Spatio-temporal distribution of allis shad (Alosa alosa L.) and twaite shad (Alosa fallax L.) in the bay of Biscay.

This study of the offshore distribution of allis shad Alosa alosa and twaite shad Alosa fallax throughout the French continental shelf area of the Atlantic ocean is based on catch data collected over the 3 - year period August 1986-March 1989, during bottom trawl surveys performed by the French research institute for exploitation of the sea (IFREMER). The survey area extends from Ouessant island to << Gouf de Capbreton >> (48degrees30' N - 43degrees45' N) down to a depth of 700 m (4degrees W - 1degrees20' W). Alosa fallax is distributed primarily at water depths < 50 m. Contrary to Alosa alosa which ranges in water deeper than 100 m, A. fallax doesn't occur over this depth. Each distribution of allis shad and twaite shad is aggregated and located in the river mouths of the most important watersheds (Gironde, Loire). For the two species and especially for twaite shad the largest fish are encountered in the deeper zone. The ages of A. alosa and A. fallax range from 2 to 5 and 2 to 6 years old. From all the campaigns, most of the allis shads (89 to 95 %) are 2 and 3 years old and most of the twaite shads (85 to 92 %) are 3 and 4 years old. Young twaite shad seem to be more dependent on estuary environment. The coefficient of condition of two years old allis shads caught in the estuary is significantly higher than for those captured at sea. The necessary adaptation period to migrate from the estuary to the marine new habitat, can explain a loss of condition.