U.S. Dept. of Commerce / NOAA / OAR / PMEL / Publications
We use the term coupled biophysical processes to mean physical processes that influence biological conditions affecting various life history stages of pollock.
In both Shelikof Strait and the Bering Sea, eddies play a role in coupled biophysical processes. These features frequently occur over the sea valley west of Kodiak Island, as revealed in infrared, synthetic aperture radar, and color scanner satellite imagery [Reed et al., 1988; Schumacher et al., 1991; Vastano et al., 1992; Liu et al., 1994], buoy trajectories [Incze et al., 1990], water property and larval distributions [Schumacher et al., 1993], and moored current records [Bograd et al., 1994]. The location of eddy formation coincides with the spawning region. Formation of three or four eddies per month during spring [Bograd et al., 1994] assures that some eggs hatch into an eddy. As a result of limited dispersion, high abundances of larvae often exist in such features. Further, since some eddies tend to remain nearly stationary for periods of weeks, they retain larvae in the sea valley [Reed et al., 1989; Vastano et al., 1992; Schumacher et al., 1993]. Some eddies have unique chemical properties [Incze et al., 1989], which may aid prey production and survival of larvae in the eddy [Schumacher and Kendall, 1991]. Nutritional condition of first-feeding larvae can be reflected by the ratio of ribonucleic acid to deoxyribonucleic acid (RNA/DNA). Concentrations of nauplii, larval gut contents, and RNA/DNA were higher for larvae in an eddy than for those in adjacent waters [Canino et al., 1991; Bograd et al., 1994].
Results from observations during May 1990 showed a connection between an eddy and larvae [Schumacher et al., 1993]. Contours of larval abundance coincide with those of salinity, and lie in close proximity to buoy trajectories. Physical data showed little or no exchange of water between the eddy and adjacent waters, permitting estimates of mortality that reflect only predation and/or starvation. The observations yield a daily mortality (4.4%) low compared to other estimates based on observations (6.3%, Reed et al. [1989]; 5-11%, Yoklavich and Bailey [1990]), or from dispersion model simulation (5.9-8.0%, Kim and Bang [1990]).
In the eastern Bering Sea, the presence of relatively small eddies (diameter
General results from modeling studies suggest that wind mixing of the upper water column can both be beneficial or detrimental to larval survival, depending on the intensity of the turbulence [Davis et al., 1991]. Many processes exist that connect pollock larval survival to mixing [Bailey and Macklin, 1994]. These authors determined a time series of abundance of larvae hatched on a given day that survived through the early feeding stage, and established a mixing index using the cube of the wind speed. Comparing these series revealed two patterns: strong wind events during the first-feeding period coincided with lower than expected survival, and periods of higher than expected larval survival were associated with calm periods of wind often bracketed by strong mixing. During a spring with moderate winds and a shallow mixed layer, concentrations of food, growth at age and mortality rates were more conducive to larval survival than during a spring when strong winds were accompanied by a deep mixed layer [Bailey et al., 1994b].
Bailey and Macklin suggest how larval survival may be related to an integration of wind-mixing, stratification within an eddy, and larval behavior. The eddy observed in 1989 had enhanced prey and feeding conditions and a low-salinity core relative to surrounding waters. Pollock larvae in the laboratory avoid turbulence [Olla and Davis, 1990] by moving deeper in the water column. Reduced light intensity with increasing depth has detrimental effects on the ability of larvae to search for and capture prey [Heath, 1989]. The vertical stratification of the eddy required more wind-induced turbulence than adjacent waters to mix to comparable depths. Thus, under similar winds, larvae within the eddy could remain higher in the water column in better feeding conditions than larvae outside the eddy. Hence, first-feeding larvae are more likely to survive in the eddy than in the surrounding waters giving similar prey fields.
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