Induced maturation and spawning of striped bass, Morone saxatilis, exposed to 6-, 9-, and 12-month photothermal regimes

This thesis was partitioned into three studies. In the first study, striped bass, Morone saxatilis, (n = 16) were monitored over a 13 month period to examine the efficacy of ultrasound to determine sex and to monitor sexual maturation. Four observers reviewed all ultrasound images (n = 208) of striped bass and were able to sex male and female striped bass with 99 and 95% accuracy, respectively throughout an annual reproductive cycle. Egg diameter was highly correlated with average (r = 0.89) and maximum (r=0.92) ovarian diameter as measured by ultrasound. Spenniating males exhibited larger maximum testicular diameters (23.5 mm) than non-spermiating males (13.1 mm). Fully mature female and male striped bass exhibited maximum ovarian diameters > 30 mm and testicular diameters > 20 mm as measured by ultrasound. The results of this study indicate that ultrasound is an effective and non-invasive method that aquaculturists can use to quickly scan, sex, and assess the sexual maturation of adult male and female striped bass year-round.

In the second study, photothennal manipulation was used to induce spawning out-of season and to increase the frequency of spawning in striped bass, Morone saxatilis. Forty adult striped bass, held in five environmental chambers, were exposed to compressed 6- and 9- month cycles and a 12-month reference cycle. Maximum egg diameters increased with cycle length, averaging 845 urn, 946 urn, and 1,073 um for females held on the 6-, 9-, and 12-month cycles,respectively. Females held on the 6-month cycle had smaller ovarian diameters (19.6 mm) than those held on the 9-month (27.4 mm) and 12-month (24.5 mm) cycles. Ovary diameter was highly correlated with egg diameter (r = 0.91). Spawning success (percent of females spawning) increased with cycle length, averaging 38%, 86%, and 100% in the 6-, 9-, and 12-month cycles, respectively. However, no differences in latent times, fecundity, or fertility were detected between cycles or between tank-spawned and strip-spawned females. Males held on the 6-month cycle had smaller testicular diameters (16.2 mm) than those held on 9-month (23.6 mm) and 12-month (19.2 mm) cycles. Testicular diameter of spenniating males (15.7 mm) was significantly larger than that of non-spenniating males (6.9 rnrn) in all cycles. Mortality increased with cycle length, averaging 28%, 38%, and 88% for fish held on the 6-, 9-, and 12-month cycles, respectively. Maturation and spawning were advanced by five and three months for fish held in the 6- and 9-month cycles, respectively when compared to a natural 12-month reference cycle. Fish held on the 6-month cycle matured twice within a year. However, the relatively low number of spawning females and diminished egg, ovarian, and testicular diameter of fish held on the 6-month cycle suggest that an abbreviated cycle length « 9 months) may inhibit maturation and constrain spawning success.