ICES Marine Science Symposia

V. Models and management strategies ICES mar. Sei. Symp., 199: 379-390. 1995 Development of a trawl fishery for deepwater metanephropid lobsters off the northwest continental slope of Australia: designing a management strategy compatible with species life history B. G. Wallner and B. F. Phillips Wallner, B. G., and Phillips, B. F. 1995. Development of a trawl fishery for deepwater metanephropid lobsters off the northwest continental slope of Australia: designing a management strategy compatible with species life history. - ICES mar. Sei. Symp., 199: 379-390. A trawl fishery has developed on the northwestern continental slope of Australia since 1983 for four species of metanephropid (Metanephrops australiensis, M. velutinus, M. boschmai, and M. sibogae). Monitoring of commercial trawl catches between 1986 and 1989 has provided information about catch and effort trends, species distributions, catch composition, growth rates, and reproduction. These data are compared with those on Nephrops norvegicus. Management and harvesting strategies for the fishery are discussed, with consideration of the available fishery statistics and biological information. B. G. Wallner: CSIRO Division of Fisheries, PO Box 20, North Beach, Western Australia 6020, Australia [tel: (+61) 92468288, fax: (+61) 92468233], B. F. Phillips: Australian Fisheries Management Authority, PO Box 7051, Canberra Mail Centre, ACT, 2610, Australia. Introduction referred to as M. andamanicus. M. velutinus is now acknowledged as the valid name following work by Chan Clawed lobsters of the genus Metanephrops occur on the and Yu (1991). In 1987 a fourth species, M. sibogae, was continental slopes of many countries. Prior to 1985, discovered to the north of Australia in waters bordering Metanephrops were commercially fished only off south Indonesia. A fifth species, M. neptunus, was also oc eastern Africa (Berry, 1969), and fished experimentally casionally caught. Controlled initial development of in the western Atlantic Ocean, the Caribbean Sea (Roe, these fisheries occurred through legislation that defined 1966), and in New Zealand waters (Pike and Cooper, the fishery boundaries, limited entry, and required com 1969). pletion of catch and effort log-books to facilitate stock The presence of metanephropids, called scampi in assessment research. Australia, on the northwest slope of Australia, was first Metanephropids do not have a long history of exploi reported in 1894 (Alcock, 1894). However, it was not tation, and consequently the biology and life histories of until 1982 that promising commercial quantities were these animals are not well known. However, manage caught (Anon., 1983). Following successful fishing trials ment measures should be designed with the known, or by a commercial trawler in 1983 (Carter et al., 1983) and inferred, life history information in mind. This article the discovery of additional fishing grounds in 1984 briefly describes the Australian fishery for Metaneph (Davis and Ward, 1984), a commercial fishery based on rops spp. and reviews the biological characteristics that three species of metanephropids (M. velutinus, M. aus may influence the population dynamics of the stocks traliensis, and M. boschmai) commenced in 1985. Prior under conditions of exploitation. Comparisons with to 1991, M. velutinus caught in Australian waters were Norway lobsters (Nephrops norvegicus) are made where 380 B. G. Wallner and B. F. Phillips ICES mar. Sei. Symp., 199 (1995) possible. Management options for the fishery that would Results be compatible with the species life history are also dis cussed. Fishery production Although metanephropids occur over a wide geographic range, the best catches have been taken within clearly Methods defined areas of the northwestern continental slope of Commercial catches were sampled aboard commercial Australia (Fig. 1). Twelve stern trawlers of 23-30 m vessels at approximately two-month intervals between length towing multiple “otter” trawls fished these areas. February 1986 and December 1988. The most abundant Because of the importance of penaeid and carid prawn species were counted and weighed, then the individuals by-catch (Wallner and Phillips, 1988), codend mesh size were sexed and their carapace lengths measured (using is never larger than 75 mm. Fishing is conducted con carapace length as defined by Berry, 1969). Length- tinuously during the 24-h period over soft, muddy bot frequency measurements collected in 1986, prior to in tom in depths of 250-500 m, although catch rates at tensive commercial fishing, were used to estimate dawn and dusk may be higher (Ward and Davis, 1987). growth rates for three species by dissection into compo Seasonal patterns of fishing effort result from seasonal nent modal groups using the maximum likelihood closures in other shallow water penaeid fisheries in method described by MacDonald and Pitcher (1979). It which these vessels also participate. Generally, meta was assumed that modes represented annual cohorts and nephropids are targeted preferentially to prawns owing that lengths within each cohort were normally distri to their higher export value; however, fluctuations in buted. Sexes were treated separately and modal values market conditions, or the presence of aggregated were constrained to fit a von Bertalanffy function with schools of the penaeid Aristaeomorpha foliacea can alter out reducing the fit estimated by the chi-square statistic. target preference. Female Metanephrops spp. were assessed macro- During the initial three years or “development scopically for the presence of developed ovaries, phase”, between 1985 and 1988, the fishery displayed a spawned ova, and recent moulting. The developmental rapid expansion in catch and effort (Fig. 2). Total catch state of the brood was noted in accordance with four increased fourfold from 3501 in 1985-1986 to 14041 in recognizable stages (after Berry, 1969). Fecundity was 1987-1988. This was produced by a trebling of effort estimated for three species by removal and counting of from 10800 to 31700 trawl hours. In 1985-1986, meta all eggs adhering to the pleopods of 249 ovigerous nephropids comprised 47% of the total catch and the females sampled during 1987. Late stage eggs were less greater proportion of the fishery value. By 1987-1988, firmly adhered to the pleopods than recently spawned metanephropids, comprised only 24% of the total catch. eggs and it was thought that capture by trawling could The reduced importance of metanephropids was partly have introduced variable losses of late stage eggs. There due to the establishment of markets for penaeid and fore, only estimates based on recently spawned eggs are caridean prawns and a consequent redirection of tar reported here. Thus, estimates are of potential fecundity geted effort away from metanephropids (Wallner and rather than effective fecundity. Results are expressed as Phillips, 1988). The prawn component of the catch in linear regression between carapace length (L, in mm) creased over this period from 50% in 1985-1986 (1741) and the number of eggs (E) as E = a + bL. Statistical to 73% in 1987-1988 (10211). During 1987-1988, four precision was tested by analysis of variance of the re deepwater penaeid species (Aristaeomorpha foliacea, siduals. Haliporoides sibogae, Aristeus virilis, Plesiopenaeus Trawl log-book data provided a breakdown of all edwardsianus) were of commercial importance and fishing activity, including position, time of day, trawl accounted for 67% of the prawn catch. Of these, A. duration, depth, fishing gear used, and catch retained. foliacea was the single most important species, compris Analysis of these records provided information on the ing 49% of the prawn catch and 43% of the total fishery composition of the catch and catch rates by species, catch. Two carid species, Heterocarpus woodmasoni and time, area, and depth of operation. Catch per unit of H. sibogae, made up the balance of the commercial effort (c.p.u.e.) was standardized according to the total prawn catch. length of net towed, by scaling to a common 73.2 m (40 In 1988-1989, total crustacean catch declined to 6551 fathom) total headrope length. Comparisons of c.p.u.e. for a fishing effort of 19 700 trawl hours. The metaneph in the two fishing grounds in this study were made only ropid catch was 1881 or 29% of the total catch. No major using data obtained from vessels known to target meta new fishing grounds were discovered in this period and nephropids. depressed world market prices, particularly for prawns, may have contributed to lower effort. ICES mar. Sei. Symp., 199 (1995) Development of a trawl fishery 381 112°E 116°E 120° E 124° E 128 °E 132°E N À 12°S Darvin ground I 16°S ground II \ LEGEND INDIAN OCEAN Broom* CATCH RATE (kg/h) □ 1 to 15 20 °S □ 15 to 30 Port Hedland m 30 to 45 Ü 45 to 60 m >60 24° S WESTERN AUSTRALIA 28 °S AREA SHOVN 32 °S AUSTRALIA Perth Figure 1. Map showing distribution and relative abundance of four combined species of Australian Metanephrops off the Western Australian continental slope. Data represent the maximum catch rate recorded between 1986 and 1989 for each 0.5° square. Fishing grounds I and II mentioned in the text are also indicated. 382 B. G. Wallner and B. F. Phillips ICES mar. Sei. Symp., 199 (1995) □ prawn catch is influenced by the patterns and intensity of prior fish ing. 1800-, □ scampi catch ,-35000 1600- effort -30000 1400- Distribution -25000 1200 - It is apparent, from c.p.u.e. records, that metanephro 'r 1000- -20000 pid species are mainly caught in a linear zone adjacent to 800- -15000 the coastline (Fig. 1). Scampi habitat is closely corre 600- lated with sediment type and grain size. McLoughlin et -10000 400- al. (1988) pointed out that areas of calcareous muddy -5000 2 0 0 - sands supported the highest concentrations of Meta nephrops spp. around the Scott Reef-Rowley Shoals 1985/86 1986/87 1987/88 1988/89 area, as reported by Davis and Ward (1984). Nephrops fishing season norvegicus also makes burrows in this type of substrate Figure 2.

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