LABORATORY STUDIES OF PREDATION BY MARINE COPEPODS ON FISH LARVAE

Abstract

A variety of marine copepods have been shown to fatally injure or capture and ingest young anchovy larvae in the laboratory. Labidocera jollae, L. trispinosa, and Pontellopsis occidentalis (family Pontel­ lidae), species common to surface waters of the California Cmrent, are effective predators of larval fish. The copepods can be attracted by the vibrations of the larval tail beat and react by biting or capturing the fish larvae. Cruising speeds for these copepods varies from 1.5 to 4 body lengths per second which, coupled with continuous swimming behavior, results in extensive searching by the copepod for prey. In laboratory exper\ments, when the ratio of larval fish prey to L. jollae female individuals was low «10:1 in 3500 ml), all of the larvae were killed in 24 hr where refers to both capture-in­ gestion and biting resulting in a fatality. If the ratio was higher, killing increased but rarely reached 100'/0 mortality of the larvae. L. trispinosa males and females never killed all the larvae offered to them in 24 hr in 3500 ml although more larvae were killed as the number offered was increased. In­ creased swimming and escape ability developed as anchovy larvae became older and were not caught or bitten as effectively as younger ones by copepods. However, killing of larvae by P. occidentalis was un­ affected by the age of the larvae up to 3.5 days old. When Artemia salina nauplii were substituted for larval fish as prey for L. trispinosa, the amount of grazing was proportional to the ratio of nauplii to copepods. If the number of Artemia nauplii was less than 11-14/liter per copepod in 3500 ml all the nauplii were killed in 24 hr. When the density of nauplii was increased, more were killed but never all of them. In experiments where nauplii and yolk­ sac larvae were offered together as prey the mortality of the larvae due to predation declined in pro­ portion to the increase in the number of nauplii provided. Caloric requirements were calculated from oxygen consumption measurements and showed that only 1 to 4 anchovy larvae are required per day per copepod to satisfy the metabolic needs of Labidocera, depending on the species and the sex. This number is far less than can be killed or captured if the density of larvae is high enough. In large (140 cm) vertical cylinders larvae and L. trispinosa were distributed within 25 cm of the surface in the dark. Data are presented which show that L. trispinosa and anchovy larvae also co­ occur in the upper few meters under the surface of the sea. This is probably also true for P. occident­ alis and L. jollae. No data are yet available on the relative density of predatory copepods and fish larvae where they co-occur or their possible predator-prey interactions in the sea.