Changes In Trophic Level Research Articles

The Effect of Fish Introductions on the Diatom and Cladoceran Communities of Lake Opeongo, Ontario, Canada

Fish introductions are one of the most widespread anthropogenic perturbations to aquatic ecosystems. Paradoxically, the effects of these introductions on aquatic ecosystems are typically poorly documented. This project studied the effect of fish introductions on Lake Opeongo, an oligotrophic lake in Algonquin Provincial Park, Ontario, Canada (45° 42′ N, 78° 22′ W), using the remains of algae (diatoms) and zooplankton (cladocerans) preserved in the sediments. It was hypothesized that the introduction of cisco or lake herring (Coregonus artedii Lesueur) in 1948, which filled the underutilized pelagic forage fish niche, should have altered nutrient availability for phytoplankton. Prior to cisco introduction, the diatom community of Lake Opeongo reflected a relatively stable oligotrophic state established before European settlement, and consisted of the Cyclotella stelligera complex with subdominants Tabellaria flocculosa IIIp and the Aulacoseira distans complex. No marked changes occurred until ca. 1962 when the diatom community shifted to an assemblage with increased total phosphorus preferences, consisting of Asterionella formosa and lesser amounts of Cyclotella bodanica var lemanica, the C. stelligera complex, Fragilaria crotonensis and T. flocculosa IIIp. The dominant cladoceran Bosmina longirostris increased significantly in relative abundance since the introduction of cisco. The most likely cause of this shift was increased nutrient recycling and/or trophic level changes caused by human manipulation of the fish community of the lake.

Recording the occurrence of trophic level changes in the lagoon of Venice over the '90s

Four areas of the Venice lagoon, placed near the Malamocco mouth (Alberoni, st. A), in the Lido watershed (Sacca Sessola, st. B) and near the mainland under the influence of freshwater and urban (San Giuliano, st. C) or industrial (Fusina, st. D) effluents were monitored in two periods: 1989–92 and 1998–99 in order to quantify some environmental changes (macroalgal and phytoplankton biomass, nutrient concentrations, physico-chemical variables) occurred in both the water column and the surface sediment over the '90s. Stations B and C, two areas particularly affected by macroalgae exhibited a biomass of ca. 20 and 8 kg WW m − 2 , respectively, during 1989–90. In 1998–99 maximum densities decreased to ca. 0.3 and 0.01 kg WW m − 2 . During that period, phytoplankton also decreased significantly both in peaks (Chl a: from 58–86 to 4.0–3.5 μg dm − 3 ) and mean values (Chl a: from 9.1–10.3 to 1.3–1.4 μg dm − 3 ), especially at sts. C and D. As far as nutrient concentrations are concerned, a different trend was observed in the water column and in the surface sediment, mainly because of the reduction of the primary producers and the disappearance of anoxic crises. In 1998–99 reactive phosphorus (RP) in the water column was up to ca. 3 times as high as in 1989–92. Conversely, dissolved inorganic nitrogen (DIN) was not significantly changed. In 1998–99 the 5 cm sediment top layer at sts. B, C, displayed a significant total nitrogen (TN) and total phosphorus (TP) decrease (TN annual mean: from 1.29 and 2.79 mg DW g − 1 to 0.69 and 1.47 mg DW g − 1 , respectively; TP: from 401 and 626 μg DW g − 1 to 360 and 455 μg DW g − 1 ). A different result was found at st. A which in 1998–99 was colonised by the seagrass Zostera marina L. That station showed a TN sediment increase from 0.25 to 0.67 mg DW g − 1 and a TP decrease from 455 to 350 μg DW g − 1 . Station D, which did not show any macrophyte biomass coverage either in 1992–93 or in 1998–99, exhibited negligible differences. Besides the monitoring of biomasses and nutrients, significant changes related to oxygen concentration, water transparency, pH and E h and sediment rates were also recorded.

Relationships between Holocene sea-level variations, trophic development, and climatic change in Negra Lagoon, Southern Uruguay

Paleolimnological data are presented on trophic development, climatic change and sea level variations in Negra Lagoon, a 142 km2 coastal lagoon in southern Uruguay. Using a sediment core that extended to the early Holocene, analyses of organic matter, carbonate, diatoms and opal phytoliths allowed us to track changes in trophic state and paleosalinity levels, which were closely related to Holocene sea level variation, temperature and humidity. Diatom Association Zones (DAZ) corresponding to transgressive events were dominated by marine/brackish taxa and relatively low organic matter values, while those DAZ corresponding to regressive events showed increases in brackish/freshwater diatoms and organic matter. Opal Phytoliths Association Zones (OPAZ) also were identified, temperature and humidity indices were calculated, and climatic trends were inferred. During the middle Holocene (i.e., ~5000–4000 yr BP), cool–dry climatic conditions were established. After ~4000 yr BP, a transition from cool to warm–humid conditions was observed, but fully warm humid conditions established only after ~1700 yr BP. Such climatic amelioration led to increases in trophic state.

Abrupt Changes in Bosmina (Cladocera, Crustacea) Assemblages During the History of the Ostrowite Lake (Northern Poland)

During the 10000-year history of the Ostrowite Lake, there have been several episodes of change in dominance amongst species of the genus Bosmina. The dominants were alternately B.longirostris and Eubosmina spp. Amongst the subgenus Eubosmina, two species prevailed in different periods:Bosmina coregoni and Bosmina reflexa. The first species, Bosmina coregoni, is characterized by long antennae and short carapace mucrones, while the second one, Bosmina reflexa, by short antennae and very long mucrones. Bosmina reflexa was dominant at the beginning of the lake's history (Preboreal, 10000--9000 BP) and during the early Subboreal Period (5000–3500 BP). Only a small number of remains of Bosmina longispina(Eubosmina spp.) were present along the entire profile. Such strong changes in the domination of Bosmina species were observed for the first time in the sediments of Polish lakes. A similar transition from one species to another has been described in a few German lakes and was linked to climate changes. However, the changes in the Ostrowite Lake do not correlate with climate changes, but most probably, with changes in the lake's trophic level. This is suggested by the simultaneous increase of Bosmina reflexa and a good indicator of eutrophy, Bosmina longirostris.

The trophic structure of the pipefish community (Pisces: Syngnathidae) from a western Mediterranean seagrass meadow based on stable isotope analysis

Syngnathus abaster andSyngnathus typhle (Pisces: Syngnathidae) from a dense Cymodocea nodesa meadow in the western Mediterranean Stagnone di Marsala (Italy) were studied using δ13N and δ15N analysis. Because of the presence of these two species in the same habitat and the specialized parental care by the male, the effect of species and sex on the isotopic composition was also studied to investigate the different feeding strategies between and within species.S. abaster andS. typhle exhibited enriched13C and15N values throughout the sampling period (mean ±SE, δ13C =−10.5±1.8‰ and−10.8±2.0‰, δ15N=11.9±0.7‰ and 10.6±1.0%., respectively), suggesting that these species receive their energy mainly from mixed sources, particularly from sedimentary and particulate organic matter and the seagrassC. nodosa as ultimate organic matter sources. ANOVA results demonstrated that the interaction between season, species, and sex significantly affected the isotopic composition of the pipefish (p<0.001 for both δ13C and δ15N). Differences between species and sex, although significant, were smaller than the values generally reported for trophic level differences (≈1‰ and ≈3.5‰ for δ15N, respectively) and change in foraging habitat, Slight isotopic differences may mirror small differences in resource exploitation and resource partitioning. Evidence from stomach content analysis from the literature coupled with stable isotope measurements, while disagreeing somewhat, provide additional knowledge of pipefish feeding strategies.

Simulated response to harvesting strategies in an exploited ecosystem in the southwestern Gulf of Mexico

The impact of some optimized harvesting strategies on ecosystem structure was investigated using a mass-balanced model of the ecosystem in the southwestern Gulf of Mexico, where there are four types of artisanal fisheries and a shrimp fishery that has collapsed. The Ecopath with Ecosim software was used to simulate harvesting strategies aimed at optimizing economic (profit), social (jobs), ecological (conservation of ecosystem structure) and shrimp-recovery criteria. As expected, the ecosystem changes that would ensue vary according to the combination of optimization goals. We found that for some scenarios, the extraction of biomass from a discrete trophic-level changes impacting ecosystem and catch structure. This was clearly observed through the tendency of the mean trophic level of the ecosystem and catch, as well as the fishing-in-balance index (FBI). A particular discussion was made about the collapsed shrimp fishery, where the impact of a specific shrimp-recovery strategy was evaluated. Collapse is strongly associated to physical variables and recovery based on trophic relationships is plausible but with a high ecosystem structure cost.

Stable isotopic comparison in otoliths of juvenile sablefish ( Anoplopoma fimbria) from waters off the Washington and Oregon coast

Genetic methods have been extensively used in the examination of fish population composition and structure. However, it is generally not easy to determine the stock structure of marine fish using the genetic markers, particularly given the large population size and wide spread distribution of long-lived fish. As an alternative, sagittal otoliths of juvenile sablefish ( Anoplopoma fimbria) along the Washington and Oregon coast were collected and analysed for stable oxygen and carbon isotope ratios. δ 18 O values of the otolith nuclei ranged from −1.3 to +0.1‰ VPDB, whereas δ 13 C of the same otoliths ranged from −8.7 to −6.2‰ VPDB. In contrast with the lower isotopic composition in early life, δ 18 O and δ 13 C values of sablefish otoliths at adult stage were levelled at about 2.0‰ VPDB and −1.0‰ VPDB, respectively. From north to south three different spawning stocks or subpopulations were recognized, with a gradual decrease in otolith δ 13 C and trophic level changes in sablefish’s diet. Therefore, stable isotopic records of otoliths may be a potential supplement for genetic markers when studying the stock structure of marine fish.

Bridging Morphological Transitions to the Metazoa

Our inability to answer many questions regarding the development of metazoan complexity may be due in part to the prevailing idea that most eukaryote "phyla" originated within a short period of geologic time from simple unicellular ancestors. This view, however, is contradicted by evidence that larger groups of eukaryotes share characters, suggesting that these assemblages inherited characters from a common ancestor. Because molecular analyses have had limited success in resolving the relationships of higher eukaryote taxa, we have undertaken a phylogenetic analysis based primarily on morphological characters. The analysis emphasizes characters considered to have a high probability of having evolved only once. Transitions between taxa are evaluated for the likelihood of character-state transformations. The analysis indicates that the evolutionary history of the clade containing the Metazoa has been complex, encompassing the gain and loss of a secondary and perhaps a primary photosynthetic endosymbiont with accompanying changes in trophic level. The history also appears to have included a hetero-autotrophic ancestor that possessed a "conoid" feeding apparatus and may have involved a transformation from a flagellate to an amoeboid body form, a trend toward increased intracellular compartmentation, and the development of complex social behavior. Such changes could have been critical for establishing the underlying complexity required for a rapid diversification of cell and tissue types in the early stages of metazoan evolution.

Long-term trends in the trophic structure of the North Sea fish community: evidence from stable-isotope analysis, size-spectra and community metrics

Fishing has wide-ranging impacts on marine ecosystems. One of the most pervasive signs of intensive fishing is "fishing down the food web", with landings increasingly dominated by smaller species from lower trophic levels. Decreases in the trophic level of landings are assumed to reflect those in fish communities, because size-selective mortality causes decreases in the relative abundance of larger species and in mean body size within species. However, existing analyses of fishing impacts on the trophic level of fish communities have focused on the role of changes in species composition rather than size composition. This will provide a biased assessment of the magnitude of fishing impacts, because fishes feed at different trophic levels as they grow. Here, we combine body size versus trophic level relationships for North Sea fishes (trophic level assessed using nitrogen stable-isotope analysis) with species–size–abundance data from two time-series of trawl-survey data (whole North Sea 1982–2000, central and northern North Sea 1925–1996) to predict long-term trends in the trophic structure of the North Sea fish community. Analyses of the 1982–2000 time-series showed that there was a slow but progressive decline in the trophic level of the demersal community, while there was no trend in the trophic level of the combined pelagic and demersal community. Analyses of the longer time-series suggested that there was no trend in the trophic level of the demersal community. We related temporal changes in trophic level to temporal changes in the slopes of normalised biomass size-spectra (which theoretically represent the trophic structure of the community), mean log2 body mass and mean log2 maximum body mass. While the size-based metrics of community structure showed long-term trends that were consistent with the effects of increased fishery exploitation, these trends were only correlated with trophic level for the demersal community. Our analysis suggests that the effects of fishing on the trophic structure of fish communities can be much more complex than previously assumed. This is a consequence of sampled communities not reflecting all the pathways of energy transfer in a marine ecosystem and of the absence of historical data on temporal and spatial changes in the trophic level of individuals. For the North Sea fish community, changes in size structure due to the differential effects of fishing on species and populations with different life histories are a stronger and more universal indicator of fishing effects than changes in mean trophic level.

Long‐term changes in the trophic level of the Celtic Sea fish community and fish market price distribution

Summary The intensive exploitation of fish communities often leads to substantial reductions in the abundance of target species, with ramifications for the structure and stability of the ecosystem as a whole. We explored changes in the mean trophic level of the Celtic Sea (ICES divisions VII f–j) fish community using commercial landings, survey data and estimates of trophic level derived from the analysis of nitrogen stable isotopes. Our analyses showed that there has been a significant decline in the mean trophic level of survey catches from 1982 to 2000 and a decline in the trophic level of landings from 1946 to 1998. The decline in mean trophic level through time resulted from a reduction in the abundance of large piscivorous fishes and an increase in smaller pelagic species which feed at a lower trophic level. Similar patterns of decline in the trophic level of both catches and landings imply that there have been substantial changes in the underlying structure of the Celtic Sea fish community and not simply a change in fishery preferences. We suggest that the reported changes in trophic structure result from reductions in the spawning stock biomass of traditional target species associated with intensive fishing, together with long‐term climate variability. The relative distribution of fish market prices has changed significantly over the past 22 years, with high trophic level species experiencing greater price rises than lower trophic level species. Although decreased abundance of high trophic level species will ultimately have negative economic consequences, the reduction in mean trophic level of the fish community as a whole may allow the system to sustain higher fishery yields. Management objectives in this fishery will depend on the relative values that society attaches to economic profit and protein production.

Stable nitrogen and carbon isotope ratios in multiple tissues of the northern fur seal Callorhinus ursinus: implications for dietary and migratory reconstructions

We investigated changes in trophic level and feeding location over time in juvenile male northern fur seals Callorhinus ursinus from the Pribilof Islands, Alaska, using stable nitrogen (δ 15 N) and carbon (δ 13 C) isotope analyses of their fur, muscle, blubber, brain, liver, and kidney tissues. Mean δ 15 N values were non-uniform between tissues and ranged from 14.9‰ (fur) to 17.1‰ (lipid extracted blubber). Mean δ 13 C values also varied with tissue type and ranged from -24.7‰ (non-lipid extracted blubber) to -17.5‰ (fur). Mean isotope values of tissues clustered into groups coincident with their estimated protein and isotope turnover times, with fur representing the most remote incor-

Temporal records of δ13C and δ15N in North Pacific pinnipeds: inferences regarding environmental change and diet

Sea lion and seal populations in Alaskan waters underwent various degrees of decline during the latter half of the twentieth century and the cause(s) for the declines remain uncertain. The stable carbon ((13)C/(12)C) and nitrogen ((15)N/(14)N) isotope ratios in bone collagen from wild Steller sea lions (Eumetopias jubatus), northern fur seals (Callorhinus ursinus) and harbor seals (Phoca vitulina) from the Bering Sea and Gulf of Alaska were measured for the period 1951-1997 to test the hypothesis that a change in trophic level may have occurred during this interval and contributed to the population declines. A significant change in δ(15)N in pinniped tissues over time would imply a marked change in trophic level. No significant change in bone collagen δ(15)N was found for any of the three species during the past 47years in either the Bering Sea or the Gulf of Alaska. However, the (15)N in the Steller sea lion collagen was significantly higher than both northern fur seals and harbor seals. A significant decline in δ(13)C (almost 2 ‰ over the 47years) was evident in Steller sea lions, while a declining trend, though not significant, was evident in harbor seals and northern fur seals. Changes in foraging location, in combination with a trophic shift, may offer one possible explanation. Nevertheless, a decrease in δ(13)C over time with no accompanying change in δ(15)N suggests an environmental change affecting the base of the foodweb rather than a trophic level change due to prey switching. A decline in the seasonal primary production in the region, possibly resulting from decreased phytoplankton growth rates, would exhibit itself as a decline in δ(13)C. Declining production could be an indication of a reduced carrying capacity in the North Pacific Ocean. Sufficient quantities of optimal prey species may have fallen below threshold sustaining densities for these pinnipeds, particularly for yearlings and subadults who have not yet developed adequate foraging skills.

Santa Giusta Lagoon (Sardinia): phytoplankton and nutrients before and after waste water diversion

The purpose of this paper was to assess the effect of the diversion of waste water on the trophic status of the Stagno di Santa Giusta, a hypertrophic lagoon on the centre-western coast of Sardinia. The data available (1990, 1992-1994) before the diversion (1995) were compared with those collected from 1995 to 1999. The lagoon has an area of about 8 km2; its mean depth is about 1 m and is well mixed as regards circulation and stratification. This study examined temperature, salinity, pH, main nutrients (reactive and total phosphorus, nitrate and ammonium nitrogen, reactive silica), chlorophyll-a, phytoplanktonic density and phytobenthic biomass. A comparison of the condition of the water before and after the diversion showed that there was an insufficient reduction in nutrient concentrations. The mean concentrations of nitrate nitrogen and ammonium nitrogen fell from 33 and 87 mg N m-3, respectively, to 28 and 66 mg N m-3, respectively. There was a more marked reduction in phosphorus (from 107 mg P m-3 as reactive phosphorus and from 190 mg P m-3 as total phosphorus to 77 and 127 mg P m-3, respectively), and in reactive silica, from 3 to 1.7 mg l-1. Chlorophyll-a decreased from 11.3 to 10.2 mg m-3; the total density of phytoplankton dropped considerably, but this was due to a different species composition. Phytobenthic biomass showed no particular variations. There were no changes in trophic level, so that dystrophic crises still occurred after the diversion. The results showed that there were no substantial improvements in the trophic status of the lagoon during the years after the diversion, except for some months in 1995. A possible explanation may lie in the structure of the diversion system, which does not completely stop the inflow of sewage into the lagoon.

The history of eutrophication in the sea area of Helsinki in the 20th century. Long-term analysis of plankton assemblages.

The aim of this study is to present a historical continuum of nearly 100 years of environmental investigations and changes in trophic levels in the sea area of Helsinki. The survey is based on a re-examination of original planktological and meteorological data; the principle being that eutrophication can be detected in plankton assemblages despite changes in methodology. The bays around Helsinki were found to be moderately eutrophic to hypereutrophic at the beginning of the 20th century. There were considerable differences in the development of the bays both in terms of degradation and recovery. The efficiency of wastewater treatment and especially the introduction of sea outfall have played an important role in decreasing eutrophication in the bay area of Helsinki.

A Monitoring and Classification System for New Zealand Lakes and Reservoirs

ABSTRACT Five variables gave strong indications of change in the trophic state of New Zealand lakes, namely; concentrations of chlorophyll a, total phosphorus and total nitrogen, as well as Secchi depth and dissolved oxygen depletion rate. The New Zealand Lake Monitoring Program, investigated 17 lakes for 3 to 4 years to develop a method for detection of small changes in these five variables and to develop a cost-effective monitoring system for lakes. Two different methods were developed which enabled trends observed separately for variables to be combined into an indicator of change (PAC, Percent Annual Change) and an index of trophic state (TLI, Trophic Level Index). The PAC is a relative indicator that gives the average percent annual change of the trophic state variables, and is an indicator of the magnitude of the change observed. It is calculated from the 5 variables listed above. The TLI is a numerical indicator of lake trophic level, which typically varies from 2 (oligotrophic) to 7 (supertrophic). The TLI can be examined for time trends that generally reflect the changes detected by the PAC relative indicator. These two indicators, when used together, give a good estimate of the probability and magnitude of trophic level change in a lake as well as the trophic level of the lake. Results for all the monitored lakes are given. The PAC and TLI indicators also gave useful results when used to study reservoir monitoring data.

The late Pleistocene ostracod record of the crater lake sediments from Lago di Albano (Central Italy): changes in trophic status, water level and climate

Four cores (ranging between ca. 9 and ca. 14 m in length) from Lago di Albano in Central Italy were studied for their ostracod content, as well as algal and bacterial pigments, CaCO3 and concentration of organic matter. Cores PALB 94 1E and PALB 94 1C from Site 1, located at the bottom of a steep slope at 70 m water depth, where oxygen concentration is below 6 mg l-1, spans the Holocene and the late Pleistocene until 28 kyr B.P. (calibrated age). The other cores, PALB 94 6A and PALB 94 6B taken at a depth of 30 m, where oxygen is 7--11 mg l-1, represent mainly Pleistocene deposits.

The trophic status of marine turtles as determined by stable isotope analysis

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 166:277-284 (1998) - doi:10.3354/meps166277 The trophic status of marine turtles as determined by stable isotope analysis B. J. Godley1,2,*, D. R. Thompson1,**, S. Waldron3, R. W. Furness1 1Division of Environmental and Evolutionary Biology, Graham Kerr Building, University of Glasgow, Glasgow G12 8QQ, United Kingdom 2Department of Veterinary Anatomy, University of Glasgow Veterinary School, Glasgow G61 1QH, United Kingdom 3Scottish Universities Research and Reactor Centre, Scottish Technology Park, East Kilbride, Glasgow G75 OQU, United Kingdom *E-mail: b.godley@udcf.gla.ac.uk **Present address: National Institute of Water & Atmospheric Research, 301 Evans Bay Parade, Greta Point, PO Box 14-901, Kilbirnie, Wellington, New Zealand Stable isotope ratios of nitrogen and carbon were determined in bone collagen, egg proteins and keratin from epidermal carapace scutes of loggerhead Caretta caretta, green Chelonia mydas, and leatherback Dermochelys coriacea turtles from the Mediterranean Sea and the European Atlantic Ocean. Isotope ratios in proteins from loggerhead turtles indicate that this species occupies a higher trophic position than green turtles. Leatherback turtles are apparently intermediate in trophic status. Within loggerhead turtles, both isotope ratios correlated positively with body size, indicating a trend of increasing trophic level with age. Within the size range of green turtles sampled, there was apparently no change in trophic level in relation to body size. In absolute terms, isotope signatures of egg proteins were markedly different from those of bone collagen. However, inter-specific differences in isotope values were consistent with those in bone protein. The novel application of this technique to marine turtles generally confirmed dietary information collected conventionally in these species and has also afforded additional insights into shifts in diet and trophic status as turtles mature. Feeding ecology · Caretta caretta · Chelonia mydas · Dermochelys coriacea · Mediterranean · Carbon · Nitrogen Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 166. Publication date: May 28, 1998 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1998 Inter-Research.

Recent lake acidification and cladoceran dynamics: surface sediment and core analyses from lakes in Norway, Scotland and Sweden

To interpret the remains of Cladocera in lake sediments in relation to pH history, fish abundance, vegetation change, trophic level change and other historic events, it is necessary to understand the balance of abiotic and biotic forces responsible for their present distribution, population dynamics and morphological types. Once these factors are understood, we can reverse the arguments to infer past lake conditions. Many cladoceran species are influenced by fish and invertebrate predation. Some species, especially in the plankton, also show a clear physiological relation to pH and aluminium levels in lakes. Moreover, several littoral-benthic species have a habitat distribution restricted to rock, sand, mud, vegetation, or a combination of some of these. Remains of littoral and planktonic cladocerans were analysed in surface sediments of 18 Norwegian lakes with pH ranging from 4.5-7.5. In addition, sediment cores from four sites in Norway, four in Scotland and one in Sweden were analysed. The majority of sites showed evidence of recent acidification. In lakes with non- planktivorous fish, analyses of cladoceran remains gave no information on past fish populations, but indicated the pH history of the lakes. In lakes with present or past populations of planktivorous fish, the cladoceran record could be used to assess past fish status as well as past pH. In some lakes changes in the cladoceran communities could be related to changes in macrophyte distribution.

Density-Dependent, Trophic Level Responses to an Introduced Run of Sockeye Salmon (Oncorhynchus nerka) at Frazer Lake, Kodiak Island, Alaska

Sockeye salmon (Oncorhynchus nerka) have developed into a major run at Frazer Lake, Kodiak Island, Alaska, reaching over 600 000 adults 34 yr after the first egg-plant in 1951. In the last nine years (1978–86), escapements have averaged over 250 000 with a record high of 486 000 adults in 1985. The increase in escapements were inversely related to seasonal macrozooplankton densities (p &lt; 0.05; r2 = 0.42) but directly correlated with increases in the cladoceran to copepod ratio (p &lt; 0.001). In addition, the average body length of the smallest macrozooplanker (Bosmina) has shifted below the observed threshold length of 0.40 mm for juvenile sockeye prey. We also found an inverse relationship between greater parental escapements and age 1. smolt length (p &lt; 0.01; r2 = 0.75). Small-sized smolts produced from a density-dependent rearing environment indicate that spawning capacity equals or exceeds rearing capacity. Juvenile recruits from larger escapements (~400 000) exerted a controlling, density-dependent influence on the secondary trophic level which drastically reduced subsequent adult returns. Moreover, adverse trophic level changes, particularly after the largest escapements, also appeared to reduce the rearing efficiency for lower densities of juveniles, as subsequent smolt sizes from smaller escapements were less than expected. Thus, these findings not only provide a rationale for suggesting that fewer spawners would provide recruits more closely matching the rearing capacity of this lake, but also provide empirical evidence for the potential adverse density-dependent effects of large escapements.

Trophic relationships in the demersal fish community off Namibia

The benthic fish community off Namibia (between the Cunene River and Walvis Bay) in 50–650 m of water was studied during three bottom trawl surveys. The community was investigated on the basis of distribution, abundance and diet of 51 species, constituting 95 per cent of the demersal fish biomass. Dietary studies revealed the existence of three major trophic groups, one containing species that prey on pelagic and nektonic organisms, a second dominated by predators that feed on benthic polychaetes and copepods and a third group containing predators of benthic crabs, demersal shrimps and fish. Well differentiated from these groups are a few species that prey mainly on jellyfish and ophiuroids. Because for most of the species, their trophic level changes with growth, they were divided into size classes which are analysed independently. To describe interactions between the size classes, a similarity index combining diet affinity with spatial coincidence was applied. Cluster analysis showed that, for Merluccius capensis, Raja straeleni, R. clavata and Trachurus trachurus, there is a large difference in both geographic distribution and dietary preference between the size classes. For species such as Hoplostethus atlanticus, Austroglossus microlepis and Coelorinchus flabellispinis, the different size classes seem to share the same ecological niche. Variation in the number of species and the parameters mean abundance and niche width are described to enhance knowledge of the general structure of the community.