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 o f 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, A C T, 2610, Australia.

Introduction

Clawed lobsters of the genus Metanephrops occur on the continental slopes of many countries. Prior to 1985, Metanephrops were commercially fished only off south­eastern Africa (Berry, 1969), and fished experimentally in the western Atlantic Ocean, the Caribbean Sea (Roe, 1966), and in New Zealand waters (Pike and Cooper, 1969).

The presence of metanephropids, called scampi in Australia, on the northwest slope of Australia, was first reported in 1894 (Alcock, 1894). However, it was not until 1982 that promising commercial quantities were caught (Anon., 1983). Following successful fishing trials by a commercial trawler in 1983 (Carter et al., 1983) and the discovery of additional fishing grounds in 1984 (Davis and Ward, 1984), a commercial fishery based on three species of metanephropids (M. velutinus, M. aus­traliensis, and M. boschmai) commenced in 1985. Prior to 1991, M. velutinus caught in Australian waters were

referred to as M. andamanicus. M. velutinus is now acknowledged as the valid name following work by Chan and Yu (1991). In 1987 a fourth species, M. sibogae, was discovered to the north of Australia in waters bordering Indonesia. A fifth species, M. neptunus, was also oc­casionally caught. Controlled initial development of these fisheries occurred through legislation that defined the fishery boundaries, limited entry, and required com­pletion of catch and effort log-books to facilitate stock assessment research.

Metanephropids do not have a long history of exploi­tation, and consequently the biology and life histories of these animals are not well known. However, manage­ment measures should be designed with the known, or inferred, life history information in mind. This article briefly describes the Australian fishery for Metaneph­rops spp. and reviews the biological characteristics that may influence the population dynamics of the stocks under conditions of exploitation. Comparisons with 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 be compatible with the species life history are also dis­cussed.

Methods

Commercial catches were sampled aboard commercial vessels at approximately two-month intervals between February 1986 and December 1988. The most abundant species were counted and weighed, then the individuals were sexed and their carapace lengths measured (using carapace length as defined by Berry, 1969). Length- frequency measurements collected in 1986, prior to in­tensive commercial fishing, were used to estimate growth rates for three species by dissection into compo­nent modal groups using the maximum likelihood method described by MacDonald and Pitcher (1979). It was assumed that modes represented annual cohorts and that lengths within each cohort were normally distri­buted. Sexes were treated separately and modal values were constrained to fit a von Bertalanffy function with­out reducing the fit estimated by the chi-square statistic.

Female Metanephrops spp. were assessed macro- scopically for the presence of developed ovaries, spawned ova, and recent moulting. The developmental state of the brood was noted in accordance with four recognizable stages (after Berry, 1969). Fecundity was estimated for three species by removal and counting of all eggs adhering to the pleopods of 249 ovigerous females sampled during 1987. Late stage eggs were less firmly adhered to the pleopods than recently spawned eggs and it was thought that capture by trawling could have introduced variable losses of late stage eggs. There­fore, only estimates based on recently spawned eggs are reported here. Thus, estimates are of potential fecundity rather than effective fecundity. Results are expressed as linear regression between carapace length (L, in mm) and the number of eggs (E) as E = a + bL. Statistical precision was tested by analysis of variance of the re­siduals.

Trawl log-book data provided a breakdown of all fishing activity, including position, time of day, trawl duration, depth, fishing gear used, and catch retained. Analysis of these records provided information on the composition of the catch and catch rates by species, time, area, and depth of operation. Catch per unit of effort (c.p.u.e.) was standardized according to the total length of net towed, by scaling to a common 73.2 m (40 fathom) total headrope length. Comparisons of c.p.u.e. in the two fishing grounds in this study were made only using data obtained from vessels known to target meta­nephropids.

Results

Fishery production

Although metanephropids occur over a wide geographic range, the best catches have been taken within clearly defined areas of the northwestern continental slope of Australia (Fig. 1). Twelve stern trawlers of 23-30 m length towing multiple “otter” trawls fished these areas. Because of the importance of penaeid and carid prawn by-catch (Wallner and Phillips, 1988), codend mesh size is never larger than 75 mm. Fishing is conducted con­tinuously during the 24-h period over soft, muddy bot­tom in depths of 250-500 m, although catch rates at dawn and dusk may be higher (Ward and Davis, 1987). Seasonal patterns of fishing effort result from seasonal closures in other shallow water penaeid fisheries in which these vessels also participate. Generally, meta­nephropids are targeted preferentially to prawns owing to their higher export value; however, fluctuations in market conditions, or the presence of aggregated schools of the penaeid Aristaeomorpha foliacea can alter target preference.

During the initial three years or “development phase” , between 1985 and 1988, the fishery displayed a rapid expansion in catch and effort (Fig. 2). Total catch increased fourfold from 3501 in 1985-1986 to 14041 in 1987-1988. This was produced by a trebling of effort from 10800 to 31700 trawl hours. In 1985-1986, meta­nephropids comprised 47% of the total catch and the greater proportion of the fishery value. By 1987-1988, metanephropids, comprised only 24% of the total catch. The reduced importance of metanephropids was partly due to the establishment of markets for penaeid and caridean prawns and a consequent redirection of tar­geted effort away from metanephropids (Wallner and Phillips, 1988). The prawn component of the catch in­creased over this period from 50% in 1985-1986 (1741) to 73% in 1987-1988 (10211). During 1987-1988, four deepwater penaeid species (Aristaeomorpha foliacea, Haliporoides sibogae, Aristeus virilis, Plesiopenaeus edwardsianus) were of commercial importance and accounted for 67% of the prawn catch. Of these, A. foliacea was the single most important species, compris­ing 49% of the prawn catch and 43% of the total fishery catch. Two carid species, Heterocarpus woodmasoni andH. sibogae, made up the balance of the commercial prawn catch.

In 1988-1989, total crustacean catch declined to 6551 for a fishing effort of 19 700 trawl hours. The metaneph­ropid catch was 1881 or 29% of the total catch. No major new fishing grounds were discovered in this period and depressed world market prices, particularly for prawns, may have contributed to lower effort.

ICES mar. Sei. Symp., 199 (1995) Development o f a trawl fishery 381

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382 B. G. Wallner and B. F. Phillips ICES mar. Sei. Symp., 199 (1995)

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Figure 2. Annual scampi catch, prawn catch, and effort levels for four fishing seasons.

Table 1. Location, depth, area, and dominant species for fish­ing grounds I and II.

G round I Ground II

Location 16°10'-17°10'S 17°40°-18°40'SDepth 420-480m 360-420mApproximate area 222 km 2 222 km2Dominant species M. australiensis M. velutinus

Catch rates

Catch rate trends were examined for two species of Metanephrops, each at a different small productive trawl ground (Table 1; Fig. 1). Thec.p.u.e. for fishing groundsI and II from the start of commercial fishing until April 1989 are shown in Figure 3. These data are superim­posed upon the monthly trawl effort. Fishing effort on both grounds was sporadic, characterized by pulses of effort when vessels enter the fishery during closures of other managed fisheries. At both grounds, initial c.p.u.e. declined rapidly in response to relatively low levels of effort as surplus standing stocks were removed. After the rapid initial depletion, the c.p.u.e. for groundII (Fig. 3, bottom) decreased further, although short­lived rises in the catch rate were apparent following periods of little or low fishing activity. The pattern for ground I (Fig. 3, top) was similar, except that effort levels between February 1987 and May 1988 were much lower than at ground II. This preceded a very large 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 obtained from fishing the virgin stock. This phenom­enon could be a result of real abundance increases due to recruitment to the fishable stock or to greater catch- ability as animals emerged from burrows for longer periods. This may indicate that fished metanephropid populations can recover and that the amount of recovery

is influenced by the patterns and intensity of prior fish­ing.

Distribution

It is apparent, from c.p.u.e. records, that metanephro­pid species are mainly caught in a linear zone adjacent to the coastline (Fig. 1). Scampi habitat is closely corre­lated with sediment type and grain size. McLoughlin et al. (1988) pointed out that areas of calcareous muddy sands supported the highest concentrations of Meta­nephrops spp. around the Scott Reef-Rowley Shoals area, as reported by Davis and Ward (1984). Nephrops norvegicus also makes burrows in this type of substrate 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 more frequently caught with mud adhering to the exo­skeleton than M. australiensis. They suggested that the latter species prefers comparatively firmer substrate, in which they build less extensive burrows and may spend considerable periods of time outside their burrows. M. velutinus and M. boschmai, on the other hand, make deeper burrows in softer sediment. Commercial catch records from sequential trawls in areas of preferred substrate during a 24-h period revealed reasonably con­stant catch rates (coefficients of variation ranged be­tween 12 and 20%), suggesting that metanephropids tend toward an even distribution on the bottom over spatial scales from 0.3 to 1.7 km2.

Australian metanephropids occur from about 260 m to about 500 m on the continental slope, each species having a clearly defined depth distribution (Fig. 4). Other species of Metanephrops are also reported to be found in continental slope waters greater than 200 m deep (Roe, 1966; Berry, 1969; Wear, 1976; Hiramoto, 1987). However, M. thomsoni is caught in waters of the Indo-west Pacific and East China Sea at depths from 120 m to 200 m (Chan and Yu, 1988). This contrasts with the wider depth distribution described for Nephrops norvegicus of 15 m to at least 800 m (Farmer, 1975), with the majority of commercial landings coming from conti­nental shelf waters in the Northeast Atlantic (Howard, 1982; FAO Yearbook, 1987).

Population structure

Size-frequency distributions of different metanephropid species were characteristically polymodal. An example, for M. australiensis, is presented in Figure 5. It is assumed this polymodality was the result of annual recruitments forming size cohorts. The figure also indi­cates the approximate 50% selection length for Neph­rops norvegicus for a 70-mm codend trawl (Briggs,

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ICES m ar. Sei. Symp.. 199 (1995) Development o f a trawl fishery 383

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H II I II i 't it t i " 'i r r m

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time (months)

Figure 3. (Top) Monthly fishing effort for trawl ground I (see Table 1) and corresponding catch per unit effort for Metanephrops australiensis from this ground; (bottom) monthly fishing effort for trawl ground II (see Table 1) and corresponding catch per unit effort for M. velutinus from this ground. Interpolated catch rates for periods of no fishing are shown by dotted lines.

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384 B. G. Wallner and B. F. Phillips

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ICES m ar. Sei. Sym p., 199 (1995)

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Figure 4. D epth distribution of Australian Metanephrops spp.

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Figure 5. Carapace length-frequency distribution for Metanephrops australiensis. (a) The 50% mesh selection size for Nephrops norvegicus using 70 mm codend trawl mesh, (b) The size at which 50% of the females sampled were ovigerous.

ICES mar. Sei. Symp., 199 (1995) Development o f a trawl fishery 385

1986). The data were collected from a sample retained in trawl nets using 51-mm codends, thus the 50% selection length in this case could be smaller than indicated. For M. australiensis it appears that size classes less than about 47 mm carapace length but well above the mesh selection length are not well represented in samples. This feature is also exhibited in samples of other Austra­lian metanephropids and was also noted for M. andama- nicus in South African waters (sensu Berry, 1969).

Due to the common occurrence of the skewed length- frequency distributions it is unlikely that poor recruit­ment is responsible; rather, it is probably attributable to reduced catchability of young animals, possibly due to greater proportions of time spent in burrows. Juvenile N. norvegicus are found in the same habitat as adults but mostly remain in burrows for at least their first year, emerging more frequently once reaching 10-15 mm carapace length and are then found with increasing fre­quency in trawl and photographic samples (Chapman, 1979,1980). Therefore, small metanephropids although occurring on the fishing grounds, may not recruit fully to the fishery until about 3+ years (M. boschmai) and 4+ years (M. velutinus and M. australiensis) (Wallner et al., 1989). Berry (1969) noted that the size at recruitment for M. andamanicus was coincident with the onset of repro­ductive maturity. This was also evident for Australian metanephropids, although for M. australiensis the year class before the length at which 50% of the female population was ovigerous was well represented in samples (Fig. 5).

Sex ratio

Sex ratios of Metanephrops australiensis and M. bosch­mai in the catch usually did not depart significantly from 1:1. However, samples of M. velutinus frequently had significantly greater numbers of females than males, particularly during periods when proportions of females carrying berry were high (Fig. 6). The most significant departure from even sex ratios occurred during October 1987, because berried females accounted for up to 72% of the total M. velutinus catch. It is hypothesized that berried female M. velutinus emerge from burrows for longer periods or more frequently and consequently suffer a higher catchability. This emergence could be to oxygenate the brood or for feeding in order to build depleted energetic reserves after spawning. This behaviour contrasts markedly with that described for ovigerous female N. norvegicus, which display very low catchability due to limited emergence from their bur­rows (Thomas and Figueiredo, 1965; Chapman, 1980).

Growth

Growth rate in metanephropids is incremental and, as for all crustaceans, a function of moult frequency. No

direct information was collected on the timing of moult­ing or frequency as recently moulted individuals occurred very infrequently in trawl samples and reliable macroscopic staging of instars was not possible. How­ever, Berry (1969) found an annual moult period in reproductively mature M. andamanicus that was coordi­nated with the reproductive cycle in the females.

Growth rates were estimated for M. australiensis, M. velutinus, and M. sibogae by dissecting sample length- frequency distributions into 6, 5, and 4 component cohorts, respectively (Table 2). Differential rates of growth between sexes have been described for N. norve­gicus. Female growth rates are retarded because of the increased metabolic requirements of reproductive matur­ation and reduced feeding outside their burrows when incubating eggs (Farmer, 1975; Hillis, 1979; Anon., 1984). This does not appear to occur in Australian species of Metanephrops, as there were only slight differ­ences in estimated growth rates between sexes, with females growing slightly faster in two of the three species. There were small differences in growth rates between species but the L„ varied more. M. australiensis is the largest species and M. sibogae the smallest, based on maximum size recorded from catch samples. M. velutinus exhibited similar rates of growth to M. sibogae, but has an intermediate maximum size. These estimates of growth are based on samples from a single location and substantial variation may occur between popu­lations due to extrinsic factors such as temperature, habitat, and population density, as has been shown for N. norvegicus (Farmer, 1975; Bailey, 1986; Bailey et al., 1986; Chapman and Bailey, 1987).

Only a single method has been used to estimate growth rates for Australian metanephropids. Tagging and captive laboratory studies were not possible as trawl-caught animals were dead or moribund upon removal from the nets. Another problem was that the sporadic nature of commercial fishing on different grounds prevented regular data collection, permitting a progression of modes to be followed through time. Un­fortunately, methods based solely on dissection of length-frequency distributions require some subjective interpretation of the data, particularly with respect to the number of component modal groups that are fitted. Nicholson (1979) argues that little confidence can be placed in estimates of length-at-age for N. norvegicus using this approach, as cohorts older than 2+ years could not be reliably separated. Similarly, there is no method for determining the age of the first modal group represented in these distributions, although Crossland et al. (1987) experimented with metabolic aging of M. andamanicus [= M . velutinus] using lipofuscin ratios. Therefore, the nominal ages of 1+ for the first group in M. sibogae and 2+ for M. australiensis and M. velutinus (a small number of 1+ animals in these distributions

386 B. G. Wallner and B. F. Phillips

1.4 "I

ICES m ar. Sei. Sym p., 199 (1995)

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Figure 6. (Top) Mean monthly sex ratio of Metanephrops velutinus samples collected during 1986-1987. Data are expressed as number of males/number of females, error bars are standard errors; * = value differs from ratio unity with probability 0.05-0.01 and ** <0.01 (G-test). (Bottom) Mean proportion of M. velutinus females of reproductive size brooding newly spawned eggs in samples collected during 1986-1987. E rror bars are standard errors.

were trimmed for the analysis; Table 2) were inter­polated. Therefore, although growth rates reported here are biologically plausible and the L„ predicted using a von Bertalanffy model are close to the maximum sizes observed in samples, they should be regarded as preliminary estimates.

Reproduction

The sexual anatomy for all Australian metanephropids is similar to that described for M. andamanicus (Berry, 1969). Therefore mating and reproduction in Metaneph­

rops spp. can be considered generically. The highest average proportions of female M. velutinus in trawl catches with recently spawned eggs occurred during Sep­tember (59.5 ± 8.9 s.e.) and October (94.2 ± 1.6 s.e.) and the lowest in May and June (Fig. 6, bottom). This pattern is consistent with the cycle described for M. andamanicus, where fertilized eggs are spawned and adhered to the pleopods during the austral spring where they are incubated for 9-10 months (Berry, 1969). The embryo is lecithotrophic and is nourished from a large yolk during the incubation period. This cycle is also similar to that of N. norvegicus around Scotland and

ICES m ar. Sei. Symp., 199 (1995) Development o f a trawl fishery 387

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 16421+ mode (mm) Truncated Truncated Truncated Truncated 25.4 25.12+ mode (mm) 33.9 34.6 33.4 35.9 35.7 36.63+ mode (mm) 42.9 44.3 43.9 44.6 43.7 44.94+ mode (mm) 50.2 51.8 51.2 50.8 49.8 51.15+ mode (mm) 56.0 57.5 56.3 55.46+ mode (mm) 60.7 61.9 59.8 58.77+ mode (mm) 64.5 65.4L „ (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 31x2 48.64 26.74 36.19 31.69 27.18 52.96Max CL (mm) observed 74 75 67 68 58 59

Ireland, except that the seasons are reversed in the northern hemisphere (Thomas, 1964; Farmer, 1974).

It is likely that different species, depths, and latitudes introduce variations from the generalized reproductive cycle described for M. velutinus. For example, spawning appears to occur later for M . sibogae, which occurs in the shallowest depth zone at the extreme north of the fishery. Sampling of the population in October 1987 revealed that the majority of females had developed ovaries, but only 0.2% carried eggs. In January 1988 a comparatively synchronous spawning had occurred, with 80.2% of females carrying newly spawned eggs. Delayed spawning and consequent compression of the incubation period also occurs at lower latitudes in TV. norvegicus and there is evidence for biennial spawning in stocks at higher latitudes (Chapman, 1980). The wide­spread occurrence of female Metanephrops carrying both gravid ovaries and late stage eggs during winter indicates that annual spawning is usual in reproductively mature metanephropids.

Australian metanephropids have lower fecundity than Nephrops norvegicus. Female N. norvegicus produce between 800 and 5000 eggs (Farmer, 1975), while the number of newly spawned eggs in the largest and most fecund Australian species (M. australiensis) is between 300 and 1500 (Fig. 7). Berry (1969) and Matsuura and Hamasaki (1987) also found that M. andamanicus and M. thomsoni produce fewer than 1500 eggs. The number of eggs spawned for all species varied with the size of the female. Linear regressions best described the relation­ship between the number of eggs spawned (E) and the carapace length of the animal (L) (Table 3). However, the effective fecundity of Australian species is about 50% lower because of attrition during the long incu­bation period (Wallner et al., 1989). Matsuura and

Hamasaki (1987) also calculated that 46% of eggs are lost during incubation in M. thomsoni. High rates of egg loss (32-75%) are a feature similarly reported for N. norvegicus (Figueiredo and Nunes, 1965; Chapman and Ballantyne, 1980; Morizur, 1981; Morizur et al., 1981; Figueiredo et al., 1983).

The larvae of M. andamanicus (sensu Berry, 1969), M. thomsoni (sensu Uchida and Dotsu, 1973), and M. challenged hatch in an advanced stage of development, do not feed and undergo only a few moults before adopting a benthic habit as juveniles after 3 -4 days (Berry, 1969; Uchida and Dotsu, 1973; Wear, 1976). Wear (1976) also concluded from examinations of eggs that M. australiensis had an abbreviated larval life. In contrast, Nephrops norvegicus has a prezoea and three free-swimming zoeal stages (Farmer, 1974). Develop­ment time until settlement is estimated at two to three weeks (Farmer, 1975), although this is temperature- dependent (Thompson and Ayers, 1989).

Discussion

Australian metanephropid species have similar life histories characterized by relatively low fecundity, a prolonged incubation period, abbreviated larval devel­opment, and require three to five years to reach repro­ductive maturity, and recruit into the fishery. Some aspects of the biology of metanephropids differ from those for Nephrops norvegicus and these features could have negative consequences for populations of meta­nephropids when exploited non-selectively, such as by trawl fishing.

Unlike N. norvegicus, ovigerous Metanephrops spp. females do not avoid trawl nets by remaining in their 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

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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 Meta­nephrops velutinus, M. australiensis, and M. boschmai. Linear regression fits (see Table 3) are plotted.

exhibiting higher catchability after spawning. This, and the generally lower fecundity of metanephropids, could mean that the reproductive potential of a population could be lowered by fishing to a point where recruitment is affected.

Metanephropids have a very abbreviated pelagic lar­val phase and it is therefore probable that settled larvae and juveniles up to several years old co-inhabit the same grounds as adult stocks. Although these animals are not directly caught by trawling activities it is possible that destruction of burrows, or removal of food species (Wassenberg and Hill, 1989), may result in a significant mortality of pre-recruit age classes. This has not been evident in Nephrops populations in the northern hemi­sphere, and it is thought that high recruit mortality is more likely to be a function of high adult density (Hill and White, 1990). However, for Metanephrops spp., the low probability of recruitment due to larval drift and the requirement for several years’ growth before recruits enter the fishery, could mean that recovery periods for

severely depleted trawl grounds would be in the order of years.

In comparison with stable and long-standing fisheries for Norway lobster, such as in Scotland and the Mediter­ranean (Farmer, 1975; Bailey et al., 1986), the brief history of the Australian scampi fishery has seen overall declining catch rates and fluctuating catches. Commer­cial interest in the fishery, as measured by the level of effort applied (Fig. 2), has lessened due to declining profitability. Therefore, some management interven­tion may be desirable with the aim of stock recovery and improving the yield from depleted areas.

Management regulations applied to Nephrops fish­eries differ among countries. Protection of ovigerous females, licence limitation, vessel restrictions, and seasonal closures are all used, in part, by a number of countries. However, almost all countries impose mini­mum size limits for landed Nephrops. This is usually accompanied by regulation of trawl net mesh sizes (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.

SpeciesNo. of

observationsa

(s.e.)b

(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 m ar. Sei. Symp., 199 (1995) Development o f a trawl fishery 389

and, in some mixed fisheries, to minimize mortality on juvenile fish stocks. Mesh-size regulation would appear to have no utility in the Australian scampi fishery, as recruit overfishing is avoided by the cryptic behaviour of small size classes and the fishery has no commercially significant fish stocks. Also, any increase in mesh size would reduce the catch of important penaeid and carid species.

Seasonal closures in fisheries are usually timed to maximize reproduction or recruitment. Female meta­nephropids are reproductively active throughout the year, either maturing ovaries or carrying berry, and the timing and patterns of recruitment are not yet known. Thus, an appropriate seasonal closure is not yet obvious.

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 by increases in c.p.u.e., than consistently fished areas. As the Australian fishery consists of areas of high meta­nephropid abundance interspersed with widespread areas that support only low densities, rotational closures appear to offer the most rational method for managing these resources. This approach requires that sufficient grounds exist, or are found, to employ the fishing fleet on a rotational basis, or that vessels can be deployed in other fisheries during rest periods. Knowledge of the rate of recovery of a fished ground is also needed to set appropriate closure periods. If the ground is continuing to produce penaeid and carid prawn catches, then the value of these products foregone during the closure should be less than the expected return from future metanephropid catches. Different patterns of effort could be evaluated for each component of the catch by modelling, as has been attempted for N. norvegicus (Brander and Bennett, 1989; Cardador and Caramelo, 1989).

Changes in fishing practices should also be considered for this fishery. Demersal trawling tends to reduce the topographic and structural complexity of the bottom and these effects may lower the productivity of this fishery. Observations made during the period of this study indi­cate some faunal changes; for example, the virtual elim­ination of once-common large hexactinellid sponges from heavily trawled areas. The long-term effects of this type of change are not yet known. Creel fishing, which is used to take N. norvegicus in Scottish and Norwegian waters, is non-destructive and selects for larger individ­uals than trawling (Howard, 1982; Bjordal, 1986). This fishing method may offer an alternative to trawl fishing in Australian slope waters; however, trials to date have shown that Metanephrops spp. will enter baited creels but catches have been disappointing (Evans, 1990). Midwater trawling may also permit the harvesting of vertically migratory penaeid and carid stocks without

disturbing the bottom or inflicting undesirable fishing mortality upon recovering metanephropid populations. This fishing method would also require research to determine the densities, behaviour, and patterns of movement of these prawns, and has yet to be attempted.

Acknowledgements

We thank Lisa Hobbs for her assistance in collecting data aboard commercial trawlers; Richard Litchfield for his length-frequency analyses of M. australiensis and M. sibogae data; Sebastian Rainer and two anonymous referees for their constructive criticism of an earlier draft of the manuscript.

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