Behavioural Characteristics of the Ayu, Plecoglossus altivelis, as Predictive Indices for Stocking Effectiveness in Rivers

Abstract

A laboratory index to predict the stocking effctiveness of ayu, Plecoglossus altivelis, in a river was determined by comparing the results of a stocking experiment in a field and laboratory tests on (1) swimming ability, (2) optokinetic response, (3) & (4) tendency to swim upstream in artificial river-1 & -2, and (5) jumping activity to a waterfall. Fish groups used here were landlocked stock (LAKE), amphidromous stock (RIVER), and reared fish from three different hatcheries (HR-A, HR-B, HR-C).
Stocking effectiveness represented by recapture rates through fishing season was positively re-lated with the tendency to migrate upstream after release. The tendency to migrate upstream in a river showed a close reiationship with jumping activity and tendency to swim upstream in artificial river-2, white it correlated negatively with swimming ability: i.e. Fish groups such as LAKE and HR-C with active jumping behaviour had a strong tendency to swim upstream in both artificial and natural rivers, resulting in a high recapture rate, whereas inactive fish in jumping behaviour such as HR-A showed a tendency to go downstream and thus, low recapture. It was concluded that both these two laboratory indices had the potential to predict stocking effectiveness of the ayu before release, however, the more appropriate index was the jumping test since the artifical river of large scale required much effort.