U.S. Dept. of Commerce / NOAA / OAR / PMEL / Publications
We have used models in many ways, including to conceptualize program strategies, to expand the time-space domain of field observations, to guide program direction through hypothesis generation, to elucidate potential linkages among sets of biological and physical factors, and to provide information into the fisheries management stream.
Ongoing modeling studies examine potential impact of interannual changes in
circulation on survival of larvae in the western Gulf of Alaska. Physical factors
that pose challenges include complex bathymetry with many islands, mesoscale
Thus far SPEM has reproduced the observed general spatial features of circulation [Stabeno et al., 1995]. A comparison between model output and measured currents yielded reasonable agreement. The model also generates eddies with similar spatial scales to those observed. Results from SPEM show that during 1978 (the strongest year class) larvae were more likely transported into coastal waters along the Alaska Peninsula, while in 1990 (a below average year class) they remained in the sea valley where currents then result in transport offshore [Stabeno et al., 1995]. This latter scenario implies loss of recruits. These results support the original transport hypothesis.
SPEM is being coupled to a spatially explicit, individual-based, probabilities model (IBM) of egg and larval development. The IBM has distinct advantages over more traditional approaches that consider only the "mean" individual [Huston et al., 1988]; since it follows the unique life history of each fish, the IBM approach yields specific information about survivors. The model employs a spatial tracking algorithm for each individual, that includes vertical migration according to life stage. Horizontal transport, growth, and behavior are governed by velocity, salinity and temperature fields generated by SPEM. Low-pass filtered velocity and scalar fields from SPEM are stored once per model day, then used as input for multiple runs of the biological model. The model-generated spatial distributions qualitatively compare favorably with observed distributions of larvae and juveniles. Interannual differences in wind and freshwater runoff lead to differences in the modeled spatial paths of individuals, and in the distributions of population attributes (e.g., growth).
The objective of this component is to adapt and extend a coupled physical biological 1-D model to investigate production dynamics of the pelagic ecosystem as it pertains to survival of larval pollock. The approach uses field observations to determine rates and appropriate species composition for several of the distinct physical [Coachman, 1986] and biological [Smith and Vidal, 1986] domains in the eastern Bering Sea. The present model includes stage-structured dynamics of copepod populations (Calanus and Neocalanus) and larval pollock feeding and growth (S. Bollens, pers. comm.). The temporal behavior of the mixed layer comes from observations (see below). Model results include that the species composition of zooplankton has a strong influence on growth of larval pollock; the presence of protozoan prey becomes important when young copepods are scarce (i.e., early spring over the slope). Results from the model also suggest that variability of the mixed-layer depth has significant impact on larval growth by affecting lower trophic level production. These results have led to the addition of a research component that addresses the potential importance of protozoans as prey for larval pollock.
Many studies attempt to examine the causes of recruitment variation in fishes and relate these to biotic and abiotic factors. Hollowed [1992] lists 47 such studies along the coast of the northeast Pacific. There are two schools of thought regarding the utility of the correlative approach. It can be considered futile because of biases, measurement error, and the near certainty of spurious correlations [Walters and Collie, 1988]. Others claim that these studies provide information on patterns that lead to testable hypothesis [Kope and Botsford, 1990]. Recent studies [Tyler, 1992; Hollowed, 1992] advocate the use of correlative studies with the constraint that the analysis be based on a sound conceptual framework and judicious use of statistical methods. This approach was applied to a recruitment time series for adult pollock for the period of 1962-1989 and egg and larval series for 1981-1989 [Megrey et al., 1995]. The physical time series included estimates of precipitation, sea-level atmospheric pressure, wind-driven mixing, transport and water properties. Statistical techniques suggest that recruitment, as well as indices of age-0 and age-1 abundance, were related to precipitation, an index of sea level pressure, and wind mixing. Given the time period and phasing of the physical factors, the results imply that stratification can influence behavior of and predation on juveniles, and that increased baroclinicity influences larval survival through its impact on eddy generation and wind mixing. The former implication supports previous studies of juvenile behavior [Olla and Davis, 1995], while the latter two have been inferred from analysis of biophysical observations.
In order to examine biophysical conditions from pre-spring bloom conditions
through the summer, we have deployed a mooring over the outer slope
Results from both Shelikof Strait and the slope waters of the eastern Bering Sea indicate that the highest abundances of pollock larvae often reside in eddies. To examine the nature of biophysical processes extant in these features and determine their influence upon survival requires in situ observations. Finding a reliable method to locate an eddy for field studies provides a challenge. Although infrared imagery has proved useful, cloud cover and generally weak sea surface temperature gradients limit this approach. High resolution Synthetic Aperture Radar (SAR) eliminates both of these constraints. Mesoscale features are imaged by SAR through several possible mechanisms that are not well understood, including modulation of the short surface waves by current shear, alteration of the stability of the surface wind across a relatively sharp sea surface temperature gradient, surface film damping of short surface waves, shifts in the Doppler frequency due to variability in the surface currents, and current-induced wave refraction. This latter mechanism has been examined for features in Shelikof Strait [Liu et al., 1994].
During April and early May 1992, three eddies
It has long been thought that processes involving nutrition and predation of larval fishes play significant roles in their survival and ultimately in the strength of year classes. Until recently, however, lack of suitable technology has hampered efforts to study these processes. The recent discovery of a record of daily growth of larvae in their otoliths (ear bones) has been applied to estimate growth and survival rates of a large number of species [Campana and Neilsen, 1985; Jones, 1986]. Immunoassay techniques, in which antibodies to particular fish species are developed and used to detect the presence of macerated remains of eggs or larvae of that species in predator guts, have also found increasing use [Theilacker et al., 1993]. Determining the nutritional state of larvae through histological assessment, and determining RNA-DNA ratios of whole larvae, or in cells of particular organs, has proven very valuable [Theilacker, 1978; Buckley, 1984; Clemmensen, 1988; Theilacker and Shen, 1993; Canino, 1994]. All of these techniques have been refined and applied in FOCI studies.
Laboratory rearing studies with larval pollock confirmed that increments are deposited daily in their otoliths [Bailey and Stehr, 1988]. Growth rates and hatch dates, based on length and age determined by otolith increments, of field-collected pollock larvae from various years and areas, have been compared [Yoklavich and Bailey, 1990]. Growth rates were found not to vary interannually, but the hatching period did. By estimating decreases in the abundance of cohorts within year classes with time during the larval period, mortality rates have been calculated [Yoklavich and Bailey, 1990]. These techniques were used to investigate differences in larval survival during the season. It was found that larvae that were at a first feeding stage during calm weather, had higher survival rates than larvae that reached first feeding during storms [Bailey and Macklin, 1994]. Growth rates of young-of-the-year juveniles have been determined, although beyond the larval period otolith growth is more complex and grinding is required to discern all of the increments [Brown and Bailey, 1992; Bailey et al., 1994a]. Besides using the record of daily growth in larval otoliths, studies have analyzed the elements deposited in the otoliths as a record of the environment experienced by the larvae at various times during their development. This technique has been investigated as a means of discriminating among juvenile pollock of various geographic/genetic origins [Severin et al., 1995].
To investigate predation on fish eggs and larvae by invertebrates that macerate their prey, antibodies against yolk proteins were developed, and potential predator gut contents have been assayed for the presence of these proteins [Bailey et al., 1993; Brodeur and Merati, 1993]. Decapod shrimp, euphausids, and gammarid amphipods were all found to consume significant numbers of pollock eggs and yolk-sac larvae. We are now developing immunoassays for later stage larvae to investigate the role of predators in their mortality (Brodeur, R.D. and N. Merati, pers. comm.).
DNA of individual cells is fairly constant, so the DNA content of whole animals increases proportional to increases in cell number (growth). However, RNA content of cells is variable, and reflects active protein synthesis. RNA/DNA ratios have been found to be accurate indicators of recent feeding of larvae: higher ratios indicate better feeding condition [Canino, 1994]. Condition of pollock larvae, measured by RNA/DNA in whole individual larvae, has been found to vary with location, time of year, and interannually [Canino et al., 1991; Bailey et al., 1994b]. Variations in larval condition were concordant with variations in prey abundances. Thus it appears that pollock larvae in Shelikof Strait may experience feeding conditions that may limit growth. A prolongation of the larval stage due to reduced growth rate likely increases mortality. Even more precise indicators of larval nutritional condition and recent growth history have been developed using flow cytometry to measure RNA and DNA contents of brain cells of individual larvae [Theilacker and Shen, 1993]. These methods can be applied to assay larvae at sea in near real time.
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