Do the effects of piscivorous largemouth bass cascade to the plankton?

Ecologists have hypothesized that an increase in the biomass of piscivorous fish in lakes will cause a decrease in populations of planktivorous fish, an increase in the size of herbivorous zooplankton and a decrease in the biomass of phytoplankton. Here we present an experimental test of whether the effects of largemouth bass (Micropterus salmoides) cascade to the planktivorous fish, zooplankton and phytoplankton of a 15-ha water storage reservoir. A pilot study indicated that the reservoir was eutrophic with dense populations of planktivorous fish dominated by threadfin shad (Dorosoma petenense). No piscovorous fish were present in the reservoir. We conducted a one-month mesocosm experiment using water and plankton from the reservoir showing that the presence of threadfin shad reduced large-sized zooplankton and increased the productivity and biomass of phytoplankton. To test whether the effects of piscivorous fish could cascade to the plankton, we assessed the effects of the addition of piscivorous largemouth bass on the planktivorous fish, zooplankton and biomass of phytoplankton of the reservoir by monitoring the reservoir during the year before and the two years after largemouth bass were stocked. In the second year after the addition of largemouth bass, the number of planktivorous fish decreased and the relative abundance of threadfin shad declined. Although the abundance of cladocerans increased after the addition of largemouth bass, the average size of zooplankton did not change. We did not detect changes in chlorophyll a, Secchi depth, or concentrations of total phosphorus and total nitrogen as a result of the addition of largemouth bass.     

Resilience of predators to fishing pressure on coral patch reefs

Numbers and biomass of piscivorous fish and their predation on other fish may often be high in undisturbed coral reef communities. The effects of such predation have sometimes been studied by removal of piscivores (either experimentally or by fishermen). Such perturbations have usually involved removal of large, highly vulnerable, mobile piscivores that are often actively sought in fisheries. The effects of fishing on smaller, demersal, semi-resident piscivores have been little studied. We studied such effects on the fish communities of patch reefs at Midway atoll by experimentally removing major resident, demersal, piscivorous fishes. First, four control reefs and four experimental reefs were selected, their dimensions and habitats mapped, and their visible fish communities censused repeatedly over 1 year. Census of all control and experimental reefs was continued for the following 39 months, during which known piscivores were collected repeatedly by hand spearing. Records were kept of catch and effort to calculate CPUE as an index of predator density. Spearfishing on the experimental reefs removed 2504 piscivorous fish from 12 families and 43 taxa (mostly species). The species richness of the catch did not show an overall change over the duration of the experiment. Spearman rank correlation analysis showed some unexpected positive correlations for density in numbers and biomass of major fished piscivorous groups (especially lizardfish) over the experiment. Only two relatively minor fished piscivorous taxa declined in abundance over the experiment, while the overall abundance of piscivores increased. Visual censuses of fish on the experimental reefs also failed to show reduction of total piscivores over the full experimental period. No significant trend in the abundance of lizardfish censused over the full period was apparent on any of the control reefs. The high resilience of piscivores on these experimental reefs to relatively intense fishing pressure could result from their protracted recruitment seasons, high immigration rates, cryptic habits, or naturally high abundances. A major factor was the high immigration rates of lizardfish, replacing lizardfish and other less mobile piscivores removed from the reefs by spearing. On the fished reefs, the removed lizardfish population replaced itself >20 times during the experiment; other piscivorous taxa replaced themselves only 5 times.     

Trophic ecology of the pikeperch (Sander lucioperca) in its introduced areas: a stable isotope approach in southwestern France

During the last decades, non-native predatory fish species have been largely introduced in European lakes and rivers, calling for detailed information on the trophic ecology of co-existing native and non-native predators. The present study describes the trophic ecology of the introduced pikeperch (Sander lucioperca) in two southwestern French rivers, using stable isotope analysis. Pikeperch could be categorized as a top-predator, and had a significantly higher trophic position (TP, mean±SE=4.2±0.1) compared to other predatory fish such as the native pike (Esox lucius, TP=3.7±0.1) and the introduced European catfish (Silurus glanis, TP=3.8±0.1). Most studies of resource use in freshwaters consider predatory fish as ecologically equivalent; however, this study showed that the pikeperch occupied a higher trophic niche compared to other predatory species in the Lot and Tarn rivers (Garonne River basin). This apparent specialization may thus have consequences upon interspecific relationships within the predatory guild and upon the functional organization of biological communities. To cite this article: D. Kopp et al., C. R. Biologies 332 (2009).     

Near-bottom depletion of zooplankton over coral reefs: III: vertical gradient of predation pressure

Many diurnal planktivorous fish in coral reefs efficiently consume zooplankton drifting in the overlying water column. Our survey, carried out at two coral reefs in the Red Sea, showed that most of the diurnal planktivorous fish foraged near the bottom, close to the shelters from piscivores. The planktivorous fish were order of magnitude more abundant near (<1.5 m) the bottom than higher in the water column. The predation pressure exerted by these fish was assessed by measuring the consumption of brine shrimps tethered at different heights above the bottom on a vertical line which was pulled over the reef. Below 1.5 m above bottom, the shrimps survival probability sharply decreased toward the bottom. Higher in the water column, survivorship was nearly 100% with little vertical variation. Our results indicate that near-bottom depletion of zooplankton in coral reefs is likely due to intense predation at that boundary layer. Risk of predation by piscivorous fish apparently restricts planktivorous fish to forage near the bottom, with a distribution pattern greatly deviating from ideal-free distribution.     

Influence of flood pulses on diet composition and trophic relationships among piscivorous fish in the upper Paraná River floodplain

The aim of this article is to evaluate whether alterations in flood pulses differentially affect diet composition, feeding niche breadth, and diet overlap of piscivorous fish. Species examined were Acestrorhynchus lacustris, Hoplias aff malabaricus, Plagioscion squamosissimus, Rhaphiodon vulpinus, and Salminus brasiliensis. These species were collected with gillnets (different mesh sizes) in the upper Paraná River floodplain, during four distinct flood events (four periods; A = 1992/1993; B = 2000; C = 2001; and D = 2002). The volumetric method was chosen to express diet results. Feeding niche breadth was calculated using Levins measure, and diet overlap was evaluated by the Pianka’s Index. Flooding was more intense and lasted longer in the first period (1992/1993—A). Diet composition of the studied species was broad (47 total items consumed). For period A, Prochilodus lineatus was the main item taken by four out of five species. In the other periods, there were relevant alterations in diet, since P. lineatus was not recorded in any stomach of the five species; rather, it was replaced by the shrimp, Macrobrachium amazonicum. Diet overlap was low in all periods. The greatest overlap was obtained in period C for P. squamosissimus and R. vulpinus, due to high consumption of shrimps. There were no significant differences in niche breadth among species. However, the species presented distinct variation patterns in niche breadth. For example, H. aff. malabaricus showed a tendency toward increasing niche over the period, but the other species presented larger niches only during period A. Therefore, it can be concluded that the intensity and duration of the flood pulse influences: (i) the diet composition of piscivores; (ii) the breadth of their niches; and (iii) feeding overlap among species. Handling editor: J. Cambray     

Feeding ecology of the waterbirds in a tropical mangrove in the southeast Gulf of Mexico

ABSTRACT

Species that exploit the same type of environmental resources are defined as a guild, which have similar needs in the use of food or habitat. It was analyzed the diet of five waterbirds’ offsprings species (Neotropic Cormorant (Nannopterum brasilianus), Reddish Egret (Egretta rufescens), Boat-billed Heron (Cochlearius cochlearius), Snowy Egret (Egretta thula) and Great Egret (Ardea alba)), by prey identification and calculated the relative importance, overlap and breadth diet. The general diet of the piscivorous guild consisted of 17 fish species from 13 genera and eight families. The highest overlap was between the Reddish Egret and Boat-billed Heron. Fish species dominated the diets of all studied waterbirds, Poecilia velifera was the most abundant prey species in each of the birds, suggesting that they are abundant in the wetlands system of northern Yucatan. Diet overlap in waterbirds species depends on the use of resources and feeding habitat. Since reproductive success largely depends on the availability of food resources, mainly of resident or estuarine fishes. The information about diet is important for the conservation of waterbirds.     

Visual feeding of fish in a turbid environment: Physical and behavioural aspects

Turbidity has both positive and negative effects on prey detection, by increasing or diminishing the contrast between prey and background due to the scattering of light. The positive effect of turbidity on prey contrast depends on the optical properties, scattering properties of suspended particles and the visual sensitivity of the predator.

The positive effect of turbidity is pronounced for larval fish, given that their visual field is short, leaving fewer particles between them and their prey to scatter light and interfere with detection. This relationship, together with a decreased risk of predation, makes turbid environments more optimal for some species and size groups of fish (planktivores and fish larvae) and less so for others (adult piscivore fish). Thus, turbidity might have a structuring effect on a fish community. Recently it has been demonstrated that UV light might have positive effects on prey detection and consumption. How UV light might interact with different kinds of particles producing turbidity is not well documented.     

Visual feeding of fish in a turbid environment: Physical and behavioural aspects

Turbidity has both positive and negative effects on prey detection, by increasing or diminishing the contrast between prey and background due to the scattering of light. The positive effect of turbidity on prey contrast depends on the optical properties, scattering properties of suspended particles and the visual sensitivity of the predator. The positive effect of turbidity is pronounced for larval fish, given that their visual field is short, leaving fewer particles between them and their prey to scatter light and interfere with detection. This relationship, together with a decreased risk of predation, makes turbid environments more optimal for some species and size groups of fish (planktivores and fish larvae) and less so for others (adult piscivore fish). Thus, turbidity might have a structuring effect on a fish community. Recently it has been demonstrated that UV light might have positive effects on prey detection and consumption. How UV light might interact with different kinds of particles producing turbidity is not well documented.