determine how environmental factors affect survival of a larval cohort. 
Environmental factors act in concert, and it is the sum of experiences over the 
entire embryo and larval stages that determines whether a good or poor year 
class results. A bioassay for 96 hours, testing one or two factors, usually can 
provide only a rough evaluation of the potential effect of the factor(s) on 
recruitment. More meaningful conclusions can be drawn from investigations 
that encompass the entire larval period. Many studies of that kind have been 
carried out on larvae of freshwater fishes (69), but the difficulties in rearing 
larvae of marine species have limited most bioassay research to embryo and 
yolk-sac larva stages. 
Experiments in large volumes of seawater, either in plastic bag enclosures, 
such as those used in recent Controlled Ecosystems Pollution Experiments 
(CEPEX) (70) or in large tanks (74) hold great promise because whole 
communities can be entrapped in such volumes. Effects of predation and 
competition can be evaluated. Direct and indirect effects of added pollutants 
on each trophic level can be observed. Recruitment success or failure by fishes 
in such systems can be interpreted in the context of observed changes that 
took place in the plankton community during the course of larval 
development. 
Other approaches include transitional studies that combine laboratory and 
field experiments. The “field bioassay” developed by Lasker (56) uses 
laboratory-reared larvae in shipboard experiments, in which larvae are reared in 
natural seawater sources to evaluate the potential of particular water masses to 
support larval survival and growth. The recent discovery, based on laboratory 
studies, that daily growth rings are present on otoliths of larvae, will allow 
better estimates of larval growth and mortality rates in the sea (26, 93), and 
also will allow comparison of growth in the laboratory with growth under 
natural conditions. 
Except for swimming-feeding behavior of a few species and behavioral 
responses to varying light levels (12, 15, 17) little is known about normal 
behavior patterns of larvae or changes in behavior induced by environmental 
effects. Behavioral studies not only can increase our understanding of how 
pollutants affect larval behavior, but they also can provide important insight 
into how predation and competition operate during the larval stage. 
There are many techniques presently available that allow environmental 
factors and their effects on marine fish larvae to be evaluated. In the next 10 
years, culture of marine fishes will be routine procedure at many laboratories; 
and as more data accumulate, some of the seemingly contradictory results 
obtained to date, especially with regard to critical food concentrations, will be 
resolved. Additional species of marine fishes need to be tested for larval 
195 
