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. Annual scampi catch, prawn catch, and effort levels for four fishing seasons. on the continental shelves of the Northeast Atlantic and in the Mediterranean Sea (Rice and Chapman, 1971; Farmer, 1975; Bailey et al., 1986). Carter et al. (1983) noted that M. andamanicus \=M . velutinus] and M. boschmai have reduced carapace spination and were Table 1. Location, depth, area, and dominant species for fish ing grounds I and II. more frequently caught with mud adhering to the exo skeleton than M. australiensis. They suggested that the Ground I Ground II latter species prefers comparatively firmer substrate, in which they build less extensive burrows and may spend Location 16°10'-17°10'S 17°40°-18°40'S considerable periods of time outside their burrows. M. Depth 420-480m 360-420m Approximate area 222 km2 222 km2 velutinus and M. boschmai, on the other hand, make Dominant species M. australiensis M. velutinus deeper burrows in softer sediment. Commercial catch records from sequential trawls in areas of preferred substrate during a 24-h period revealed reasonably con Catch rates stant catch rates (coefficients of variation ranged be tween 12 and 20%), suggesting that metanephropids Catch rate trends were examined for two species of tend toward an even distribution on the bottom over Metanephrops, each at a different small productive trawl spatial scales from 0.3 to 1.7 km2. ground (Table 1; Fig. 1). Thec.p.u.e. for fishing grounds Australian metanephropids occur from about 260 m I and II from the start of commercial fishing until April to about 500 m on the continental slope, each species 1989 are shown in Figure 3. These data are superim having a clearly defined depth distribution (Fig. 4). posed upon the monthly trawl effort. Fishing effort on Other species of Metanephrops are also reported to be both grounds was sporadic, characterized by pulses of found in continental slope waters greater than 200 m effort when vessels enter the fishery during closures of deep (Roe, 1966; Berry, 1969; Wear, 1976; Hiramoto, other managed fisheries. At both grounds, initial 1987). However, M. thomsoni is caught in waters of the c.p.u.e. declined rapidly in response to relatively low Indo-west Pacific and East China Sea at depths from levels of effort as surplus standing stocks were removed. 120 m to 200 m (Chan and Yu, 1988). This contrasts with After the rapid initial depletion, the c.p.u.e. for ground the wider depth distribution described for Nephrops II (Fig. 3, bottom) decreased further, although short norvegicus of 15 m to at least 800 m (Farmer, 1975), with lived rises in the catch rate were apparent following the majority of commercial landings coming from conti periods of little or low fishing activity. The pattern for nental shelf waters in the Northeast Atlantic (Howard, ground I (Fig. 3, top) was similar, except that effort 1982; FAO Yearbook, 1987). levels between February 1987 and May 1988 were much lower than at ground II. This preceded a very large Population structure increase in the c.p.u.e. during June and July 1988, when the mean catch rate returned to a level of 70% of that Size-frequency distributions of different metanephropid obtained from fishing the virgin stock. This phenom species were characteristically polymodal. An example, enon could be a result of real abundance increases due to for M. australiensis, is presented in Figure 5. It is recruitment to the fishable stock or to greater catch- assumed this polymodality was the result of annual ability as animals emerged from burrows for longer recruitments forming size cohorts. The figure also indi periods. This may indicate that fished metanephropid cates the approximate 50% selection length for Neph populations can recover and that the amount of recovery rops norvegicus for a 70-mm codend trawl (Briggs, CSmr e.Sm. 199(1995) Symp.. Sei. mar. ICES australiensis Figure 3. (Top) Monthly fishing effort for trawl ground I (see Table 1) and corresponding catch per unit effort for for effort unit per catch for corresponding effort 1)and Table (see I ground trawl for effort fishing Monthly (Top) 3. Figure effort (trawl h) effort (trawl h) 1000 1200 0 0 2 1 1000 800 600 800 200 400 . velutinus M.
from this ground; (bottom) monthly fishing effort for trawl ground II (see Table 1) and corresponding catch per unit unit per catch corresponding 1)and Table (see II ground trawl for effort fishing monthly (bottom) ground; this from - - - a 18 Jn 95 a 18 Jn 97 a 18 Jn 1989 Jan 1988 Jan 1987 Jan 1986 Jan 1985 Jan 1984 Jan a 18 Jn 95 a 18 Jn 97 a 18 Jn 1989 Jan 1988 Jan 1987 Jan 1986 Jan 1985 Jan 1984 Jan 1 1 1
from this ground. Interpolated catch rates for periods of no fishing are shown by dotted lines. dotted by shown are fishing no of periods for rates catch Interpolated ground. this from 1 1 ...... T"cn"| i 11 i'l'rir H II I II ii'iVrr ie (months) time ie (months) time i i ii rifn ii t 11 i1 m r ii n rw effort trawl ac rate catch rw effort trawl ac rate catch Development of a trawl trawl a fishery of Development rtrr| i+ n rnrr| frrt i ' tti t it t
' " m r r i 1-60 Metanephrops 40 383 catch rate (kg/h) catch rate (kg/h)
384 Figure 5. Carapace length-frequency distribution for for distribution length-frequency norvegicus Carapace 5. Figure
abundance index (kg / h) Phillips F.B. and Wallner G. B. using 70 mm codend trawl mesh, (b) The size at which 50% of the females sampled were ovigerous. were sampled females the of 50% which at size The (b) mesh, trawl codend 70 mm using 40' 0 2 30 10 ■fc (1) O" (1) c 3 o >. - o o o o o o o o 00 o ? c J C 0 2 0 4 0 6 80 C\J C\J i— CD 20 - - - 00 o - T Figure 4. Depth distribution of Australian Australian of distribution Depth 4. Figure 25 J y— Oc? c CO o c OC OC CO CO CO CO CO o V DC V CD CVJ o CO CD CVJ 035 30
J O —T- T T— aaaelnt ls (mm) class length carapace Metanephrops australiensis. Metanephrops 1 et itra (m) interval depth 045 40 O■ r ■< CO - T CD 00 ■r- 055 50 Metanephrops V CD CVJ o T— (b) I (a) The 50% mesh selection size for for size selection mesh 50% The (a) - T 1 / / / / / 06 70 65 60 spp. O o o T— / / / / / / / females (n=1070) females males ■ □ □ ü 00 Nephrops ICES mar. Sei. Symp., 199 (1995) Development of a trawl fishery 385 1986). The data were collected from a sample retained in direct information was collected on the timing of moult trawl nets using 51-mm codends, thus the 50% selection ing or frequency as recently moulted individuals length in this case could be smaller than indicated. For occurred very infrequently in trawl samples and reliable M. australiensis it appears that size classes less than macroscopic staging of instars was not possible. How about 47 mm carapace length but well above the mesh ever, Berry (1969) found an annual moult period in selection length are not well represented in samples. reproductively mature M. andamanicus that was coordi This feature is also exhibited in samples of other Austra nated with the reproductive cycle in the females. lian metanephropids and was also noted for M. andama- Growth rates were estimated for M. australiensis, M. nicus in South African waters (sensu Berry, 1969). velutinus, and M. sibogae by dissecting sample length- Due to the common occurrence of the skewed length- frequency distributions into 6, 5, and 4 component frequency distributions it is unlikely that poor recruit cohorts, respectively (Table 2). Differential rates of ment is responsible; rather, it is probably attributable to growth between sexes have been described for N. norve reduced catchability of young animals, possibly due to gicus. Female growth rates are retarded because of the greater proportions of time spent in burrows. Juvenile increased metabolic requirements of reproductive matur N. norvegicus are found in the same habitat as adults but ation and reduced feeding outside their burrows when mostly remain in burrows for at least their first year, incubating eggs (Farmer, 1975; Hillis, 1979; Anon., emerging more frequently once reaching 10-15 mm 1984). This does not appear to occur in Australian carapace length and are then found with increasing fre species of Metanephrops, as there were only slight differ quency in trawl and photographic samples (Chapman, ences in estimated growth rates between sexes, with 1979,1980). Therefore, small metanephropids although females growing slightly faster in two of the three occurring on the fishing grounds, may not recruit fully to species. There were small differences in growth rates the fishery until about 3+ years (M. boschmai) and 4+ between species but the L„ varied more. M. australiensis years (M. velutinus and M. australiensis) (Wallner et al., is the largest species and M. sibogae the smallest, based 1989). Berry (1969) noted that the size at recruitment for on maximum size recorded from catch samples. M. M. andamanicus was coincident with the onset of repro velutinus exhibited similar rates of growth to M. sibogae, ductive maturity. This was also evident for Australian but has an intermediate maximum size. These estimates metanephropids, although for M. australiensis the year of growth are based on samples from a single location class before the length at which 50% of the female and substantial variation may occur between popu population was ovigerous was well represented in lations due to extrinsic factors such as temperature, samples (Fig. 5). habitat, and population density, as has been shown for N. norvegicus (Farmer, 1975; Bailey, 1986; Bailey et al., Sex ratio 1986; Chapman and Bailey, 1987). Only a single method has been used to estimate Sex ratios of Metanephrops australiensis and M. bosch growth rates for Australian metanephropids. Tagging mai in the catch usually did not depart significantly from and captive laboratory studies were not possible as 1:1. However, samples of M. velutinus frequently had trawl-caught animals were dead or moribund upon significantly greater numbers of females than males, removal from the nets. Another problem was that the particularly during periods when proportions of females sporadic nature of commercial fishing on different carrying berry were high (Fig. 6). The most significant grounds prevented regular data collection, permitting a departure from even sex ratios occurred during October progression of modes to be followed through time. Un 1987, because berried females accounted for up to 72% fortunately, methods based solely on dissection of of the total M. velutinus catch. It is hypothesized that length-frequency distributions require some subjective berried female M. velutinus emerge from burrows for interpretation of the data, particularly with respect to longer periods or more frequently and consequently the number of component modal groups that are fitted. suffer a higher catchability. This emergence could be to Nicholson (1979) argues that little confidence can be oxygenate the brood or for feeding in order to build placed in estimates of length-at-age for N. norvegicus depleted energetic reserves after spawning. This using this approach, as cohorts older than 2+ years behaviour contrasts markedly with that described for could not be reliably separated. Similarly, there is no ovigerous female N. norvegicus, which display very low method for determining the age of the first modal group catchability due to limited emergence from their bur represented in these distributions, although Crossland et rows (Thomas and Figueiredo, 1965; Chapman, 1980). al. (1987) experimented with metabolic aging of M. andamanicus [=M . velutinus] using lipofuscin ratios. Growth Therefore, the nominal ages of 1+ for the first group in Growth rate in metanephropids is incremental and, as M. sibogae and 2+ for M. australiensis and M. velutinus for all crustaceans, a function of moult frequency. No (a small number of 1+ animals in these distributions 386 B. G. Wallner and B. F. Phillips ICES mar. Sei. Symp., 199 (1995) 1.4 "I 1.2 - 1.0 & tn 0 CO E ai 0 8 LL . - àr 'S 0. 6 - 0.4 J— i 1 '------1------1------|- — r----- 1------1------1------1— Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 100 - ■ 80 ■; ‘ -'I.; -o Cl> c I 6 0 “ ,. cQ.n C i 0 o 40- mm «/) Table 2. Summary of length-frequency analysis for Metanephrops australiensis, M. velutinus, and M. sibogae. Mean carapace lengths (mm) for modal groups identified are shown for males and females. The von Bertalanffy growth curve parameters, goodness-of-fit, and maximum carapace length observed in any sample are also given for each group. Values in parentheses are standard errors. M. australiensis M. velutinus M. sibogae Males Females Males Females Males Females No. in sample 2659 2501 709 798 1707 1642 1+ mode (mm) Truncated Truncated Truncated Truncated 25.4 25.1 2+ mode (mm) 33.9 34.6 33.4 35.9 35.7 36.6 3+ mode (mm) 42.9 44.3 43.9 44.6 43.7 44.9 4+ mode (mm) 50.2 51.8 51.2 50.8 49.8 51.1 5+ mode (mm) 56.0 57.5 56.3 55.4 6+ mode (mm) 60.7 61.9 59.8 58.7 7+ mode (mm) 64.5 65.4 L„ (s.e.) 81.31 (7.89) 76.82 (2.75) 67.97(1.61) 67.44(1.87) 69.91 (4.50) 67.51 (6.32) ti-to (s.e.) 2.57 (0.23) 2.29(0.19) 1.88 (0.13) 2.37 (0.18) 1.71 (0.02) 1.47(0.21) k (s.e.) 0.21 (0.04) 0.26 (0.03) 0.36 (0.03) 0.32 (0.03) 0.26 (0.04) 0.32 (0.07) df 37 33 28 27 28 31 x2 48.64 26.74 36.19 31.69 27.18 52.96 Max CL (mm) observed 74 75 67 68 58 59 Ireland, except that the seasons are reversed in the Hamasaki (1987) also calculated that 46% of eggs are northern hemisphere (Thomas, 1964; Farmer, 1974). lost during incubation in M. thomsoni. High rates of egg It is likely that different species, depths, and latitudes loss (32-75%) are a feature similarly reported for N. introduce variations from the generalized reproductive norvegicus (Figueiredo and Nunes, 1965; Chapman and cycle described for M. velutinus. For example, spawning Ballantyne, 1980; Morizur, 1981; Morizur et al., 1981; appears to occur later for M. sibogae, which occurs in the Figueiredo et al., 1983). shallowest depth zone at the extreme north of the The larvae of M. andamanicus (sensu Berry, 1969), fishery. Sampling of the population in October 1987 M. thomsoni (sensu Uchida and Dotsu, 1973), and M. revealed that the majority of females had developed challenged hatch in an advanced stage of development, ovaries, but only 0.2% carried eggs. In January 1988 a do not feed and undergo only a few moults before comparatively synchronous spawning had occurred, adopting a benthic habit as juveniles after 3-4 days with 80.2% of females carrying newly spawned eggs. (Berry, 1969; Uchida and Dotsu, 1973; Wear, 1976). Delayed spawning and consequent compression of the Wear (1976) also concluded from examinations of eggs incubation period also occurs at lower latitudes in TV. that M. australiensis had an abbreviated larval life. In norvegicus and there is evidence for biennial spawning in contrast, Nephrops norvegicus has a prezoea and three stocks at higher latitudes (Chapman, 1980). The wide free-swimming zoeal stages (Farmer, 1974). Develop spread occurrence of female Metanephrops carrying ment time until settlement is estimated at two to three both gravid ovaries and late stage eggs during winter weeks (Farmer, 1975), although this is temperature- indicates that annual spawning is usual in reproductively dependent (Thompson and Ayers, 1989). mature metanephropids. Australian metanephropids have lower fecundity than Discussion Nephrops norvegicus. Female N. norvegicus produce between 800 and 5000 eggs (Farmer, 1975), while the Australian metanephropid species have similar life number of newly spawned eggs in the largest and most histories characterized by relatively low fecundity, a fecund Australian species (M. australiensis) is between prolonged incubation period, abbreviated larval devel 300 and 1500 (Fig. 7). Berry (1969) and Matsuura and opment, and require three to five years to reach repro Hamasaki (1987) also found that M. andamanicus and ductive maturity, and recruit into the fishery. Some M. thomsoni produce fewer than 1500 eggs. The number aspects of the biology of metanephropids differ from of eggs spawned for all species varied with the size of the those for Nephrops norvegicus and these features could female. Linear regressions best described the relation have negative consequences for populations of meta ship between the number of eggs spawned (E) and the nephropids when exploited non-selectively, such as by carapace length of the animal (L) (Table 3). However, trawl fishing. the effective fecundity of Australian species is about Unlike N. norvegicus, ovigerous Metanephrops spp. 50% lower because of attrition during the long incu females do not avoid trawl nets by remaining in their bation period (Wallner et al., 1989). Matsuura and burrows with the females of one species (M. velutinus) 388 B. G. Wallner and B. F. Phillips ICES mar. Sei. Symp., 199 (1995) 1 500 c/3 1000 - O)CD 0) **—o <5 ■Q E 3 0 M. velutinus Z 5 0 0 - M. australiensis ■* M. boschmai 35 40 50 55 60 65 7045 Carapace length (mm) Figure 7. Relationships between the numbers of newly spawned eggs carried on the pleopods and carapace lengths for Meta nephrops velutinus, M. australiensis, and M. boschmai. Linear regression fits (see Table 3) are plotted. exhibiting higher catchability after spawning. This, and severely depleted trawl grounds would be in the order of the generally lower fecundity of metanephropids, could years. mean that the reproductive potential of a population In comparison with stable and long-standing fisheries could be lowered by fishing to a point where recruitment for Norway lobster, such as in Scotland and the Mediter is affected. ranean (Farmer, 1975; Bailey et al., 1986), the brief Metanephropids have a very abbreviated pelagic lar history of the Australian scampi fishery has seen overall val phase and it is therefore probable that settled larvae declining catch rates and fluctuating catches. Commer and juveniles up to several years old co-inhabit the same cial interest in the fishery, as measured by the level of grounds as adult stocks. Although these animals are not effort applied (Fig. 2), has lessened due to declining directly caught by trawling activities it is possible that profitability. Therefore, some management interven destruction of burrows, or removal of food species tion may be desirable with the aim of stock recovery and (Wassenberg and Hill, 1989), may result in a significant improving the yield from depleted areas. mortality of pre-recruit age classes. This has not been Management regulations applied to Nephrops fish evident in Nephrops populations in the northern hemi eries differ among countries. Protection of ovigerous sphere, and it is thought that high recruit mortality is females, licence limitation, vessel restrictions, and more likely to be a function of high adult density (Hill seasonal closures are all used, in part, by a number of and White, 1990). However, for Metanephrops spp., the countries. However, almost all countries impose mini low probability of recruitment due to larval drift and the mum size limits for landed Nephrops. This is usually requirement for several years’ growth before recruits accompanied by regulation of trawl net mesh sizes enter the fishery, could mean that recovery periods for (Dow, 1980), to protect undersize animals from capture Table 3. Results of regressions for Australian Metanephrops spp. according to the linear model E = a + bL where E is the number of eggs spawned and L is the length of the female carapace. Values in parentheses are standard errors. No. of a b Species observations (s.e.) (s.e.) r2 F ratio P M. velutinus 60 -1162.64 36.2 0.683 125.03 0.0001 (170.39) (3.24) M. australiensis 26 -970.75 31.11 0.648 44.14 0.0001 (264.43) (4.68) M. boschmai 35 -638.55 26.49 0.432 25.06 0.0001 (238.67) (5.29) ICES mar. Sei. Symp., 199 (1995) Development of a trawl fishery 389 and, in some mixed fisheries, to minimize mortality on disturbing the bottom or inflicting undesirable fishing juvenile fish stocks. Mesh-size regulation would appear mortality upon recovering metanephropid populations. to have no utility in the Australian scampi fishery, as This fishing method would also require research to recruit overfishing is avoided by the cryptic behaviour of determine the densities, behaviour, and patterns of small size classes and the fishery has no commercially movement of these prawns, and has yet to be attempted. significant fish stocks. Also, any increase in mesh size would reduce the catch of important penaeid and carid species. Acknowledgements Seasonal closures in fisheries are usually timed to maximize reproduction or recruitment. Female meta We thank Lisa Hobbs for her assistance in collecting nephropids are reproductively active throughout the data aboard commercial trawlers; Richard Litchfield for year, either maturing ovaries or carrying berry, and the his length-frequency analyses of M. australiensis and M. timing and patterns of recruitment are not yet known. sibogae data; Sebastian Rainer and two anonymous Thus, an appropriate seasonal closure is not yet obvious. referees for their constructive criticism of an earlier draft of the manuscript. Long-term management strategies should incorporate substantial periods of little or no fishing, because fishing grounds that were permitted periods of rest between pulses of fishing effort showed some recovery, indicated References by increases in c.p.u.e., than consistently fished areas. Alcock, A. 1894. Natural history notes from H.M. Indian As the Australian fishery consists of areas of high meta Marine Survey Steamer “Investigator” , ser. II, no. 1. On the nephropid abundance interspersed with widespread results of deep sea dredging during the season 1890-91. Annual Magazine for Natural History, 13: 225-245. areas that support only low densities, rotational closures Anony., 1983. CSIRO finds scampi and carid prawns on the appear to offer the most rational method for managing N.W. Shelf. Austr. Fish., March 1983: 13. these resources. This approach requires that sufficient Anony., 1984. Abundance and size composition of Norway grounds exist, or are found, to employ the fishing fleet lobster (Nephrops) populations. Department of Agriculture and Fisheries for Scotland, Marine Laboratory Aberdeen, on a rotational basis, or that vessels can be deployed in Triennial Review of Research, 1982-1984: 16-20. other fisheries during rest periods. Knowledge of the Bailey, N. 1986. Why does the size of Nephrops vary? Scott. rate of recovery of a fished ground is also needed to set Fish. Bull., 49: 31-36. appropriate closure periods. If the ground is continuing Bailey, N., Howard, F. G., and Chapman, C. G. 1986. Clyde to produce penaeid and carid prawn catches, then the Nephrops: biology and fisheries. Proc. Roy. Soc. Edinburgh, 90B: 501-518. value of these products foregone during the closure Berry, P. F. 1969. The biology of Nephrops andamanicus should be less than the expected return from future Wood-Mason (Decapoda, Repantia). Oceanogr. Res. Inst. metanephropid catches. Different patterns of effort (Durban), Invest. Rep., 22. 55 pp. could be evaluated for each component of the catch by Bjordal, A .A . 1986. The behaviour of Norway lobster towards modelling, as has been attempted for N. norvegicus baited creels and size selectivity of creels and trawls. FiskDir. Skr. Havunders., 18:131-137. (Brander and Bennett, 1989; Cardador and Caramelo, Brander, K. M., and Bennett, D. B. 1989. Norway lobsters in 1989). the Irish Sea: modelling one component of a multispecies Changes in fishing practices should also be considered resource. In Marine invertebrate fisheries: their assessment for this fishery. Demersal trawling tends to reduce the and management, pp. 183-204. Ed. by J. F. Caddy. Wiley- Interscience, New York. 752 pp. topographic and structural complexity of the bottom and Briggs, R. P. 1986. A general view of mesh selection for these effects may lower the productivity of this fishery. Nephrops norvegicus (L.). Fish. Res., 4: 59-73. Observations made during the period of this study indi Cardador, F., and Caramelo, A. M. 1989. Effects of manage cate some faunal changes; for example, the virtual elim ment strategies on the catch of hake, horse mackerel, blue ination of once-common large hexactinellid sponges whiting and Nephrops in ICES Divisions VIIIc and IXa. ICES CM 1989/H: 39. from heavily trawled areas. The long-term effects of this Carter, D .,Maxwell, J. G.H.,andBotwell,C. 1983. “Cautious type of change are not yet known. Creel fishing, which is optimism" over potential for scampi fishery on N.W. Shelf. used to take N. norvegicus in Scottish and Norwegian Austral. Fish., November 1983: 2-12. waters, is non-destructive and selects for larger individ Chan, T. Y., and Yu, H. P. 1988. Clawed lobsters (Crustacea: Decapoda: Nephropidae) of Taiwan. Bui. Inst. Zool. Acad. uals than trawling (Howard, 1982; Bjordal, 1986). This Sin., 27: 7-12. fishing method may offer an alternative to trawl fishing Chan, T. Y., and Yu, H. P. 1991. Studies on the Metanephrops in Australian slope waters; however, trials to date have japonicus group (Decapoda, Nephropidae), with descrip shown that Metanephrops spp. will enter baited creels tions of two new species. Crustaceana, 60:18-51. Chapman, C. J. 1979. Some observations on populations of the but catches have been disappointing (Evans, 1990). 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