Reproductive Conditions of the Klamath River Green Sturgeon

Reproductive characteristics of the adult Klamath River green sturgeon Acipenser medirostris were studied during the spawning migration. The locations of captures were from the mouth of the Klamath River upstream to river kilometer 72. A total of 82 females and 118 males were sampled for age, sex, body size, gonad weight, fecundity, egg size, and gonadal histology during April–July for five consecutive years (1999–2003). All fish sampled were considered mature adults, except for two immature males (body weight, 10 and 16 kg) captured close to the mouth of the river. The average body weight for males and females was 32 and 46 kg, respectively. The condition factor ranged from 0.60 to 0.92 for males and from 0.65 to 0.94 for females. The long tapered body conformation for both sexes made it difficult to sex individuals by external morphology, but in general, the females had a slightly more robust conformation. The fork length range was 139–199 cm in males and 151–223 cm in females. The majority (.90%) of males were 15–28 years old, and females were 19–34 years old. In all females the preovulatory condition was distinguished by the migration of the germinal vesicle to the animal pole, and the mean polarization index (distance of the germinal vesicle from the animal pole divided by oocyte diameter) was 0.042. The gonadosomatic index for females ranged from 7% to 17% and that for males from 2% to 8%. Individual and relative fecundity ranges were 59,000–242,000 and 2,000–4,000 eggs, respectively. The fully grown eggs were the largest recorded for North American sturgeon, averaging 4.33 mm in diameter. Although this study indicates that the Klamath River supports an important and potentially stable spawning migration, continued monitoring of the population and identification of spawning and nursery sites are critical for the long-term preservation of this species.