Seasonal cycles in the sonic muscles of the weakfish, Cynoscion regalis

College of Marine Studies, University of Delaware 700 Pilottown Road, Lewes, Delaware 19958 Present address: UT-Houston Medical School. Laboratory for Neuroendocrinology, P.O. Box 20708, Houston, Texas 77225 Abstract.-Male weakfish, Cynoscion regalis, were collected from the southwest portion ofDelaware Bay from April through September in 1990 and 1991. Morphometric measurements ofthe sonic muscles, testis size (gonadosomatic index, or GSIl, and plasma androgen concentrations were recorded to obtain data on the seasonality of sonic muscle condition and its relationship with the timing of reproduction in this population. The sonic muscles were bilaterally symmetrical and showed no significant seasonal differences in length or width across both collecting periods. Sonic muscle thickness did change significantly across both collecting periods and there was a threefold increase in sonic muscle mass during the course of each collecting period. GSI and levels of both plasma testosterone and 11ketotestosterone also varied significantly across both collecting seasons. Changes in sonic muscle mass followed but lagged one to three weeks behind the rise and fall in plasma androgen levels. Pertinent models of skeletal muscle hypertrophy and atrophy are discussed as is the possibility that increased sonic muscle mass during the spawning season may increase the reproductive fitness of male weakfish.