Straying of Adult Pink Salmon from their Natal Stream following Embryonic Exposure to WeatheredExxon ValdezCrude Oil

Numbers of strays (adult salmon returning to a nonnatal stream), straying rates, and distribution of strays were estimated for pink salmon incubated in oil-contaminated gravel and for an unexposed control group. The treatment groups were incubated in oiled gravel, which resulted in initial aqueous exposures for total polynuclear aromatic hydrocarbons (TPAHs) of 5 and 19 μg/L for a low and high dose, respectively; the control group was incubated in gravel without oil. Fry from each treatment group were marked with coded wire tags (CWTs). The numbers of tagged fish released were 65,409 from the control; 69,441 from the low-dose group; and 70,414 from the high-dose group. A total of 288,492 pink salmon were sampled for CWTs when returning to spawn in the natal stream of the experimental fish, when returning to streams within 60 km of the natal stream, and when returning to two hatcheries within 100 km of the natal stream. The frequencies of observed strays were 0.023, 0.030, and 0.025 for the control, low-dose, and high-dose groups, respectively. Although the frequency of observed strays was 30% and 9% (respectively) higher than the controls for the low- and high-dose groups, the differences among treatments were not statistically significant, and the rates did not increase with TPAH dose. Estimates of the adult straying rates (with 95% confidence intervals) within a 35-km radius of the natal watershed were as follows: 5.3% (3.4–7.1%) for the control group; 9.2% (5.1–13.2%) for the low-dose group; and 5.7% (2.8–8.5%) for the high-dose group. Most (90%) strays were recovered within 10 km of the natal watershed. Exposed fish tended to be recovered at a greater distance from the natal stream than were control fish. The estimated percentage of strays recovered within 10 km was higher for the control group (95%) than for either the low-dose (81%) or high-dose (83%) groups, and only fish from oiled groups were recovered in distant fishing areas. However, these differences were also not statistically significant. Our results do not support the hypothesis that oil exposure of embryos in intertidal spawning grounds was responsible for the high rates of straying of wild-stock pink salmon that were observed in Prince William Sound after the Exxon Valdez oil spill.