U.S. Dept. of Commerce / NOAA / OAR / PMEL / Publications
Prior to FOCI, only very limited knowledge existed of either the physical environment or the life history of pollock in the western Gulf of Alaska. What was known suggested the following hypothesis: optimum survival and subsequent recruitment result when larvae are transported to nursery grounds in coastal regions along the Alaska Peninsula, rather than into the Gulf of Alaska. Further, biophysical processes occurring during transport have significant impact on survival. Initially studies focussed on the life history of pollock and the nature of the regional circulation. As these aspects became known, field operations switched to maintenance time series of selected biological and physical characteristics, studies of biophysical processes, and development of methods to examine preferential survival. Simultaneously, we began implementation of coupled biophysical and correlative models, and development of methods to transfer FOCI results to the assessment and prediction models used to provide biomass scenarios to management.
Separation of pollock larvae from those of similar species in field samples
was first accomplished in 1981 [Dunn
and Matarese, 1987]. At the same time, the strong year-classes in the
mid-1970's resulted in a large fishery [Megrey,
1990], and hydroacoustic surveys were conducted to estimate the adult population
for input to assessment and prediction models. The surveys showed that large
concentrations of pre-spawning walleye pollock migrate from the southwest end
of Shelikof Strait (taken to include the sea valley that extends from the Barren
Islands southwestward to the slope off Chirikof Island, Figure 1) primarily
to the area near Cape Kekurnoi. Spawning takes place mainly in the deep sea
valley in early April [Kim
and Nunnallee, 1991], at depths between 150-
Figure 1. Upper Panel: The FOCI study area in the western Gulf of Alaska with a schematic showing the location of pollock life history stages. The outlined arrows represent potential transport of larvae off the shelf. Lower Panel: Circulation in the study area as derived from satellite-tracked buoys (51) drogued at 40 m and deployed between 1986 and 1994 (after Schumacher and Kendall [1991]).
The larvae are about
By late May considerable year-to-year variation exists in abundance and location
of larvae [Kendall
and Picquelle, 1990] as well as the zooplankton that produce their prey
[Incze
and Ainaire, 1994]. Two primary tracks characterize westward larval
drift: along the Alaska Peninsula in the relatively slow moving flow
We have established the life history pattern between spawning migration and young of the year (Figure 1). Using results from egg and larval surveys, together with estimates of the spawning adult population from hydroacoustic observations and indices of juvenile abundance [Bailey and Spring, 1992], the stages when year-class strength is established have been determined. A low abundance of larvae results in weak recruitment. A large abundance of larvae, however, does not imply strong recruitment. This suggests that processes during the juvenile stage can also be critical [e.g., Bailey et al., 1994b]. Recently, we have focussed more research on understanding young of the year juveniles since interannual variability in their survival also affects year class strength [Bailey and Spring, 1992].
Much of the variability in the physical environment in the Gulf of Alaska results from large scale atmospheric phenomena. Global patterns in the upper level atmospheric pressure generate climatic conditions that include an annual cycle in the number of low pressure centers traversing the region [Niebauer, 1988]. The consistent passage of storms along the Aleutian Island chain (the Aleutian Low) dominates wintertime atmospheric circulation. Interaction of frequent storms with the mountainous coastline results in a high precipitation rate (>200 cm yr-1) along the coastal region. The discharge rate of freshwater reflects seasonal variations in air temperature, precipitation, runoff and storage from the previous winter [Royer, 1982].
Along the Alaska Peninsula, along with a deep
The dominant circulation feature is the Alaska Coastal Current (ACC), a distinct
flow that only 20 years ago was unknown. FOCI research has elucidated many of
the characteristics of the ACC, which extends for
In Shelikof Strait, horizontal density gradients and vertical shear in the
mean flow create the baroclinic instability, evident in satellite images [Vastano
et al., 1992; Schumacher
et al., 1991] and in analysis of current records [Mysak
et al., 1981], which dominates flow patterns and generates eddies [Schumacher
et al., 1993]. The ACC does not span the sea valley, and estimates of
coherence become insignificant for separations
FOCI research has found and elucidated the dominant circulation and mesoscale features: the ACC, eddies generated by baroclinic instability, and an estuarine-like flow of slope waters into the sea valley. These features must be simulated in any numerical model of the region. Further, results show that the timing and location of hatching determines whether larvae enter an eddy, or are transported with either the slow-moving coastal flow or the rapid ACC. Modeling studies [Stabeno et al., 1995] suggest that the location of late larvae varies greatly year to year depending on advection. The phasing between biological and physical processes determines transport of larvae and presumably their eventual recruitment.
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