Effect of nursery habitat degradation on flatfish population: Application to Solea solea in the Eastern Channel (Western Europe)
Introduction
Estuaries and coastal areas are essential fish habitat because they act as nursery grounds for various marine fish species (Able, 2005, Beck et al., 2001, Miller et al., 1984). Juvenile growth and survival are determined by both the capacity and quality of these nursery habitats (Gibson, 1994, Iles and Beverton, 2000, Rijnsdorp et al., 1992). The increase of human activities along rivers, estuaries and in coastal areas affects ecosystems, in particular by pollution and habitat destruction (Coleman et al., 2008, Diaz and Rosenberg, 2008, Halpern et al., 2008). Habitat degradation is one of the most serious threats for the recovery of fish stocks (Hall, 1998). Quantitative and qualitative factors related to anthropogenic disturbance (Meng et al., 2000, Phelan et al., 2000) influence the quality of coastal and estuarine nurseries and thus recruitment and renewal of populations (Burke et al., 1993, Cowan et al., 2000, Ferber, 2001, Johnson et al., 1998, Peterson et al., 2000, Scharf, 2000, Thrush et al., 2008).
The inshore waters of the Eastern Channel (ICES Division VIId; Fig. 1) support nursery areas for several commercially important species, especially the common sole (Solea solea, L.) (Riou et al., 2001). The Bay of Seine is the largest estuary in the Eastern Channel and is thereby a potentially important nursery ground. However, this sector is highly disturbed through channel dredging, constructions of dikes and harbour extensions, which have substancially reduced the estuary area and subsequently suitable juvenile fish habitats, such as shallow muddy areas (> 75% decrease; Lesueur, 1999). In addition, concentrations of organic chemical contaminants are high in this estuary, it being among the most contaminated in Europe (Gilliers et al., 2006), with episodes of oxygen depletion (Billen and Poulin, 1999). Thus, human disturbances have affected, perhaps reduced the nursery function of this estuary (Le Pape et al., 2007): its contribution to the renewal of flatfish stocks in the Eastern Channel is currently low (Riou et al., 2001) and fish growth performance and population density are significantly lower than in other nursery areas (Gilliers et al., 2006).
The present study aims to quantify the effects of human disturbance on sole nursery grounds in the Seine estuary and establish their consequences on the sole population of the Eastern Channel. A habitat suitability model, constructed for the recent period, was used in combination with historical habitat maps to provide backward projections of the potential juvenile sole densities. A two-step approach was used: 1) A quantitative map of sole nursery habitats in the Eastern Channel during recent times was first developed in a Geographic Information System (GIS). A database was built by collating trawl surveys undertaken throughout the coastal and estuarine parts of the Eastern Channel during the three last decades. A generalized linear model (GLM) was developed to identify key factors explaining variations of juvenile sole densities in various habitats, featured by different subregions, bathymetry and sediment structure. Using this GLM and a GIS, key sites for the early life-history stages of sole were identified. 2) Historical maps of the Seine estuary were used to construct habitat suitability maps for key dates in the modifications of this estuary since 1850. Combining the historical suitability maps and the GLM model of sole densities, the historical productions of the Seine estuarine nursery were calculated and related to its contribution to the sole population.
Section snippets
Beam trawl survey data
From 1974 to 2007, different beam trawl surveys focused on juvenile flatfish species have been undertaken throughout the Eastern Channel coastal zone (Fig. 1, Table 1). Riou et al. (2001) collated these survey data until 1998. Data used in the present study were for a large part identical to this database, with an upgrade with the surveys undertaken between 1999 and 2007 (UK surveys on the English coast, French surveys in the downstream of the Seine estuary and in the Bay of Somme). In 2006,
Distribution of YoY in the Eastern Channel at the present time
All factors except the “sediment” had a significant effect in the two sub-models (Table 4). The “Sediment” effect was not significant (p = 0.4) in the sub-model for positive densities (Table 4). This factor was then excluded from this sub-model.
When considering the effect of the different factors obtained from the delta model (Fig. 4), differences between sectors are markedly pronounced (Fig. 4a): on average, the highest YoY densities (Rye Bay) reached tenfold the smallest ones (South Downs). The
An approach based on several hypotheses: overview of possible bias
To model the effect of habitat degradation in the Seine estuary, a habitat suitability model based on scientific surveys undertaken during the past 30 years was coupled with a GIS that incorporated actual and historical maps. Several methodological points could be discussed and improved in future research.
Acknowledgements
This project was supported by the French National Research Agency (ANR, Project SoleBeMol-Pop, ANR Vulnérabilité, Milieux et climat. Special thanks to Franck Coppin (Ifremer, Boulogne sur Mer, France) and Marie Laure Cochard (Ifremer, Port en Bessin, France) for their help on collection of trawl survey data, Mario Lepage and his colleagues (Cemagref, Bordeaux, France) for their help in gathering trawl data of the Water Framework Directive, Michel Bellouis and Benoit Loubrieu (Ifremer, Plouzané,
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