Jean-Pierre Bergeron

Comparison of growth and condition indices of juvenile flatfish in different coastal nursery grounds

Coastal and estuarine ecosystems in the eastern channel and southern bight of the North Sea provide nursery habitats for juvenile fishes. We examined the age 0-group juveniles of three flatfish speciesSolea solea, Limanda limanda and Pleuronectes platessa, collected in five nursery areas with different characteristics (three sites located near small estuaries and two affected by important inputs of industrial effluents), to evaluate habitat influence on their growth and condition. We measured a biochemical index (RNA:DNA ratio), a morphometric index (Fulton’s K condition index), plus a recent growth index (marginal otolith increment width) on each individual (about 3 months old), collected during surveys in autumn 1999. The three indices displayed few significant differences among the five nursery sites and different patterns for each species. We suggest that the investigated nursery areas provide habitats of equivalent quality for the age 0-group juvenile flatfishes in spite of different anthropogenic disturbances. On the other hand, this study focuses on the importance of using different biological indicators to assess habitat quality and environmental stressors in coastal areas.

Change in the shoaling behaviour and nutritional condition of anchovies (Engraulis encrasicolus L.) during a wind-induced water column disturbance: a natural event test of a general hypothesis

Fishermen have long observed the sudden change of shoaling behaviour of small pelagic fish following a strong wind event. The phenomenon was thereafter confirmed by fisheries scientists using acoustic tools. In this study, we report our observations made with acoustics to measure disorganization of the previously observed spatial distribution and shape of shoals that occurred after a wind event. The measurements were made during a fisheries research cruise in the Bay of Biscay (NE Atlantic) aimed at determining (1) small pelagic fish species abundance and (2) the individual nutritional state of anchovies (Engraulis encrasicolus L.) via evaluation of their RNA/DNA ratio. A spatial fine-scale study was conducted in two of the main breeding areas for anchovy, one located near the plume of a large river (haline stratification) and the other in an open sea location over the continental shelf break (thermal stratification). After a strong wind event, a change of the spatial and bathymetric distribution of the fish community, including anchovy, was observed in the open sea area, whereas no effect was detected in the near shore area. The RNA/DNA ratios determined in anchovy white muscles were quite similar in both areas before the wind event. Afterwards, they remained unchanged near the shore but were significantly reduced in the open sea location. This difference can be explained by the disparity in the strength of the stratification (i.e. haline versus thermal), but a change in the species composition of the fish community may also have played a role.