FISHERY BULLETIN: VOL, 85, NO. 2 



3.0 -, 



300 



500 



600 700 



DEPTH (m) 



Figure 2.— Catch rate of three species of Heterocarpus by depth. 



in large numbers at depths between 549 and 777 m 

 (300 and 425 fathoms). A maximum catch rate of 

 2.33 kg/trap-night was obtained at 777 m. Hetero- 

 carpus longirostris was the deepest dwelhng of the 

 Heterocarpus species taken, ranging from 823 m 

 (450 fathoms) down past 1,097 m (600 fathoms), 

 which was the greatest depth targeted in this study. 

 Maximum catch rates of H. longirostris were ob- 

 tained at 1,052 m (575 fathoms), but the deeper end 

 of this species' range was not sampled and, as with 

 H. ensifer, it is uncertain that the depth of maximum 

 catch rate obtained here is indicative of depth of 

 maximum abundance. 



Heterocarpus ensifer was the smallest of the three 

 major species taken. Carapace lengths of 3,401 in- 

 dividuals ranged from 11 to 38 mm with a mean of 

 26.4 mm. Heterocarpus laevigatus was the largest 

 species taken. The mean carapace length was 38.2 

 mm and the range was 13 to 61 mm {N = 16,405). 

 The mean carapace length of H. longirostris was 

 similar to that oiH. laevigatus (i = 37.5 mm), but 

 the size range of 1,443 individuals was more re- 

 stricted (20 to 50 mm). The mean size of H. longi- 

 rostris taken in this study is probably higher than 

 the mean of catchable shrimp of this species for all 

 depths. We set traps only in the shallower end of 

 this species' depth range (<1,143 m) where the 

 population is dominated by females that grow to a 

 larger size than the males. 



It has been suggested that the size of shrimp 



varies with depth, i.e., larger shrimp occur within 

 the range of maximum abundance while smaller in- 

 dividuals were found in shallower or deeper water 

 (Clarke 1972; Wilder 1977). This type of distribu- 

 tion was not observed for any of the three major 

 species of Heterocarpus in the Marianas. Linear 

 regressions of mean carapace length with depth by 

 sex were computed for each of the three species. A 

 significant decrease in carapace length with increas- 

 ing depth was obtained for H. longirostris females, 

 although the full depth range of this species was not 

 sampled. In all other cases, no significant change 

 in size with depth was observed (Table 2). 



Wilder (1977) reported high male to female ratios 

 of 3-4 to 1 for H. ensifer and H. laevigatus taken 

 around Guam. He also stated that small individuals 

 were nearly all males and large individuals almost 

 all female. This was not true for the shrimp ex- 

 amined in this study. The overall sex ratios (ex- 



Table 2. — Results of mean carapace length by depth regressions 

 for three species of Heterocarpus by sex. 



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