| Abstract |
Considering the alarming abundance level of the Allis shad stock (Alosa alosa) in the Gironde-Garonne-Dordogne basin, causes of stock’s decline have to be determined. This work, thanks to the modelling of the reproduction process (using a “stock-recruitment” relationship) and mortality process (using a “recruitment-stock” relationship), gave a plausible explanation of this decline. The “recruitment-stock” models confirmed that most of the Allis shad spawners return in rivers to spawn after 5 years in ocean and also enabled to estimate the natural mortality ( M ) and the fishing mortality ( F ) coefficients. The obtained values were very high ( M = 0, 74 and F = 2, 6 ) but don’t show any tendency during the period analysed, despite inter-annual fluctuations. On the other hand, the modelling of the “stock-recruitment” dynamics gave rise to a regime shift in the middle of the 90’s with a decreasing of the maximum production level and the appearance of a depensation effect (Allee effect). The combination of these two approaches led to put forward assumptions about the dynamics of the population’s crash. At last, a theoretical approach of the spawner run process, taking into account the homing process, gave perspectives which could allow integrating variable recruitments in the “stock-recruitment” relationship according to the river (Garonne or Dordogne). |