Are young sea bass, Dicentrarchus labrax L. (Teleostei : Pisces) adapted to mussel cultures?

In Mont Saint-Michel Bay, one of the most abundant food items in young sea bass diets was Mytilus edulis. This invertebrate is an abundant cultured bivalve in many coastal systems and it is known to be in direct trophic competition with the natural food items of sea bass. This study shows that mussel culture can play the role of a nursery area, providing food for young opportunistic fish. Quantification of these interactions appears to be of a major importance in order to enable sustainable management of coastal areas

Feeding ecology of 0-group sea bass Dicentrarchus labrax in salt marshes of Mont Saint-Michel bay (France)

0-group sea bass, Dicentrarchus labrax, colonize intertidal marsh creeks of Mont Saint Michel Bay, France, on spring tides (e.g., 43% of the tides) during flood and return to coastal waters during ebb. Most arrived with empty stomachs (33%), and feed actively during their short stay in the creeks (from 1 to 2 h) where they consumed on average a minimum of 8% of their body weight. During flood tide, diet was dominated by mysids, Neomysis integer, which feed on marsh detritus. During ebb, when young sea bass left tidal marsh creeks, the majority had full stomachs (more than 98%) and diet was dominated by the most abundant marsh (including vegetated tidal flats and associated marsh creeks) resident amphipod, Orchestia gammarellus. Temporal and tidal effects on diet composition were shown to be insignificant. Foraging in vegetated flats occurs very rarely since they are only flooded by about 5% of the tides. It was shown that primary and secondary production of intertidal salt marshes play a fundamental role in the feeding of 0-group sea bass. This suggests that the well known nursery function of estuarine systems, which is usually restricted to subtidal and intertidal flats, ought to be extended to the supratidal, vegetated marshes and mainly to intertidal marsh creeks

Towards a standardized characterization of the potentially migrating silver European eel (Anguilla anguilla L.)

We defined a standardized method for discriminating candidate silver eels that may undergo catadromous migration in the following season from the sedentary fraction of a population. A combination of two qualitative criteria (state of differentiation of the lateral line and colour contrast) and one quantitative criterion (Ocular Index OI) was used to determine the development toward silvering. In the non-migratory phase, we found a gradient of the three criteria between yellow (0 criterion), presilver (1 to 2 criteria) and silver (3 criteria) eels. In the migrant phase, silver eels had ended their metamorphosis process and were characterized at the same time by the presence of the 3 silvering criteria. A mark-recapture survey using PIT-tags provided evidence that only identified silver eels (3 silvering criteria present) in the catchment actually emigrated the following season. Moreover, the use of a single criterion of silvering among the three generated large variation in the estimated proportion of candidates for emigration which varied between –22% and +63 %. Such a result confirmed that a multicriteria approach is needed to characterize in a standard way the potentially migrating silver eel

Biodiversity in salt marshes: from the patrimonial value to the ecosystem functioning. The case study of the Mont Saint-Michel bay.

Until 1979, European salt marshes were known only through the inventories of fauna and especially of flora. On such criteria, the salt marshes of the Mont-Saint-Michel bay (France) were regarded as most significant of the French coasts. However, it took 20 years of research on the role of these wetlands of the estuaries-salt marsh systems to highlight the ecological, social and economic interest of this ecotone, between continental and marine systems, a long time considered as territory “without value”, except for stock breeders or hunters

Role of fish communities in particulate organic matter fluxes between salt marshes and coastal marine waters in the Mont Saint-Michel bay.

Among the 90 fishspecies censused in the Mont Saint-Michel Bay (France), 23 colonise and forage in the salt marshes during flood. Therefore, this environmentmay play an important trophic and nursery role for these species. This community is largely dominated by mullets (81% of the biomass), Liza ramada and secondarily L. aurata. But gobies (mainly Pomatoschistus minutus and P. lozanoï) and sea bass (Dicentrarchus labrax) are also present; they represent respectively 11% and 4% of the biomass. During the tide cycles, mullets export from salt marshes about 7% of their body weight (FW) containing a mixture of sediment (43%), organic matter (24%) and water (33%). Gobies and sea bass mainly feed on dwelling macro-invertebrates, and they export respectively 4.5% and 10% of their body weight during a tide cycle. Thus, we estimated that 50 tonnes year−1 of particulate organic matter (dry weight POM) are exported from the 4000 ha of salt marshes to the marine coastal waters. These fish communities appear to be POM transporters and could play a significant role in the global energy budgets of coastal environments such as Mont Saint-Michel Bay. According to the seasons and the years, the energy exported by fish communities is assumed to range between 0 and 10% of the total POM output

Can thin-lipped mullet directly exploit the primary and detritic production of European macrotidal salt marshes?

Juveniles and adults (>100 mm) of Liza ramada colonize macrotidal salt marsh creeks of Mont Saint-Michel bay (France)between March and November, during spring tide floods (43% of the tides) and return to coastal waters during the ebb. This fish species actively feeds during its short stay in the creek (from 1 to 2 h). On average, each fish swallows sediment including living and inert organic matter, which amounts to 8% of its fresh body weight. Their diet is dominated by small benthic items (especially diatoms and salt marsh plant detritus), that correspond to the primary and detritic production of this macrotidal salt marsh creek. Despite very short submersion periods, mullets filter and ingest large quantities of sediment and concentrated organic matter (on average organic matter in stomach content is 31%) produced by these coastal wetlands. European salt marshes are thus shown to act as trophic areas for mullets, which are well adapted to this constraining habitat which is only flooded for short periods during spring tides

European salt marshes diversity and functioning: the case study of the Mont Saint-Michel bay, France

The macrotidal Mont Saint-Michel bay has been studied intensively since 1990. The objectives of this study, supported by the European Union, was to understand various processes underlying the functioning of this hydrosystem with a special focus on organic matter and nutrient fluxes between saltmarshes and marine waters. This paper presents a synopsis of these studies. The tidal flats are unvegetated and primary production is exclusively due to microphytobenthos communities dominated by diatoms. Halophile plant communities colonize the top parts of the tidal flats. Their composition and production vary according to a maturity gradient and sheep grazing. In ungrazed saltmashes, production ranged from 1080 gDW m−2yr−1 in the lower marsh to 1990 gDW m−2yr−1 in the upper marsh whereas it was only 200 to 500 gDW m−2yr−1 in Salicornia spp. dominated pioneer zones and sheep grazed areas. Most of this organic matter (OM) was trapped in situ, processed by fungi and bacteria, and then released seaward via tidal fluxes, groundwater and runoff as particulate OM and nutrients: –497 kg N, –1200/–1000 kg P-PO4 and –9900/–4200 kg inorganic carbon). A small amount of OM was exported to the bay as macrodetritus. Fatty acids and stable isotopes, used as markers, showed that OM produced by the marsh halophytes contributed to the diet of all the tidal flats invertebrates that were studied. Transient fish species were shown to colonize the saltmarshes to forage or graze, exporting about 50 tons POM (DW)y−1. Therefore, it is assumed that the saltmarsh production enhances the production of the whole bay. But the functioning is still poorly known because the nutrient sinks have not all been identified. Part of the nutrients input was provided by precipitation (+327 kg y−1), but the contribution of the catchments was not quantified despite the fact that their influence was shown by the presence of lindane in all the compartments of the system. Dynamics of saltmarshes are mainly influenced by natural sedimentation (1.5 million m3y−1 in the bay), plant community succession, and management (i.e., reclamation and agricultural activities)

Feeding competition between two goby species, Pomatoschistus lozanoi (de Buen) and P. minutus (Pallas), in a macrotidal satmarsh.

Two transient goby species (Pomatoschistus minutus and P. lozanoi) occurred in saltmarshes of the macrotidal Mont Saint Michel bay. They colonised the tidal creeks during each spring tide all along the study conducted in 1997. P. minutus swallowed a least 4% of its body weight. Its diet was dominated by the most abundant resident amphipod of the saltmarshes, Orchestia gammarellus. P. lozanoi ingested less food during their stay in the creek (2.4% of its body weight). O. gammarallus also occurred in its diet, but mysids were the main food items both during flood and ebb. In addition to this trophic segregation, a temporal segregation was observed. In fact, P. minutus stayed longer than P. lozanoi in the saltmarsh ; it colonised creeks first and returned to coastal waters the last. These two predatory and sympatric species had a different trophic behaviour that limited interspecific competition both from the trophic and temporal point of view

Do fish communities function as biotic vectors of organic matter between salt marshes and marine coastal waters?

The contribution of fish communities to organic matter (OM) fluxes, especially between salt marshes and adjacent marine coastal waters are reviewed. For this a data set fromthe bay ofMont Saint-Michel and literature is examined and discussed. In a range of macro-tidal coasts of Europe, salt marshes are only flooded at spring tides for a short time. Many animals, including fish, then invade the salt marshes through tidal creeks. They forage there for up to a few hours and swim back to sea at ebb. Meanwhile, organic matter is exported as gut content. In the 4000 ha of salt marshes of the bay of Mont Saint-Michel mullets were responsible for the export of about 8 kg of dry weight OM h

Composition du peuplement piscicole d'un petit estuaire anthropisé (Le Couesnon, France)

Dans un petit estuaire du nord de la Bretagne (France), l’ichtyofaune a été échantillonnée lors de chaque saison d’une année sur six stations selon une répartition amont-aval. 28 espèces ont ainsi été capturées à l’aide de deux engins de pêche, une senne de plage couplée à un tramail. Selon les indices d’abondance numériques et pondéraux, trois zones peuvent être identifiées. Une zone estuarienne marine en aval qui est composée essentiellement d’espèces marines et résidentes de la baie adjacente. Le peuplement est dominé par les gobies buhottes et par les juvéniles de plies, de bars et de mulets. La diversité piscicole et les indices d’abondance sont plus élevés que dans les autres zones. Les stations les plus en amont correspondent à la zone estuarienne dulçaquicole qui est presque exclusivement composée d’espèces dulcicoles. Ce peuplement est dominé par les brèmes, les gardons et les chevaines. Mais les indices d’abondance sont les plus faibles à cause de la banalisation et de la chenalisation des berges. La troisième zone correspond à la zone estuarienne intermédiaire. La présence d’espèces d’origine marine et littorale y est limitée par un barrage estuarien. Cet aménagement hydraulique, situé très en aval, réduit ainsi la fonction de nurserie de cet estuaire vis à vis des espèces marines. La répartition spatiale du peuplement piscicole est donc le résultat d’un facteur abiotique (la salinité) et de l’action anthropique (le barrage et la perte des habitats)