Hormone profiles of captive striped bass Morone saxatilis during spermiation, and long-term enhancement of milt production

Plasma profiles of reproductive and thyroid hormones were studied in captiv e striped bass Morone saxatilis during an Il-wk period encompassing the spa wning season, and the effect of a sustained-release gonadotropin-releasing hormone agonist (GnRHa)-delivery system (GnRHa-implant) on milt production was evaluated. The highest percentage of spermiating fish was observed betw een mid-April and mid-May, and mean total expressible milt ranged from 3.5 to 6.0 mL/kg, Plasma gonadotropin II (GtH II) increased significantly, thou gh inconsistently, during the spermiation period, whereas testosterone and 11-ketotestosterone levels declined continually. Plasma 17,20 beta-dihydrox y-4-pregnen-3-one and 17,20 beta,21-dihydroxy-4-pregnen-3-one remained low and unchanged during the peak of the spermiation period, while thyroid horm ones were high and fluctuated without exhibiting a trend consistent with sp ermiation, The observed endocrine profiles suggest that captivity can dimin ish plasma GtH II and triiodothyronine levels in striped bass; Transfer of spermiating males from large holding tanks to small spawning tanks reduced total expressible milt after 14 d, but treatment with a GnRHa-implant resto red milt volume, presumably due to the prolonged elevation of plasma GnRHa and GtH II induced by the GnRHa-implant. Also, treatment with the GnRHa-imp lant induced a two- to four-fold elevation of expressible milt for at least 20 d compared to control fish, while resulting in only a 5 to 15% decrease in sperm density. It appears that captivity and hatchery operations can di minish milt production in striped bass, and that GnRHa-delivery systems, vi a sustained elevation of plasma GtH II, can induce long-term enhancement in milt volume without affecting sperm density greatly.