Experimental predation by sticklebacks on larval mackerel and protection of fish larvae by Zooplankton alternative prey

Abstract

Three-spined sticklebacks Gasterosteus aculeatus L. strongly prefer larvae of Atlantic mackerel Scomber scombrus L. over smaller wild invertebrate Zooplankton in experiments where the sticklebacks were allowed to feed ad libitum and to voluntary cessation. The relative preference for fish larvae increased as their proportion declined, a “counter-switching” behavior. In spite of this increasing preference, the presence of Zooplankton as alternative prey strongly protected the fish larvae from predation. This occurred because at high plankton abundance (or low ratio of fish larvae to zooplankton) the total number of fish larvae (and total biomass) ingested by the sticklebacks during their voluntarily limited foraging periods decreased, despite lack of satiation. The mechanism for this interaction appears to be interference by the zooplankton (nonpreferred prey) in the stickleback's search for fish larvae. We interpret the stickleback's response (cessation of feeding before satiation) as equivalent to control of total ingestion by mean prey size, which would have the effect of maximizing the rate of food intake while minimizing foraging time. This result and the counter-switching are consistent with adaptations for minimizing risk from its own predators while foraging.