Resistance of naturally spawned pink salmon eggs to mechanical shock

Routine hydraulic sampling of pink salmon eggs (Oncorhynchus gorbuscha) is the subject of a long-running dispute over impacts of the Exxon Valdez oil spill on embryo survival in Prince William Sound, Alaska, because relationships between the time of spawning, sensitivity of eggs to mechanical damage, and sample timing were unclear. Previous laboratory or hatchery studies demonstrate that resistance of eggs to mechanical damage increases with maturity, but applicability to natural populations requires an understanding of embryo age distributions and the ability to discriminate between sampler-induced egg mortality and natural mortality. Resistance of naturally spawned eggs to hydraulic shock, determined six times between late September and mid-November in a southeastern Alaska stream, increased sigmoidally from less than 2% to 98%. In contrast, the number of eggs that died from natural causes was unrelated to sample time. Rapid removal of all eggs from the water allowed accurate discrimination between shocked and eggs dead prior to sampling, an improved method we recommend for future studies. The rate of shock resistance increase was slower in naturally spawned eggs than in uniform-age embryos subjected to the same hydraulic shock. We caution that combining shocked and dead eggs into one 'dead' category does not accurately describe natural mortality.

Publications:
Thedinga, J. F., M. G. Carls, J. M. Maselko, R. A. Heintz, R. E. Thomas and S. D. Rice. 2003. Shock resistance and observer classification of pink salmon eggs. Exxon Valdez Oil Spill Restoration Project Final Report (Restoration Project 01492), National Oceanic and Atmospheric Administration, National Marine Fisheries Service, Auke Bay Laboratory, Juneau,Alaska.