EFFECT OF TEMPERATURE ON THE ESCAPE RESPONSES OF LARVAL HERRING, CLUPEA-HARENGUS

Herring (Clupea harengus L.) larvae from spring and autumn spawning stocks were reared at different constant temperatures from 5-degrees to 17-degrees-C. At equivalent developmental stages, the spring larvae were longer than the autumn larvae and the larvae reared at low temperatures were longer than those reared at high temperatures. At hatching and at the end of the yolk-sac stage, the larvae were induced, by a probe, to make C-start escape responses, which were recorded and analysed using a high-speed video recording at 400 frames s-1. The response was rapid and of short duration. The tail-beat frequency and swimming speed were measured during the burst of swimming following the C-start at different test temperatures and in larvae with different temperature histories. The tail-beat frequency was strongly temperature-dependent, rising from 19 Hz at 5-degrees-C to 37 Hz at 17-degrees-C with no effect of temperature history, season or developmental stage. The burst-swimming speed ranged at hatching from 75 to 90 mm s-1 at 5-degrees-C to 110 to 160 mm s-1 at 17-degrees-C and at yolk resorption from 90-115 mm s-1 at 5-degrees-C to 175 190 mm s-1 at 17-degrees-C. The longer, spring-spawned larvae swam faster than the shorter autumn-spawned larvae. When the swimming speeds were expressed as body lengths (L) s these differences disappeared. Larvae swam from 7-9 L s-1 at 5-degrees-C to 15-20 L s-1 at 17-degrees-C at hatching, and from 8-9 L s-1 at 5-degrees-C to 15-17 L s-1 at 17-degrees-C at yolk resorption. There was, however, a significantly faster specific swimming speed by the larvae reared at 12-degrees-C in spring 1991.