U.S. Dept. of Commerce / NOAA / OAR / PMEL
An Overview of Shelikof Strait FOCI
Since 1984, the FOCI program has been conducting fisheries oceanographic
research in order to understand variability in recruitment to the pollock
stock in Shelikof Strait, Alaska. This research was based on a foundation
that included some fundamental knowledge of the physical environment and,
to a lesser degree, knowledge of the fisheries biology. Our focus has been
on transport of the planktonic stages toward nursery grounds and processes
affecting mortality as the animals are being transported. Shelikof Strait
FOCI is now generating information that is being considered in management
of the fishery. We consider the following results to be of major importance
toward developing an understanding of regional recruitment of pollock:
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Circulation is dominated by the Alaska Coastal Current which is one of
the strongest and most persistent currents found along the coasts of North
America. While the majority of volume transport throughout the water column
is directed seaward through the Shelikof sea valley, approximately 75%
of the water in the upper 50 m appears to stay on the continental shelf.
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There is a persistent, relatively weak flow of slope water into the sea
valley. This feature is a nutrient source which lies under and (at the
near surface) to the south of the ACC. Interannual variations in bottom-water
properties have been related to anomalies in the circulation in the Gulf
of Alaska.
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Baroclinic instability and wind-driven flow out of Wide Bay are mechanisms
which form features that tend to retain larvae on the shelf. The role of
these features in larval retention and the potential impact on secondary
production/larval mortality are ongoing topics of research.
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Strong spatial variations in the wind field can result in an area of convergence
of the surface-layer currents. The scale of this feature is consistent
with the observed size of a patch of pollock larvae.
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During the 1980's, several adjacent strong year classes (1975-1979) moved
through the Shelikof Strait population of walleye pollock causing a variation
in numbers of adults from 6.0 to 1.3 billion. Until recently, only weak
or moderate year classes have occurred since 1979. Thus during our studies
we have not had the opportunity to observe conditions leading to a strong
year class. The interannual variations in the physical environment and
in the early life history of pollock have resulted in only minor changes
in year class size. Focus on the strong year class of 1994 may lead to
advances in our understanding.
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There is little interannual variation in the timing and location of pollock
spawning and egg and larval development within the Shelikof Strait system.
Spawning occurs primarily in early April over the deep trench near Cape
Kekurnoi. The eggs reside mainly below 150 m in the water column and are
advected little during their 2-wk incubation period. The larvae develop
in the upper 50 m of the water column and are advected to the southwest
as they grow at about 2 mm/d during late April and May. During this time
the larvae are often in large patches that have been found entrained in
eddies in lower Shelikof Strait. Entrainment increases their residence
time in the area and lessens their chances of being advected offshore.
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Observable growth of surviving larvae varies little. Condition of larvae
varies with season and location. Sufficient food may be unavailable to
larvae early in the spawning season. Larvae in eddies seem to be in better
feeding condition than those outside eddies.
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Year class size seems to be largely determined by the end of the larval
period, although events during the following summer may reduce the year
class also. Storms during the early larval period may be particularly detrimental
to larval survival, although the mechanism is not yet clear. An index of
storminess in the Gulf of Alaska may provide a way of predicting year class
size.
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