SPAWNING OF 2 POMATOSCHISTUS SPECIES (GOBIIDAE) IN RELATION TO NEST AVAILABILITY AND DEPTH - A FIELD EXPERIMENT

The spawning of two sympatric gobiid fishes, the sand goby Pomatoschis tus minutus and the common goby Pomatoschistus microps was investigate d in relation to nest availability and depth. In two field experiments , carried out in the northern Baltic proper during summer, empty shell s of the mussel Mya arenaria were divided into three size groups and p rovided as spawning substrates. The shells were set out at different d epths and at different densities. Spatial habitat partitioning by dept h was found. The sand goby preferred the deeper bottoms (2.8 m) and th e common goby spawned only in areas shallower than 1 m. The spawning o f the two species was separated in time by about 6 weeks with an overl ap at the beginning of July. Low water temperatures (approximately 10- degrees-C) in June probably delayed spawning in the common goby. The s and goby preferred the larger shells. The larger the shells, the large r was the number of eggs, i.e. the male potential reproductive success . The common goby preferred the smaller shells, and their potential re productive success did not show a clear trend with increasing shell si ze. In both species, the mean number of shells used for spawning incre ased with increasing substrate density. The maximum nesting density wa s 21 nests per m2 for the sand goby and 20 nests per m2 for the common goby. The maximum total density for both species together was 23 nest s per m2. Interspecific competition for spawning substrates was minimi zed, as spawning was separated in time and each species preferred diff erent sizes of shell. The intensity of spawning under the shells provi ded indicates that a lack of suitable substrates limits spawning by bo th 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.