What are Ichthyoplankton?

Ichthyoplankton in a petri dish Icosteus egg close up Fish larvae as seen under microscope

Ichthyoplankton are the eggs and larvae of fish found mainly in the upper 200 meters of the water column, also called the near-surface waters. The eggs are passive and drift in the ocean along with the water currents. Most fish larvae have almost no swimming ability initially, however by half way through their development they are active swimmers. Ichthyoplankton are a relatively small but vital component of total zooplankton. They feed on smaller plankton and are prey themselves for larger animals.

Why do we study them?
It is important to study ichthyoplankton because the abundances of eggs and larvae of several species have been demonstrated to be a good indicators of the transient spawning population size of the adults. Determining the abundance of eggs and larvae in an area is usually less expensive to do than sampling the adults. For species such as sardine and anchovy, egg and larval counts are a good indicator of population size. Thus, we can use the egg and larval data as a way to monitor trends in population abundance of the adults. We are able to tell when populations are declining, often more rapidly than we could if we were just monitoring adults.

Are ichthyoplankton indicators of ecosystem health?
For species that aren’t captured by a fishery, monitoring their population trends by monitoring their eggs or larvae can provide an indication of a healthy or stressed ecosystem. It is unlikely that we would have an idea of the abundance, growth or decrease of these species in any other way.