Effects of larger heterospecifics and structural refuge on the survival of a coral reef fish, Thalassoma hardwicke

Structural refuge and competition can influence the survival of coral reef fishes; however, the effect of structural refuge on the outcome of competition remains unclear. In Moorea, French Polynesia, surveys were used to quantify habitat association for juvenile Thalassoma hardwicke and 7 sympatric labroid species. Having identified the 3 species most similar to focal T. hardwicke in habitat association, a field assay was used to determine if the presence of larger individuals of these species adversely affected the survival of focal individuals. Finally, a field experiment was used to test how competition with larger residents and structural refuge interact to influence survival of transplanted T. hardwicke. Surveys demonstrated that Gomphosus varius, Pseudocheilinus hexataenia, and T. quinquevittatum were most similar to T. hardwicke in habitat association. In the field assay, the presence of T. quinquevittatum had the greatest negative effect on survival of T. hardwicke. The field experiment revealed that competition with T. quinquevittatum and structural refuge both altered T. hardwicke survival, although their effects were not interactive. Survival of T. hardwicke was 2.3 times greater in treatments without T. quinquevittatum relative to those with T. quinquevittatum, and 2.8 times greater in treatments with structural refuge relative to those without structural refuge. The additive effect may have occurred via the establishment of social dominance hierarchies between transplanted T. hardwicke and resident T. quinquevittatum independently of structural refuge, suggesting that it is important to account for interactions occurring independently of resources when examining resource competition.