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For 4 years after the Exxon Valdez
oil spill, Alaska Department of Fish and Game (ADF&G) biologists
recorded elevated mortalities among pink salmon embryos incubating in
contaminated sections of affected streams. The persistence of these
observations was contrary to expectations because streambed
contamination was thought to be minimal, and salmon eggs were not
considered sensitive to polynuclear aromatic hydrocarbons (PAH).
In 1992, we began examining the plausibility of the ADF&G
observations by incubating pink salmon eggs in gravel contaminated with
known quantities of oil. After following three brood years, we concluded
that pink salmon eggs are sensitive to low concentrations of the PAH
that characterize weathered oil and that effects can be observed in eggs
incubating downstream from the oil source.
Embryos exposed to aqueous PAH concentrations of 1.0 ppb demonstrated a
twofold increase in mortality compared with unexposed embryos and a 10%
reduction in growth during their first 6 months in salt water. Fish
initially exposed as embryos to aqueous PAH concentrations of 19 ppb
from the 1993 and 1995 broods experienced marine survivals 15% to 60%
lower, respectively, than unexposed fish. Estimates reveal that exposed
cohorts produced 38% and 42% fewer mature adults than unexposed cohorts
in the 1993 and 1995 broods, respectively. Observations of reduced
growth suggest further reductions in reproductive potential through
Our laboratory observations indicate the
plausibility of the ADF&G observations. These conclusions are
further supported by knowing that the minimum effective concentrations
of PAH identified in our studies are bracketed by the range of
contamination levels reported in and around pink salmon streams in PWS
for at least 4 years after the spill.
ABL Ocean Carrying Capacity - Research sidebar