ranged from 1.31 × 10−2 to 2.00 × 100; it was larger in abundant prey conditions and smaller in sparse preys. Making some plausible assumptions about physiological activity on both organisms, every ratio meet the quantitative restriction that prey production should be equal to or larger than ciliate consumption. However, prey candidates would be so sparsely distributed that ciliate plankton could not capture sufficient prey organisms in its random filter-feeding manner. Even though planktonic ciliates must have some extraordinary mechanisms to capture preys efficiently, this quantitative imbalance might be one of the reasons for decreasing ciliate/prey ratio in sparse prey conditions.

The dynamics of top-down and bottom-up effects in food webs of varying prey diversity, composition, and productivity

The dynamics of top-down and bottom-up effects in food webs of varying prey diversity, composition, and productivity

Linking terrestrial and aquatic ecosystems: The role of woody habitat in lake food webs

Trees that fall into lakes from riparian forest become habitat for aquatic organisms, and are potentially an important link between terrestrial and aquatic ecosystems. Coarse woody habitat (CWH) promotes the production of benthic invertebrate prey and offers refuge for prey fishes, which are in turn consumed by piscivorous fishes. We used a simulation model to explore responses of an aquatic food web to changes in CWH caused by lakeshore residential development and windstorm, as well as to harvest of adult piscivores. Residential development had a negative effect on fishes, and could lead to extirpation of benthivorous prey fish species. In contrast, pulsed addition of CWH following a windstorm had little effect on the aquatic food web. Our results suggest that CWH is more important as shelter for prey fishes than as a substrate for benthic invertebrate production. However, piscivore harvest can supersede the role of CWH as prey shelter, leading to piscivore collapse and prey persistence even when CWH levels are low enough to promote piscivore dominance. Thus, the effects of lakeshore residential development on fishes can be masked by angler harvest of top piscivores.

Multi-locus phylogeography of the dusky dolphin (Lagenorhynchus obscurus): passive dispersal via the west-wind drift or response to prey species and climate change?

BackgroundThe dusky dolphin (Lagenorhynchus obscurus) is distributed along temperate, coastal regions of New Zealand, South Africa, Argentina, and Peru where it feeds on schooling anchovy, sardines, and other small fishes and squid tightly associated with temperate ocean sea surface temperatures. Previous studies have suggested that the dusky dolphin dispersed in the Southern Hemisphere eastward from Peru via a linear, temperate dispersal corridor provided by the circumpolar west-wind drift. With new mitochondrial and nuclear DNA sequence data, we propose an alternative phylogeographic history for the dusky dolphin that was structured by paleoceanographic conditions that repeatedly altered the distribution of its temperate prey species during the Plio-Pleistocene.ResultsIn contrast to the west-wind drift hypothesis, phylogenetic analyses support a Pacific/Indian Ocean origin, with a relatively early and continued isolation of Peru from other regions. Dispersal of the dusky dolphin into the Atlantic is correlated with the history of anchovy populations, including multiple migrations from New Zealand to South Africa. Additionally, the cooling of the Eastern Equatorial Pacific led to the divergence of anchovy populations, which in turn explains the north-south equatorial transgression of L. obliquidens and the subsequent divergence of L. obscurus in the Southern Hemisphere.ConclusionOverall, our study fails to support the west-wind drift hypothesis. Instead, our data indicate that changes in primary productivity and related abundance of prey played a key role in shaping the phylogeography of the dusky dolphin, with periods of ocean change coincident with important events in the history of this temperate dolphin species. Moderate, short-term changes in sea surface temperatures and current systems have a powerful effect on anchovy populations; thus, it is not infeasible that repeated fluctuations in anchovy populations continue to play an important role in the history of coastal dolphin populations.

Direct uptake of nitrogen by Pfiesteria piscicida and Pfiesteria shumwayae, and nitrogen nutritional preferences

The rates of uptake of a range of forms of nitrogenous nutrients were measured in cultures of Pfiesteria piscicida and Pfiesteria shumwayae maintained at varying physiological states. The measured rates of dissolved N uptake under some conditions approached the rates of N uptake that are achieved through phagotrophy. Rates of dissolved N uptake by P. piscicida contributed <10% of the cellular N of flagellated cells feeding on algae, but were equal to or greater than phagotrophic N acquisition in cells recently removed from fish cultures. Specific N uptake rates (V, h −1) were higher for cells that were maintained on algal prey for long periods (months) than those that were grown with live fish. However, rates of N uptake on a cellular basis for cells grown on or recently removed from fish were comparable to those maintained on algal prey, likely reflecting differences in the sizes of cells of different physiological condition. Preferences for form of N generally followed a decreasing trend of amino acids > urea > NH 4 + > NO 3 −. Nitrate consistently was not a preferred form of N. Although Pfiesteria spp. are often found in eutrophic environments, the relationship between Pfiesteria spp. and nutrient availability is likely to be primarily indirect, mediated through the production of various prey on which Pfiesteria spp. feed. These findings also confirm, however, that when dissolved N concentrations are elevated, they can contribute to the supplemental nutrition of these cells, and thus may provide a significant source of N to Pfiesteria spp. in nature.

HOW DO PREY PERSIST?

Understanding predator–prey relations is critical for management and conservation of species. Common descriptions of predator–prey dynamics often imply that low population density of prey, prey switching by predators, and high fecundity or productivity of prey are important in allowing prey species to coexist with predators. Studies of the effects of introduced predators on prey species do not support the idea that low prey density, switching by predators, and fecundity of prey are vital to coexistence of predator and prey. More likely, prey have a suite of morphological and behavior adaptations, including uses of specific habitat features, that render some individuals far less vulnerable to predation and allow predators and prey to coexist. Management activities designed to maintain desired prey species should include maintaining or enhancing features of habitat that reduce prey vulnerability.

Availability of Clupeid Prey to Primary Piscivores in Smith Mountain Lake, Virginia

AbstractPrey supply to predators is limited by the morphology, behavior, and spatial distributions of both predator and prey. To be eaten, prey must co‐occur spatially and temporally with the predator (distribution), the predator must recognize the prey as potential food and successfully capture it (behavior), and the predator must have a mouth large enough to ingest its target (morphology). We quantified the cumulative effect of these three factors on clupeid prey availability to both largemouth bass Micropterus salmoides and striped bass Morone saxatilis in Smith Mountain Lake, Virginia. As expected, morphological constraints were more severe for small predators. Alewives Alosa pseudoharengus were morphologically more available than gizzard shad Dorosoma cepedianum. Incongruent spatial distributions, the result of lake stratification, prevented striped bass from feeding on gizzard shad during summer and limited largemouth bass consumption of alewives. For unknown behavioral reasons, largemouth bass and striped bass consumed only clupeids between 40 and 160 mm total length. For largemouth bass, the available supply of alewives was equivalent to demand and that of gizzard shad exceeded consumption by 53%. For striped bass, the available supply of gizzard shad surpassed consumption by 26% and that of alewives was 4% greater than consumption. Annual consumption by all clupeid predators, including largemouth bass and smallmouth bass Micropterus dolomieu, striped bass, walleye Sander vitreus, flathead catfish Pylodictis olivaris, channel catfish Ictalurus punctatus, white catfish Ameiurus catus, and crappies Pomoxis spp., was 94% of the total available clupeid supply. Our analysis indicates that increased predation on the clupeid forage base of Smith Mountain Lake would lead to decreased prey production and produce prey shortages for piscivores that are dependent on this food resource.

THE “SEAL PRICK”: A LOW INVASIVE METHOD FOR BLOOD SAMPLING IN MALE ELEPHANT SEALS

D. PAULY 1997. competition between fisheries and marine mammals for prey and primary production in the Pacific Ocean. Journal of Northwest Atlantic Fishery Science 22:173-187. WADA, K. 1971. Food and feeding habits of northetn fur seals along the coast of Sanriku. Bulletin of Tokai Regional Fisheries Research Laboratory 64:l-37. (in Japanese with English abstract). YONEZAKI, S., M. KIYOTA, N. BABA, T. KOIDO

Prey productivity effects on the impact of predators of the mussel, Mytilus californianus (Conrad)

The relationship of increasing prey productivity, a measure incorporating prey settlement and body growth, to changes in the relative impact of two predator groups, birds and the sea star, Pisaster ochraceus (Brandt) on a competitively dominant mussel, Mytilus californianus were examined. The purpose of this experiment was twofold, 1) to determine if the separate effects of each predator group on prey abundance increased as prey productivity increased and 2) to determine if the relative impacts of the two predator groups diverged as prey productivity increased. In this experiment, the separate impact of each predator group increased with increasing prey productivity. However, the relative impact of each predator group did not diverge with increasing prey productivity. Unlike previous studies that suggested with increasing prey productivity the relative effect of two predator groups should diverge, this experiment suggested that communities can have more redundant predator groups than originally thought. The results of an analysis using a proportional hazards model suggested that despite increasing prey productivity, birds and the sea star were equal in their ability to curb population increases by M. californianus. These results highlight the need to carefully consider what type of species to species comparisons to make when attempting to discern the relative roles of different predator groups in a community.

The influence of episodic events on transport of striped bass eggs to the estuarine turbidity maximum nursery area

The estuarine turbidity maximum (ETM) is an important nursery area for anadromous fish where early-life stages can be retained in high prey concentrations and favorable salinities. Episodic freshwater flow and wind events could influence the transport of striped bass (Morone saxatilis) eggs to the ETM. This hypothesis was evaluated with regression analysis of observational data and with a coupled biological-physical model of a semi-idealized upper Chesapeake Bay driven by observed wind and freshwater flow. A particle-tracking model was constructed within a numerical circulation model (Princeton Ocean Model) to simulate the transport of fish eggs in a 3-dimensional flow field. Particles with the sinking speed of striped bass eggs were released up-estuary of the salt front in both 2-d event-scale and 60-d seasonal-scale scenarios. In event scenarios, egg-like particles with observed specific gravities (densities) of striped bass eggs were transported to the optimum ETM nursery area after 2 d, the striped bass egg-stage duration. Wind events and pulses in river discharge decreased the number of egg-like particles transported to the ETM area by 20.9% and 13.2%, respectively, compared to nonevent conditions. In seasonal scenarios, particle delivery to the ETM depended upon the timing of the release of egg-like particles. The number of particles transported to the ETM area decreased when particles were released before and during wind and river pulse events. Particle delivery to the ETM area was enhanced when the salt front was moving up-estuary after river pulse events and as base river flow receded over the spawning season. Model results suggest that the timing of striped bass spawning in relation to pulsed events may have a negative (before or during events) or positive (after river flow events) effect on egg transport. Spawning after river flow events may promote early-stage survival by taking advantage of improved transport, enhanced turbidity refuge, and elevated prey production that may occur after river pulse events. In multiple regression analysis of observed data, mean spring freshwater flow rates and the number of pulsed freshwater flow events during the striped bass spawning season explained 71% of the variability in striped bass juvenile abundance in upper Chesapeake Bay from 1986 to 2002. Positive parameter estimates for these effects support the hypothesis that pulsed freshwater flow events, coupled with spawning after the events, may enhance striped bass early-stage survival. Results suggest that episodic events may have an important role in controlling fish recruitment.

Trophodynamic modeling of walleye pollock (Theragra chalcogramma) in the Doto area, northern Japan: model description and baseline simulations

AbstractAn age‐structured trophodynamic model was constructed to quantitatively analyze factors affecting post‐settlement mortality and growth of walleye pollock (Theragra chalcogramma) in the Doto area, the main nursery ground of the Japan Pacific population. The model included (i) multiple age classes of pollock, (ii) a generic predator, (iii) fisheries, and (iv) major prey of pollock. Major processes considered were (i) recruitment, (ii) bottom‐up control of somatic growth, (iii) mortality because of predation, cannibalism and fishing, (iv) size‐selective prey selection, (v) temperature‐dependent bioenergetics such as conversion efficiency and daily consumption rate, and (vi) production and advective supply of prey. By assuming that pollock select prey based upon both relative abundance and predator–prey size relationships, the model accurately simulated seasonal and ontogenetic variations in the diet. However considering ontogenetic segregation, the model showed that, due to cannibalism, newly recruited fish would be totally consumed within 6 months after settlement. By considering segregation (10% overlap during spring and 0.1% during other seasons), an agreement of diet between the simulation and empirical data averaged 82.7% for the different seasons and fish sizes. Euphausiids, the most important prey of pollock, suffered the highest predation impact (22.2 ± 5.3 WWg m−2 yr−1) exceeding annual production in the model domain (17.2 ± 0.1 WWg m−2 yr−1), indicating that an advective supply of prey is necessary to support the pollock population. The daily ration of pollock during spring and summer averaged at 1.2 and 0.6% BW day−1 for small (≤200 mm) and large (&gt;200 mm) pollock, respectively; this daily ration was reduced by half during autumn and winter.

BOUNDARIES OF OPEN MARINE ECOSYSTEMS: AN APPLICATION TO THE PRIBILOF ARCHIPELAGO, SOUTHEAST BERING SEA

We applied ecosystem energetics and foraging theory to characterize the spatial extent of the Pribilof Archipelago ecosystem, located in the southeast Bering Sea. From an energetic perspective, an ecosystem is an area within which the predatory demand is in balance with the prey production. From a foraging perspective, an ecosystem boundary should at least include the foraging range of the species that live within it for a portion of their life cycle. The Pribilof Islands are densely populated by species that adopt a central place foraging strategy. Foraging theory predicts that the area traveled by central place foragers (CPF) should extend far enough so that their predatory demands are in balance with prey production. Thus, the spatial extent of an ecosystem, as defined by energetics and the foraging range of constituent species, will require a similar energy balance, and independent assessments should yield similar results. In this study, we compared the area of maximum energy balance, estimated with a food web model during the decade 1990– 2000, with estimates of the foraging range of northern fur seals (the farthest traveling CPF in the Pribilof Islands community) obtained from the literature. From the food web simulations, we estimated that a circle of 100 nautical miles (NM), or 185.2 km, radius encloses the area of highest energy balance and lowest biomass import and that it represents a switch from a piscivorous‐dominated (smaller areas) to a zooplanktivorous‐dominated (larger areas) community. The distance from the breeding site to locations recorded at sea for lactating female fur seals, during the years 1995–1996, ranged from 5.0 to 172.2 NM (9.3–318.9 km), with a median of 97 NM (179.6 km). Thus, ∼50% of the locations recorded for lactating fur seals occurred beyond the area of energy balance estimated by the model, indicating that additional factors can motivate their foraging extent. We propose that energetic constraints set the minimum extent of the Pribilof ecosystem, while the foraging distance of fur seals may indicate the maximum extent. In discussing these results, we highlight the limitations of current definitions of the spatial extent in ecosystems, when related to open oceanic environments, and discuss viable alternatives to characterize boundaries of aquatic systems that are not physically separated from adjacent areas. We believe that these arguments, though controversial, are very timely given the increased emphasis currently placed on the management and protection of entire marine ecosystems.

Modelling Production and Biomasses of Prey and Predatory Fish in Lakes

This work presents a new dynamic model to predict two fundamental functional categories of fish in lakes, prey and predatory fish. The model has been developed within the framework of a more comprehensive lake ecosystem model, LakeWeb, which also accounts for phytoplankton, bacterioplankton, two types of zooplankton (herbivorous and predatory), zoobenthos, macrophytes and benthic algae. The new fish model gives seasonal variations (the calculation time is 1 week). It is meant to account for all factors regulating the production of fish for lakes in general. The model has not been calibrated and tested in the traditional way using data from a few well investigated lakes. Instead, it has been tested using empirical data and regressions based on data from many lakes. The basic aim of the model is that it should capture typical functional and structural patterns in many lakes. It accounts in a relatively simple manner for many complicated processes, like fishing (by birds, animals and man), fish migration to and from lakes and how macrophyte cover gives shelter to small fish and reduces predation pressure. Food choices are handled by distribution coefficients regulating how much of the different available food sources a given organism would consume. Beside these distribution coefficients, and the way the food choices are structured (the food choice panel), fundamental concepts in the fish model are: (1) metabolic efficiency ratios, which express how much of the food consumed by the predator that will increase the biomass of the predator and how much that will be lost by respiration and faeces, (2) actual consumption rates, which are defined from the ratio between the actual biomass of the predator and the normal biomass of the predator, and the normal consumption rates, which are related to the turnover time of the predator. We have demonstrated that the new model gives predictions which agree well with the values given by the empirical regressions, and also expected and requested divergences from these regression lines when they do not provide sufficient resolution.

The necessity for intertidal foraging by estuarine populations of subadult dungeness crab,Cancer magister: Evidence from a bioenergetics model

Complex intertidal habitats characteristic of northeastern Pacific coastal estuaries provide critical nursery environments for young-of-the-year Dungeness crab,Cancer magister, yet their role in supporting subsequent year classes remains unclear. SubadultC. magister (40–130 mm; 1+ and >1+ year classes), which reach densities as high as 4,300 crabs ha−1 in subtidal channels during low tides, migrate during flood tides from subtidal refuges into intertidal habitats to forage. As with other brachyuran species that undertake extensive tidally-driven migrations, intertidal foraging may contribute significantly to the energy budget of subadultC. magister. In order to explore the energetic incentive for intertidal migrations by subadult crabs, we developed an ontogenetically-based bioenergetics model for crabs within Willapa Bay, Washington. The model showed that energetic demand varied spatially across the bay, with the highest average energetic demand of a population of subadult crabs (2.13×106 kJ ha−1) occurring in a habitat stratum termed lower side channel (LSC) and characterized by relatively little subtidal area and extensive intertidal flats. Comparison of model results with subtidal prey production revealed that the latter could not satisfy subadultC. magister energetic demands, especially in LSC where modeled crab predation depleted subtidal prey biomass within 17 simulation days. We estimate that 1 ha of subtidal crabs from LSC would minimally require an additional 1.6 ha of intertidal area to satisfy energetic demands without depleting prey biomass. Our model results support the assertion thatC. magister make regular migrations to forage on productive intertidal flats, and suggest that intertidal foraging may contribute significantly to the diet of subadult crabs in coastal estuaries.

Habitat restoration as mitigation for lost production at multiple trophic levels

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 264:233-247 (2003) - doi:10.3354/meps264233 Habitat restoration as mitigation for lost production at multiple trophic levels Deborah P. French McCay*, Jill J. Rowe Applied Science Associates, 70 Dean Knauss Drive, Narragansett, Rhode Island 02882, USA *Email: dfrench@appsci.com ABSTRACT: Seagrass beds and salt marshes are important in the life histories of many fish, invertebrates, and aquatic birds, such that population size and production of these organisms can be limited by the loss of these habitats. Therefore, restoring seagrass or salt marsh would be expected to benefit species dependent on these resources. Following the ŒNorth Cape¹ oil spill of January 1996 in Rhode Island (northeastern USA), habitat restoration was used to compensate for losses of fish, invertebrates, and aquatic birds. A habitat restoration model based on food-chain transfers was used to estimate equivalent production at the same trophic level as the losses, a novel approach that uses energetic efficiencies to scale across trophic levels. Benefits of habitat to each trophic level were estimated by assuming that the production of consumers is proportional to prey production gained by the restoration of habitat. The habitat restoration model balanced the losses with trophically equivalent production, discounting future gains in compensatory production relative to present losses such that interest is paid, analogous to economic discounting. Results showed that restoration of seagrass beds would be more productive than salt marsh restoration in southern New England and, likely, other temperate zone locations. Moreover, benthic faunal production in salt marshes could be accounted for by in situ primary production of angiosperms and benthic microalgae, whereas in seagrass beds, benthic faunal production exceeded that expected from in-bed primary production. The trophic model provides a methodology to estimate the scale of a restoration project that will provide production (ecological services) equivalent to losses of organisms at multiple trophic levels, applicable to natural resource damage assessments and other environmental assessments. KEY WORDS: Restoration · Habitat · ŒNorth Cape¹ oil spill · Natural resource damage assessment · Trophic transfer · Food web modeling Full text in pdf format PreviousNextExport citation Mail this link - Contents Mailing Lists - RSS Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 264. Online publication date: December 15, 2003 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2003 Inter-Research.

Pelagic food web as the basis of fisheries in Lake Tanganyika: A bioenergetic modeling analysis

Fisheries in Lake Tanganyika, East Africa, are mainly based on two predominantly planktivorous clupeids (Stolothrissa tanganicae and Limnothrissa miodon) and a centropomid predator (Lates stappersi), caught with lift nets, purse seines, and beach seines by traditional, artisanal, and industrial fishers. The biological basis and sustainability of the present fisheries were assessed in a comprehensive project “Research for the Management of the Fisheries on Lake Tanganyika” in 1992–1998. Production in the whole lake was estimated for the entire pelagic food chain leading to the commercially important fish species. Preliminary calculations based on a constant production efficiency suggested that while the crustacean zooplankton production was sufficient to maintain the estimated planktivorous fish production, the food requirements of piscivorous fish exceeded the production of potential prey. Here, the food consumption by pelagic fish was estimated with a bioenergetic model using actual diet data for different size classes. In order to reveal potential differences in the food web sustaining the fisheries, separate calculations were made for different parts of the lake, which harbor different fish community compositions and size structures. According to the bioenergetic calculations, the food requirements of the planktivorous fish were a reasonable fraction (25–38%) of the zooplankton production. In contrast, very high predation pressure was indicated on shrimps (73–104%), and especially on prey fish (&amp;gt;100%), suggesting that the total biomass of the prey planktivores had been underestimated, or that the predatory fish biomass was overestimated. Annual catch of Stolothrissa tanganicae was 18–35% of estimated production in individual countries and 25% in the whole lake. For Limnothrissa miodon , the corresponding ratio was moderately low (19–22%) in Tanzania and Zambia, but high (55–61%) in Democratic Republic of Congo and Burundi; in the whole lake, the exploitation rate was 30%. For Lates stappersi , the catch/production ratios were very high (76–112%) in all parts of the lake, and even the lakewide average exploitation rate was as high as 94%. These figures suggest that the present clupeid fishery is on a sustainable basis, while the Lates populations are clearly overexploited.

HISTORY AND CURRENT DEVELOPMENT OF A PARADIGM OF PREDATION IN ROCKY INTERTIDAL COMMUNITIES

A paradigm is a set of mutually supportive hypotheses that provides a frame of reference within a field. In 1962, Kuhn proposed that paradigms form within the dual contexts of empirical evidence and intellectual history. Facts potentially contradictory to a paradigm may not be recognized until they are observed repeatedly and incorporated as supportive evidence into a new theory. Support for this interpretation can be found in the history of a paradigm of predation in rocky intertidal communities. Hypotheses were developed in the contexts of innovative field experiments and historical arguments of competition theory. The resulting paradigm proposed that predators restrict populations of competitively dominant prey to refuges. Different types of refuge, or no refuge, prevail in different areas of the intertidal zone, accounting for patterns of prey distribution, the coexistence of natural enemies, and the local maintenance of diversity. An ensuing period of criticism made reference to potential contradictions. Rather than by predation alone, prey abundances are determined largely by an interplay of varying rates of predation and prey production. Furthermore, prey refuges are neither necessary nor sufficient to explain all observed instances of local coexistence of predators and prey. We present a model in which intertidal boundaries of prey are set by equilibria between predation and prey production. Predation and prey productivity vary with environmental gradients and with explicit spatial configurations of the prey. This synthesis relies on insights of the original paradigm, incorporates the contradictory observations, and depends on novel capabilities afforded by spatially explicit computer simulations. The resulting synthesis provides explanations for distinctive aspects of zonation, including abrupt prey boundaries in continuous gradients of predation, and converging upper and lower prey boundaries in gradients of decreasing wave exposure.