LABORATORY STUDIES OF PREDATION BY MARINE COPEPODS 



ON FISH LARVAE 



Kurt Lillelund'- - and Reuben Lasker' 



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. trit^piyiosa, and Pontellopsis occidentnlis (family Pontel- 

 lidae), species common to surface waters of the California Current, 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 experiments, 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 "killing" refers to both capture-in- 

 gestion and biting resulting in a fatality. If the ratio was higher, killing increased but rarely reached 

 100% 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 Arteinia salina nauplii were substituted for larval fish as prey for L. tri<spinosa, 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 lar\'ae 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. 



Huge mortalities of larval fish are known to oc- 

 cur in the sea. From the time of Hjort (1914) 

 these have been attributed mainly to the lack 

 of the proper food when the larvae begin to feed 

 (see review by Blaxter, 1969). Undoubtedly 

 other biotic and abiotic factors are also involved 



' Institut fiir Hydrobiologie und Fischereiwissen.schaf t 

 der Universitat Hamburg, 2 Hamburg 50, Olbersweg 24, 

 Germany. 



" This research was supported in part by a Grant 

 from the Deutsche Forschungs Gemeinschaft. 



' National Marine Fisheries Service, Fishery-Ocean- 

 ography Center, La Jolla, Calif. 92037. 



Manuscript accepted April 1971. 



FISHERY BULLETIN: VOL. 69. NO. 3, 1971. 



in larval fish mortality, but com])aratively little 

 work has been done to measure their effect. 

 Among the possible causes of larval-fish mortal- 

 ity, predation by other zooplankters may be an 

 important factor. Freshwater aquarists have 

 known for some time that copepods must be elim- 

 inated from fish rearing tanks or high mortal- 

 ities of young larvae or fry will occur (Davis, 

 1959). Lillelund (1967) reviewed the literature 

 pertaining to predation by freshwater copepods 

 on fish larvae and described the predatory be- 

 havior of cyclopoid copepods as he observed them 



655 



