Experimental and observational patterns of density-dependent settlement and survival in the marine fish Gobiosoma

Dynamics of reef fishes are influenced by settlement and post-settlement processes. How these processes co-vary or interact is often overlooked due to a focus on one process or study design. We approached this problem by conducting both observational and experimental studies of settlement and post-settlement survival in two goby species, Gobiosoma evelynae and G. prochilos, in Tague Bay of St. Croix, U.S. Virgin Islands. Settlement to coral heads on patch reefs was spatially variable, but the spatial patterns were relatively consistent through time: certain coral heads repeatedly received many settlers, while others received few. We investigated factors that could influence settlement and therefore contribute to the patchy distribution of settlers. Coral heads that received more settlers were larger and further from the center of the reef than heads with few settlers. The heads with many settlers also had more resident fishes, suggesting that resident fishes might attract settlers. We conducted a large-scale fish removal experiment and found that variation in settlement rates to coral heads was mostly driven by inherent properties of the coral heads and not by the presence of resident fishes. We then conducted a replicated, smaller-scale experiment, which showed that increasing density of resident fishes decreased settlement and post-settlement survival. Observational data, however, showed that settlers survived at comparable rates on coral heads with high and low fish density. We discuss several alternative hypotheses that might explain this discrepancy in the strength of density-dependence between the observational and experimental studies. One of these explanations, which has not been considered in the literature, is that the difference in the results arose through a correlation between settlement intensity and the quality of either the larvae or the site to which they settled. As a result, higher settlement may not necessarily lead to higher mortality. We develop a graphical model that shows how density-dependence and habitat or larvae quality may interact. Such a model may help to resolve some of the contradictory results obtained among studies of reef fishes.