Assessment of stone crab (Lithodidae) density on the South Georgia slope using baited video cameras

M. A. Collins 0 C. Yau 0 F. Guilfoyle 0 P. Bagley 0 I. Everson 0 I. G. Priede 0 D. Agnew 0 0 M. A. Collins , C. Yau, F. Guilfoyle, P. Bagley, and I. G. Priede: Department of Zoology, University of Aberdeen , Tillydrone Avenue, Aberdeen, AB24 2TZ. I. Everson: British Antarctic Survey, High Cross, Madingley Road, Cambridge, CB3 0ET. D. Agnew: Renewable Resources Assessment Group, Imperial College, Royal School tel: During January 2000 a baited video camera system was deployed fifteen times at depths of 719-1518 m around the Subantarctic island of South Georgia. Four species of lithodid (Anomura: Lithodidae) crab (Paralomis formosa, P. spinosissima, Lithodes sp., and Neolithodes diomedeae) were attracted to the baits of which Paralomis formosa was the most abundant. Using arrival rate at baits, predictions of odour plume size, and observations of walking speed the abundance of the stone crab, Paralomis formosa, was estimated. Numbers of crabs increased rapidly following bait emplacement, with total numbers observed in the 4.9 m2 field of view exceeding 50 within 200 minutes on three occasions. Current speed was used to predict the area of the odour plume, and by integrating the area to account for scavenger speed the effective area of the odour plume was obtained. The density of crabs, estimated from the increase in crab numbers per unit area of odour plume, averaged 8313 individuals km 2 (range 1100-25 600). Density was not significantly correlated with depth, temperature, or current speed and variability was attributed to substrate form. 1054-3139/02/040370+10 $35.00/0 - Stone crabs (lithodids) are cold-water species, found in shallow water at high latitudes, but limited to deepwater in temperate and tropical areas. Lithodids are scavengers, which are commercially exploited by potfisheries and world-wide catches amounted to approximately 47 000 metric tonnes in 1998 (FAO, 1999). The principal fishery is for king crabs off Alaska, which targets red king crab, Paralithodes camtschaticus, but also takes blue king crab, Paralithodes platypus, and golden king crab, Lithodes aequispinus (Zheng and Kruse, 2000). Red king crab attain the highest value and landings peaked at 83 000 tonnes in 1980, but have since declined markedly as a consequence of poor recruitment (Zheng and Kruse, 2000), high harvest rates and increased natural mortality (Collie and Kruse, 1998). In the South Atlantic, fisheries have developed in the Beagle Channel and off Tierra del Fuego for South American king crab, Lithodes antarcticus syn. Lithodes santolla (Vinuesa et al., 1995; Wyngaard and Iorio, 1995), and false southern king crab, Paralomis granulosa, which has also been targeted around the Falklands (Hoggarth, 1993). From the early 1990s there has been interest in exploiting stone crabs in South Georgia waters, and following a workshop on crabs held by the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR) in 1993 precautionary catch limits have been set for some species. The crab fishery is currently an exploratory fishery regulated by CCAMLR and subject to an exploratory fishing plan, with a catch limit of 1600 tonnes per annum and minimum size limits of 102-mm carapace length for Paralomis formosa, and 90 mm for P. spinosissima. Although some information is available on the biological characteristics of these crabs and their fisheries (Lo pez-Abellan and Balgueras, 1994; Otto and Macintosh, 1996), absolute estimates of crab biomass have proven difficult to derive (Watters, 1997). Moreover, most information is currently limited to the shallower species, P. spinosissima, which has been the target of the current exploratory fishery (Otto and Macintosh, 1996; Watters, 1997). A potential fishery species needs to be marketable, abundant and relatively easy to capture. One of the first tasks in investigating the potential of a new fishery is to determine the size and location of the resource. The assessment of crustacean populations is notoriously difficult (Haefner, 1985) and in the deep water around South Georgia the bottom topography precludes a trawl survey and depletion methods such as the DeLury method require an active fishery. An alternate approach to estimating the abundance of scavengers is to monitor their rates of arrival at a bait placed on the sea floor and, using assumptions about the size and shape of odour plumes and estimates of walking speed (Sainte-Marie and Hargrave, 1987), density can be estimated. These methods have been applied to amphipods (Sainte-Marie and Hargrave, 1987) and hagfish (Martini et al., 1997), whilst a simpler method using first arrival time at bait has been used to estimate abundance of scavenging deep-sea fish (Priede and Merrett, 1996). During a 1997 photographic study, directed at estimating densities of the toothfish Dissostichus eleginoides around South Georgia, large numbers of lithodid crabs were attracted to bait at depths greater than 700 m (Yau et al., 2001). The present paper describes the results of a subsequent study aimed at estimating densities of crabs at depths of 7191518 m around South Georgia using baited video cameras. Experimental procedure As part of the biannual South Georgia groundfish survey in January 2000, two AUDOS (Aberdeen University Deep Ocean Submersibles) rigs were deployed from the Falklands registered fishing vessel Argos Galicia. The AUDOS are autonomous lander vehicles designed to photograph and track scavenging fish and invertebrates on the seafloor (Priede and Bagley, 2000). Two AUDOS lander vehicles were operated around South Georgia in January 2000. AUDOS-I consisted of an aluminium frame onto which was mounted a programmable digital video camera (JVC Colour Video Camera, TK-C1380 in housing with controller), current meter (Sensortec), twin acoustic releases (Mors AR and RT), a battery and two 50 watt deep-sea lights (Deep-Sea Power & Light, Inc.). AUDOS-II was a simplified version lacking the current meter and equipped with a single acoustic release (Mors RT). Buoyancy was provided by glass spheres (Benthos, Inc., each giving 24 kg positive buoyancy) attached to a 100 m mooring line. The AUDOS rigs descended by free-fall with 100 kg of ballast, which held the rigs in position on the seafloor. The ballast, with a graduated cross and baits attached, remained on the seafloor and was connected to the AUDOS vehicle by a 2-m length of wire. The cross therefore rested 3050 cm above the seafloor on top of the ballast, and the positive buoyancy of the mooring line held the AUDOS 2 m above the cross. Each deployment was baited with four squid, Illex argentinus, hung from the cross, with sardines inserted in the mantle cavity and attached to the ballast (total 800 g). Each experiment lasted 6 h and the AUDOS video cameras recorded a total of one hour of video. The camera was programmed to record 45 consecutive seconds in each 2.5 min for the first two hours; 45 s in each 5 min for the next t (...truncated)