Tank spawning of first generation domestic green sturgeon

Green sturgeon Acipenser medirostris from hatchery spawned wild‐caught broodfish matured under aquaculture conditions. The first females (three out of four) matured at age 8 years, were successfully induced to ovulate by hormonal injections, and their eggs collected by caesarean surgery for fertilization. While the incision healed completely, unknown complications resulted in mortalities 5–12 months later. To minimize handling stress and to eliminate mortalities associated with surgery, a semi‐natural tank spawning technique was used in subsequent years. A female was placed into a 3.7 m diameter flat‐bottom circular fiberglass tank, depth 0.7 m, with two or three males, and the fish were injected with gonadotropin‐releasing hormone analog (GnRHa) and white sturgeon pituitary extracts to induce spawning. Fuller’s Earth was continually added to the spawning tank using a belt feeder to minimize egg adhesion to other eggs and to the tank surfaces. The tank was checked hourly for oviposition, and the eggs collected using a fine mesh dip net, every 1–3 h. Spawning lasted 15–21 h, and 53–80 000 eggs were collected from individual spawning females. Direct observation of spawning activity was not possible because of the high turbidity caused by Fuller’s Earth. However, the female and males occasionally breached at the water surface and appeared to be interacting. After oviposition was completed, viable milt was collected from all males, and fertilization success ranged from 7% to 83% in five spawning events. Over three consecutive years (2009–2011) all five females and 11 males used in semi‐natural tank spawning ovulated or spermiated, and were not subject to post‐spawning mortality. The relatively slow deposition of eggs and the continuous addition of Fuller’s Earth minimized the adhesion of eggs into large clumps. While the egg fertility and hatchability varied greatly among individual females, the tank spawning technique presented a feasible alternative to other more invasive methods for obtaining fertilized eggs of green sturgeon, and may be applicable for other sturgeons, particularly endangered species.