Chapman et al.: Reproductive condition of Acipenser transmontanus 



631 



ovaries (groups 2 and 3) increased and stabilized at 

 an overall average of 16% (= 32/197) in group 2 and 

 at a overall average of 25% (= 50/197) in group 3; the 

 remaining 59% consisted of group- 1 females. The 

 average proportion of ripe females (group 3) in size 

 groups less than 135.1 cm was statistically less than 

 that observed in the combined size class of 135.1 to 

 195 cm (chi-square, P<0.05). A similar result holds 

 for maturing (group-2) females (chi-square, P<0.05). 

 Together, these data indicated that proportions of 

 reproductively active females in the sturgeon popu- 

 lation may be determined by a combination of two 

 factors: 1) the recruitment of females into puberty 

 (first vitellogenic cycle) occurs over a range of body 

 sizes beginning at 95. 1 cm; and 2 ) female white stur- 

 geon require more than a one-year interval between 

 consecutive spawnings. If one conjectures a predomi- 

 nant 4-year spawning period in iteroparous females 

 (because of the percentage [25%] of ripe females), we 

 could expect to see a ratio of immature to mature to 

 ripe around 2:1:1 in size classes above 135 cm. Our 

 data reject this ratio (chi-square, P<0.005). Unfor- 

 tunately, we were not able to clearly distinguish be- 

 tween the first and second spawning cycles by histo- 

 logical examination of vitellogenic and ripe ovaries. 



Male samples, in contrast to female samples, had 

 overall low proportions of immature (premeiotic 

 phase) individuals (5% in group 1), and recruitment 

 into the meiotic phase of spermatogenesis (group 2) 

 occurred at a smaller size, beginning at 75.1 cm 

 (Table 2). Sampled fish from 105.1 to 195 cm were 

 practically all (294 of 304) in group 2 (57%) or group 

 3 (40%). If one conjectures a predominant biennial 

 spawning interval, we would expect the data to show a 

 1:1 ratio of meiotic and postmeiotic testicular stages. 

 Our data reject this ratio (chi-square, P<0.001). 



Relative fecundity (egg production) 



Egg production from artificially spawned wild white 

 sturgeon females ranged from 3,192 to 8,582 eggs/ 

 kg of body weight. The average size of ova was 3.7 

 mm ±0.02 mm. The average weight of these females 

 was 36 ±2 kg and fork length was 153 ±4 cm («=40). 

 The estimated hatchery production was 203,328 ova 

 for a female of average size; individual fish produced 

 between 63,840 and 469,432 eggs. The relationship 

 between the egg diameter and fork length was not 

 statistically significant. 



Discussion 



We found that white sturgeon do not have external 

 sexual dimorphism and, except for ripe fish on spawn- 



ing grounds, there are no apparent morphological 

 differences between the sexes. The 1:1 sex ratio (over- 

 all) of white sturgeon in San Francisco Bay is in 

 agreement with data for the Columbia River system 

 (Beamesderfer and Rien 5 ) and Eurasian acipenserid 

 species (Chugunov and Chugunova, 1964; Zubova, 

 1971; Persov, 1975). A greater proportion of males in 

 smaller size classes and a predominance of females 

 in larger size classes have also been observed in other 

 species of sturgeon (e.g. A. sinensis, Xinetal., 1991). 

 One possible explanation for this difference in sizes 

 is that growth rates diverge between the sexes dur- 

 ing the reproductive phase of life. Kohlhorst et al. 

 ( 1980) found no difference in growth rates between 

 sexes in white sturgeon of the San Francisco Bay; 

 however, their study was limited to a narrow size 

 and age range. An alternative explanation may be a 

 higher rate of mortality in males and a resultant 

 shorter life span in comparison with females. High 

 rates of mortality for males may be associated with 

 earlier puberty, more frequent breeding, and longer 

 residence on the spawning grounds. Assuming that 

 the population was sampled randomly, both differ- 

 ences in growth and mortality may have contributed 

 to the substantial divergence in modal size classes 

 between the males (105.1-115 cm FL) and females 

 (135.1-145 cm FL). Although the predominance of 

 females among larger fish may be explained by these 

 factors, the predominance of males among smaller 

 fish appears to contradict a chromosomally deter- 

 mined (gonochoristic) sex ratio and should be inves- 

 tigated further. 



Although subject to high individual variation, the 

 relative fecundity among sturgeon species appears 

 to be generally similar. For example, for the large 

 anadromous species Huso huso, Raspopov (1987) re- 

 ported a relative fecundity of 2,750 to 10,500 eggs/ 

 kg of body weight. Smaller species such as shortnose 

 sturgeon, A. brevirostrum, had an average relative 

 fecundity of 11,568 eggs/kg of body weight (Dadswell, 

 1979). Although we could not directly estimate rela- 

 tive fecundity for white sturgeon females in San 

 Francisco Bay, egg production from artificially 

 spawned females averaged 5,648 eggs/kg of body 

 weight and 203,328 eggs for an average length ( 153 

 cm) female. Relative fecundity for white sturgeon 

 females in the Columbia River was not reported; 

 however, the number of eggs produced was from 



5 Beamesderfer, R. C, and T. A. Rien. 1993. Dynamics and 

 potential of white sturgeon populations in three Columbia River 

 reservoirs. In R. C. Beamesderfer and A. A. Nigro(eds), Sta- 

 tus and habitat requirements of the white sturgeon populations 

 in the Columbia River downstream from McNary Dam, vol.1, 

 report H, p. 175-204. Final Report (Project 86-50) to 

 Bonneville Power Admin.. Portland, Oregon. 



