Small Pelagic Fish Abundance Research Articles

Interactions between the common dolphin, Delphinus delphis, and the Portuguese purse seine fishery over a period of 15 years (2003–2018)

Abstract Interactions between cetaceans and the purse seine fishery in coastal waters of mainland Portugal were investigated using onboard observations over a period of 15 years (2003–2018). In 10% of the fishing sets, there were interactions with one of three species of cetaceans: common dolphin (Delphinus delphis), bottlenose dolphin (Tursiops truncatus), and harbour porpoise (Phocoena phocoena). The common dolphin was the most frequently observed species, occurring in 89% of all interaction events, and the only species with observed mortality. Therefore, this study focused only on the interactions with common dolphins. A generalized additive model (GAM) was applied to observer data to investigate how the fishing activity and the abundance of small pelagic fish influenced the probability of occurrence of interactions between the common dolphin and the purse seine fishery between 2003 and 2018. The results revealed significant annual and geographic variation in the occurrence of interactions, and a significant influence of fishing effort and catch per unit effort of sardine (Sardina pilchardus). A second GAM in a hurdle model was used to model the number of common dolphins that interacted with the fishing activity as a function of fishing seasonality and the local abundance of pelagic fish. There was a significant effect of the seasonality of the fishing activity and catch per unit effort of sardine and chub mackerel (Scomber colias) on the number of common dolphins interacting with the fishery. The results of this study suggest that the probability of interactions and the number of common dolphins interacting with the fishery were affected by the local abundance of sardine and chub mackerel. As the common dolphin population appears to be increasing in the study area, the risk of by‐catch also increases, thus the monitoring of the Portuguese purse seine fishery is important to inform the implementation of management plans, and also as a response to global efforts for sustainable fisheries and marine mammal conservation.

Trophic importance of small pelagic fish to marine predators of the Mediterranean Sea

Small pelagic fish (SPF) species such as European sardine Sardina pilchardus, European anchovy Engraulis encrasicolus, round sardinella Sardinella aurita and European sprat Sprattus sprattus play important ecological roles in the Mediterranean Sea marine ecosystem. However, changes in SPF abundance and biomass have recently been observed in the basin. Therefore, uncovering the trophic importance of SPF to their predators is key to unravelling potential ecological causes and consequences of SPF population changes related to predator dynamics. Here, we reviewed 101 published studies reporting the presence of the SPF species named above in the diet of marine predators inhabiting the Mediterranean Sea. We found that the number of species preying on SPF was highest for teleost fish species, followed by elasmobranchs, seabirds, marine mammals, cephalopods and sea turtles. These predators were both commercial and non-commercial taxa, and vulnerability ranged from medium to very high. From a conservation point of view, most predator species were considered Least Concern by the IUCN, with few exceptions such as the Balearic shearwater Puffinus mauretanicus, a Critically Endangered species. Geographic information showed that the European sardine was more important in the diet of predators from the western Mediterranean basin, while European anchovy consumption was higher on the eastern side. Round sardinella was consumed in high amounts in the Tyrrhenian Sea, and European sprat was anecdotally consumed. The importance of SPF in the diet of Mediterranean predators may indicate bottom-up control of the marine ecosystem; however, limited temporal information precluded the evaluation of temporal changes, highlighting the need to develop new studies in this area.

Characterisation of season-wise spatial distribution of small pelagic fishery in Kerala, south-west coast of India

In this study, we attempted to identify and characterise the spatial variability of small pelagic fish abundance using geostatistical methods. The small pelagic fish abundance was estimated in terms of spatial parameters using generalised additive models (GAMs). The ring seine fishing grounds in the south-eastern Arabian Sea exhibited seasonal variability in distribution. Results of the study would help in prediction of the major pelagic fishing grounds for traditional fishers which would help in reducing fuel and time spent on searching for the fishing ground leading to sustainable exploitation. The prediction model can contribute to the management of pelagic fishery along the Kerala coast.

Fish debris in sediments from the last 25 kyr in the Humboldt Current reveal the role of productivity and oxygen on small pelagic fishes

Upwelling of cold, nutrient-rich water from the oxygen minimum zone (OMZ) off Peru sustains the world’s highest production of forage fish, mostly composed of anchovy (Engraulis ringens). However, the potential impacts of climate change on upwelling dynamics and thus fish productivity in the near future are uncertain. Here, we reconstruct past changes in fish populations during the last 25,000 years to unravel their response to changes in OMZ intensity and productivity. We quantified and identified fish scales and bones deposited in laminated sediments from Pisco (Peru) with an average sampling resolution of 20.4 years (±7.1). The records span the Last Glacial Maximum to the recent Holocene and thus encompass a variety of combinations of productivity, oxygen, and global temperature. Our results reveal that productivity appears to be the main factor controlling small pelagic fish abundance, while sub-surface oxygenation affects mainly anchovy and likely sardine populations. Lower productivity and higher oxygen concentrations during the glacial resulted in lower total fish productivity, whereas higher productivity and a stronger OMZ in some time intervals during the Holocene resulted in higher fish abundances. A variety of different conditions between these two oceanographic end members indicate preferred environmental conditions for a variety of small pelagic fishes. There is no evidence in our record for an out of phase relationship between anchovy and sardine at the timescales examined in the present study. Anchovy have been the predominant small pelagic fish throughout the record, at least over centennial to millennial timescales. Its abundance reached a maximum during the Current Warm Period, an era characterized by high productivity and intense OMZ conditions. Thus, industrial fisheries developed during a period of exceptional productivity in relation to that of the last 25 kyr. The records reveal that dramatic decreases in pelagic fish abundances have occurred in response to past large-scale climate changes than those observed in the instrumental period, which suggests that future climate change may result in substantial changes in ecosystem structure.

Effect of environmental conditions on the seasonal and inter‐annual variability of small pelagic fish abundance off North‐West Africa: The case of both Senegalese sardinella

AbstractThe objective of this study was to assess the effect of environmental variations on the abundance of Sardinella aurita and Sardinella maderensis in Senegalese waters in the upwelling system. Monthly data indicating the abundance of sardinella were first estimated from commercial statistics, using Generalized Linear Model from 1966 to 2011. Abundance indices (AIs) were then compared with environmental indices, at the local scale, a Coastal Upwelling Index (CUI) and a coastal Sea Surface Temperature (SST) index, and on a large scale, the North Atlantic Oscillation (NAO), the Atlantic Multidecadal Oscillation (AMO) and the Multivariate El Niño Southern Oscillation Index (MEI), using correlations and times series analyses. The results showed that the abundance of sardinella is determined by a strong seasonal pattern and inter‐annual fluctuations. The abundance of S. aurita peaked in spring and in autumn, whereas that of S. maderensis peaked in the warm season (July–September). The trend of the sardinella abundance was significantly correlated with the CUI, especially in autumn and spring. Interannual fluctuations of S. maderensis and S. aurita abundance are, respectively, driven by the precocity and the duration of the upwelling season that is attributed to distinct migration patterns. Both sardinella species also respond with a delay of around 4 years to the winter NAO index and the autumn CUI, and the AMO index, respectively, both related to migration patterns. The wide variations in sardinella biomass are caused by variations in environmental conditions, which should be considered in the implementation of an ecosystem‐based approach in sardinella stocks management.

Variable selection and accurate predictions in habitat modelling: a shrinkage approach

Habitat modelling is increasingly relevant in biodiversity and conservation studies. A typical application is to predict potential zones of specific conservation interest. With many environmental covariates, a large number of models can be investigated but multi‐model inference may become impractical. Shrinkage regression overcomes this issue by dealing with the identification and accurate estimation of effect size for prediction. In a Bayesian framework we investigated the use of a shrinkage prior, the Horseshoe, for variable selection in spatial generalized linear models (GLM). As study cases, we considered 5 datasets on small pelagic fish abundance in the Gulf of Lion (Mediterranean Sea, France) and 9 environmental inputs. We compared the predictive performances of a simple kriging model, a full spatial GLM model with independent normal priors for regression coefficients, a full spatial GLM model with a Horseshoe prior for regression coefficients and 2 zero‐inflated models (spatial and non‐spatial) with a Horseshoe prior. Predictive performances were evaluated by cross‐validation on a hold‐out subset of the data: models with a Horseshoe prior performed best, and the full model with independent normal priors worst. With an increasing number of inputs, extrapolation quickly became pervasive as we tried to predict from novel combinations of covariate values. By shrinking regression coefficients with a Horseshoe prior, only one model needed to be fitted to the data in order to obtain reasonable and accurate predictions, including extrapolations.

Feeding ecology of some fish species occurring in artisanal fishery of Socotra Island (Yemen)

The demersal species Lethrinus borbonicus, Lethrinus mahsena, Lethrinus microdon, Lethrinus nebulosus, Lutjanus bohar, Lutjanus gibbus, Lutjanus kasmira, Epinephelus fasciatus, Epinephelus stoliczkae, Carangoides gymnostethus and Euthynnus affinis are important coastal fishes species of the northern coast of Socotra (Yemen), exploited by local fishery. The biology and feeding ecology of these species are poorly known in the area. A total of 1239 specimens were sampled from the main fishing landing site of the island (Hadibo). Total length and weight were measured, stomach contents were analyzed, diet overlap, Fulton's Condition index, and trophic levels were estimated. C. gymnostethus, L. microdon and L. kasmira occupied the highest position (T=4.50), L. nebulosus occupied the lower one (TL=3.41). The role of the increasing abundance of small pelagic fish in the diet of many species after the upwelling event is evident, but also different feeding strategies are reported, according to fish ecology.

Intraguild predation between small pelagic fish in the Bay of Biscay: impact on anchovy (Engraulis encrasicolus L.) egg mortality

Small pelagic fish can play an important role in various ecosystems linking lower and upper trophic levels. Among the factor behind the observed inter-annual variations in small pelagic fish abundance, intra- and inter-specific trophic interactions could have a strong impact on the recruitment variability (e.g. anchovy). Egg cannibalism observed in anchovies has been postulated to be a mechanism that determines the upper limit of the population density and self-regulates the population abundance of the species. On the other hand, predation by other guild species is commonly considered as a regulation mechanism between competing species. This study provides empirical evidence of anchovy cannibalism and predation of the main small pelagic fish species on anchovy eggs and estimates the effect of intraguild predation on the anchovy egg mortality rate. Results show that, depending on the year (2008–2009), up to 33 % of the total anchovy egg mortality was the result of sardine predation and up to 4 % was the result of egg cannibalism together with predation by Atlantic and Atlantic Chub mackerel and sprat. Results also indicate that in the Bay of Biscay, fluctuations in the survival index of the early life stages of anchovy are likely to be attributable at least in part to egg cannibalism and especially to a high sardine predation on anchovy eggs.

Synchronous patterns of fluctuations in two stocks of anchovy Engraulis ringens Jenyns, 1842 in the Humboldt Current System

Summary Most of the studies investigating synchrony in fluctuations of abundance of small pelagic fish have been based on catch data only, which do not describe the dynamics of populations as a relative abundance index. In this paper, catch, biomass, recruitment and recruitment rate, were used to compare synchronous changes for two stocks of anchovy (Engraulis ringens) from 1982 to 2004. One is the North Central Peru stock (NCP) and the other is the shared South Peru-Northern Chile stock (SPNC). Correlation analysis demonstrated a significant association between population time series, particularly during the growing phase of the stocks. Thus, the synchronous fluctuation pattern of the two stocks is due to the recovery phase and probably driven by density-independent effects of simultaneous favorable environmental conditions occurring in the two regions. The conclusions were: (i) both NCP and SPNC anchovy stocks are in an overall positive phase of synchrony, (ii) higher correlations in the synchronous pattern of fluctuations occurred during simultaneous increase of biomass, and (iii) short-term fluctuations were negative when the abundance of the stock was lower and/or impacted by El Nino events.

Jellyfication of Marine Ecosystems as a Likely Consequence of Overfishing Small Pelagic Fishes: Lessons from the Benguela

Changes in two contrasting ecosystems of the Benguela upwelling region, one dominated at mid-trophic level by jellyfishes (Namibia, northern Benguela ecosystem, where small pelagic fish abundance has been severely depleted) and one still dominated by small pelagic fishes (South Africa, southern Benguela) were compared in an effort to determine ecosystem trajectories under different exploitation regimes. The role of small pelagic fishes (clupeoids) was highlighted in the context of their importance in maintaining interactions in marine ecosystems. In particular, we examined trophic cascades and possible irreversible changes that promote the proliferation of jellyfishes in marine systems. We found that the presence of large populations of small pelagic fishes has a fundamental role in preserving beneficial trophic interactions in these marine ecosystems. The implications of trophic cascades, such as those observed in the northern Benguela, for ecosystem-based management were apparent. In addition, this comparison provides contrasting case studies to inform the development of management scenarios that avoid ecosystem shifts that affect predators and reduce the value of fisheries production. Changes in two contrasting ecosystems of the Benguela upwelling region, one dominated at mid-trophic level by jellyfishes (Namibia, northern Benguela ecosystem, where small pelagic fish abundance has been severely depleted) and one still dominated by small pelagic fishes (South Africa, southern Benguela) were compared in an effort to determine ecosystem trajectories under different exploitation regimes. The role of small pelagic fishes (clupeoids) was highlighted in the context of their importance in maintaining interactions in marine ecosystems. In particular, we examined trophic cascades and possible irreversible changes that promote the proliferation of jellyfishes in marine systems. We found that the presence of large populations of small pelagic fishes has a fundamental role in preserving beneficial trophic interactions in these marine ecosystems. The implications of trophic cascades, such as those observed in the northern Benguela, for ecosystem-based management were apparent. In addition, this comparison provides contrasting case studies to inform the development of management scenarios that avoid ecosystem shifts that affect predators and reduce the value of fisheries production.

Ecosystem status and functioning: searching for rules of thumb using an intersite comparison of food-web models of Northeast Atlantic continental shelves

AbstractLassalle, G., Lobry, J., Le Loc'h, F., Mackinson, S., Sanchez, F., Tomczak, M. T., and Niquil, N. 2013. Ecosystem status and functioning: searching for rules of thumb using an intersite comparison of food-web models of Northeast Atlantic continental shelves. – ICES Journal of Marine Science, 70:135–149. This work aimed to provide a better understanding of how the structure and function of marine ecosystems and trophic control mechanisms influence their response to perturbations. Comparative analysis of Ecopath models of four Northeast Atlantic ecosystems was used to search for rules of thumb defining the similarities and differences between them. Ecosystem indicators, related to the ecology of species interactions, were derived from these models and compared. Two main questions were addressed. (i) What are the main energy pathways and mechanisms of control? (ii) Do these ecosystems exhibit the widespread and potentially stabilizing food-web structure such that top predators couple distinct energy pathways? A strong bentho-pelagic coupling operated over the Bay of Biscay Shelf, while energy reached higher trophic levels mostly through pelagic compartments, in northern areas. Zooplankton was demonstrated to be trophically important in all ecosystems, acting as a regulator of the abundance of small pelagic fish. A latitudinal pattern in flow control was highlighted by this analysis, with a significant contribution of top-down effect at higher latitudes. This top-down control of the Baltic Sea, combined with the fact that this ecosystem did not exhibit the potentially stabilizing two-channel structure, suggested a non-stable environment.

Hydroacoustical Survey of Near-Surface Distribution, Abundance and Biomass of Small Pelagic Fish in the Gulf of California

Abstract: Near-surface distribution (10–40 m depth), abundance, and biomass of small pelagic fish were estimated using an echo sounder (EY-60, 120 kHz) during January 2007 in central and northern areas of the Gulf of California. Small pelagic fish distribution was spatially correlated with simultaneous, continuous, high-resolution measurements of 4 m depth temperature, salinity, turbidity, chlorophyll a, and dissolved oxygen concentration to characterize water and habitat conditions. Small pelagic fish shoals were concentrated in five areas of northern and central parts of the gulf, along the coast of Sonora, and south of Isla Tiburon. About 62% of small pelagic fish abundance and 57% of the biomass was echo-detected during the night, when small pelagic fish tend to assemble near the surface. Fish abundance in these areas (11,210 km2) was estimated at 6.16 ×109 individuals. Echo-integrated median biomass was 2.6 ×105t. Projecting this biomass to the surveyed area (80,102 km2), median biomass of multispeci...

A prey-related shift in the abundance of small pelagic fish in eastern Tasmania?

Abstract McLeod, D. J., Hobday, A. J., Lyle, J. M., and Welsford, D. C. 2012. A prey-related shift in abundance of small pelagic fish in eastern Tasmania? – ICES Journal of Marine Science, 69: 953–960. Shifts in the relative abundance of small pelagic fish species have signalled a change in the ocean environment in a number of locations. Here we show that the replacement of jack mackerel, Trachurus declivis, with redbait, Emmelichthys nitidus, as the dominant small pelagic species from eastern Tasmania, following a period of high fishing pressure on jack mackerel, is consistent with altered zooplankton communities and long-term climate change. Stomach contents analysis and morphology measurements were conducted on both species to determine if they were functionally equivalent with regard to zooplankton prey. Diet varied between species and with fish size. Krill (Nyctiphanes australis) was consumed by both species, with redbait feeding more heavily on small copepods. The diet overlap and morphometric characteristics indicated that these species are not equivalent with regard to prey and therefore changes in prey availability may have contributed to the observed shifts in relative abundance. The continued poleward extension of the East Australian Current is expected to favour small warm-water copepods; thus, redbait may have an advantage over jack mackerel due to prey preferences. An increase in relative abundance of redbait has decreased effort in surface fisheries and may impact on surface-feeding higher predators in this region.

Foraging ecology and ecophysiology of Cape gannets from colonies in contrasting feeding environments

Using a multi-disciplinary approach, we evaluated the potential consequences of long-term contrasting prey availability on the condition Cape gannets Morus capensis. We compared breeding adults from a decreasing colony on Malgas Island off the west coast of South Africa, where the abundance of small pelagic fish has decreased, with an increasing colony on Bird Island off the south coast, where pelagic fish are more abundant. We investigated (1) pelagic fish density using data from a hydro-acoustic survey, (2) gannet diet (stomach content analyses, fatty acid and stable isotope analyses), (3) gannet foraging effort and at-sea feeding areas (nest attendance patterns, GPS-tracking and blood haematocrit), (4) chick growth rates, and (5) adult body condition (morphometric measurements and breast muscle thickness). Our data confirmed contrasting prey availability between colonies, although pelagic fish were more abundant on the west coast than in previous years. Gannets exhibit dietary plasticity, feeding on pelagic fish and trawler discards, but favour natural prey when available. Stomach content samples showed that gannets from both islands mainly ate natural prey in 2009, but there were differences in their stable isotope and fatty acid signatures, supporting evidence of long-term diet differences. Nevertheless, chick growth rates and adult body condition were similar at both colonies, which was surprising for breeding adults from the west coast colony that had been feeding extensively on energy-poor fishery waste for several years. Breeding gannets' behavioural flexibility seems to have succeeded in maintaining body condition. However, this might have long-term costs, as adult survival at Malgas Island has decreased in recent years. Population decreases at this colony are exacerbated by low juvenile survival and perhaps differential recruitment to Bird Island. Higher foraging effort by adult breeding on the large colony of Bird Island compared to birds from the west coast, despite greater pelagic fish abundance on the south coast, probably suggests a greater intra-specific competition there. Monitoring of population trends, population health indices and foraging behaviour, as well as prey availability is necessary to better understand the mechanisms underlying the population trends.

Identifying essential fish habitat for small pelagic species in Spanish Mediterranean waters

Populations of small pelagic fish support important fisheries in Spanish Mediterranean waters, particularly sardine (Sardina pilchardus) and anchovy (Engraulis encrasicolus), and are distributed along the entire length of the Spanish continental shelf. Using annual acoustic survey data for the years 2003–2005, a GIS-based environmental modelling approach was used to investigate the distribution and abundance of small pelagic fish in relation to environmental variables. Multivariate analysis was applied to provide a preliminary picture of relationships between fish and environmental conditions, followed by application of Generalised Additive Models (GAMs). GAMs showed the presence/absence of fish to be related to bathymetry, sea surface chlorophyll-a concentration and sea surface temperature (SST). The strength and significance of these relationships varied spatially and temporally. High resolution Essential Fish Habitat (EFH) maps were generated for sardine and anchovy, based on the predicted probability of presence of each species. Substantial inter-annual variability in the distribution and quality of EFH was observed, particularly for anchovy. Identification of EFH is of great importance to assess and manage sardine and anchovy resources as it provides a natural link between population dynamics features and geographical scenarios.

Food-web dynamics in the South Catalan Sea ecosystem (NW Mediterranean) for 1978–2003

An ecosystem model representing the continental shelf and upper slope of the South Catalan Sea (NW Mediterranean) is calibrated and fitted to the available time series data from 1978 to 2003. We use a process-oriented model to explore the extent to which changes in marine resources and the ecosystem were driven by trophic interactions, environmental factors and fishing activities. Fishing effort and fishing mortality are used to drive the model, while observed (absolute and relative) biomasses and catches are compared with the predicted results. A reduction in the sum of the squared deviations of the observed and predicted values of the biomass is used as a metric for calibrating and assessing the fit of the model. A posteriori trophodynamic indicators are used to explore the ecosystem's structural and functional changes from 1978 to 2003, and a generalized least squares regression is used to assess the significance of the predicted trends. In general, a high proportion of the variability in the time series data is explained by the main trophic interactions (37–53%), fishing activities (14%), and indirectly by considering the environment (6–16%), as driving factors. The model's predictions match satisfactorily with the yearly data on the biomass for anglerfish, adult hake, demersal sharks, anchovy and mackerel, which show a statistically significant decrease over time, while the biomass of flatfish and seabirds are observed to increase. Catch data show a significant decrease in anglerfish, demersal sharks, anchovy and sardine, while there is an increase in red mullet, flatfish, juvenile hake and horse mackerel. These changes in biomass are predicted to have direct and indirect impacts on the ecosystem mediated by the trophic web, such as the proliferation of non-commercial species with lower trophic levels (e.g., benthic invertebrates) or higher turnover rates (e.g., cephalopods and benthopelagic fish). This is consistent with anecdotal information from the Mediterranean and is likely caused by trophic cascades due to the removal of demersal and pelagic higher trophic level organisms (predator release), and a decrease in small pelagic fish (competitor release). Trophodynamic indicators suggest a degradation pattern over time: both the mean trophic level of the community (mTL co, excluding primary producers and detritus) and a modified version of Kempton's index of biodiversity decrease with time, while the total flow to detritus and the loss of production due to fishing increase from 1978 to 2003. Additionally, the demersal/pelagic ratio increases due to an overall decrease in the abundance of small pelagic fish in the ecosystem.

Pacific sardine behaviour related to tidal current dynamics in Bahía Magdalena, México

The behaviour, distribution and abundance of the Pacific sardine Sardinops sagax, detected by acoustics, was studied in relation to the pelagic environmental conditions of water temperature, fluorescence and dissolved oxygen concentration induced by tidal currents in Bahía Magdalena, México (24°32′ N; 112°01′ W). Sampling was along an 18 km long transect covering an area inside the bay (mean depth 20 m), the main entrance to the bay (mean depth 35 m), and the continental shelf (mean depth 100 m). The main entrance to the bay was the most dynamic area along the hydrographic transect, where the Pacific sardines tended to aggregate, probably to feed on the phytoplankton accumulated during the receding tide. With the flood tide, a high‐speed tidal current is generated, introducing cool water with low oxygen concentrations into the bay. During the ebb tide, a tidal wave is also formed, but the warm and relatively well oxygenated bay water mass is transported towards the open sea. Acoustic records showed that whatever the tidal current direction (inflow or outflow), no fish schools were detected within the area of more intense tidal current speeds (>120 cm s−1). Visual observations of surface Pacific sardine schools during the flood tide indicate that sardines were out of the transducer signal detection range (<10 m depth), suggesting that Pacific sardines undergo vertical migration, probably to avoid the high water current speed with low dissolved oxygen content. This behaviour may have a relevant effect on the overall hydroacoustic estimation of small pelagic fish abundance in shallower pelagic environments such as this subtropical bay.

Structure and functioning of the eastern Scotian Shelf ecosystem before and after the collapse of groundfish stocks in the early 1990s

The fishery-induced collapse of the Atlantic cod (Gadus morhua) stock on the eastern Scotian Shelf has altered the species composition of this ecosystem. Ecopath mass-balance models of the ecosystem before and after the collapse were developed to explore how the structure, function, and key species of the ecosystem had changed. For the first time, an analysis of uncertainty was conducted to examine the effects of the uncertainty on model estimates. A comparison of the two Ecopath models indicated that although total productivity and total biomass of the ecosystem remained similar, there were changes in predator structure, trophic structure, and energy flow, many of which were robust to uncertainty. Biomass has significantly increased at trophic levels 3 and 4, and the composition of these trophic levels has changed as a result of the mean increase in trophic level of many species-groups. Piscivory has increased, presumably because of the high abundance of small pelagic fish, and the ratio of pelagic feeders to demersal feeders has increased from 0.3 to 3.0. Thus, the ecosystem has changed from a demersal-feeder-dominated system to a pelagic-feeder-dominated system. Although uncertainty remains concerning some model estimates, the ecosystem has been profoundly altered and exhibits classic symptoms of "fishing down the food web". However, overall system properties were generally conserved.

Impact of environmental perturbation on a tropical fish community

Tissawewa is a tropical reservoir in which annually recurring water level fluctuations are the main source of environmental variations. Aspects of a major perturbation studied related to (i) a progressive water level decrease that ultimately caused the reservoir to (ii) dry completely and a radical change of environment after refilling. The first aspect caused a decrease in the abundance of small pelagic fish but an increase in large bottom-dwellers. Main causes responsible for these changes are a decrease in the food available to the fish community resulting in weight loss and increased mortality of smaller bodied species. Also, the concentration of suspended silt and seston increased, probably damaging fish gills, thereby adversely affecting oxygen intake of the fish. Apparently, bottom-dwellers are better adapted to these conditions than pelagic species. The second aspect caused the composition of the food source to change, resulting in diet shifts and changes in relative abundance of some fish species (often compensated for by an inverse change of another species of the same feeding guild). In contrast with the annually recurring minor perturbations, the major perturbations caused the species composition and size structure of the fish community to change.

Impact of environmental perturbation on a tropical fish community

Tissawewa is a tropical reservoir in which annually recurring water level fluctuations are the main source of environmental variations. Aspects of a major perturbation studied related to (i) a progressive water level decrease that ultimately caused the reservoir to (ii) dry completely and a radical change of environment after refilling. The first aspect caused a decrease in the abundance of small pelagic fish but an increase in large bottom-dwellers. Main causes responsible for these changes are a decrease in the food available to the fish community resulting in weight loss and increased mortality of smaller bodied species. Also, the concentration of suspended silt and seston increased, probably damaging fish gills, thereby adversely affecting oxygen intake of the fish. Apparently, bottom-dwellers are better adapted to these conditions than pelagic species. The second aspect caused the composition of the food source to change, resulting in diet shifts and changes in relative abundance of some fish species (often compensated for by an inverse change of another species of the same feeding guild). In contrast with the annually recurring minor perturbations, the major perturbations caused the species composition and size structure of the fish community to change. [KEYWORDS: Tilapia]