Spacing and Jumping Behaviour of the Ayu Plecoglossus altivelis

Abstract

Interstock differences in spacing pattern and jumping activity were compared for 5 groups (stocks) of juvenile ayu: Landlocked lake stock (Wild-L), amphidromous river stock (Wild-R), and reared fish from 3 hatcheries (Reared-A, -B, -C). Reared-A and -B formed a tight aggregation (ϕ 15–30 cm; diameter) in the daytime, while Wild-L dispersed throughout an experimental tank (ϕ 108 cm), and Wild-R exhibited behaviour intermediate between Wild-L and hatchery fishes, forming a loose aggregation and/or schooling in a circle (ϕ 70–80 cm). At night, however, there was no difference between the 4 groups; i.e. all fish moved randomly and were inactive. If assuming that a spacing pattern was maintained by an interindividual distance (Optimum Distance to the Nearest Neighbour: ODNN) peculiar to a fish group, Wild-L appeared to have a large ODNN, while Reared-A and -B, a small ODNN, and Wild-R, intermediate value. Jumping activity was highest in Wild-L (77.9: Percentage of the first-time jumpers), followed by Wild-R (60.4), Reared-B (56.3), Reared-C (56.0) and Reared-A (33.6). Diel variations in jumping behaviour of the 5 fish groups were synchronized with each another, showing remarkable peaks from 14:00 till 20:00 hours, corresponding to a rise in temperature of river water. In conclusion, spacing and jumping behaviour were closely related to each other: I.e. fish with the larger ODNN in spacing presented the more active jumping behaviour. © 1990, The Japanese Society of Fisheries Science. All rights reserved.