Advanced Methods and Technology

FOCI uses simulation models to help conceptualize program strategies, expand the time-space domain of field observations, guide the direction of the program in generating hypotheses, help determine relationships between biological and physical factors, and provide information to fishery managers. Information on current speed and direction, along with meteorological data such as wind speed and direction, is used to produce simulations of ocean circulation. Ongoing modeling studies examine the potential impact of interannual changes in circulation on survival of larvae in the western Gulf of Alaska.

To simulate ocean circulation patterns, FOCI developed a semispectral primitive equation model (SPEM) that can accommodate complex bathymetry, mesoscale meanders and eddies, strong vertical shear, strong forcing winds, and freshwater runoff. The model determines the influence of phasing between physical features such as eddies and meanders of the ACC and location and timing of larval hatching and transport toward the nursery grounds. Results show that for strong year classes, larvae were more likely transported into coastal waters along the Alaska Peninsula, while for weak year classes they remained in the sea valley where currents transported them offshore (implying a loss of recruits). The SPEM can also be coupled to a probabilistic, individual-based biological model (IBM) of egg and larval development which follows the unique life history of each fish and yields specific information about survivors. Horizontal transport, growth, and behavior of the larvae are governed by ocean water velocity, salinity, and temperature generated by the SPEM. The model-generated spatial distributions compare favorably with observed distributions of larvae and juveniles.

To examine field conditions and provide information for models, FOCI scientists developed a moored platform (Peggy Bering Sea) that measures a suite of biophysical parameters (solar radiation, air temperature and humidity, wind, ocean temperature, salinity, currents, acoustic backscatter and chlorophyll). Selected parameters are telemetered via satellite to the laboratory for near real-time analysis. Physical measurements describe the atmospheric and oceanic environment. Biological measurements show the timing of phytoplankton and zooplankton production which can be related to walleye pollock larvae sampled during simultaneous shipboard larval surveys.

Scientists have long considered that processes involving nutrition and predation of larval fishes play significant roles in larval survival and, ultimately, in the strength of each year class. However, until recently the lack of suitable techniques to test this relationship has hampered efforts to study these processes. FOCI scientists are working to refine and apply new techniques in their research.

Discovery of a record of daily growth of larvae in fish otoliths (ear bones) has allowed scientists to estimate growth and survival rates based on length and age determined by growth ring increments. Growth rates and hatch dates of field-collected walleye pollock were found to vary interannually. Techniques to estimate mortality rates are used to investigate differences in larval survival during the season. Researchers found that larvae reaching the first-feeding stage during calm weather had higher survival rates than larvae that reached the first-feeding stage during storms.

To investigate predation on fish eggs and larvae by invertebrates, antibodies against yolk proteins were developed and potential predator gut contents were examined for the presence of these proteins. A number of invertebrates were found to consume significant numbers of walleye pollock eggs and yolk-sac larvae.

The nutritional state of larvae is determined through histological assessment and determination of RNA/DNA ratios of whole larvae or cells of organs. RNA/DNA ratios have been found to be accurate indicators of feeding larvae: higher ratios indicate better feeding conditions.

Condition of larvae, measured by these techniques, varies locally, seasonally, and interannually. Variations in condition were similar to variations in prey abundance. A lengthening of the larval period due to reduced growth may increase the time larvae are vulnerable to predation. More precise indicators of larval condition and recent growth have been developed using flow cytometry to measure RNA and DNA contents of brain cells of individual larvae.