Size matters: predation of fish eggs and larvae by native and invasive amphipods.

Invasive predators can have dramatic impacts on invaded communities. Extreme declines in macroinvertebrate populations often follow killer shrimp () invasions. There are concerns over similar impacts on fish through predation of eggs and larvae, but these remain poorly quantified. We compare the predatory impact of invasive and native amphipods ( and ) on fish eggs and larvae (ghost carp and brown trout ) in the laboratory. We use size-matched amphipods, as well as larger reflecting natural sizes. We quantify functional responses, and electivity amongst eggs or larvae and alternative food items (invertebrate, plant and decaying leaf). , especially large individuals, were more likely than to kill trout larvae. However, the magnitude of predation was low (seldom more than one larva killed over 48 h). Trout eggs were very rarely killed. In contrast, carp eggs and larvae were readily killed and consumed by all amphipod groups. Large had maximum feeding rates 1.6-2.0 times higher than the smaller amphipods, whose functional responses did not differ. In electivity experiments with carp eggs, large consumed the most eggs and the most food in total. However, in experiments with larvae, consumption did not differ between amphipod groups. Overall, our data suggest will have a greater predatory impact on fish populations than , primarily due to its larger size. Higher invader abundance could amplify this difference. The additional predatory pressure could reduce recruitment into fish populations.