Fish larva

Ichthyoplankton (from Greek: , ikhthus, "fish"; and πλαγκτός, planktos, "drifter") are the eggs and larvae of fish. They are usually found in the sunlit zone of the water column, less than 200 metres deep, which is sometimes called the epipelagic or photic zone. Ichthyoplankton are planktonic, meaning they cannot swim effectively under their own power, but must drift with the ocean currents. Fish eggs cannot swim at all, and are unambiguously planktonic. Early stage larvae swim poorly, but later stage larvae swim better and cease to be planktonic as they grow into juveniles. Fish larvae are part of the zooplankton that eat smaller plankton, while fish eggs carry their own food supply. Both eggs and larvae are themselves eaten by larger animals.

Fish can produce high numbers of eggs which are often released into the open water column. Fish eggs typically have a diameter of about 1 millimetre (0.039 in). The newly hatched young of oviparous fish are called larvae. They are usually poorly formed, carry a large yolk sac (for nourishment) and are very different in appearance from juvenile and adult specimens. The larval period in oviparous fish is relatively short (usually only several weeks), and larvae rapidly grow and change appearance and structure (a process termed metamorphosis) to become juveniles. During this transition larvae must switch from their yolk sac to feeding on zooplankton prey, a process which depends on typically inadequate zooplankton density, starving many larvae.

Ichthyoplankton can be a useful indicator of the state and health of an aquatic ecosystem. For instance, most late stage larvae in ichthyoplankton have usually been preyed on, so ichthyoplankton tends to be dominated by eggs and early stage larvae. This means that when fish, such as anchovies and sardines, are spawning, ichthyoplankton samples can reflect their spawning output and provide an index of relative population size for the fish. Increases or decreases in the number of adult can be detected more rapidly and sensitively by monitoring the ichthyoplankton associated with them, compared to monitoring the adults themselves. It is also usually easier and more cost effective to sample trends in egg and larva populations than to sample trends in adult fish populations.

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