Fish larvae
Introduction

In 1865 the Norwegian scientist G. O. Sars discovered that eggs of cod, haddock, and gurnard are planktonic (Russell, 1976). Since then, English and German scientists described early life stages of fishes from the North Sea, and Ehrenbaum (1909) published the first comprehensive ichthyoplankton book for the Northeast Atlantic. After World War II, the early life stages of fish were investigated intensively, resulting in a large accumulation of information on the morphology of fish eggs and larvae worldwide. Now we know that most marine fish spawn pelagic eggs and larvae. Some coastal fish (e.g. herring) spawn demersal eggs on the sea bottom, but the larvae float to the surface after hatching and pass into a planktonic stage. During early life stages they remain in the upper layer of the ocean with other planktonic organisms, thus permitting quantitative collections in plankton nets.

Regardless of their adult depth habitat, almost all demersal and pelagic fish spend the planktonic stage together in surface layer. To live in surface water, larval body forms must be adapted to planktonic life, and thus develop a variety of morphological specializations. The great morphological diversity of adult fish, plus this specialization, result in a remarkable array of fish larval forms.

Hempel (1979) recognized the main objectives of ichthyoplankton surveys as follows:
1) General knowledge of fish eggs and larvae themselves: morphogenesis, physiology, behavior, taxonomy, systematics, and zoogeography;
2) Ichthyoplankton as an important element of the aquatic food web;
3) Culture of fish eggs and larvae as an important requisite for aquaculture, including selective breeding, toxicity tests, physiological and genetic studies, and identification of undescribed species;
4) Knowledge of fish population dynamics, and causes of large fluctuations in fish stocks.

Furthermore, one of the most important goals of recent ichthyoplankton surveys is to evaluate the spawning stock biomass, using quantitative sampling of fish eggs and larvae (Hunter and Lo, 1993).