Size-related differences in diel feeding activity, prey selection and nocturnal migration strategy for the planktonic larvae of Gobiomorphus cotidianus in Lake Rotoiti (NI), New Zealand

The diel distribution and feeding patterns of planktonic bullies Gobiomorphus cotidianus (total length 5-18 mm) were determined in Lake Rotoiti in summer 1994. By day, planktonic bullies inhabited the metalimnion and after sunset moved to the epilimnion where water temperatures were close to the preferred temperature for growth of this species. Feeding activity increased at night but more so for the larger (13-18 mm) bullies, which preyed mainly on Bosmina, than for the smaller (5-12 mm) ones, which preyed mainly on copepods. The diel vertical migration of planktonic bullies was therefore associated with both increased feeding at night as well as with a thermoregulatory benefit, although smaller fish may have obtained more of a thermoregulatory benefit than a feeding one.