Ontogenic Change and Stock Differences in Schooling Behavior of Juvenile Masu Salmon Oncorhynchus masou, before and after Smoltification

Abstract

Ontogenic change and stock differences in schooling behavior, smoltification and growth among two wild and two hatchery stocks of the masu salmon Oncorhynchus masou, each 100 juveniles, was observed in 1, 000l circular tanks for one year and nine months.
Hatchery stocks grew faster than wild stocks. Onset of smoltification occurred in age O⁺ fish of one hatchery stock four months earlier than in the other three stocks. All stocks showed schooling behavior or aggregation during the daytime, while at night, they dispersed throughout the tank and stayed relatively still. Increased schooling behavior was observed in the two wild stocks just before and during smoltification and the swimming layer became shallower, whereas such tendencies were not observed in the two hatchery stocks. Inter-individual distance increased in the two hatchery stocks in direct proportion to body size, whereas the two wild stocks showed a constantly lower distance value. The smoltification period was also characterized by the occurrence of anti-clockwise schooling in every stock.
These behavioral changes occurring with the onset of smoltification in wild stocks seem to be adaptive preparations for seaward migration and marine life. Such distinct differences among stocks may be derived from the effects of domestication and/or the genetic characteristics of each parental strain.