Fitness consequences of habitat use and competition among coral-dwelling fishes.

Differences in individual fitness among habitats may explain patterns of habitat selection and why individuals compete for habitats. Transplant experiments at two widely separated locations on the Great Barrier Reef were used to examine growth and survival of two competing species of coral-dwelling fish (Gobiodon histrio and G. brochus) that inhabit two species of coral (Acropora nasuta and A. loripes). At Lizard Island on the northern Great Barrier Reef, growth of G. histrio was 3 times higher and survival was 5 times higher on A. nasuta than on A. loripes. These fitness-related advantages may explain why G. histrio mostly inhabits and competes strongly for A. nasuta in the field. Growth of G. brochus was 2.5 times higher on A. nasuta than on A. loripes and survival was approximately equal on each species of coral. However, G. brochus mostly inhabits A. loripes in the field and is excluded from A. nasuta as a result of competition with G. histrio. Reduced growth in A. loripes demonstrates a cost of competition with G. histrio. These results also demonstrate a trade-off between competitive ability and the costs of using alternative habitats for G. histrio and G. brochus. Patterns of growth and survival on A. nasuta and A. loripes at One Tree Island on the southern Great Barrier Reef were generally similar to those at Lizard Island. However, growth rates for both species of fish on A. loripes and survival of G. histrio on A. loripes were lower at One Tree Island. Growth was closely correlated with the interbranch space of the coral species inhabited at each location Therefore, habitat structure appears to be the mechanism underlying habitat-related differences in growth.