Puy Lim

Modelling the factors that influence fish guilds composition using a back-propagation network: Assessment of metrics for indices of biotic integrity

Abstract Fish assemblages are reckoned as indicators of aquatic ecosystem health, which has become a key feature in water quality management. Under this context, guilds of fish are useful for both understanding aquatic community ecology and for giving sound advice to decision makers by means of metrics for indices of biotic integrity. Artificial neural networks have proved useful in modelling fish in rivers and lakes. Hence, this paper presents a back-propagation network (BPN) for modelling fish guilds composition, and to examine the contribution of five environmental descriptors in explaining this composition in the Garonne basin, south west France. We employed presence–absence data and five variables: altitude, distance from the river source, surface of catchment area, annual mean water temperature, and annual mean water flow. We found that BPN performed better for predicting species richness of guilds than multiple regression models. The standardised determination coefficient of observed values against estimated values was used to characterise model performance; it varied between 0.55 and 0.82. Some models showed high variability which was presumably due to spatial heterogeneity, temporal variability or sampling uncertainty. Surface of catchment area and annual mean water flow were the most important environmental descriptors of guilds composition. Both variables imply human influence (i.e. land-use and flow regulation) on certain species which are of interest to environmental managers. Thus, predicting guilds composition with a BPN from landscape variables may be a first step to assess metrics for water quality indices in the Garonne basin.

Modelling of microhabitat used by fish in natural and regulated flows in the river Garonne (France)

Abstract The aim of our study was to compare the microhabitat used by three fish species: brown trout ( Salmo trutta L.), European minnow ( Phoxinus phoxinus L.) and stone loach ( Barbatula barbatula L.), in natural and regulated flows of a section of the river Garonne (France). Six Artificial Neural Network (ANN) models were set up, one for each fish species in each flow condition. Models were run and tested with 1107 observations obtained by point abundance sampling performed by electrofishing. Each model had thirteen independent environmental variables (distance from the bank, water depth, water velocity, percentage of different substratum fractions defined as large boulders, small boulders, large pebbles, small pebbles, gravels, sand, mud and bedrock, flooded vegetation cover, and presence or absence of ‘blockage’ which is one or several pieces of wood providing shelter), and one dependent variable (fish density for the considered population). A cross-validation testing procedure (leave-one-out bootstrap) was performed to validate the ANN models. Finally, we used a method based on the first partial derivatives of the networks output with respect to each input to focus on the sensitivity of some of the variables selected. During the training phase, all models were judged satisfactory with Mean Squared Errors (MSE) ranging from 0.40 to 1.93, and Performance Indexes (PIs) from 60 to 89%. After the testing procedure, MSE ranged between 1.53 and 8.23, and PIs between 51 and 80%. With the exception of brown trout in regulated flow, patterns of microhabitat use obtained revealed that fish densities were highly connected to one major influencing variable: water depth for brown trout and stone loach, and water velocity for European minnow, other variables accounting for lower individual contributions. Analysis of the partial derivatives brought into relief some differences when comparing microhabitat use in natural and regulated flows for some of the variables tested, and no differences when comparing others. The results are discussed with regard to the biology and the ecology of each fish species at microhabitat and macrohabitat scales, and according to the relationship between microhabitat utilization and microhabitat availability.

Multi-residues analysis of pre-emergence herbicides in fluvial sediments : application to the mid-Garonne River

Contamination of man and ecosystems by pesticides has become a major environmental concern. Whereas many studies exist on contamination from agriculture, the effects of urban sources are usually omitted. Fluvial sediment is a complex matrix of pollutants but little is known of its recent herbicide content. This study proposes a method for a fast and reliable analysis of herbicides by employing the accelerated solvent extractor (ASE). The aim of the study is to show the impact of a major town (Toulouse) on the herbicide content in the river. In this study, three herbicide families (i.e.s-triazine, substituted ureas and anilides) were analysed in fluvial sediment fractions at 11 sampling sites along the mid-Garonne River and its tributaries. River water contamination by herbicides is minor, except for at three sites located in urban areas. Among the herbicidal families studied, urban and suburban areas are distinguished from rural areas and were found to be the most contaminated sites during the study period, a winter low-water event. The herbicide content of the coarse sediment fractions is about one third of that found in the fine fractions and usually ignored. The distribution of pesticide concentrations across the whole range of particle sizes was investigated to clarify the role of plant remains on the significant accumulation in the coarse fractions.

Longitudinal diversity patterns in streams: comparing invertebrates and fish communities

The aim of our study was to compare the longitudinal pattern of invertebrate EPTC (Ephemeroptera-Plecoptera-Trichoptera-Coleoptera) taxa richness and fish species richness, at the Adour-Garonne drainage basin scale (SW France). We considered 177 sampling sites for invertebrates and 109 sampling sites for fish, spread over the whole stream system. The statistical treatment consisted in simple linear regressions including bootstrap simulations, performed between the logarithm of EPTC taxa or fish species richness and the logarithm of the distance from the source. The model obtained for EPTC fitted a parabola with a maximal taxa richness in the intermediate part of the river continuum, while the model obtained for fish corresponded to a linear increase in species richness with the distance from the source. This difference in spatial distribution pattern is discussed with respect to the benthic nature of invertebrates vs. the pelagic nature of most fish species.

Habitat selection in amphidromous Gobiidae of Reunion Island: Sicyopterus lagocephalus (Pallas, 1770) and Cotylopus acutipinnis (Guichenot, 1863)

Freshwater populations of the Indo-pacific region are characterized by a large proportion of amphidromous species. In this paper, we analyse habitat selection by two amphidromous sympatric Gobiidae: Sicyopterus lagocephalus and Cotylopus acutipinnis in Reunion Island. A sampling method using Point Abundance Sampling (PAS) was conducted in 12 rivers. We used mixed logistic models in order to examine the presence probability of species according to location, downstream-upstream gradient, microhabitat variables (depth, velocity and predominant substrata) and presence of conspecifics and sympatric species. Presence probabilities varied between the sampled rivers. We observed a positive attraction between identical developmental stages of both species, which suggests that social interactions or similar preferences for environmental cues influenced their distribution. The presence probabilities of both species’ juveniles decreased from downstream to upstream. We showed that traditional microhabitat variables weakly explained the spatial distribution of both S. lagocephalus and C. acutipinnis, in Reunion Island. We suggest that weak habitat selection for these species is consistent with the amphidromous life style because of the unpredictability of juvenile settlement and the extreme hydrological variations in tropical rivers.

Study of a vertical profile of pre-emergence herbicide contamination in Middle Garonne sediments.

The fate of pre-emergence herbicides was investigated in sediment cores at sites selected for their nested sedimentation rates. Sediment was segregated in the coarse fraction (>63 µm) and in clays and silts (<63 µm) from the surface to a 45-cm depth. The fine fraction was more contaminated than the coarse fraction, mainly because of desethylatrazine (DEA). However, pesticides were observed in sediments at depths dated to before these compounds were used. Contaminated ancient sediments showed that contaminated elements migrated into the sediment. The fine fraction DEA profile was associated with the chronology of atrazine use. Comparing the sediment age, the half-life of the pesticides studied and banning dates showed that a non-degraded pesticide stock can be established in sediment.

Fish diversity conservation and river restoration in southwest France: a review

The evidence of human pressure on freshwater ecosystems has been largely recognised (Dynesius and Nilsson 1994). But it was only recently that water quality guidelines have laid a stronger emphasis on aquatic ecosystem health (Hart et al. 1999). For example, the European Water Framework Directive (EWFD) acknowledges that a good condition of ecosystems is essential for sustainable development (Kallis and Butler 2001). Nevertheless, in order to recognise an aquatic ecosystem in good conditions, we need first to both characterise and identify its biotic communities (Bryce et al. 1999). Consequently, analysing diversity patterns and distributions of aquatic communities has become a critical aspect for water quality management (Boulton 1999, Jenerette et al. 2002). One important element in aquatic ecosystem management is the spatial characterisation of riverine fish communities. Indeed, fish are considered as indicators of both aquatic quality and aquatic restoration success (Angermeier and Schlosser 1995, Paller et al. 2000, Oberdorff et al. 2001). Furthermore, there is a need to understand how fish assemblages vary within ecoregions, basins or physiographic regions (Naiman et al. 1988, Smogor and Angermeier 2001). This chapter deals with fish diversity and conservation in the Garonne basin and is heavily based on work by Aguilar Ibarra (2004). We present a review of the research carried out on fish ecology and we discuss fisheries policies and options on fish conservation and river restoration. We organise our chapter as follows: we first discuss why the Garonne basin is an interesting case for studying riverine fish diversity. Second, we review the main results of research on fish ecology in the area of study and the main water fisheries policies. Third, options on fish conservation and river restoration are given, and finally we list perspectives for further management and research.