CONSTRAINTS IMPOSED BY HIGH-DENSITIES ON BEHAVIORAL SPAWNING STRATEGIES IN THE BARBEL, BARBUS-BARBUS

Hancock et al. (1976) documented spawing failure (''abandoned strategy '') in the barbel Barbus barbus (L.) when more than 5 males court a ma ture female. This paper analyses barbel spawning behaviour in the Rive r Ourthe (River Meuse Basin) where large populations with an unbalance d sex ratio and a relative scarcity of suitable spawning grounds impos e density-related constraints. Under these high densities (up to 600 s pawners per 150 m2), males shift from courting to non-courting behavio ur, when they only participate to the gametes shedding phase of a spaw ning sequence. Fifty % of the 189 spawning episodes observed in 1989-1 990 were successfully completed, despite spawning aggregates of 8-37 c ourting males, significantly (p < 0.05) above the limit apparently tri ggering the abandoned strategy in females. Spawning efficiency (the ra tio of successfully completed episodes) was significantly (p < 0.05) d ependent on female size, on the spatial location of spawning site (dis tance from non-courting males) and on the differences between the pref erences of the species for spawning habitat and characteristics of spa wning site (depth and current speed). Behavioural strategies of males and females are discussed within the context of sexual selection, cons idering the interactions between specific and individual interests.