Environmental influences on recruitment in the saucer scallop {Amusium balloti) fishery of Shark Bay, Western Australia

Transcription

1 ICES mar. Sei. Symp., 199: Environmental influences on recruitment in the saucer scallop {Amusium balloti) fishery of Shark Bay, Western Australia Lindsay M. Joll and Nicolavito Caputi Joll, L. M., and Caputi, N Environmental influences on recruitment in the saucer scallop (A m usium balloti) fishery of Shark Bay, Western Australia. - ICES mar. Sei. Symp., 199: The saucer scallop (Am usium balloti) fishery in Shark Bay is part of a multispecies fishery in which some vessels also fish for prawns. The scallop fishery, which has been fully exploited since 1984, has shown large variations in annual catch (from 120 to 7301 meat weight), reflecting variations in annual recruitment. The strength of recruitment is significantly correlated (r = 0.86, n = 7 years) with a major oceanographic process off the West Australian coastline, the Leeuwin Current, during the spawning season. The strength of this current is strongly influenced by El Nino/Southern Oscillation (ENSO) events. Possible modes of action of the current include the flushing of larvae away from suitable grounds and/or reduced spawning stimulus. The stock recruitment-environment relationship indicates that the spawning stock has not significantly affected recruitment after taking into account the environmental effect. The high correlation (0.81) between the abundance of pre-recruit scallops and the catch in the following year allows the catch to be predicted from surveys of pre-recruits. L. M. Joll and N. Caputi: Western Australian Fisheries Department, W. A. Marine Research Laboratories, PO Box 20, North Beach, W A 6020 Australia [fax: +61 (09) ], Introduction Catches of the saucer scallop A m usium balloti (Bernardi) in Shark Bay have shown a greater than sixfold variation over the period since Surveys designed to measure the abundance of pre-recruits to the Shark Bay scallop population each year have indicated that these catch variations were primarily the result of interannual variations in recruitment. Scallop populations throughout the world are acknowledged as having highly variable recruitment as a result of the influence of environmental factors (Hancock, 1973). Environmental factors which may be responsible for the variations in recruitment of A. balloti in Shark Bay were therefore investigated. Secondly, to enable the proper assessment of the stock-recruitment relationship, the effect of the environment needs to be taken into account, especially during the vulnerable larval phase of the life history. Probably the major oceanographic influence on the West Australian coastline is the Leeuwin Current. This southward flowing current of relatively warm, low salinity tropical water appears to be strongest during the southern autumn and winter months of April to August (Pearce and Phillips, 1988). The strength of this current is strongly influenced by El Nino/Southern Oscillation (ENSO) events. Satellite imagery of Shark Bay indicates that tongues of warmer water, derived from the Leeuwin Current, enter Shark Bay and are likely to be a major influence on the hydrology of the bay. This article examines 7 years of data to enable hypotheses to be generated on the interrelationships between the strength of the Leeuwin Current, the indices of abundance of spawning stock, pre-recruit and residual scallops, and the catch taken by the fishery. Life history and fishery The life history of A. balloti has been described in Joll (1987, 1989a). Briefly, gametogenesis commences in late March/early April and spawning occurs from April/ May until December. The planktonic larval life lasts days (Rose et a i, 1988). Growth of new recruits is rapid, with scallops spawned in at least the first 3 to 4 months of the spawning season being catchable in survey trawls (50 mm mesh) in November and reaching a size suitable for commercial harvest (90 mm shell height) by

2 48 L. M. Joll and. N. Caputi March or April the following year. These newly recruited scallops develop reproductively at this time and enter the breeding stock. Because of their highly developed swimming capabilities (Joll, 1989b), fisheries for saucer scallops are based on trawling rather than dredging (Young and Martin, 1989). The Shark Bay scallop fishery has been fully exploited since 1984 and access to the fishery is limited to 14 trawlers licensed to fish only for scallops (using 100 mm mesh) and 35 vessels licensed to fish for both prawns and scallops (using 50mm mesh). S ICES m ar. Sei. Symp., 199 (1995) \ \ \ N MAIN FISHING GROUNDS I-! DENHAM SOUND GROUND: CARNARVON S H A R K, BAY Methods Recruitment abundance indices Surveys of pre-recruits (0+ scallops) and residual (1 + + scallops) to determine indices of abundance have been conducted in November each year since 1983 using the 20 m research vessel Flinders. The largest pre-recruit scallops are rarely larger than 75 mm shell height in November, while the smallest residual animals are rarely smaller than 75 mm. The abundance of prerecruits in the November survey is therefore defined as the abundance of scallops less than or equal to 75 mm shell height. However, the upper limit of pre-recruit sizes and the lower limit of residual sizes is checked each year by examining the daily growth rings present on the left valve (Joll, 1988). The pre-recruits are juveniles derived from the spawning which commenced in April/ May and will recruit to the fishery and the spawning stock in the following year. Surveys are conducted using systematic sample trawling to cover the approximately 1000 km2 of the two known fishing areas (Fig. 1). Approximately 70 trawls of min duration are carried out at night (to standardize for possible diurnal variations in catchability) using twin 11m headrope trawls with 50 mm mesh nets and 45 mm codends and the catches standardized for the distance travelled. Since virtually all the catch has come from the main grounds, with little recruitment to the Denham Sound ground from 1985 on, the indices of abundance have only been based on the results from the main grounds. Catchability of scallops is affected by trawling speed and adjustments for variations in vessel speed (J. W. Penn, unpubl. data) are made to the catches by the survey trawls. Vulnerability to the gear is also likely to be affected by size, because of the relationship between size and swimming capability (Joll, 1989b). Scallops smaller than 30 mm have limited swimming capabilities as well as an apparently high threshold to swimming stimuli and therefore have a low vulnerability to trawls. Swimming capabilities increase and apparent thresholds to stimuli decrease with size, until animals reach about S. 260 ïtt T Figure 1. Shark Bay scallop fishing areas. 80 mm shell height, and remain relatively constant from this size up to the maximum size of around 125 mm. However, because the mean size of recruits has been relatively constant from year-to-year, the influence of size variations on the measure of recruitment strength over the period of the surveys is not considered to be a major source of bias. Catch data and spawning-stock index Data from the fishery are obtained through research logbooks completed voluntarily by all vessels operating in the scallop and prawn/scallop fishery. The logbooks are completed on a daily basis and provide records of hours fished, area of operation, and total estimated catch per trawl. Landing data providing exact records of catch per trip are completed when the catch is unloaded and these are checked against receivals by wholesale buyers. An annual index of the spawning stock (kg.h-1 per fathom of trawl headrope) was obtained from the research logbooks of the scallop fleet by averaging the commercial catch rate from the logbook data for the months of April to July for the main trawl grounds. If a catch rate was unavailable for a particular month,

3 ICES m ar. Sei. Symp., 199 (1995) The saucer scallop fishery o f Shark Bay, Western Australia 49 because of a management ban on fishing for that period, then the catch rate for the adjacent month was substituted. Environmental data The environmental variable examined as a possible influence on the recruitment is the Leeuwin Current. Pearce and Phillips (1988) demonstrated that the strength of the Leeuwin Current is influenced by El Nino/Southern Oscillation events. During ENSO events the Leeuwin Current appears to weaken as a result of a decrease in the sea levels in the western part of the Pacific Ocean relative to those in the eastern Pacific. Reid and Mantyla (1976) indicate that the strength of the Leeuwin Current should be reflected in the coastal sea level and, as described by Pearce and Phillips (1988), this study uses the sea-level measurements from the Fremantle (32 S 116 E) tide gauge as an index to the flow of the Leeuwin Current. While the sea level at Carnarvon (25 S 114 E) may have more accurately reflected the influence of the Leeuwin Current in the Shark Bay area, these data were not available for the whole period of this study. The major spawning activity occurs mainly in the period April to July (L. M. Joll, unpubl. data). Consequently, in considering the effects of the Leeuwin Current on scallop recruitment, it is the strength of the current in these months that is likely to be important. As the peak in the sea level at Fremantle occurred about a -, month after the peak at Carnarvon, as a result of the southward moving current, the sea level over the months of May to August at Fremantle was used to examine the influence of the Leeuwin Current in Shark Bay during April to July. Results Predicting catches from annual surveys In all years except 1983, when the abundance index for residual stock in the November survey was 269 scallops per nautical mile (spnm), the residual stock indices have been in the range 24 to 90 spnm. The abundance indices for recruit stock over the period from 1983, however, have varied from 40 to 604 spnm. Despite the variations in total stock constitution, the prediction of the catch by the index of total abundance of scallops (Recruit index + Residual index) was as good as using an equation involving the indices of abundance of both recruit and residual stocks separately. Total abundance of scallops, T in the main trawl grounds during the November survey of year t was related to the catch of the following year, Ct+1, resulting in the following fitted model (Fig. 2): Ct+1 = 4.64Tt0'781 where Ct+1 is the predicted value of Ct+1. The model was fitted using a logarithmic transformation and has a corre- 87/ /87 83/84 g « /85 85/86 88/ RECRUIT + RESIDUAL INDEX (spnm) Figure 2. Relationship between the recruitment plus residual catch rate (scallop per nautical mile) from the November research survey and the catch in the following season. The year of the survey and the year of the catch are shown next to the points.

4 50 L. M. Joll and N. Caputi ICES mar. Sei. Symp., 199 (1995) 700-, E S o 200 U J GC SEA LEVEL (MAY-AUGUST) (cm) Figure 3. Relationship between the average Fremantle sea level (cm) during May to August and the level of recruitment (scallops per nautical mile) measured in the November research survey of the same year. The year is shown next to the points. lation coefficient of 0.81 (p < 0.05) and a residual mean square (RMS) of A preliminary version of this model, established prior to the 1989 season, predicted the low 1989 catch. Based on the total abundance index for the November 1989 survey of 168 spnm, the model indicates that a catch of 2501 should be taken in Environmental relationships (i) Recruitm ent-en vironment The relationship between the average Fremantle sea level (cm) over the months of May to August for a given year, FSLt, and the subsequent abundance of recruits (spnm) as measured by the November survey of that year, Rt, was examined and resulted in the following model (Fig. 3): R, = exp( FSLt) This model, fitted using a logarithmic transformation, resulted in a correlation of 0.86 (p < 0.05) and an RMS of The 1989 data point can be regarded as the first independent check of this relationship as the hypothesis of the relationship between the strength of the Leeuwin Current as measured by the sea level and the scallop recruitment was established prior to (ii) Catch-environment Because the fishery has been fully exploited since 1984, the catch is highly dependent on the strength of the recruitment. Because of the relationship between recruitment and the strength of the Leeuwin Current (as measured by the Fremantle sea level), there is also a strong relationship between the Fremantle sea level and the catch in the following year (Fig. 4). The model for the relationship is: Ct+1 = exp( FSLt) This model, fitted using a logarithmic transformation, resulted in a correlation of (p < 0.05) and an RMS of The 1982 sea level and 1983 catch provide an additional, independent data point, as the hypothesis about the relationship between the Leeuwin Current and recruitment is based on data derived from the recruitment surveys which commenced in (iii) Spawning stock-recruitm ent-environm ent The effect of the spawning stock, S on the recruitment level was examined using a Ricker stock-recruitment

5 ICES mar. Sei. Symp., 199 (1995) The saucer scallop fishery o f Shark Bay, Western A ustralia 51 87/88 86/87 83/ / SEA LEVEL (MAY-AUGUST) (cm) Figure 4. Relationship between the average Fremantle sea level (cm) during May to August with the catch (t) in the following year. The year of the sea level data and the year of the catch are shown next to the points. relationship with an environmental variable incorporated, and resulted in the following model (Fig. 5): ft, = S, exp( St FSLt). This resulted in a multiple correlation of 0.91, with an inverse relationship between the spawning stock and recruitment over the range of spawning stocks observed, though the effect of spawning stock was not significant (p > 0.05) after the sea level was taken into account. Discussion Effects of environmental variability on recruitment and catch Irregular recruitment as a result of environmental variation was considered by Hancock (1973) to be a general characteristic of scallop populations. Massive increases in the abundance of the Chilean scallop Argopecten purpuratus were noted to be associated with the El Nino event of 1982/1983 (Arntz, 1984; Wolff, 1987). Wolff considered that factors improving the survival of recruits may include a shortened larval period due to increased temperatures, maintenance of adequate oxygen levels and reduction in the abundance of many predators. Dickie (1955) observed that recruitment of Placopecten magellanicus in the Bay of Fundy area was related to water temperature. He explained the action of the environment in terms of the influence of water temperature on the duration of the larval period and the fact that low water temperatures reflected a high level of exchange between the Bay of Fundy and outside water masses. This high level of exchange was considered to lead to heavy losses of larvae. Caddy (1979) noted 9-year cycles in landings of P. magellanicus from the Bay of Fundy, which corresponded to similar cycles in tidal features. He hypothesized that these tidal influences controlled the degree of retention of larvae within a gyre and this influenced recruitment to scallop grounds in its vicinity. Hydrological flushing was considered by Caddy (1989) as an important environmental influence on recruitment success in sedentary molluscs. Retention of larvae within the water mass is of particular importance to the recruitment success of estuarine species (Andrews, 1983), but has also been shown to be important for pelagic fish (lies and Sinclair, 1982). The higher abundances of juvenile Pecten maximus, and P. maximus spat in artificial spat collectors, were considered by Thouzeau and Lehay (1988) and Thouzeau (1991) to be partly related to hydrographic factors causing larval retention. The mechanism of action of the environment on the recruitment success of A. balloti has yet to be identified. It may be that there is hydrological flushing in years of strong Leeuwin Currents or there could be a temperature effect on spawning or fertilization because of warmer waters. Because the Shark Bay scallop fishery is fully

6 52 L. M. Joll and N. Caputi ICES mar. Sei. Symp., 199 (1995) SPAWNING STOCK INDEX (kg h~1fathom 1) Figure 5. Relationship between spawning stock (kg h 1 per fathom of trawl headrope) and recruitment (scallops per nautical mile) from the November research survey of the same year. Year of spawning and the Fremantle sea level (cm) are shown next to the points. The three curves are for Fremantle sea levels, 75, 80, and 85 cm, which range from high to low sea-level conditions. 4 exploited, the variations in recruitment resulting from the environmental effects of the Leeuwin Current also give rise to variations in the catch in the following year. This study indicates that the annual survey of pre-recruit and residual scallops in November may provide the ability to predict the catch for the following year. A correlation of 0.81 has been found between the survey abundance and the catch. However, more years of data are required to confirm the relationship. In years of El Nino/Southern Oscillation (ENSO) events the sea level is low (Reid and Mantyla, 1976; Pearce and Phillips, 1988). The inverse relationship between the sea level and subsequent recruitment and catch indicates that the recruitment will be high in the year of an ENSO event and the catch will be high in the year after an ENSO event. During the period on which this model is based, 1983 to 1989, there was one ENSO event in 1987 and this corresponded to the highest recruitment measured in the November surveys and a record catch of 7311 in the following year (Figs. 2, 3). Another ENSO event occurred in 1982, the year before the November surveys began, and this corresponded to the very high catch of 7031 in Stock-recruitment-environm ent relationships Dickie (1955) and Hancock (1973) noted that stockrecruitment relationships are not apparent for scallop stocks. Gulland (pers. comm, in Hancock, 1979) considered that the high fecundity of scallops would result in recruitment being independent of stock size. Nevertheless, poor recruitment following a reduction in the abundance of adult stock has been reported in a variety of stocks. Dredge (1988) stated that reduced recruitment of A. japonicum balloti in Queensland was the result of extreme reductions in the density of adult scallops as a result of fishing effort. Young and Martin (1989) noted that recruitment failure of most beds of Pecten fum atus in southern Australia occurred after a period of commercial fishing and suggested that a certain minimum adult density was required to ensure successful spawning and high levels of recruitment. Based on studies of spatfall strength of P. fum atus in artificial collectors, Young et al. (1989) concluded that the number of larvae competent to settle is related to the parent stock, but that the number actually settling is dependent on unknown physical or biological factors at the settlement site. On the other hand, Joll (1989a) noted that recruitment to the stock of A. balloti at the Abrolhos Islands could be high when adult stocks were low and conversely. In this fishery, fishing occurs after spawning, which reduces the possibility of recruitment overfishing. The recruitment data for the Abrolhos Islands population of A. balloti suggested that the stock-recruitment relationship may be described by a Ricker curve and that the recruitment is subject to a high degree of environmental control.

7 ICES m ar. Sei. Symp., 199 (1995) The saucer scallop fishery o f Shark Bay, Western A ustralia 53 Vahl (1982) and Orensanz (1986) also noted a negative relationship between year-class strength and the density of adult scallops. Orensanz (1986) suggested that recruitment of Chlamys tehuelcha, in the San Jose Gulf, Argentina, was constrained by the carrying capacity of the grounds, in contrast to the adjacent ground in the San Matias Gulf where the stock collapsed as a result of intensive fishing. These studies suggest that the level of recruitment in scallops can be both positively and negatively related to the spawning stock, depending on the level and timing of fishing. That is, when there is heavy fishing pressure on stocks, especially if the fishing occurs before spawning, then recruitment overfishing is possible. However, if stocks are lightly fished and/or fishing occurs after spawning, then density-dependent effects may play an important role in determining the level of recruitment. The examination of the stock-recruitment-environment relationship for the Shark Bay stock reported here indicates that there is a major environmental effect in determining recruitment success in this population. The identification of an environmental variable which affects recruitment has allowed the development of an hypothesis regarding the relationships between stock, recruitment, and the environment. This hypothesis has now been incorporated into a model of the fishery and will be tested as more years of data become available. The data do not indicate any significant effect of breeding stock size on recruitment, but the management regime is designed to prevent the fishery from driving the breeding stock to low levels. Acknowledgements Tidal data for Fremantle and Carnarvon were supplied by the National Tidal Facility, The Flinders University of South Australia, Copyright reserved. Surveys of the scallop grounds were conducted with the assistance of the Master and crew of the RV Flinders. The authors thank Dr J. W. Penn and other research scientists of the Western Australian Marine Research Laboratories for critically reading the manuscript. References Andrews, J. D Transport of bivalve larvae in James River, Virginia. J. Shellfish Res., 3: Arntz, W. E El Nino and Peru: positive aspects. Oceanus, 27: Caddy, J. F Long-term trends and evidence for production cycles in the Bay of Fundy scallop fishery. Rapp. P.-v. Réun. Cons. int. Explor. Mer, 175: Caddy, J. F Recent developments in research and management for wild stocks of bivalves and gastropods. In Marine invertebrate fisheries: their assessment and management, pp Ed. by J. F. Caddy, John Wiley, New York. 752 pp. Dickie, L. M Fluctuations in abundance of the giant scallop, Placopecten magellanicus (Gmelin), in the Digby area of the Bay of Fundy. J. Fish. Res. B dc an., 12: Dredge, M. C. L Recruitment overfishing in tropical scallop fishery. J. Shellfish Res., 7: Hancock, D. À The relationship between stock and recruitment in exploited invertebrates. Rapp. P.-v. Réun. Cons. int. Explor. Mer, 164: Hancock, D. A Population dynamics and management of shellfish stocks. Rapp. P.-v. Réun. Cons. int. Explor. Mer, 175: Iles, T. D., and Sinclair, M Atlantic herring: stock discreteness and abundance. Science, 215: Joll, L. M The Shark Bay scallop fishery. Fisheries Management Paper No. 11. Fisheries Dept., Western Australia, 123 pp. Joll, L. M Daily growth rings in juvenile saucer scallops, A m usium balloti (Bernardi). J. Shellfish Res., 7: Joll, L. M. 1989a. Recruitment variation in stocks of the saucer scallop A m usium balloti in the Abrolhos Island area, pp In Proceedings of the Australasian scallop workshop. Ed. by M. C. L. Dredge, W. F. Zacchrin, and L. M. Joll. Tasmanian Government Printer, Hobart, Australia. 290 pp. Joll, L. M. 1989b. Swimming behaviour of the saucer scallop A m usium balloti (Mollusca: Pectinidae). Mar. Biol., 102: Orensanz, J. M Size, environment, and density: the regulation of a scallop stock and its management implications. In North Pacific Workshop on stock assessment and management of invertebrates. Ed. by G. S. Jamieson and N. Bourne. Can. Spec. Publ. Fish, aquat. Sei., 92: Pearce, A. F., and Phillips, B. F ENSO events, the Leeuwin Current, and larval recruitment of the western rock lobster. J. Cons. int. Explor. Mer, 45: Reid, J. L., and Mantyla, A. W The effect of the geostrophic flow upon coastal sea elevations in the northern North Pacific Ocean. J. Geophys. Res., 81: Rose, R. A., Campbell, G. R., and Sanders, S. G Larval development of the saucer scallop A m usium balloti (Bernardi) (Mollusca: Pectinidae). Aust. J. Mar. Freshwat. R es., 39: Thouzeau, G Experimental collection of postlarvae of Pecten maximus (L.) and other macrofaunal species in the Bay of Saint-Brieuc, France. I. Settlement patterns and biotic interactions among the species collected. J. exp. mar. Biol. Ecol., 148: Thouzeau, G., and Lehay, D Variabilité spatiotemporelle de la distribution, de la croissance et de la survie des juvéniles de Pecten maximus (L.) issus des pontes 1985, en baie de Saint-Brieuc. Oceanologica Acta, 11: Vahl, O Long-term variations in recruitment of the Iceland scallop, Chlamys islandica from Northern Norway. Neth. J.S ea Res., 16: Wolff, M Population dynamics of the Peruvian scallop Argopecten purpuratus during the El Nino phenomenon of Can. J. Fish, aquat. Sei., 44: Young, P. C., and Martin, R. B The scallop fisheries of Australia and their management. Rev. Aquat. Sei., 1: Young, P. C., Martin, R. B., McLoughlin, R. J., and West, G Variability in spatfall and recruitment of commercial scallops (Pecten fum atus) in Bass Strait, pp In Proceedings of the Australasian Scallop Workshop. Ed. by M. C. L. Dredge, W. F. Zacchrin, and L. M. Joll. Tasmanian Government Printer, Hobart, Australia. 290 pp.