SPAWNING SUCCESS OF THE MALE PLAINFIN MIDSHIPMAN .2. SUBSTRATUM AS A LIMITING SPATIAL RESOURCE

Two series of field experiments were performed to test potential substratum limitation of spawning plainfin midshipman fish Porichthys notatus Girard by a guild of four taxa of sessile invertebrates. This guild occupied space on the undersides of rocks that toadfish used for egg attachment. At the time of the experiments, a mud tube-dwelling terebellid polychaete (Thelepus sp.) dominated the coverage by this guild beneath rocks at the study site. Removals of foulers from beneath rocks indicated that foulers sometimes compete with toadfish embryos for attachment space. Space freed prior to the toadfish spawning season was subsequently used by toadfish for egg attachment. In 1988, removal of foulers from the undersides of treatment rocks subsequently produced 75% greater area cover of toadfish embryos than control rocks where foulers had not been removed. In 1989, a similar experiment resulted in no difference between treatment and control rocks. However, less extensive fouling beneath control rocks in 1989 suggests that embryo-fouler interactions were then less important. Manipulations of space occupied by toadfish embryos beneath concrete plates suggested that the mating success of individual males was limited by the amount of substratum suitable for embryo attachment. Plates from which embryos had been completely or partially removed subsequently acquired more embryos than control plates on which embryos had not been removed. The magnitude of subsequent spawning was directly proportional to the magnitude of embryo removal. Additional variables (area of plate, initial embryo coverage, and male body size) also influenced results, but inconsistently and to less extent. Experimental results support the hypothesis that space limitation dampens the amplitude of variation in mating success among males.