SNOUT DIMORPHISM IN WHITE STURGEON, 



ACIPENSER TRANSMONTANUS, FROM THE 



COLUMBIA RIVER AT 



HANFORD, WASHINGTON 



Several publications (Bajkov 1951; Semakula 

 1963; Vladykov and Greeley 1963; Semakula and 

 Larkin 1968; Haynes et al. 1978; Haynes and 

 Gray 1981) describe the behavior, natural his- 

 tory, and taxonomy of white sturgeon, Acipenser 

 transmontanus. Although differences in snout 

 length and shape between young and adult white 

 sturgeon are known, morphological divergence 

 in snout type of similar sized individuals has not 

 been reported. We observed and documented a 

 morphological dimorphism in the snouts of juve- 

 nile and adult white sturgeon in the Hanford 

 reach of the Columbia River. 



Materials and Methods 



Sturgeon were collected in April and June 

 1977 with 100 m trammel nets at White Bluffs 

 Pool (River Mile 371) on the Columbia River. 

 Specimens were examined in the field, and total 

 length (TL), physical characteristics, and snout 

 types were recorded. A subsample of 10 sturgeon 

 (41-92 cm TL), 5 designated "long snout" and 5 

 "short snout," was taken to the laboratory where 

 detailed snout and head measurements were 

 made and sex was determined (these fish were 

 <100 cm TL to facilitate storage). All other fish 

 were released after examination. Morphometric 

 and meristic measurements followed Hubbs and 

 Lagler (1958). A discriminate analysis (Morrison 

 1976) was used to select separating characteris- 

 tics. 



Results and Discussion 



Field observations on 99 white sturgeon 

 ranging from 35 to 205 cm TL showed two snout 

 types based on size and shape. Although both 



snout types fit the basic characteristics of white 

 sturgeon summarized in Scott and Crossman 

 (1973), the short snout specimens captured at 

 White Bluffs were more representative of that 

 description: "...snout in adults short, depressed, 

 bluntly rounded, shorter than postorbital length, 

 in young sharper, longer than postorbital length; 



"The long snout was morphologically similar 



except for a long, slender, and pointed snout. The 

 long snout characteristic was more pronounced 

 in smaller immature fish measuring 35-120 cm 

 TL, but was still evident in larger more mature 

 individuals (Figs. 1, 2). Of the sampled popula- 

 tion, 48% exhibited the long snout and 47% the 

 short snout characteristics, and 5% could not be 

 identified with either group. 



Mean total length of white sturgeon examined 

 in the laboratory was 65.50+20.63 cm for long 

 snout and 70.62+18.79 cm for short snout fish 

 (mean snout lengths were 6.77+1.65 cm and 

 6.58+0.91 cm, respectively). Of 21 morphometric 

 and meristic measurements made on the sub- 

 sample, 6 were deemed appropriate to demon- 

 strate snout differences(Table 1). Although sam- 

 ple size was small, the data suggest a correlation 

 between snout length and sex. Of the 10 sturgeon 

 examined in the laboratory, all long snout speci- 

 mens were males while 4 of 5 short snout speci- 

 mens were females. 



Snout type differences in white sturgeon have 

 not been reported in other areas of the species 

 range. The occurrence of this dimorphism at 

 Hanford may reflect isolating mechanisms, such 

 as physical barriers which block white sturgeon 

 movements. Bajkov (1951) suggested that hydro- 

 electric dams along the Columbia River isolate 

 white sturgeon populations, preclude immigra- 

 tion and lead to divergent evolution. White 

 Bluffs Pool is in the Hanford reach of the Colum- 

 bia River which is bounded upstream by Priest 

 Rapids Dam and downstream by McNary Dam. 

 The adaptive significance of this dimorphism at 

 Hanford is unknown. 



Table 1.— Measurements used to demonstrate differences between five long snout and five 



five short snout white sturgeon. 



158 



FISHERY BULLETIN: VOL. 80, NO. 1. 1982. 



