Fish Size Structure Research Articles

Normalized abundance spectra of fish community reflect hydro-peaking on a Mediterranean large river

The European Water Framework Directive requires the integration of body size characters as an important part of fish-based bioassessment tools for freshwaters ecological status determination. The study of the entire fish community size-structure provides valuable information about food web capacity, food web stability and ecological efficiency of aquatic ecosystems. One of the most used representations of community size structure is the Normalized Abundance Spectra (NAS) that provides an approximation of the total fish abundance and food web capacity (through the parameter y-intercept) and an estimation of food web efficiency (through the slope of the linear regression). In this study we explored NAS of the lower Ebro River fish community by integrating data from monthly electrofishing samplings during a whole year (November 2014- October 2015). We found that the percentage of total alien and alien-prey individuals were directly related with y-intercept and inversely related with slope of NAS. This is because the bulk of the community consists of introduced species of small body length. Furthermore, we detected significant relationships between NAS-related parameters and the hydrological variables describing diel flow oscillations and daily flow variability. Based on this, we suggest that high flow variability conditions and, above all, high hydro-peaking conditions, caused a diminution of the total abundance of fish and a decrease of the proportion of small sized fish (i.e. lower y-intercept and flatter slopes of NAS, respectively). Finally, a significant interaction between hydro-peaking and the percentage of alien-predators suggests that high hydro-peaking conditions benefit predation by facilitating predator-prey encounters. This is reflected by strong linear relationship between NAS parameters and percentage of piscivorous at high hydropeaking conditions that disappears at low hydro-peaking fluctuations. We concluded that the high proportion of alien fishes and the presence of a hydropower generation plant that operates by hydro-peaking are important factors determining fish size structure in the lower Ebro River.

Habitat selection and reproductive success of coot Fulica atra on ponds under different fish size and density conditions

Fish may influence habitat selection and reproductive success in waterfowl. We investigated the effects of common carp Cyprinus carpio on breeding coots Fulica atra along a gradient of fish size structure and density, created by separate stocking of age cohorts in ponds in eastern Poland. Coot breeding densities were higher on ponds with low biomass of small-sized, young-of-the-year fish than on ponds with medium- or large-sized fish, stocked at high biomass densities; they also increased with increasing submerged vegetation biomass (an effect correlated with water transparency) and emergent vegetation cover. Densities of nektonic and epibenthic macroinvertebrates and of amphibian larvae were also negatively influenced by fish size/density gradient, while densities of emerging insects were not affected. However, coot breeding success per pair was similar among pond types, while positively related to submerged vegetation, indicating that either plant food abundance was more important than the overall trophic impact of fish or fledgling production was additionally limited by factors other than food. Carp may adversely affect pond habitats of waterfowl both via trophic interactions and through abiotic disturbance of ecosystem processes. In coots, however, the effects can be mitigated by maintenance of abundant emergent vegetation and of submerged macrophytes resistant to fish.

Is Recovery of Large-Bodied Zooplankton after Nutrient Loading Reduction Hampered by Climate Warming? A Long-Term Study of Shallow Hypertrophic Lake Søbygaard, Denmark

Nutrient fluctuations and climate warming can synergistically affect trophic dynamics in lakes, resulting in enhanced symptoms of eutrophication, thereby potentially counteracting restoration measures. We performed a long-term study (23 years) of zooplankton in Danish Lake Søbygaard, which is in recovery after nutrient loading reduction, but now faces the effects of climate warming. We hypothesized that the recovery of large-bodied zooplankton after nutrient loading reduction would be hampered by climate warming through indirect effects on fish size structure. We found a shift in macrozooplankton from initial dominance of Daphnia spp. towards Bosmina spp. as well as a decline in the body size of copepods and an increase in the abundance of nauplii. These changes coincided with the increase in small sized fish as a result of rising water temperature. Despite a reduction in body size, the total biomass of cladocerans increased coinciding with a diminished fish catch per unit effort (CPUE), and likely then an overall reduction in the predation on zooplankton. A cascading effect to phytoplankton was evidenced by enhanced zooplankton:phytoplankton and cladoceran:phytoplankton ratios and a decrease in Chl-a:TP and Chl-a:TN ratios. Our results indicate that climate warming, through changes in the size structure of fish community, has major effects on zooplankton size structure. In Lake Søbygaard, the decline in zooplankton size did not prevent, but modulated, the positive cascading effect on phytoplankton through an expected diminished fish CPUE related to nutrient loading reduction.

Influences of depth and pelagic subsidies on the size-based trophic structure of Beaufort Sea fish communities

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 549:153-166 (2016) - DOI: https://doi.org/10.3354/meps11709 Influences of depth and pelagic subsidies on the size-based trophic structure of Beaufort Sea fish communities Ashley D. Stasko1,*, Heidi Swanson1, Andrew Majewski2, Sheila Atchison2, James Reist2, Michael Power1 1Department of Biology, University of Waterloo, 200 University Avenue West, Waterloo, Ontario N2L 3G1, Canada 2Fisheries and Oceans Canada, 501 University Crescent, Winnipeg, Manitoba R3T 2N6, Canada *Corresponding author: astasko@uwaterloo.ca ABSTRACT: The quality and quantity of food reaching the seafloor is negatively related to water column depth. Lower food availability at depth may in turn affect trophic structure and result in depth-related patterns in biomass and body size (size spectra) in demersal fish communities. Here, we used demersal fishes collected from 4 habitat depths in the Beaufort Sea, spanning 18 to 1000 m, to investigate depth-related changes in the slopes of biomass size-spectra relationships and whether they are related to benthic resource use. Significant positive relationships between biomass-weighted trophic level (estimated from δ15N) and log2 body mass classes demonstrated strongly size-structured fish food webs at all habitat depths, with larger individuals occupying higher trophic levels. Total fish community biomass peaked between 350 and 500 m. The decline in biomass production beyond 500 m was greater for smaller size classes, resulting in significantly shallower (less negative) size-spectra slopes with increasing habitat depth. Large size classes in deep habitats were dominated by species known to obtain pelagic subsidies. Lower isotopic distinction between benthic and benthopelagic fish species, based on 13C measurements, suggested lower dietary differentiation between feeding guilds in deeper habitats. We suggest pelagic subsidies allow high relative biomass production in larger size classes of fishes in deeper habitats, whereas decreasing availability of benthic resources limit biomass production in smaller size classes. Size spectra of demersal fish communities may provide an important indicator of structure in both pelagic and benthic Arctic environments, and be a useful parameter to include in long-term monitoring programs. KEY WORDS: Benthic-pelagic coupling · Biomass · Size distribution · Demersal fishes · Arctic marine ecology · Pelagic subsidies Full text in pdf format PreviousNextCite this article as: Stasko AD, Swanson H, Majewski A, Atchison S, Reist J, Power M (2016) Influences of depth and pelagic subsidies on the size-based trophic structure of Beaufort Sea fish communities. Mar Ecol Prog Ser 549:153-166. https://doi.org/10.3354/meps11709 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 549. Online publication date: May 10, 2016 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2016 Inter-Research.

Density-dependent effects as key drivers of intraspecific size structure of six abundant fish species in lakes across Europe

We studied fish size structure by using mean size, size diversity, and the slope of linear size spectra of six common European fish species along large-scale environmental gradients. We further analyzed the response of these three size metrics to environmental variables and to density-dependent effects, i.e., relative estimates of abundance (catch per unit effort, CPUE). We found differences in the strength of main predictors of size structure between the six species, but the direction of the response was relatively similar and consistent for most of the size metrics. Mean body size was negatively related to temperature for perch (Perca fluviatilis), roach (Rutilus rutilus), and ruffe (Gymnocephalus cernuus). Lake productivity (expressed as total phosphorus concentration) and lake depth were also predictors of size structure for four of six species. Moreover, we found a strong density dependence of size structure for all species, resulting in lower mean body size and size diversity and steeper size spectra slopes when density dependence increases. This suggests that density dependence is a key driver of fish size structure.

Comparing model predictions for ecosystem-based management

Ecosystem modeling is becoming an integral part of fisheries management, but there is a need to identify differences between predictions derived from models employed for scientific and management purposes. Here, we compared two models: a biomass-based food-web model (Ecopath with Ecosim (EwE)) and a size-structured fish community model. The models were compared with respect to predicted ecological consequences of fishing to identify commonalities and differences in model predictions for the California Current fish community. We compared the models regarding direct and indirect responses to fishing on one or more species. The size-based model predicted a higher fishing mortality needed to reach maximum sustainable yield than EwE for most species. The size-based model also predicted stronger top-down effects of predator removals than EwE. In contrast, EwE predicted stronger bottom-up effects of forage fisheries removal. In both cases, the differences are due to the presumed degree of trophic overlap between juveniles of large-bodied fish and adult stages of forage fish. These differences highlight how each model’s emphasis on distinct details of ecological processes affects its predictions, underscoring the importance of incorporating knowledge of model assumptions and limitation, possibly through using model ensembles, when providing model-based scientific advice to policy makers.

Implications of scaled δ15N fractionation for community predator-prey body mass ratio estimates in size-structured food webs.

Nitrogen stable isotope ratios (δ(15) N) may be used to estimate community-level relationships between trophic level (TL) and body size in size-structured food webs and hence the mean predator to prey body mass ratio (PPMR). In turn, PPMR is used to estimate mean food chain length, trophic transfer efficiency and rates of change in abundance with body mass (usually reported as slopes of size spectra) and to calibrate and validate food web models. When estimating TL, researchers had assumed that fractionation of δ(15) N (Δδ(15) N) did not change with TL. However, a recent meta-analysis indicated that this assumption was not as well supported by data as the assumption that Δδ(15) N scales negatively with the δ(15) N of prey. We collated existing fish community δ(15) N-body size data for the Northeast Atlantic and tropical Western Arabian Sea with new data from the Northeast Pacific. These data were used to estimate TL-body mass relationships and PPMR under constant and scaled Δδ(15) N assumptions, and to assess how the scaled Δδ(15) N assumption affects our understanding of the structure of these food webs. Adoption of the scaled Δδ(15) N approach markedly reduces the previously reported differences in TL at body mass among fish communities from different regions. With scaled Δδ(15) N, TL-body mass relationships became more positive and PPMR fell. Results implied that realized prey size in these size-structured fish communities are less variable than previously assumed and food chains potentially longer. The adoption of generic PPMR estimates for calibration and validation of size-based fish community models is better supported than hitherto assumed, but predicted slopes of community size spectra are more sensitive to a given change or error in realized PPMR when PPMR is small.

Evaluation and management implications of uncertainty in a multispecies size-structured model of population and community responses to fishing.

Implementation of an ecosystem approach to fisheries requires advice on trade-offs among fished species and between fisheries yields and biodiversity or food web properties. However, the lack of explicit representation, analysis and consideration of uncertainty in most multispecies models has limited their application in analyses that could support management advice.We assessed the consequences of parameter uncertainty by developing 78 125 multispecies size-structured fish community models, with all combinations of parameters drawn from ranges that spanned parameter values estimated from data and literature. This unfiltered ensemble was reduced to 188 plausible models, the filtered ensemble (FE), by screening outputs against fish abundance data and ecological principles such as requiring species' persistence.Effects of parameter uncertainty on estimates of single-species management reference points for fishing mortality (FMSY, fishing mortality rate providing MSY, the maximum sustainable yield) and biomass (BMSY, biomass at MSY) were evaluated by calculating probability distributions of estimated reference points with the FE. There was a 50% probability that multispecies FMSY could be estimated to within ±25% of its actual value, and a 50% probability that BMSY could be estimated to within ±40% of its actual value.Signal-to-noise ratio was assessed for four community indicators when mortality rates were reduced from current rates to FMSY. The slope of the community size spectrum showed the greatest signal-to-noise ratio, indicating that it would be the most responsive indicator to the change in fishing mortality F. Further, the power of an ongoing international monitoring survey to detect predicted responses of size spectrum slope was higher than for other size-based metrics.Synthesis and applications: Application of the ensemble model approach allows explicit representation of parameter uncertainty and supports advice and management by (i) providing uncertainty intervals for management reference points, (ii) estimating working values of reference points that achieve a defined reduction in risk of not breaching the true reference point, (iii) estimating the responsiveness of population, community, food web and biodiversity indicators to changes in F, (iv) assessing the performance of indicators and monitoring programmes and (v) identifying priorities for data collection and changes to model structure to reduce uncertainty.

Effectiveness of fishing gears to assess fish assemblage size structure in small lake ecosystems

AbstractMeasurement of fish body‐size distributions is increasingly used as a management tool to assess fishery status. However, the effects of gear selection on observed fish size structure has not received sufficient attention. Four different gear types (experimental gill nets, fine mesh bag seine, and two different sized mesh trap nets), which are commonly employed in the study area for fisheries surveys, were used to fish in five small (< 200 ha) lakes to evaluate differential catch in terms of species composition and assemblage size distributions. Kolmogorov–Smirnov tests revealed that, out of the five lakes and six comparisons, the four gear types captured fish of statistically similar size distributions in only one instance. Non‐metric multi‐dimensional scaling followed by a multi‐response permutation procedure revealed that the species composition of fish captured by these gears also differs. These results support the notion that multiple gear types should be used to assess body‐size distributions as well as fish assemblage composition.

Preference for Cannibalism and Ontogenetic Constraints in Competitive Ability of Piscivorous Top Predators

Occurrence of cannibalism and inferior competitive ability of predators compared to their prey have been suggested to promote coexistence in size-structured intraguild predation (IGP) systems. The intrinsic size-structure of fish provides the necessary prerequisites to test whether the above mechanisms are general features of species interactions in fish communities where IGP is common. We first experimentally tested whether Arctic char (Salvelinus alpinus) were more efficient as a cannibal than as an interspecific predator on the prey fish ninespine stickleback (Pungitius pungitius) and whether ninespine stickleback were a more efficient competitor on the shared zooplankton prey than its predator, Arctic char. Secondly, we performed a literature survey to evaluate if piscivores in general are more efficient as cannibals than as interspecific predators and whether piscivores are inferior competitors on shared resources compared to their prey fish species. Both controlled pool experiments and outdoor pond experiments showed that char imposed a higher mortality on YOY char than on ninespine sticklebacks, suggesting that piscivorous char is a more efficient cannibal than interspecific predator. Estimates of size dependent attack rates on zooplankton further showed a consistently higher attack rate of ninespine sticklebacks compared to similar sized char on zooplankton, suggesting that ninespine stickleback is a more efficient competitor than char on zooplankton resources. The literature survey showed that piscivorous top consumers generally selected conspecifics over interspecific prey, and that prey species are competitively superior compared to juvenile piscivorous species in the zooplankton niche. We suggest that the observed selectivity for cannibal prey over interspecific prey and the competitive advantage of prey species over juvenile piscivores are common features in fish communities and that the observed selectivity for cannibalism over interspecific prey has the potential to mediate coexistence in size structured intraguild predation systems.

Effects of Summer Drawdown on the Fishes and Larval Chironomids in Beulah Reservoir, Oregon

Summer drawdown of Beulah Reservoir, Oregon, could adversely affect fish and invertebrate production, limit sport fishing opportunities, and hinder the recovery of threatened species. To assess the impacts of drawdown, we sampled fish and Chironomidae larvae in Beulah Reservoir in the springs of 2006 to 2008. The reservoir was reduced to 68% of full pool in 2006 and to run-of-river level in 2007. From spring 2006 to spring 2007, the catch per unit effort (CPUE) of fyke nets decreased significantly for dace [Rhinichthys spp.] and northern pikeminnow [Ptychocheilus oregonensis], increased significantly for suckers [Catastomus spp.] and white crappies [Pomoxis nigromaculatus], and was similar for redside shiners [Richardsonius balteatus]. CPUE of gillnets either increased significantly or remained similar depending on genera, and the size structure of redside shiners, suckers, and white crappies changed appreciably. From 2007 to 2008, the CPUE of northern pikeminnow, redside shiners, suckers, and white crappies decreased significantly depending on gear and the size structure of most fishes changed. Springtime densities of chironomid larvae in the water column were significantly higher in 2006 than in 2008, but other comparisons were similar. The densities of benthic chironomids were significantly lower in substrates that were frequently dewatered compared to areas that were partially or usually not dewatered. Individuals from frequently dewatered areas were significantly smaller than those from other areas and the densities of benthic chironomids in 2008 were significantly lower than other years. Summer drawdown can reduce the catch and alter the size structure of fishes and chironomid larvae in Beulah Reservoir.

Size‐ and food‐dependent growth drives patterns of competitive dominance along productivity gradients

Patterns of coexistence among competing species exhibiting size- and food-dependent growth remain largely unexplored. Here we studied mechanisms behind coexistence and shifts in competitive dominance in a size-structured fish guild, representing sprat and herring stocks in the Baltic Sea, using a physiologically structured model of competing populations. The influence of degree of resource overlap and the possibility of undergoing ontogenetic diet shifts were studied as functions of zooplankton and zoobenthos productivity. By imposing different size-dependent mortalities, we could study the outcome of competition under contrasting environmental regimes representing poor and favorable growth conditions. We found that the identity of the dominant species shifted between low and high productivity. Adding a herring-exclusive benthos resource only provided a competitive advantage over sprat when size-dependent mortality was high enough to allow for rapid growth in the zooplankton niche. Hence, the importance of a bottom-up effect of varying productivity was dependent on a strong top-down effect. Although herring could depress shared resources to lower levels than could sprat and also could access an exclusive resource, the smaller size at maturation of sprat allowed it to coexist with herring and, in some cases, exclude it. Our model system, characterized by interactions among size cohorts, allowed for consumer coexistence even at full resource overlap at intermediate productivities when size-dependent mortality was low. Observed shifts in community patterns were crucially dependent on the explicit consideration of size- and food-dependent growth. Accordingly, we argue that accounting for food-dependent growth and size-dependent interactions is necessary to better predict changes in community structure and dynamics following changes in major ecosystem drivers such as resource productivity and mortality, which are fundamental for our ability to manage exploitation of living resources in, e.g., fisheries.

Temperature-Driven Regime Shifts in the Dynamics of Size-Structured Populations

Global warming impacts virtually all biota and ecosystems. Many of these impacts are mediated through direct effects of temperature on individual vital rates. Yet how this translates from the individual to the population level is still poorly understood, hampering the assessment of global warming impacts on population structure and dynamics. Here, we study the effects of temperature on intraspecific competition and cannibalism and the population dynamical consequences in a size-structured fish population. We use a physiologically structured consumer-resource model in which we explicitly model the temperature dependencies of the consumer vital rates and the resource population growth rate. Our model predicts that increased temperature decreases resource density despite higher resource growth rates, reflecting stronger intraspecific competition among consumers. At a critical temperature, the consumer population dynamics destabilize and shift from a stable equilibrium to competition-driven generation cycles that are dominated by recruits. As a consequence, maximum age decreases and the proportion of younger and smaller-sized fish increases. These model predictions support the hypothesis of decreasing mean body sizes due to increased temperatures. We conclude that in size-structured fish populations, global warming may increase competition, favor smaller size classes, and induce regime shifts that destabilize population and community dynamics.

The feeding habit of the CyprinidaeRastrineobola argenteain its new habitat, lakes Bulera and Ruhondo, two Rwandan lakes (Eastern Africa)

Rastrineobola argentea Pellegrin 1904, a small pelagic endemic cyprinid from lake Victoria was introduced into lake Bulera (Rwanda) in 1991 in order to develop a fishery. From there, it accidentally colonized lake Ruhondo. To study its biology in its new habitat, samples were collected in 2007 and size and the feeding habits of the fish were analyzed. R. argentea is smaller in lake Bulera (41.8 ± 6 mm) than in lake Ruhondo (mean total body length: 72.6 ± 15 mm). The mean total body length in lake Bulera is less than its size at the first maturity in lake Victoria (47 mm). Differences in size structure of fish between lakes Bulera and Ruhondo is attributed mainly to the food availability in the two lakes. In lake Bulera, the fish feeds on six food items with plant remnants dominant. In lake Ruhondo, R. argentea is a zooplanktivorous feeder like in lake Victoria. The dominant prey is a cladoceran species, Daphnia sp. which has not been observed in lake Bulera during the sampling period.

Consequences of the size structure of fish populations for their effects on a generalist avian predator

Size-structured interspecific interactions can shift between predation and competition, depending on ontogenetic changes in size relationships. I examined the effects of common carp (Cyprinus carpio), an omnivorous fish, on the reproductive success of the red-necked grebe (Podiceps grisegena), an avian gape-limited predator, along a fish size gradient created by stocking distinct age-cohorts in seminatural ponds. Young-of-the-year (0+) carp were an essential food source for young grebes. Only adult birds were able to consume 1-year-old (1+) fish, while 2-year-old (2+) fish attained a size refuge from grebes. Amphibian larvae were the principal alternative prey to fish, followed by macroinvertebrates, but the abundance of both dramatically decreased along the carp size gradient. Fledging success was 2.8 times greater in ponds with 0+ versus 1+ carp; in ponds with 1+ carp, chicks received on average 2.6–3 times less prey biomass from their parents, and over 1/3 of broods suffered total failure. Breeding birds avoided settling on 2+ ponds. These results show that changes in prey fish size structure can account for shifts from positive trophic effects on the avian predator to a negative impact on the predator’s alternative resources. However, competition did not fully explain the decrease in grebe food resources in the presence of large fish, as carp and grebes overlapped little in diet. In experimental cages, 1+ carp totally eliminated young larvae of amphibians palatable to fish. In field conditions, breeding adults of palatable taxa avoided ponds with 1+ and older carp. Non-trophic interactions such as habitat selection by amphibians or macroinvertebrates to avoid large fish may provide an indirect mechanism strengthening the adverse bottom-up effects of fish on birds.Electronic supplementary materialThe online version of this article (doi:10.1007/s00442-010-1862-3) contains supplementary material, which is available to authorized users.

Biomass, Density, and Size Distributions of Fishes Associated with a Large-Scale Artificial Reef Complex in the Gulf of Mexico

The Freeport Sulphur Mine Artificial Reef (FSMAR) is a decommissioned oil and gas platform and serves as the largest artificial reef complex in the northern Gulf of Mexico (NGOM). Given the increasing numbers of artificial reefs in the NGOM, yet the paucity of information that exists, the goals of this study were to evaluate the biomass, density, and size structure of fishes associated with FSMAR. Mobile acoustic surveys were used to assess both horizontal and vertical distribution and abundance of fishes associated with the shallow water (16 m depth) reef complex and adjacent soft-bottom habitats extending 1 km from the reef complex. Highest acoustic estimates of fish biomass and density were found directly over the reef with a five-fold and 16-fold decrease at 10 and 30 m distances from the structure, respectively. In addition, fish biomass and density were highest in the bottom water column (> 10 m), followed by mid-water (6.1–10 m), and lowest in the upper water column (1.5–6 m). Findings suggest that fish distribution at the FSMAR is much greater than previously surveyed decommissioned oil and gas platforms and natural reef habitats in the NGOM. We consider the potential importance of this unique nearshore complex as an important habitat to fishes utilizing nearby estuarine and inner shelf systems in the NGOM.

Habitat selection by breeding waterbirds at ponds with size-structured fish populations

Fish may significantly affect habitat use by birds, either as their prey or as competitors. Fish communities are often distinctly size-structured, but the consequences for waterbird assemblages remain poorly understood. We examined the effects of size structure of common carp (Cyprinus carpio) cohorts together with other biotic and abiotic pond characteristics on the distribution of breeding waterbirds in a seminatural system of monocultured ponds, where three fish age classes were separately stocked. Fish age corresponded to a distinct fish size gradient. Fish age and total biomass, macroinvertebrate and amphibian abundance, and emergent vegetation best explained the differences in bird density between ponds. Abundance of animal prey other than fish (aquatic macroinvertebrates and larval amphibians) decreased with increasing carp age in the ponds. Densities of ducks and smaller grebes were strongly negatively associated with fish age/size gradient. The largest of the grebes, the piscivorous great crested grebe (Podiceps cristatus), was the only species that preferred ponds with medium-sized fish and was positively associated with total fish biomass. Habitat selection by bitterns and most rallids was instead strongly influenced by the relative amount of emergent vegetation cover in the ponds. Our results show that fish size structure may be an important cue for breeding habitat choice and a factor affording an opportunity for niche diversification in avian communities.

Size-structured effects of common carp on reproduction of pond-breeding amphibians

The role of fish in driving amphibian communities has been widely recognized. However, little is known about size-structured interactions between amphibian and fish populations. This study compared the taxonomic occurrence and densities of larval amphibians between unstocked ponds and ponds stocked with different age cohorts of common carp Cyprinus carpio differing in average body size. The average total densities of early and late breeding anurans known to be vulnerable to fish were by 1–2 orders of magnitude greater in the presence of young-of-the-year carp than that of older cohorts. The probabilities of occurrence of the most common taxa did not differ between ponds stocked with young-of-the-year fish and ponds free of carp, but were significantly larger in those ponds than in ponds stocked with large-size cohorts. No significant differences between pond categories were found for densities of unpalatable Bufo bufo larvae. In aquatic systems harbouring size-structured fish populations, a fish age/size gradient may explain differential habitat suitability for breeding amphibians better than the fish presence/absence dichotomy. When dominated by young cohorts incapable of predation or of adverse habitat alteration, fish-abundant waters are suitable for amphibian reproduction. Conversely, even a ‘non-predatory’ fish, after attaining large body size, may exert a detrimental impact on amphibian breeding success. These findings may be particularly important for amphibian conservation at pond fisheries characterized by spatial separation of age/size distributed stocks.

Indicators of change in the size structure of fish communities: A case study from the south coast of South Africa

Heavy fishing is believed to change the size structure of fish communities through the differential removal of larger sized individuals and species. The size structure of the demersal fish communities off the south coast of South Africa is assessed using a variety of size-based indicators. The communities on this coast are subjected to different fisheries with wide ranging impacts. Changes in the size structure were assessed over the period from 1986 to 2003 for the whole south coast and two depth strata. There was a reduction in the mean length, mean maximum length ( L max), and proportion of large fish in these fish communities, and a corresponding increase in the proportion of small fish. Species with a large L max declined in mean catch rate and mean length in contrast to species with a small L max. The trend in the size-based indicators confirms that there has been a change in the size structure of the communities. This change is characterized primarily by a reduction in the abundance of larger sized individuals and a reduction in species with a large maximum length.

The Repeatability of Fish Biomass and Size Distribution Estimates Obtained by Hydroacoustic Surveys Using Various Sampling Strategies and Statistical Analyses

AbstractSuccessive hydroacoustic surveys were conducted on a small lake in the Jura Mountains (eastern France) to test several sampling strategies and methods of calculation. The results show that night samples give a better picture of the fish populations than daytime ones; that parallel transect surveys are the ones easiest to conduct, and that, all the survey designs tested, gave statistically equivalent biomass estimations and size distributions. The results obtained are also similar for all the statistical calculation methods used. Repeated samplings on several successive days gave similar results for the estimated biomass and fish size structure in the hypolimnion, but gave different results for the fish populations in the epilimnion. As part of the Water Framework Directive monitoring of fish populations, these hydroacoustic methods can be used to estimate fish stocks, and to track their change over time and spatial differences, but they need to be combined with complementary sampling methods. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)