Estimating the abundance of Patagonian toothfish Dissostichus eleginoides using baited cameras: a preliminary study

Abstract

The Patagonian toothfish Dissostichus eleginoides has been the object of a rapidly expanding longline fishery in the Southern Ocean. Little is known about the biology of D. eleginoides and traditional methods of estimating stock size using trawling techniques have proved ineffective because the adult fish are found in deep waters on the continental slope at depths of 700–2500 m. During September 1997, a preliminary study was undertaken using arrival times at an autonomous baited camera vehicle, the Aberdeen University Deep Ocean Submersible (AUDOS), to estimate the abundance and size of toothfish in waters around South Georgia (SG) and the Falkland Islands (FI). These are the first attempts at estimating the abundance of toothfish that are independent of catch data from the commercial fishery.

Introduction

The Patagonian toothfish, Dissostichus eleginoides Smitt, 1898, is a commercially important species caught with longlines in the Southern Ocean. It belongs to the family Nototheniidae, the so-called Antarctic cods. It is distributed around the southern coast of Chile, the Patagonian shelf, and the sub-Antarctic islands of Kerguelen, South Georgia, and Macquarie at depths of 70–2500 m (DeWitt et al., 1990, Kock et al., 1985, 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 of the large pelagic eggs (diameters of 4.3–4.7 mm) is believed to take place on the continental slope at about 500 m depth, with hatching occurring between August and November (Kock, 1993). The juveniles probably remain pelagic for a year until they reach 15–20 cm TL when they become demersal (Kellermann, 1990). Sub-adult fish (<50 cm TL) are frequently caught in trawls as an incidental bycatch on the Patagonian shelf (Des Clers et al., 1996), particularly in the squid fishery for Loligo gahi (pers. obs.), though the adults are confined to the deeper waters of the continental slope. 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) (DeWitt et al., 1990, Zhivov and Krivoruchko, 1990). Adults may attain total lengths in excess of 220 cm (Falkland Islands Government (FIG), 1998, unpublished data).

A commercial longline fishery for toothfish began around South Georgia in 1989, prior to which toothfish were caught mainly as a bycatch in trawls. Between 1988 and 1989, Soviet vessels reported catches of 4136 t of toothfish, and 8311 t in the subsequent year (CCAMLR, 1998). As a consequence of these high catches, the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR) established annual total allowable catches of around 3000 t for South Georgia (CCAMLR Subarea 48.3). A more recent licensed longline fishery has operated in the Falkland Islands Outer Conservation Zone (FOCZ) since 1994, with catches of nearly 3000 t in 1994 and 1995 (Falkland Islands Government, 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.

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. Previous attempts at modelling the population dynamics of toothfish have been based on fishery catch and effort data. However, the very high variability in the catch per unit effort between vessels targeting toothfish has meant that conventional assessment methods using depletion models are proving difficult to apply (see Des Clers et al., 1996). Traditional methods of estimating stock size using trawl swept area techniques are fraught with difficulties owing to rough topography and the depths at which toothfish are found. Therefore, alternative methods of assessment must now be considered.

Priede and Merrett (1996) demonstrated that the mean first arrival time of scavenging fish to bait could be used to obtain an estimate of abundance. This method, however, requires knowledge of both current velocity and fish swimming speed, as well as an understanding of the spatial distribution of the species. The Aberdeen University Deep Ocean Submersible (AUDOS) is an autonomous lander that has been used to investigate the biology and behaviour of deep-sea fish in the North Pacific and Atlantic Oceans (Armstrong et al., 1991, Armstrong et al., 1992, Collins et al., 1998). The aim of the present study was to test the feasibility of using the AUDOS to estimate the abundance of toothfish around South Georgia and the Falkland Islands independent of fishery data. Fish tracking was also undertaken in the Falkland Islands in an attempt to determine swimming speeds of toothfish necessary for calculating abundance.