Summary
The behavioural repertoire of subyearling American shad (Alosa sapidissima) was studied in the laboratory to examine pathways leading to Aggregation and School formation at various group sizes (3–100 fish per 500-liter tank). Three hypotheses were tested: (1) schooling is the dominant behavioural activity of subyearling American shad, (2) development and maintenance of cohesive social behaviour is affected b group size, and (3) survival is a function of group size. Seventeen discrete behaviour patterns were observed. In relation to group size, the amount of time spent Schooling varied directly whereas Following and Aggregating varied inversely. The number of bouts of Parallel orient, Parallel swim, and Pivot also varied inversely with group size. A conceptual model was developed and tested to show how normal schooling behaviour is achieved in small or fragmented groups through intermediary modal action patterns. Sequence analysis showed behavioural diversity to diminish with group size, thereby increasing group cohesion and uniformity of response to stimuli. Probability of survival, determined from weekly mortality in experimental tanks, increased significantly with group size (0.87–0.98 for 3–100 fish). It is hypothesized that survival improves with an increase in the proportion of time spent Schooling, which may increase feeding efficiency, reduce stress, or reduce metabolic expenditures.
