Vulnerability of carp larvae to copepod predation as a function of larval age and body length

Abstract

Several marine and freshwater predatory copepods (Labidocera, Pontellopsis, Mesocycops and Acanthocyclops) exhibit a wide dietary niche, ranging from protists, rotifers, cladocerans, copepods, to insect and fish larvae. Predation-induced fish larval mortality in natural environments and hatcheries reduces larval abundance and may limit fish recruitment. However, few studies quantified the losses incurred by copepod predation. Therefore, this study quantified vulnerability, susceptibility, and loss of two crucial fish, Labeo rohita and Catla catla, to cyclopoid predation in relation to larval age and body length under laboratory conditions. In addition, we compared susceptibility of fish larvae to the copepod (Mesocyclops thermocyclopoides and Mesocyclops aspericornis) predation in the laboratory, based on probability of survival after attack in relation to age and size of the larvae. We also evaluated the modulatory role of alternate zooplankton species as common prey of fish larvae and copepods. Both cyclopoid species were able to subdue and kill both species of fish larvae. The two cyclopoid species did not differ considerably in their predation efficiency on larvae; the larvae of C. catla and L. rohita did not differ considerably in their respective vulnerability to cyclopoid predation. In both species of fish larvae, the copepod-imposed mortality was inversely related to larval age (R2≥0.97) and body length (R2≥0.93). The effect of alternate prey on copepod-imposed mortality was species-specific to prey. The presence of Paramecium caudatum, Brachionus calyciflorus and Ceriodaphnia cornuta resulted in decreased mortality of fish larvae by the copepod; however, the larvae were more susceptible to copepod predation in the presence of either Hexarthra mira or Daphnia similoides as alternate prey. The copepod-induced mortality of carp larvae was recorded, even in multispecies prey environment; however the mortality was considerably less than that in control. Overall, the maximal reduction (40–100%) in copepod-imposed larval mortality was achieved by the Rotifera B. calyciflorus, followed by the Cladocera C. cornuta and ciliate P. caudatum. The modulatory effects of the presence of alternate prey were more pronounced after fish larvae reached 4days post hatch (dph). The susceptibility of carp larvae to copepod predation differed ontogenetically with maximal susceptibility between 0 and 4dph. Threshold level for copepod-imposed mortality was estimated with bilinear model, which ranged from 5.2 to 8.4dph and 6.8 to 7.8mm body length. Our results provide insight and valuable inputs for enhancing the efficiency of nursery rearing.