Patterns of ultraviolet radiation exposure in bluegill nests over the course of the spawning season

Ultraviolet radiation (UVR; wave length, 280-400 nm) can be an important source of mortality for fish, particularly during early life history stages. Because UVR exposure varies seasonally owing to solar angle, climatic conditions, and changes in the UV transparency of water, UVR-induced mortality among temperate fishes may vary as a function of spawning time. We examined patterns of UVR exposure in nests of bluegill Lepomis macrochirus over the course of a spawning season in a highly transparent lake in northeastern Pennsylvania. At the beginning, middle, and end of the spawning season, we measured nest depth and estimated the cumulative UVR exposure of all bluegill nests along three 250-m transects. Cumulative exposure was then used to predict the UVR-induced mortality for bluegill larvae in each nest. The median UVR exposure in bluegill nests was highest at the start of the spawning season, and the predicted UVR-induced larval mortality was at least 10% in 38% of the nests. As the season progressed, the median UVR exposure decreased, and only 4% of the nests were predicted to have larval mortality of at least 10% by the middle and end of the spawning season. This decrease in exposure was partly due to a seasonal decline in UVR transparency. In addition, mean nest depth increased over the course of the spawning season, as bluegills increasingly nested at depths of 4 m or more. Because UVR attenuates with depth, deeper nests received less UVR exposure than did nests closer to the surface. For many species, reproductive success is strongly dependent on spawning time. Our results suggest that in some systems a consequence of early spawning may be an increased risk of UVR-induced mortality.