Evidence for density-dependent mortality in recruitment of a temperate reef fish, cunner Tautogolabrus adspersus, among similar reefs in the vicinity of an anthropogenic disturbance

Determining the factors that regulate recruitment of cunner Tautogolabrus adspersus, a temperate reef fish, can help assess the effect of an anthropogenic disturbance on population abun- dance. The relative abundance of cunner recruits was tracked over 1 reproductive season via visual counts on line transects across similar habitats at 4 sites across 3 boulder reefs in the vicinity of the Pilgrim Nuclear Power Station (PNPS) in Plymouth, Massachusetts, USA. Habitat characteristics including macroalgae structure, canopy height, rugosity (a measure of structure complexity), tem- perature, and visibility were measured at each site to assess the influence of these factors on recruit abundance. We assumed that immigration and emigration were not responsible for recruit abun- dance patterns through time due to the sedentary life history of cunner recruits. We observed differ- ences in recruit abundance over the settlement period among reefs which were likely the result of differences in settlement. However recruitment at the end of the post-settlement period was similar across reefs. Recruit mortality rates among reefs differed in the post-settlement period, with the reef that had the highest density also having the highest mortality rate. Post-settlement mortality rates provided evidence for density-dependent mortality, since habitat and abiotic factors failed to explain the difference in mortality among reefs. Consequently, because of the existence of density-depen- dent processes in the post-settlement stages, entrainment mortality of the pre-settlement egg and lar- val stages by the PNPS will have a diminished effect on the local cunner population. An additional assessment of within-reef variability in recruitment also suggests recruitment success is sensitive to small changes in habitat structure which can occur over the recruitment season.