The motivations for investigations have changed little over this period, being mainly the assessment of adult spawning biomass and distribution, and the desire to understand how environmental variations and changes in the abundance of other species interact to regulate the abundance in particular fish populations. The factors affecting recruitment, in particular those that affect the survival of fish eggs and larvae, are of key importance in this context. The process of recruitment, in spite of about 100 years of research, is still not fully understood. Fish eggs and larvae are usually sampled with plankton nets. One of the fundamental events in the development of most fishes is the flexion of the notochord that accompanies the hypochondral development of the homocercal caudal fin.
Metamorphosis occurs when the fish assumes the general features of the juvenile.