189 Abstract.–The distribution and abun- Assemblage of deep-sea sharks dance of deep-sea sharks on Chatham Rise, New Zealand, are described. on Chatham Rise, New Zealand Sharks were collected as bycatch in two deep-water trawl fisheries at a total of 390 stations, which ranged in depth Bradley M. Wetherbee from 740 to 1503 m. Sixteen species Zoology Department of shark were caught; Deania calcea, 2538 The Mall Centroscymnus crepidater, Etmopterus University of Hawaii granulosus, and Centroscymnus owstoni Honolulu, Hawaii 96822 accounted for the largest portion of Present address: Northeast Fisheries Science Center the shark catch. Species that would National Marine Fisheries Service, NOAA provide the highest yield of commer- 28 Tarzwell Dr., Narragansett, Rhode Island 02882 cially important liver lipids were not E-mail address: [email protected] abundant in trawls. All sharks com- bined formed only 4.2% of overall bio- mass captured in trawls. Depth is a major determinant of the composition of the shark assemblage; both density of sharks (kg/km2) and species diversity Sharks are common bycatch in deep Nichols, 1995). The lipid composi- were inversely proportional to depth. water fisheries around the world, tion of liver oil is quite variable Distributional patterns of the shark community varied with location on Cha- forming as much as 50% of the among and within species, and tham Rise, and species composition of catch in deep-sea trawls in areas consequently the most desirable the shark catch varied with the spe- such as New Zealand and Austra- sharks are those individuals and cies of teleost targeted in deep-water lia (Deprez et al., 1990; Clark and species that have the highest poten- trawls. King1). Most sharks captured in the tial as a source for these valuable New Zealand and Australian deep- lipids (Davenport and Deprez, 1989; water fisheries are dead by the time Bakes and Nichols, 1995). There- they are brought to the surface and fore, understanding the distribution are discarded, but some sharks are and abundance of different species retained for their liver oil. In Japan of deep-sea shark, in conjunction and Australia, several species of with knowledge of the lipid compo- deep-sea shark in the family Squal- sition of their liver oil, is important idae are targeted in fisheries and for optimal use of these resources. their liver oil is utilized. Although Some deep-sea sharks prey upon the short-term potential of fisheries commercially important teleosts directed towards deep-sea sharks (Clark et al., 1989; Clark and King1), has been investigated for a few spe- but the impacts of shark predation cies (Summers, 1987; Davenport on fish populations in terms of the and Deprez, 1989), little informa- overall economic impact on the fish- tion on even basic biology is avail- ery are unknown. Diet varies consid- able for the species captured in erably among even closely related these fisheries. Thus, the effects species of deep-sea shark (Com- that deep-water fisheries have on pagno et al., 1991; Ebert et al., shark populations that are either 1992), and the level of predation targeted directly or caught inciden- on commercially important species tally are unknown. Information on of teleost by sharks also varies abundance, distribution, commu- among species (author’s unpubl. nity structure, reproduction, and data). Information on the distri- age and growth of deep-sea sharks bution and abundance of deep-sea would improve understanding of sharks, in conjunction with knowl- these effects. Shark liver oil contains commer- 1 Clark, M. R., and K. J. King. 1989. Deep cially important lipids, such as water fish resources off the North Island, squalene and diacyl glycerol ether, New Zealand: results of a trawl survey, which are used in cosmetic, phar- May 1985 to June 1986. New Zealand Fisheries Technical Report 11, 56 p. MAF Manuscript accepted 12 July 1999. maceutical, and other industries Fisheries Research Center, P.O. Box 297, Fish. Bull. 98:189–198 (2000). (Deprez et al., 1990; Bakes and Wellington, New Zealand. 190 Fishery Bulletin 98(1) edge of their feeding habits, would improve our under- 42 S standing of interactions be- tween sharks and commer- cially important teleosts. 43 S A variety of species of shark inhabit the deep wa- ter off New Zealand, where 44 S they form part of the by- catch of deep-sea fisheries 45 S that target teleosts such as orange roughy (Hoploste- thus atlanticus) and smooth oreo (Pseudocyttus macula- tus) (Clark and Tracey, 1994; Clark and King1). Ac- cess to this bycatch pro- Figure 1 vided an opportunity to ex- Map of Chatham Rise, New Zealand, showing depth contour lines and locations of trawls amine a multispecies com- in a 1990 orange roughy survey and a 1993 smooth oreo survey. Trawls were grouped into plex of sharks, which might 10 areas on the basis of major latitude and longitude meridians. be termed an assemblage— “a group of co-occurring pop- ulations—not necessarily interacting” as defined by mately 26 m. Codend mesh sizes were 110 mm for Crowder (1990). The purpose of this study was to orange roughy trawls and 100 mm for smooth oreo investigate the abundance and distribution of sharks trawls. Towing speed for both vessels was approxi- on Chatham Rise to increase understanding of the mately 3.0 kn. Orange roughy trawls were roughly effects of fishing on shark populations, the poten- 1 h in duration, and smooth oreo trawls ranged from tial of shark fisheries and utilization of bycatch, several minutes to 45 min. For density estimates (kg and interactions between sharks and commercially shark/km2) it was assumed that herding by, and escape important teleosts. from, nets were minimal, and that trawls sampled dif- ferent species of shark with equal effort. For each trawl, the catch was sorted into bins by Materials and methods species, and the total weight of each species caught at each station was recorded. Latitude, longitude, Data for this study were collected from deep-water water temperature, minimum and maximum depth bottom trawls during two cruises conducted by the of fishing, towing speed, and start and end time were Ministry of Agriculture and Fisheries on Chatham also recorded for each trawl. When the author was Rise, New Zealand (Fig. 1). The first survey (15 June present on the research vessel (at all times other to 5 August 1990) consisted of 281 trawls for orange than from 15 June to 10 July 1990), all individuals of roughy (H. atlanticus) and was conducted primarily each species of shark were weighed and measured, on the north of Chatham Rise from the FV Cordella. except when large numbers of sharks were caught The second survey (24 October to 9 November 1993) and a lack of time prevented examination of every consisted of 109 trawls for smooth oreo (P. macula- shark. Because of size varied among species, an esti- tus), primarily on the south of Chatham Rise from mate of the total number of individuals captured in the RV Tangaroa. Fishing during both surveys was all fishing was derived by using the average weight conducted at depths of 740–1503 m throughout the for each species. Because there were differences in day and night (Fig. 2). fishing methods (net characteristics, trawl duration) Each survey consisted of a stratified random trawl and time (season, year) between the two surveys, design intended to provide relative biomass estimates catch data from surveys were examined separately. and to illustrate patterns of distribution of deep-water When possible, comparisons were made between species on Chatham Rise. Six-panel bottom otter- common areas fished during both surveys. For com- trawls with cut-away lower wings were used in each parison of the composition of the shark community survey. The door-spread was 75 m for orange roughy at different locations on Chatham Rise, ten areas trawls and 119 m for smooth oreo trawls, and distance were designated based on major latitude and longi- between the net wings for both trawls was approxi- tude meridians (Fig. 1). Wetherbee: Assemblage of deep-sea sharks on Chatham Rise, New Zealand 191 Consideration of sharks as an assemblage, which is separate from the rest of Chatham Rise commu- nity, is an artificial division. However, because the primary interest of this study was to describe the abundance and distribution of the sharks on Cha- tham Rise, several ecological indices were employed to compare different locations, depths, and species of shark. Abundance was expressed as density (kg shark/km2) and was calculated for each species within each trawl based on the total weight of sharks caught, net width, towing speed, and trawl duration. Three features of distribution were examined for sharks: diversity, similarity, and randomness. Diversity was expressed as the number of species of sharks per trawl (Stephens et al. 1984; Garcia et al. 1998). The Bray-Curtis similarity index was used for compari- sons among the ten areas on Chatham Rise, depth intervals, and between the two surveys: s s SY=1 − ∑ ij −YYik /(∑ ij + Yik ), i=1 i=1 th th where Yij = score for i species in the j sample; and th th Yik = score for the i species in the k sample (Field et al. 1982; Sedberry and Van Dolah 1984). This index ranges from 0 (no species in common) to 1 (identical species in each sample). Morisita’s index of dispersion (Id) was calculated for each species Figure 2 of shark as an indicator of the randomness of their Number of trawls in a orange roughy survey (1990) and distributions: in a smooth oreo survey (1993) on Chatham Rise, New Zealand, at various intervals of (A) depth and (B) time of day.