Quantifying ultraviolet radiation mortality risk in bluegill larvae: effects of nest location.

Ultraviolet (UV) radiation (280-400 nm) is an increasing threat to aquatic organisms due to stratospheric ozone depletion and reductions in concentrations of dissolved organic carbon. Because fish are most vulnerable to UV during the egg and larval stages, parental spawning site selection can strongly influence mortality risk. We examined the role of nest location in determining UV-induced mortality risk for bluegill (Lepomis macrochirus) in Lake Giles, Pennsylvania, USA. In a series of five short-term incubation experiments, we found that survival of yolk sac larvae across the range of depths at which bluegill spawn was significantly lower in the presence of ambient-UV levels relative to larvae that were shielded from UV radiation. In addition, survival decreased as a function of cumulative UV exposure, as measured by the number of cyclobutane pyrimidine dimers per megabase DNA in DNA dosimeters. Although UV had the potential to significantly reduce larval survival, DNA dosimeters placed in bluegill nests concurrently with incubation experiments indicated that most nests were exposed to relatively low levels of UV. Only 19% of nests had predicted UV-induced mortality greater than 25%. Consequently, current levels of UV may be an important mortality source at the level of individual nests, but not at the population level. One reason for the weak predicted effect of UV on bluegill survival is that many nests were located at depths by which much of the incident UV had been attenuated. In addition, many of the shallower nests were protected by overhanging trees or other submerged structures. It is important to note that Lake Giles is highly transparent and therefore not representative of all lakes in which bluegill are found. Nevertheless, Lake Giles is a natural system and may be representative of north temperate lakes in the future.