Effect of light on juvenile walleye pollock shoaling and their interaction with predators

Abstract

Research was undertaken to examine the influence of light lntenslty on the shoaling behavior, activity and anti-predator behavior of juvenlle walleye pollock Theragra chalcogramrna. Under a 12 h light/l2 h dark photoperiod, juveniles displayed a diurnal shoaling and activity pattern, characterized by fish swimming in cohesive groups during the day, with a cessation of shoaling and decreased swlmmlng speeds at nlght. Prior studies of school~ng fishes have demonstrated distinct light thresholds below which school~ng abruptly ceases. To see if this threshold effect occurs in a predominantly shoaling species, like juvenile walleye pollock, another experiment was undertaken in which illumination was lourered by orders of magnitude, glrrlng fish 20 mln to adapt to each light intensity Juvenlle walleye pollock were not characterized by a d~stinct light threshold for shoaling; groups gradually dispersed as light levels decreased and gradually recoalesced as light levels increased. At light levels below 2.8 X 1 0 . ~ pE SS' m-" juvenile walleye pollock were so dispersed as to no longer constitute a shoal. Exposure to simulated predation risk had differing effects upon fish behavior under light and dark cond~tions Brief exposure to a mndc! p r e r l s t ~r :E !he .'ark c;i;ssd fish to ~W I I I I idsier, ior 5 or 6 min, than fish which had been similarly startled In the light. Chronic exposure to a l~ving predator produced simllar results; fish tended to swlm slower when a predator was present in the light, but faster when a predator was present in the dark. In the light, shoallng and/or school~ng provide protection against predators. But in the dark, with f ~s h unable to see one another, increased prey activlty resulting from predator disturbance may lead to accelerated dispersal of prey shoals Thus, perceived predation risk may have different effects upon the spatial d~stnbution of luvenlle walleye pollock under light and dark conditions. This has implications for surv~val, as f ~s h w h ~c h have become widely scattered durlng the darkness may take longer to reform shoals at dawn, resulting in greater predation nsk.