International Council for the Exploration of the Sea CM 1998/0:54 Deepwater Fish and Fisheries A NEW METHOD FOR ESTIMATING THE ABUNDANCE OF PATAGONIAN TOOTHFISH (DISSOSTICHUS ELEGINOIDES) C. Yau', M. A. Collins', I. Everson', and C. P. Nolan3 1. Department a/Zoology, University 0/ Aberdeen, Tillydrone Avenue, Aberdeen, AB242TZ, UK 2. British Antarctic Survey, HighCross, MadingleyRoad, Cambridge, CB3 OET, UK 3. Falkland Islands Government Fisheries Department, Stanley, Falkland Islands Abstract The Patagonian toothfish Dissostichus eleginoides has been the subject of a rapidly expanding longline fishery in the Southern Ocean, Traditional methods of estimating stock size have proved difficult to apply because the fish are found on the continental slope at depths of 1000 m. During September 1997, the Aberdeen University Deep Ocean Submersible (AUDOS) was used to estimate numerical density and size of toothfish in waters around South Georgia and the Falkland Islands. These are the first estimates that are independent of the commercial fishery. Introduction The Patagonian toothfish, Dissostichus eleginoides Smitt 1898, is a commercially important species caught with 10nglines in the Southern Ocean. It belongs to the family Nototheniidae, the so-called Antarctic cods, and has an amphiaustral distribution being found at depths of 70-2500 m around the Kerguelen Islands (Indian Ocean), South Georgia, the southern Patagonian shelf, and the coast of Chile (De Witt et al., 1990; Kock, 1992). The biology of toothfish is poorly known, but it is thought to be a relatively long-lived benthopelagic or midwater species. In the Patagonian area, spawning is believed to take place on the continental slope at about 500 m depth. The large eggs are pelagic and hatch between August and November. The juveniles probably remain pelagic for a further year until they reach 15-20 cm TL when they become demersal (Kellerman, 1990). Subadult fish «50 cm TL) are frequently caught in trawls as incidental bycatch on the Patagonian shelf, particularly in the squid fishery for Loligo gahi (pers. obs.), though the distribution of the adults is confined to deeper waters. Sexual maturity in the females is reached at a size of 90-100 cm TL (9-12 years), whereas males mature at 64- 94 cm TL (7-11 years) (De Witt et aI., 1990; Zhivov and Krivoruchko, 1990). Adults reach total lengths in excess of220 cm (FIG, 1998 unpublished data). A commerciallongline .fishery for toothfish began around South Georgia in 1989, prior to which toothfish were caught mainly as bycatch in trawls. Between 1988-1989, Russian vessels caught 4136 tonnes of toothfish, in the subsequent year 8311 tonnes were caught (CCAMLR, 1998). As a consequence, the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR) has established armual Total Allowable Catches (TACs) of around 3000 tonnes for Subarea 48.3. A licensed longline fishery has operated in the Falkland Islands Outer Conservation Zone (FOCZ) since 1994, with catches of nearly 3000 tonnes in 1994 and 1995 (FIG, 1997). The longliners use either the Spanish system or an automated system baited with sardines or squid, usually fishing at depths of 1000-2000 m depth. In order to manage the toothfish stocks effectively, an accurate estimate of abundance is essential. Concern has been expressed about the ability of the toothfish stocks to withstand high catch levels and conventional assessment methods are proving difficult to apply (Des Clers et al, 1996), therefore, alternative methods must now be considered. Priede and Merrett (1996) demonstrated that the mean first arrival time of scavenging fish could be used to obtain an estimate of abundance. This method, however, requires knowledge of both current velocity and fish swimming speed, and requires an understanding of the spatial distribution of the species. The Aberdeen University Deep Ocean Submersible (AUDOS) is an autonomous lander vehicle, it has been used for the last 10 years to investigate the biology and behaviour of deep-sea fish in the North Pacific and Atlantic Oceans (Armstrong et ale, 1991; Armstrong et ai., 1992; Collins et ai., 1998). The aims of the present study were to test the feasibility of using the AUDOS system to estimate the abundance of toothfish around South Georgia and the Falkland Islands. Fish tracking was also undertaken in the Falkland Islands in an attempt to obtain data on swimming speeds oftoothfish necessary for calculating abundance. Methods The AUDOS was deployed on eighteen occasions around South Georgia in September 1997 from the F.V. "Argos Galicia", a commercial stem trawler chartered for a scientific groundfish survey of the area. In October 1997, ten deployments were made in the FOCZ from the Falklands Fishery Protection Vessel M.V. "Cordelia". An additional long-terrndeployment, using alO kg toothfish carcass as bait, was deployed from "ArgoS Galicia" east of Stanley on the return voyage to the Falklands, and was recovered after 7 days by "Cordelia". The positions of the AUDOS deployments are shown in Figure 1 and are summarized in Tables I and 2. The Aberdeen University Deep Ocean Submersible (AUDOS) For the South Georgia work a simplified version ofthe AUDOS system (see Bagley and Priede, 1997) was used. The basic AUDOS system consisted of an aluminium (Grade HE 30) frame onto which were mounted a deep-sea camera (Ocean Instrumentation), an acoustic doppler current meter (Sensortec),· and twin acoustic releases (Mors). Buoyancy 2 was provided by glass spheres (Benthos Inc. 17") attached to a mooring line. A dhan buoy incorporating a VHF radio (Novatech), satellite beacon (SIS) and a large pink flag were attached to the end of the mooring to aid recovery. Ballast, in the form of scrap chain, was used to anchor the AUDOS and hold it in position. The ballast was attached to an aluminium (non-marine grade) graduated cross, marked off at 10 cm intervals, which provided calibration for the photographs. For fish-tracking deployments in Falklands waters, three arms fold down from the main fame to support the hydrophones used for triangulating fish position. Code-activated transponders (CATs) (see Bagley, 1992) were inserted into the bait so that fish swallowing the bait could be located every minute, and the direction and swimming speed of the fish could then be determined. The bait used consisted of squid (Illex argentinus) and either an. icefish or myctophids; these were tied onto the graduated cross positioned 2.5 m below the camera. Photographic Analysis The camera was loaded with Ektachrome 200 ASA colour reversal film and was set on a I minute time lapse. A 10 minute interval, however, was programmed for the long-term deployment. Small strips of film were developed on board ship using developing kits (Chrome 3-Bath). The remainder of the film was developed in the UK (Kenton Film Laboratories). Following processing, the film was viewed on a microfilm viewer. The field of view obtained from the photographs covered a mean area of 4.3 m'. Fauna were identified using relevant texts (Norman, 1937; Peden and Anderson, 1978; Macpherson, 1988; Gon and Heemstra, 1990). Total lengths (TL) of fish were measured when they were level with the graduated cross or when they were on the seafloor. Abundance Estimations Priede and Merrett (1996) demonstrated that the abundance of scavenging fish can be estimated from their first arrival time at baited cameras. The model assumes that the fish are evenly distributed across the ground and requires knowledge of fish swimming speed and current speed. The distance from which the first fish was attracted to the bait (r) is estimated from the current speed and the fish swimming speed: t r = arT 1 1 1 (-+-) Vw VF Where: t"" = first arrival time at bait (seconds) r = the distance of the fish from the bait (m) V w = current velocity (m s-') Vf = fish swimming velocity (m s·') The area occupied by an individual fish is assumed to be a hexagon of radius r, so that: 2 The abundance (n m-') is then: 3 1 n=-- 3 Aindiv Substituting equations 1 and 2 into equation 3: (_1 +_.1_)2 4 V Vw n= F 2 2.598ta" Hence: 0.3849(-1 + _1_)2 5 V Vw n = ----'--:---".-F 2 tarr The results will be sensItive to vanatlOns in current speed and swimming speed measurements. It is also important to make repeated deployments in order to obtain an average first time of arrival for scavenging fish in any location. Results Numbers and first arrival times of toothfish and other species from South Georgia (SG) and the Falkland Islands (FI) are given in Tables 1 and 2. Deployments ranged in depth from 269 m to 1525 m, but most were located close to the 1000 m isobath in both areas. Some problems with camera failure were encountered during the South Georgia cruise so that only 13 of these deployments were successfuL The shallowest deployment made in South Georgia was at 269 m within the entrance to Cumberland Bay and did not form part of the survey, but the extremely high suspended sediment load in the water column made interpretation of this particular film impossible. Current and temperature measurements were obtained for SGI-5. However, the current meter was damaged during SG6, which meant ensuing current data were unusable, although the current meter still logged temperatnre data every minute for the remainder of the cruise. In subsequent deployments, the relative strength of the current was noted from the telltale ribbons on the cross and the tilt of the AUDOS rig, evidenced by the position of the cross in the photographic frame. Current data obtained while the current meter was still functional recorded mean velocities of 0.089 m S·1 in the Shag Rocks area of South Georgia. Much higher current velocities were evident around the Falkland Islands.