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The quantitative importance of the Scheldt estuary for marine fish populations: an analysis for the goby species Pomatoschistus minutus using geochemical markers

Dutch title: Het kwantitatieve belang van het Schelde-estuarium voor mariene vispopulaties: een analyse voor de grondelsoort Pomatoschistus minutus aan de hand van geochemische merkers
Reference no: 23552+21552
Period: January 2003 till February 2008
Status: Completed

Thesaurus terms: Ecology; Estuaries
Taxonomic term: Pomatoschistus minutus (Pallas, 1770) [WoRMS]

Institutes (3)  Top 
  • Katholieke Universiteit Leuven; Departement Biologie; Afdeling Dierenecologie en -systematiek; Aquatische Ecologie en Evolutiebiologie, more
  • Vrije Universiteit Brussel; Faculteit Wetenschappen & Bio-ingenieurswetenschappen; Vakgroep Chemie; Analytical, Environmental and Geochemistry (AMGC), more
  • Vlaamse overheid; Beleidsdomein Economie, Wetenschap en Innovatie; Agentschap voor Innovatie door Wetenschap en Technologie (IWT), more, sponsor

Estuaries perform a crucial role in the population dynamics of several fish species. They provide a migratory route for anadromous and catadromous fish and an environment for truly estuarine teleosts. Freshwater fish involved colonize the upper region of estuaries, while marine species use the estuarine mouth. Above all, these marine fish enter and remain in estuaries for a short period, mostly in very large numbers, particularly during early stages of their life cycle. A number of mechanisms have been suggested to explain this feature, including avoidance or attraction by abiotic factors or gradients, the reduction in predation levels and the increased food availability.
This proposal aims to clarify the spatial and functional role of estuaries for juvenile marine fish. The question is raised which proportion of estuarine migrants contributes to the next cohort, compared with their conspecific marine residents. Hereto fish have to be assigned to their respective habitats and exchange rates have to be determined. The Scheldt estuary–Southern Bight of the North Sea gradient is used as a model system and the target fish species is the marine demersal sand goby (Pomatoschistus minutus; Gobiidae).
The objectives of this proposal are
1. identification of estuarine migrants and marine residents in a spawning population of Pomatoschistus minutus using geochemical trace elements in otoliths (ear stones).
2. Reconstruction of the individual life history of these spawners using geochemical tracers in otoliths (coupled with aging).
3. Assessment of migrations between the marine spawning and the estuarine nursery grounds using population dynamics and stable isotopes of fish tissue and stomach contents.

Sampling is organized in function of the life-history pattern of the target species Pomatoschistus minutus. Sand gobies usually live for one year, at most for two years. They spawn in early summer on shallow sand banks. Larvae are pelagic at first; young fish only start to live at the bottom when they reach 17-18 mm. Juveniles occur along the coast and in estuaries; before winter, they migrate to deeper waters. Sampling will be conducted during the spawning period (March-June) at different inshore locations between Dunkirk (France), the Westerscheldt (The Netherlands and Belgium) and Goeree (The Netherlands). To quantify migrations along the estuarine gradient, samples are taken monthly at different estuarine sampling stations (upper estuary at Doel (B) and lower estuary at Borssele (NL)). Catch per unit effort data, resulting from all the samples taken along the estuarine-inshore-offshore gradient, will be used to estimate stock size and the length-frequency structure of the population at the different sampling stations. Seasonal differences in number caught along the estuarine gradient will be used to infer fish movements

Otoliths (earstones) are used to assess age from the daily concentric growth rings. They are metabolically inert and their building stones (Ca and trace elements) are derived from the ambient water. The concentration of trace elements (mainly Sr) changes along a freshwater-saltwater gradient. As a result, crossing the freshwater seawater boundary during the life history can be read individually from the otoliths. The Ca/Sr ratio is determined using microprobe technology and linked to age. fish caught in the estuary will be used for interpretation of the Sr signals. Migrations of gobies between the coastal spawning sites and the estuarine nursery can be quantified using stable isotopes because the food web isotopic signatures are substantially different between these two environments. Organisms moving between isotopically distinct food webs carry with them information about the location of previous feeding over a period of several months. At each sampling station in the estuary 15 gobies are randomly selected and analyzed for C and N isotope ratios in their muscular tissue, as well as in their stomach contents.

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