TIMING OF MIGRATION AND EXCEPTIONAL GROWTH OF YOY ALOSA FALLAX RHODANENSIS (ROULE, 1924) IN A LAGOON IN SOUTHERN FRANCE.

Abstract

The Vaccarès lagoon in the diked Rhône delta is the only lagoon in southern France out of 28 lagoons where a large population of YOY (young of the year) shad occurs every year from June-July to November-March depending on the year. This peculiar case can be explained on one hand by the nearby spawning ground in the River Rhône and its tributaries and on the other hand by a nocturnal recruitment of the YOY shad from the River Rhône to the diked Rhône delta through water pumped for rice farming. The timing of the appearance and growth of YOY shad were studied for six years. YOY began to enter through pumps in mid-June or mid-July depending on the year, with fork lengths ranging between 20-55 mm. Growth was nearly linear from mid-July to mid-December, and the growth rate (regression slope) varied significantly between years, from 0.9 to 1.4 mm.d-1. Maximum observed total lengths of YOY were 213 mm, and mean total length of YOY at the end of the growing season was 182.9 mm ± 24.3. Growth seemed to stop in winter. The date at which YOY disappeared from catches in the Vaccarès lagoon varied between years, from mid-November to mid-March. YOY seemed to leave the lagoon after the first cold weather (air temperature < 0°C). YOY growth observed in the Vaccarès lagoon is much higher than the growth recorded in Atlantic France and in northern Europe.

Keywords

• Alosa fallax
• YOY
• French Mediterranean lagoon
• growth