Aurore Baisez

A Review of Upstream Migration and Movements in Inland Waters by Anguillid Eels: Toward a General Theory

Anguillid eels are well known as catadromous highly migratory species, with marine spawning grounds well separated from inshore and inland growth areas. However, invasion mechanisms of river systems are poorly documented. This theory is based upon the identification of four distinct movement behaviors: „founders“ that colonize rivers until they settle in the first available suitable habitat they encounter; „pioneers“ that migrate upstream to the upper boundaries of the system; „home range dwellers“ that establish in a given area for several months to several years; and „nomads;“ which are erratic eels that perform a general upstream shift as they search for suitable areas to forage or to settle. These behaviors are not mutually exclusive, any eel shifting from one behavior to another, depending upon ontogenetic attributes (age, experience, morphology, physiological stage, etc.), population parameters that determine density-dependent movements; and environmental parameters such as food availability, temperature, flow, and the carrying capacity of the ecosystem.

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 environment may 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 % 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.

Habitat preferences of different European eel size classes in a reclaimed marsh: a contribution to species and ecosystem conservation

Freshwater reclaimed marshes along the European Atlantic coast are highly suitable for European eels (Anguilla anguilla). However, European eel stocks have declined, and the coastal marshes have been subjected to major disturbances. The objective of our study was to analyze the processes governing patterns of European eel microhabitat distribution of four eel size classes (from <160 mm to >360 mm) in a reclaimed marsh (France). Analyses were conducted using artificial neural network (ANN) techniques and ecological profiles. Our ANN results showed that eel densities were significantly related to three major influencing variables: the width of ditch section, the silt depth, and the density of emergent plants. Such ecological profiles were significantly different between small (<240 mm) and large eels (>360 mm): small eels were more widespread than large eels. Large eels were absent or at low densities in shallow ditches with a high aquatic plant cover obstructing the water column and a large quantity of silt. These characteristics seem to define the ditches not directly connected with the main river where dredging operations were rare. Management of regular dredging operations in the channels by maintaining a mosaic of permanent aquatic habitats and avoiding the heavy silt loads in most ditches should be promoted. This dredging operation was probably one of the most promising ways for restoring inland eel stocks.

Trends of a bitterling (Rhodeus sericeus) population in a man-made ditch network

This study describes the trends of a bitterling population in the Bourgneuf Marsh in relation to human activities. Although there were no bitterling during the period 1987-91, the species rapidly colonized the ditch network sampled (density reaching 61.7 +/- 47.0 individuals/100 m2 and frequency of occurrence around 70-75%) during a 5-year period (1997-2001). Clearance operations probably favoured the expansion of this vulnerable fish species because of positive associations of bitterling with large and deep ditches that had been cleared in the last 2-3 years and characterized by a small cover of submerged plants. Management recommendations are discussed for conserving the bitterling population.