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

Differences in individual Fitness among habitats may explain patterns of ha bitat selection and why individuals compete for habitats. Transplant experi ments 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 (A cropora nasuta and A. loripes). At Lizard Island on the northern Great Barr ier 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 ma y explain why G. histrio mostly inhabits and competes strongly for A. nasut a 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 cora l. However, G. brochus mostly inhabits A. loripes in the field and is exclu ded from A. nasuta as a result of competition with G. histrio. Reduced grow th in A. loripes demonstrates a cost of competition with G. histrio. These results also demonstrate a trade-off between competitive ability and the co sts of using alternative habitats for G. histrio and G. brochus. Patterns o f 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 Isla nd. However, growth rates for both species of fish on A. loripes and surviv al of G. histrio on A. loripes were lower at One Tree Island. Growth was cl osely correlated with the interbranch space of the coral species inhabited at each location Therefore, habitat structure appears to be the mechanism u nderlying habitat-related differences in growth.