Spawning of two Pomatoschistus species (gobbidae) in relation to nest availability and depth: A field experiment

Abstract

The spawning of two sympatric gobiid fishes, the sand goby Pomatoschistus minutus and the common goby Pomatoschistus microps was investigated in relation to nest availability and depth. In two field experiments, carried out in the northern Baltic proper during summer, empty shells of the mussel Mya arenaria were divided into three size groups and provided as spawning substrates. The shells were set out at different depths and at different densities. Spatial habitat partitioning by depth was found. The sand goby preferred the deeper bottoms (2.8 m) and the common goby spawned only in areas shallower than 1 m. The spawning of the two species was separated in time by about 6 weeks with an overlap at the beginning of July. Low water temperatures (∼10°C) in June probably delayed spawning in the common goby. The sand goby preferred the larger shells. The larger the shells, the larger was the number of eggs, i.e. the male potential reproductive success. The common goby preferred the smaller shells, and their potential reproductive success did not show a clear trend with increasing shell size. In both species, the mean number of shells used for spawning increased with increasing substrate density. The maximum nesting density was 21 nests per m 2 for the sand goby and 20 nests per m 2 for the common goby. The maximum total density for both species together was 23 nests per m 2. Interspecific competition for spawning substrates was minimized, as spawning was separated in time and each species preferred different sizes of shell. The intensity of spawning under the shells provided indicates that a lack of suitable substrates limits spawning by both species in this area. The larger sand goby is limited in particular by the lack of larger substrates, and is probably forced to spawn in shallow water when suitable shells do not occur in its preferred depth range.