FISHERY BULLETIN: VOL. 84, NO. 4 



of both sexes ranging in size from 10 to 165 mm 

 carapace width (CW) were captured using two types 

 of trawl gear: a 9 m semi-baloon Gulf of Mexico 

 shrimp trawl and a 3 m beam trawl (Carey and 

 Heyamoto 1972). The stretched dimension of the 

 mesh for both trawl nets was 38 mm (1.5-in), and 

 the cod ends were lined with 12.7 mm (0.5-in) mesh. 

 In addition 47 of the smallest crabs (3-10 mm CW) 

 were found in the gut contents of sable fish, Ano- 

 plopoma fimbria, and Dover sole, Microstomias 

 pacificus, caught in these trawls. The Smithsonian 

 Institution provided another 306 juvenile tanner 

 crabs taken near the type location for C. tanneri 

 west of the Farallon Islands (lat. 37°30'N, long. 

 122°59'W) (Rathbun 1925) at 500-783 m. 



Size at Maturity 



The size at maturity for both male and female C. 

 tanneri was based on allometric measurements. 

 Allometry compares the difference in the propor- 

 tions of specific body parts with changes in absolute 

 size of a major body axis (Gould 1966). In Brachyura 

 the allometric growth of secondary sex characters 

 is well documented (Tessier 1960; Hartnoll 1969). 

 In the genus Chionoecetes it takes the form of dif- 

 ferential enlargement of the abdomen and modifica- 

 tion of pleopods in females whereas the size and 

 shape of the chelae are modified in the males 

 (Watson 1970; Brown and Powell 1972). 



Carapace width for both male and female crabs 

 was measured at its widest part (mesobranchial 

 region) exclusive of spines (Fig. 1A). The male 

 carapace width was compared with the length of the 

 chelar propodus (CPL) which is measured from the 

 joint between the carpus to the tip of the fixed finger 

 of the propodus (Fig. IB), whereas the female 

 carapace width was compared with the width of the 

 abdomen (AW) which is measured at its widest part 

 (fifth segment) (Fig. 1C). Males with worn or broken 

 chelipeds were not used. All measurements were 

 made to the nearest 0.01 mm using precision dial 

 calipers, and numbers were rounded to the first 

 decimal for plotting. Plots of the measurements of 

 CW vs. CPL and CW vs. AW were used to identify 

 size at maturity for males and females. 



Size-Frequency Histograms, 

 Growth, and Sex Ratio 



Measurements of the carapace width were taken 

 from the 1,978 crabs available. Size-frequency histo- 

 grams were constructed and seven juvenile instars 

 were identified from dominant modes. Adult C. tan- 



FlGURE 1.— Body dimensions of Chionoecetes tanneri measured for 

 size-frequency and allometric analyses. (A) Carapace width (o* and 

 9) measured at its widest part across the mesobranchial region 

 and exclusive of spines. (B) Chelar propodus length (cr) measured 

 from the joint between the carpus and the tip of the fixed finger 

 of the propodus. (C) Abdomen width (9) measured at its widest 

 part, across the fifth segment. 



neri are sexually dimorphic with respect to body size 

 (Pereyra 1972). Since we did not know at which molt 

 this size dimorphism was first evident, the data for 

 males and females was shaded differently in the size- 

 frequency histogram. The juvenile sex ratio was 



974 



