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 Neilson, 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., 1995a]. 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., 1995b]. 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.