|
Résumé :
|
Autumn-winter and spring migration of the brown shrimp are highly flexible processes in time, duration, distance and participation. Relative differences in the water temperature cause migration. The sensitiveness for this kind of stimuli varies for different categories of shrimps; berried females are on average more sensitive than non-berried females of the same size, spent males seem to be on average less sensitive than fertile males. Also individual shrimps within these categories are differently sensitive, probably because of differences in the physiological condition of the shrimps. As other important physiological processes in the life cycle of the brown shrimp are closely connected with moulting, the phase within the moulting cycle was used as a measure for the physiological condition of the shrimp. The physiological condition of individual shrimps in relation to moulting was estimated by measuring the surface area of certain hard parts of the carapace and the body-length. Their mutual relation expresses the distance in time a shrimp is away from its next moult. With this method, rigid patterns in migration and distribution of the brown shrimp could be demonstrated. The autumn-winter migration counteracts the transport of shrimp larvae to shallow inshore waters, where a mass grow up of juveniles takes place during summer. The spring migration though opposite in general direction, has the same function as the autumn-winter migration as both migrations are effective in a redistribution of the stock.
|
