190 



Fishery Bulletin 98(1) 



o 1990 

 A 1993 



600in 



1200m 

 1500m 



edge of their feeding habits, 

 would improve our under- 

 standing of interactions be- 

 tween sharks and commer- 

 cially important teleosts. 



A variety of species of 

 shark inhabit the deep wa- 

 ter off New Zealand, where 

 they form part of the by- 

 catch of deep-sea fisheries 

 that target teleosts such as 

 orange roughy (Hoploste- 

 thus atlanticus ) and smooth 

 oreo (Pseudocyttus macula- 

 tus) (Clark and Tracey, 

 1994; Clark and KingM. Ac- 

 cess to this bycatch pro- 

 vided an opportunity to ex- 

 amine a multispecies com- 

 plex of sharks, which might 

 be termed an assemblage — 

 "a group of co-occurring pop- 

 ulations — not necessarily interacting" as defined by 

 Crowder (1990). The purpose of this study was to 

 investigate the abundance and distribution of sharks 

 on Chatham Rise to increase understanding of the 

 effects of fishing on shark populations, the poten- 

 tial of shark fisheries and utilization of bycatch, 

 and interactions between sharks and commercially 

 important teleosts. 



Materials and methods 



Data for this study were collected from deep-water 

 bottom trawls during two cruises conducted by the 

 Ministry of Agriculture and Fisheries on Chatham 

 Rise, New Zealand (Fig. 1). The first survey (15 June 

 to 5 August 1990) consisted of 281 trawls for orange 

 roughy iH. atlanticus) and was conducted primarily 

 on the north of Chatham Rise from the FV Cordelia. 

 The second survey (24 October to 9 November 1993) 

 consisted of 109 trawls for smooth oreo (P. macula- 

 tus), primarily on the south of Chatham Rise from 

 the RV Tangaroa. Fishing during both surveys was 

 conducted at depths of 740-1503 m throughout the 

 day and night (Fig. 2). 



Each survey consisted of a stratified random trawl 

 design intended to provide relative biomass estimates 

 and to illustrate patterns of distribution of deep-water 

 species on Chatham Rise. Six-panel bottom otter- 

 trawls with cut-away lower wings were used in each 

 survey. The door-spread was 75 m for orange roughy 

 trawls and 119 m for smooth oreo trawls, and distance 

 between the net wdngs for both trawls was approxi- 



Figure 1 



Map of Chatham Rise, New Zealand, showing depth contour lines and locations of trawls 

 in a 1990 orange roughy survey and a 1993 smooth oreo survey. Trawls were grouped into 

 10 areas on the basis of major latitude and longitude meridians. 



mately 26 m. Codend mesh sizes were 110 mm for 

 orange roughy trawls and 100 mm for smooth oreo 

 trawls. Towing speed for both vessels was approxi- 

 mately 3.0 kn. Orange roughy trawls were roughly 

 1 h in duration, and smooth oreo trawls ranged from 

 several minutes to 45 min. For density estimates (kg 

 shark/km''^ ) it was assumed that herding by, and escape 

 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 

 species, and the total weight of each species caught 

 at each station was recorded. Latitude, longitude, 

 water temperature, minimum and maximum depth 

 of fishing, towing speed, and start and end time were 

 also recorded for each trawl. When the author was 

 present on the research vessel (at all times other 

 than from 15 June to 10 July 1990), all individuals of 

 each species of shark were weighed and measured, 

 except when large numbers of sharks were caught 

 and a lack of time prevented examination of every 

 shark. Because of size varied among species, an esti- 

 mate of the total number of individuals captured in 

 all fishing was derived by using the average weight 

 for each species. Because there were differences in 

 fishing methods (net characteristics, trawl duration) 

 and time (season, year) between the two surveys, 

 catch data from surveys were examined separately. 

 When possible, comparisons were made between 

 common areas fished during both surveys. For com- 

 parison of the composition of the shark community 

 at different locations on Chatham Rise, ten areas 

 were designated based on major latitude and longi- 

 tude meridians (Fig. 1). 



