Recruitment, loss and coexistence in a guild of territorial coral reef fishes

Eupomacentrus apicalis, Plectroglyphidodon lacrymatus, and Pomacentrus wardi are territorial pomacentrid fishes which occupy contiguous individual territories within rubble patches on the shallow reef slope. Loss of residents, which is non-seasonal (except for juvenile Po. wardi), results in reallocation of space in rubble patches among the species. This reallocation is random in the sense that sites previously held by one species will not be more likely than any other sites to be reoccupied by that species or to be occupied by any other particular species. The results of a 38 month study of three neighbouring patches are used to determine patterns of recruitment, survivorship, and loss for each species. The rate of recruitment of fish is proportional to the area of the rubble patch, and is seasonal in at least one of the species. About half the recruits are juveniles and young adults from other sites. The others are newly settled from the plankton. Total space used in a rubble patch does not vary significantly during the 38 month period. Po. wardi recruits and is lost at higher rates than the other species and its survivorship is significantly lower. Juvenile Po. wardi are lost at a greater rate than are adults, and their loss rate varies seasonally. The other species are similar to one another in having low recruitment and loss rates, and correspondingly high survivorship. The data are used in a critical assessment of several competing hypotheses to explain the coexistence of these fish. The available data are most closely compatible with the chance allocation or lottery hypothesis, but a definitive conclusion is not possible, and is probably beyond current experimental techniques.