PREDATION BY BIRDS AFFECTS POPULATION-STRUCTURE IN BREEDING SAND GOBY, POMATOSCHISTUS-MINUTUS, MALES

Sand gobies breed in water that ranges in depth from 15 cm to a few metres. Hunting piscivorous birds are very common on shallow beaches, thus suggesting that bird predation may have an important effect on the breeding goby population. Birds attacked non-cryptic nests more frequently than cryptic nests, indicating that they used the nest to locate their prey. It also suggests that male sand gobies cover their nests with sand to decrease the risk predation. Males colonized new nest sites faster at a depth of 40 cm than at 20 cm. Samples of breeding males from different water depths showed that the length of males increased with increasing depth. In an exclosure experiment, it was shown that males breeding in sites unavailable to birds were on average 1.2 mm larger than males breeding in control sites. The absence of predation in deeper waters apparently allows an accumulation of large males that exclude smaller individuals. This will lead to strong intra-sexual selection against small size.