Wetherbee: Assemblage of deep-sea sharks on Chatham Rise, New Zealand 



197 



tiirus spp. in the catch increased at depths greater 

 than 1200 m. The depths fished in the present study 

 did not appear to reveal the minimum depth of 

 occurrence for any species of shark. Compagno et 

 al. ( 1991 ) noted the importance of determining mini- 

 mum depth of occurrence describing the depth dis- 

 tribution of a particular species. 



On Chatham Rise, density (kg/km-) of all sharks 

 combined was fairly constant up to about 1200 m but 

 dropped drastically beyond this depth. Nakaya and 

 Shirai (1992) observed a similar dramatic decrease 

 in shark density at 500 m, and Merrett and Marshall 

 (1981) at 1000-1100 m. An inverse relation between 

 shark abundance and depth has been described for a 

 number of species in other regions of New Zealand, 

 and in other parts of the world (Merrett and Mar- 

 shall, 1981; Kobayashi, 1986; King and Clark, 1987; 

 Yano and Kugai, 1993). 



Distribution 



Diversity (species of shark/trawl) was higher for the 

 orange roughy surveys than for the oreo sui-veys, but 

 there was no significant difference in mean diversity 

 between areas common to both surveys. These obser- 

 vations support the contention that differences in 

 the shark catch between surveys are related to sam- 

 pling location, rather than to temporal or method- 

 ological differences between surveys. However, there 

 were generally more trawls within each area during 

 the orange roughy survey than for the oreo survey, 

 which may have increased the total number of spe- 

 cies caught on the north of Chatham Rise. Hill ( 1973) 

 predicated that as the size of a sample is increased, 

 so almost without limit will the diversity. Diversity 

 also declined with increased depth on Chatham Rise, 

 and Crowder (1990) suggested that such a decline 

 in species diversity might be due to changes in the 

 level of competition, predation, or environmental 

 homogeneity. 



In this study, only 16 species of shark were caught, 

 although many more species of deep-sea shark have 

 been captured in New Zealand (Paulin et al., 1989). 

 The total of only 16 species caught in the present 

 study is also low in comparison to numbers (>30) 

 of species caught in deep-water surveys in other 

 parts of the world (Kobayashi, 1986; Compagno et 

 al., 1991; Nakaya and Shirai, 1992; Yano and Kugai, 

 1993). Much of the fishing on Chatham Rise was 

 conducted at depths of greater than 1000 m, which 

 may be beyond the depth limit of a number of spe- 

 cies of squalid sharks found in New Zealand waters 

 (Yano, 1985; Compagno et al., 1991). 



The index of similarity between the orange roughy 

 and oreo surveys was high, again suggesting that 



differences introduced as a result of variable fishing 

 methods or time were probably not substantial. The 

 nearly identical indices of similarity between depth 

 intervals for each survey also support this conclu- 

 sion. The high indices of similarity between areas 

 for most species, along with the fairly low indices of 

 dispersion, indicate that although their abundance 

 is variable, most species have fairly wide distribu- 

 tions on Chatham Rise. 



Sharks within the genera Etmopterus and Centro- 

 phorus are thought to segregate by species in Jap- 

 anese waters (Kobayashi, 1986; Baba et al., 1987; 

 Yano and Tanaka, 1983). In the present study there 

 was little evidence to suggest that any two species 

 displayed such segregation. Compagno et al. (1991) 

 found that Centroscymnus spp. were sympatric but 

 had very different food habits. An examination of 

 dietary overlap among sharks common on Chatham 

 Rise may reveal whether these sympatric species 

 compete for the same food resources. 



Although the sharks captured during this study 

 were incidental to commercially important fishes, 

 such as orange roughy and smooth oreo, the data 

 collected from these trawls have provided informa- 

 tion on the abundance and distribution of a number 

 of species of deep-sea shark on Chatham Rise. Dis- 

 tributional patterns of sharks vary among species, 

 and the composition of the deep-sea shark commu- 

 nity varies with depth and location. Therefore, the 

 overall impact of deep-water trawl fisheries on shark 

 populations would be expected to vary among spe- 

 cies and to depend on the particular fishery, which in 

 turn influences the location and depth where fishing 

 is concentrated. 



Acknowledgments 



I thank A. Bush, T. Clarke, K. Holland, S. Kajiura, 

 C. Lowe, C. Meyer, C. Mostello, and J. Parrish for 

 their comments on the manuscript. P. Grimes, P. 

 McMillan, and K. Mulligan were tremendously help- 

 ful with collection of specimens and access to trawl 

 data. K. Fields, J. Fenaughty, M. Clarke, A. Hart, 

 and the captains and crews of the RV Tangaroa and 

 FV Cordelia were instrumental in collection of data. 

 J. Parrish, D. Yount, and B. Flannigan made funds 

 available for travel to New Zealand. 



Literature cited 



Baba, O., T. Taniuchi, and Y. Nose. 



1987. Depth distribution and food habits of three species of 

 small squaloid sharks off Chosi. Nippon Suisan Gakkai- 

 shi 53(31:417-424. 



