The effect of early and late hatching on the escape response of walleye pollock (Theragra chalcogramma) larvae

Hatching of fish eggs fertilized at the same time occurs over a period of several days. Differences in the escape response of fish larvae during the hatching period have not hitherto been studied. In this study, the escape response of walleye pollock (Theragra chalcogramma) larvae over the hatching period was examined. Escape speed, response to multiple touches with a fine probe, response to water currents generated by a predator and predation by euphausiids (Thysanoessa inermis) and amphi- pods (Pleusirus secorrus) were measured in the laboratory. Otolith measurements of field-collected larvae support a broad hatching period for walleye pollock eggs in the sea similar to that observed in the laboratory. The escape response of walleye pollock larvae was affected by rank in the order of hatching, thus with respect to predation, hatching order may affect the survival of larvae in the sea. Early hatching larvae were smaller, less sensitive to tactile stimulation, had a slower, weaker escape response and higher laboratory rates of predation mortality than those that hatched later.