Off-season spawning of sunshine bass ( Morone chrysops× M. saxatilis) exposed to 6- or 9-month phase-shifted photothermal cycles

Abstract

Annually-spawning sunshine bass ( Morone chrysops× M. saxatilis) were exposed to 6-, 9- or 12-month photothermal regimes. Abbreviated (phase-shifted) cycles were produced by excising photothermal conditions corresponding to May 17 to August 17 (9-month cycle), or May 17 to November 17 (6-month cycle), from a simulated annual cycle. Phase-shifted cycles reduced the interval between spawning and ovarian recrudescence, advancing gonadal maturation 2 to 3 months, and permitting off-season (December and January) spawning. Rapid gonadal growth began in June (6-month), July (9-month) and August (12-month cycle), coincident with decreasing artificial photoperiods and water temperatures. The offset in initiation of maturation persisted through final maturation, as females in the 6-, 9- or 12-month cycles matured after different periods of photothermal exposure (206, 240 and 268 days, respectively). Maximum oocyte diameters (855, 893 and 844 μm) were similar among females on all cycles. All females underwent final oocyte maturation (FOM) and ovulated after human chorionic gonadotropin (hCG) injection. Testicular growth among males on the three cycles began simultaneously (June). Maximum testicular diameters (11.2, 11.9 and 11.5 mm, respectively) were achieved after 153, 174 and 199 days in the 6, 9 and 12-month cycles, respectively. Milt was produced by >90% of the males in each treatment, and the duration of production was proportional to the cycle length, lasting 38, 42 and 91 days in the 6, 9 and 12-month cycles, respectively. Sperm concentration, duration of motility, and seminal fluid pH differed among males on the three cycles, but these differences produced no changes in fertilities, which averaged 69, 59, and 68% for fish on the 6-, 9- or 12-month cycles, respectively. All sunshine bass held on a second, sequential 6-month cycle developed mature gonads and were induced to spawn. However, peak oocyte and ovarian diameters were reduced and fertility was low (29%), suggesting additional time was needed for gonadal maturation. The good reproductive performance of fish exposed to 6- and 9-month phase-shifted periods indicates that shortened cycles are effective for inducing off-season maturation of sunshine bass, and suggests merit in evaluating the efficacy of phase-shifted cycles to advance maturation of higher-value striped bass broodstock.