SIZE SPECIFIC PREDATION BY HERONS AND ITS EFFECT ON THE SEX-RATIO OF NATURAL-POPULATIONS OF THE MOSQUITO FISH GAMBUSIA-AFFINIS BAIRD AND GIRARD

Sex-ratios of G. affinis populations in freshwater marshes in the Camargue (Rhone Delta France) are highly biased in favor of males; the sex-ratios in ditches are close to unity. Studies of the diet of free-living birds and experimental studies on prey size selection in captivity show that the abnormal sex-ratios in marshes can be attributed to differential heron predation. Ditches are relatively free from predation. Mature female Gambusia are larger and have an energy content 5-25 times greater than that of mature males. Handling times of grey heron (Ardea cinerea) and little egret (Egretta garzetta) are only slightly longer for female Gambusia than males. Hence females represent a much more profitable prey. Analysis of nestling regurgitates show that Gambusia makes up a considerable proportion of the diet of 4 spp. of Camargue herons, and that the majority of Gambusia taken are females. Under experimental conditions, captive herons consume almost exclusively female fish, even when offered in ratios where they are heavily outnumbered by males. The relevance of these results to optimal diet theory is discussed.