Role of fish (Mugilidae) in the organic matter transfer between tidal mudflats and coastal waters through biofilm consumption: in situ experimentation on spatial, seasonal and ontogenic variability.




Post-doctorant, ANR VASIREMI program, 2008-2009


University of La Rochelle

Avenue Michel Crépeau
17042 La Rochelle, France

seconded to


MNHN – CRESCO (Centre de Recherche et d’Enseignement sur les Systèmes COtiers)

38, rue du Port Blanc

35800 DINARD, France

Scientific advisor:

Eric FEUNTEUN: Centre de Recherche et d’Enseignement sur les Systèmes COtiers  (CRESCO) - UMR BOME 5178, MNHN, CNRS, IRD, University of Paris VI, 38, rue du Port Blanc, 35800 Dinard, France

Christine DUPUY: Centre de Recherche sur les Ecosystèmes Littoraux Anthropisés – UMR6217 - Unité Mixte de recherche CNRS, IFREMER, University of La Rochelle ; Avenue Michel Crépeau, 17042 La Rochelle, France


Keywords: foraging behaviour, organic matter transfer, mudflat food-web, Mugilidae, Liza aurata, Liza ramada, Marennes Oleron, Mont St-Michel Bay, functional ecology



  Scientific Context                                           

© E. Feunteun

The primary productivity of intertidal mudflats sustains sedentary invertebrates which are in their turn predated by a number of transient species, including mammals, birds and fishes. Fish invade salt marshes during flood and forage there during the short submersion period (from few minutes to a few hours according to the location on the mudflat and the duration of the flood). Then they leave the mudflat at ebb, it is assumed that at least a part of the ingested food is exported and the non assimilated OM is deposited in marine coastal waters as faeces. Therefore, these transient species participate in energy fluxes between adjacent systems: they act as biotic vectors of OM (sensus Forman, 1981) (see Laffaille et al. (1998) for the particular case of tidal salt marshes).



Mullets are among the most common species of all marine waters in the world (Nelson, 1994) and account for a fundamental protein resource for a number of human populations living in coastal areas. In Mont Saint-Michel’s bay ( France ), Liza ramada and L. aurata are the most abundant limno-benthofagous species both in tidal mudflats and estuaries (Legendre 1984, Feunteun & Laffaille 1997, Laffaille et al. 1999a).

Liza ramada and L. aurata are considered as “marine estuarine dependent” species since they only occurred in estuaries, tidal mud flats and tidal salt marshes from the beginning of spring until the beginning of autumn, at temperatures of 14-21°C. It is hypothesized that they migrate in offshore warmer water during the cold season (Claridge & Potter 1983, Aprahamian & Barr 1985, Rosecchi & Crivelli 1995). It has been shown that these different habitats play an important nursery role for these species, young of the year dominating the sampled populations, exploited food sustaining an important part of their growth.

Mullets are able to select smallest items (Thomson 1954, 1966, Almeida 1996). Wood (1953) assumes that 99% of bacteria and dinoflagellids associated to sediments (among the smallest available preys) are absorbed by mullets. The diatoms which occur in the mullets’ stomachs are mainly small (<100µm) whereas larger diatoms (> 100 µm) are available and dominate the diatom community of the creeks sediment (Brosse 1996, Radureau et al. 1999). Similar conclusions are maid for mineral particles: a majority of 20 µm diameter grain are present in the stomach contents, whereas 30 to 50 µm diameter particles dominate in the sediment available marsh. Nevertheless % of particulate organic matter ingested strongly varies according to site, season and fish behaviour.


© P. Richard

Despite very short submersion periods, mullets filter and ingest large quantities of sediments, concentrate and assimilate organic matter produced by these coastal habitats which therefore act as key trophic areas for these species. More than all the other teleosts dwelling estuaries in temperate areas, mullets probably contribute significantly to the ecological functioning of these systems because they use directly the primary production and are involved in energy fluxes in and between coastal marine systems (Odum 1970, Laffaille et al. 1998, Lefeuvre et al. 1999). Spatial and temporal variation in exploitation of these habitats may reduce trophic competition between species and between stages that have similar diets and consequently favour their growth.

Understanding i) the function of tidal mudflat and especially their effects on abundance, movements and growth of principal fish species (mullets) and ii) the role of these fish in the organic matter transfer through biofilm consumption, appear essential for management of entire ecosystems such as Marennes Oléron or Mont Saint Michel bays.





          The consumption of biofilm-associated micro-organisms and their consumers (e.g. meiofauna) by mullets will be assessed, and their export from the intertidal zone to coastal waters will be estimated for the Brouage site of Marennes-Oléron Bay and Mont Saint Michel Bay through field studies. Mullets will be sampled with stop and trammel nets during three progressive seaward distances from the mean low-tide limit, to identify their feeding preferences and to quantify their migration patterns between intertidal and coastal waters. The fish samples will be analysed for three main goals:


                       i) Quantify spatio-temporal movements of mullets between mudflats and subtidal areas (number and mass of mullets per hour and per m of net) according to season (Summer and Autumn), site (2 sampled bays) and mesohabitat (3 progressive seaward distances).

                       ii) Determine individual behavioural and life history traits of mullets according to their displacements and populations structures (age determination by scale analysis) using otolithometry (microstructure/scanning electron microscopy) and microchemical analysis for individuals life history (ICPMS and laser ablation).

                       iii) Estimate the amount of organic matter transferred by the fish from intertidal to coastal waters through stomach content analysis (diet composition analysis for organic matter, diatoms, foraminifers and nematods), treatments being conducted in collaboration with C. Dupuy (University of La Rochelle).

Secondary objectives depending on preliminary results and time available could be envisaged:

                       iv) Identify the contribution of this food consumption to the growth of mullets using energy models applied in accordance with the identified foraging patterns by a complementary approach aiming to investigate the contribution of these diet items to mullet growth using stable isotope (13C and 15N) analysis of fish and prey items. Stable isotope analysis will be performed in collaboration with P. Richard (University of La Rochelle).

In a second time, specific behaviour, functional role and interspecific competition for food resources between different mullet species (Liza ramada and Liza aurata) would be assessed, depending on the possibility of taxonomic distinction (young of the year).

These in situ results will be compared and completed with parallel experimental studies conducted by C. Lefrançois (University of La Rochelle) on grazing pressure of mullets on reproduced mudflats natural condition in mesocosms.


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