Spatial organisation of European eel (Anguilla anguilla L.) in a small catchment

Abstract –  Modelling-governing patterns of European eel ( Anguilla anguilla L.) distribution of four eel size classes (<150, 150–300, 300–450 and >450 mm) in the Frémur basin (northwest France) was done using artificial neural network (ANN) techniques and ecological profiles. Our results demonstrate the high predictive power of the ANN models. Some macro- and microscale factors, such as distance from the sea, depth and flow velocity, have the most significant influence on the models. Influence of distance from the sea appears to be very different from the spatial organisation usually described in river systems. In fact, the general tendencies of total eel densities according to the distance from the sea showed that densities increase weakly upstream. Another outcome was the variations in habitat preference according to the eel size, even if this species is spread over practically every type of microhabitat. Small eels were mainly found in shallow habitats with strong abundance of aquatic vegetation, whereas large eels tend to be found in intermediate to high depth with small to intermediate abundance of aquatic vegetation. Finally, we hypothesise that European eels change behaviour and microhabitat characteristic preference around a size of 300 mm.     

Temporal changes in European eel, Anguilla anguilla, stocks in a small catchment after installation of fish passes

Abstract  Changes in the abundance of European eel, Anguilla anguilla L., in the River Frémur, France, were examined over an 8-year period. Natural connectivity of the river was disturbed by three high dams that inhibited eel upstream migration and reduced recruitment by elvers and yellow eels. After eel passes were installed, fish became more abundant upstream (mean density 0.5 eel m⁻²). Moreover, except in the more upstream areas, no decline in eel numbers and biomass was found, in contrast to the general decline of eel throughout its distribution range. It was concluded that eel passes are important to conserve and/or to recover eel stocks.     

Patterns of silver eel (Anguilla anguilla L.) sex ratio in a catchment

Abstract –  Changes in the numbers and size-class structure of European silver eels, Anguilla anguilla , in the River Frémur (France) were examined over a 9-year period after installation of downstream eel passes. The number of silver eels migrating downstream peaked in 1999, then decreased strongly and steadily after 2000, reaching relatively low levels. At the same time, a gradual shift in the silver eel sex ratio from a dominance of males (size from 270 to 442 mm, age from 3 to 6 years) to females (size from 366 to 1112 mm, age from 4 to 9 years) was recorded. Possible explanations for the escapement patterns observed are environmental sex determination and the installation of eel passes on the main hydraulic engineering structures in 1992 and 1996.     

Turbidity, velocity and interspecific interactions affect foraging behaviour of rosyside dace (Clinostomus funduloides) and yellowfin shiners (Notropis lutippinis)

Abstract –  Fish diversity is strongly affected by habitat degradation (e.g., increased turbidity) and invasive species. We examined the effects of turbidity, velocity, length, dominance and intra- and interspecific competition on focal point depth, movement rate, dominance and aggression rate in native rosyside dace ( Clinostomus funduloides ) and invasive yellowfin shiners ( Notropis lutipinnis ) in a southern Appalachian stream (NC, USA). We compared results for intra- and interspecific groups of fishes at two densities (two and four fishes), three turbidity levels (10, 20 and 30 nephelometric turbidity units), and two velocities (12, 18 cm·s⁻¹). Dominance was significantly correlated with length in intraspecific groups of both species, and dominant fish held more profitable foraging positions about 75% of the time. Yellowfin shiners were dominant more often than rosyside dace in interspecific trials. Akaike's Information Criterion indicated that models containing turbidity, velocity, species and intraspecific competition, explained the greatest amount of information in focal point depth data. By contrast, movement and aggression rates were best explained by models based on dominance and velocity. Finally, aggression rate was best explained by models containing fish length and turbidity. These results indicate that habitat degradation, intra- and interspecific interactions influence the foraging behaviour and future success of these species in the Little Tennessee River drainage.     

Do instream habitat variables and the abundance of brown trout Salmo trutta (L.) affect the distribution and growth of stone loach,Barbatula barbatula (L.)?

Abstract –  Along a stream, we investigated whether the abundance of stone loach ( Barbatula barbatula , L.) was related to the presence of brown trout ( Salmo trutta , L.) and instream habitat variables. First, a field survey was carried out where different habitat variables and the densities of both species were quantified and subjected to principal components analysis. Then the abundance of stone loach was related to the scores of the retained axes (eigenvalues >1). The abundance of stone loach was positively correlated to substrate particle size, amount of shade, temperature, discharge and current velocity, but negatively correlated to brown trout abundance. Secondly, a month-long field enclosure experiment in a stream was performed to test for any negative effects of brown trout on stone loach growth. Four treatments were used: intraspecific competition (stone loach at double density), interspecific competition (stone loach + small trout), predation (stone loach + large trout) and a control (stone loach alone). The results showed that large trout tended to have negative effects on final stone loach biomass. The absence of a negative effect of large trout on resource density suggests that nonlethal effects rather than resource competition caused this trend.     

Generalist versus specialist: the performances of perch and ruffe in a lake of low productivity

Abstract –  To elucidate the performances of perch and ruffe in oligotrophic lakes, we carried out a field study in reoligotrophic Upper Lake Constance. Both these percids used the same habitat, albeit with different activity patterns. Interspecific competition for food was relevant only in summer when both species fed on zoobenthos. Even then, niche overlap was low, while intraspecific diet overlap was moderate to high throughout the season. Perch did not perform fixed, ontogenetic diet shifts, but used a wide range of prey. During spring and early summer, all size classes were planktivorous, then switched to benthivory and cannibalism in summer, and part of the population reverted to planktivory in autumn. Ruffe, by contrast, fed mainly on chironomid larvae and pupae throughout the year. It is suggested that in lakes of low productivity the euryphagous characteristics of perch, including cannibalism, provide a clear advantage over the benthivorous specialist ruffe in two ways: (i) it allows perch to switch to alternative prey types if one prey type becomes scarce; and (ii) reduces both intra- and interspecific competition for food.     

Migratory history of the giant mottled eel (Anguilla marmorata) in the Bonin Islands of Japan

Chino N, Arai T. Migratory history of the giant mottled eel ( Anguilla marmorata ) in the Bonin Islands of Japan.
Ecology of Freshwater Fish 2010: 19: 19–25. © 2009 John Wiley & Sons A/S
Abstract –  In order to elucidate the migratory history and habitat use of the giant mottled eel Anguilla marmorata, in the Bonin Islands of Japan, strontium (Sr) and calcium (Ca) concentrations in otoliths were examined by X-ray electron microprobe analysis. The X-ray intensity map and line analyses showed three fluctuation patterns: (i) freshwater residence, (ii) continuous residence in brackish water, and (iii) residence in freshwater after recruitment, while returning to brackish water. Based on these results and previous studies, the migratory behaviours of A. marmorata appear to differ in each habitat in response to inter- and intra-specific competition. The results suggest that A. marmorata has a flexible pattern of migration, with an ability to adapt to various habitats and salinities.     

One century of eel growth: changes and implications

Abstract – A cooperative effort gathered a large European length‐at‐age data set (N = 45,759, Lat. 36S–61N Long. 10W–27E) for Anguilla anguilla, covering one century. To assess the effect of global warming during the last century and habitat effects on growth, a model was fitted on the data representing the conditions met at the distribution area scale. Two GLMs were designed to predict eel log(GR): one model was fitted to the whole data and the other was fitted to the female data subset. A model selection procedure was applied to select the best predictors among sex, age class, five temperature parameters and six habitat parameters (depth, salinity and four variables related to the position in the catchment). The yearly sum of temperatures above 13 °C (TempSUP13), the relative distance within the catchment, sex, age class, salinity class and depth class were finally selected. The best model predicted eel log(GR) with a 64.46% accuracy for the whole data and 66.91% for the female eel data. Growth rate (GR) was greater in habitats close to the sea and in deep habitats. TempSUP13 variable had one of the greatest predictive powers in the model, showing that global warming had affected eel growth during the last century.     

A comparison of habitat use and habitat-specific feeding efficiency by Eurasian ruffe (Gymnocephalus cernuus) and yellow perch (Perca flavescens)

Abstract –  Eurasian ruffe are invading habitats in the North American Great Lakes watershed occupied by commercially important native yellow perch. We conducted laboratory experiments to evaluate potential overlap in habitat (macrophytes, mud, cobble) and food (benthic invertebrates) use. Ruffe and yellow perch both preferred macrophytes > cobble > mud in the light, but only ruffe increased their use of mud in the dark. Neither fish density nor food availability affected habitat preferences, and competition for habitat was not evident. For both species, feeding rates were marginally lower in macrophytes but did not differ between species. Our experiments suggest that if ruffe and yellow perch share a habitat (e.g., during invasion or because of predation risk), competition for space will be weak or absent. However, within a shared habitat, competition for food may occur when food is limiting because neither species has a clear advantage in its ability to consume invertebrates in any habitat.     

Diet overlap and consumption patterns suggest seasonal flux in the likelihood for exploitative competition among piscivorous fishes

Abstract –  Competition, both intraspecific and interspecific, among piscivorous fishes (predators) may be more pronounced in reservoirs because of the artificial nature of these systems and the continuous anthropogenic influences that may not allow natural processes to regulate their densities. Most studies attempting to measure the extent of competition are based on predator–prey balance indices that are annual snapshots and do not account for seasonal changes. We sought to identify times during the year when intra- and interspecific exploitative competition for prey was most likely among dominant predators in a large reservoir of the south-eastern United States. The possibility of interspecific competition was measured by examining the seasonal extent of diet overlap based on samples of stomach contents. The likelihood or severity of this competition was indexed by how well predators met their energy needs. We found predators were realising less of their energy requirements during spring and autumn, and diet analyses showed predators overlapped substantially in their use of prey during those seasons. Thus, exploitative competition was most likely or severe during spring and autumn; furthermore, this competition had the potential to be interspecific as evidenced by resource overlap.