Effects of shelter and predators on reef fishes

Abstract

The abundance of reef fishes is often influenced by the availability of potential shelter sites. The most prevalent hypothesis explaining this relationship between fish density and shelter is that post-settlement predation causes greater mortality in areas with little shelter vs. areas with abundant shelter. This hypothesis was tested with a set of field experiments, in which the availability of shelter and exposure to predators were manipulated orthogonally. The effects of shelter and predators were measured for recruitment and survival of two temperate reef fishes: the bluebanded goby ( Lythrypnus dalli Gilbert) and the blackeye goby ( Coryphopterus nicholsii Bean). Shelter was manipulated by creating isolated 1-m 2 reefs that were composed of three densities of rocks (16, 32 and 64 rocks/m 2), which both species of goby used for shelter. A preliminary set of experiments done on reefs exposed to predators revealed that recruitment and survival of both fishes were positively affected by the density of rocks. The main set of experiments revealed the proximate causes of these effects of shelter. Replicate reefs of all three shelter treatments were either kept free of predators (enclosed in predator-exclosure cages) or exposed to predators (enclosed in partial cages). In Coryphopterus, survival and recruitment varied among shelter treatments on reefs exposed to predators, but not on reefs free of predators. This result indicates that shelter influenced the density of Coryphopterus by altering the impact of predators, thus supporting the prevailing hypothesis. In contrast, the effects of shelter on Lythrypnus were not caused solely by altering the impact of predation: survival and recruitment both varied among shelter treatments even when predators were absent. For survival of Lythrypnus, differences among shelter treatments were larger on reefs exposed to predators than on reefs free of predators, indicating that shelter did also modify the impact of predation. However, recruitment of Lythrypnus was affected similarly by shelter treatments regardless of whether predators were present or absent. Therefore, the effects of shelter were not caused by altering the impact of predators, and instead, it appeared that recruits responded directly to the abundance of shelter. These results indicate that for some reef fishes, positive relationships between abundance and shelter may not be caused by lower rates of predation in areas with abundant shelter. Instead, such relationships between abundance and shelter may be driven by prey preferences (exerted at or after the time of settlement) for areas with abundant shelter, or by use of shelter sites for purposes other than refuge from predators. Therefore, in the absence of any other evidence supporting a role of predation, positive relationships between the density of reef fishes and the abundance of shelter should not be construed as proof of the importance of predation in determining the abundance of reef fishes.