Observations on the Pituitary-Gland of 3 Stocks of Atlantic Salmon During the Development of Bimodality in Length Frequency

Abstract

Pituitaries from La Have, River Philip, and Stewiacke stocks of Atlantic salmon, Salmo salar, were studied histologically during the appearance of bimodality in length frequency during autumn. La Have and Stewiacke animals were sampled again during the winter. Each stock has characteristically different growth rates; Stewiacke has a high incidence of precocious sexual maturity. Upper and lower mode fish were compared with particular emphasis on the pituitary cell types which have hitherto been implicated in smolting. Somatotropes of upper mode fish were more active in all stocks and the number of PIPAS (pars intermedia, Schiff-positive) cells was higher than in the respective lower mode fish. Mature individuals were present throughout both modal groups of Stewiacke fish. Greater stimulation of somatotropes, particularly, and an advanced PIPAS cell proliferation may have positioned upper mode fish to become earlier smolts.