Reproductive biology and per-recruit analyses of striped trumpeter (Latris lineata) from Tasmania, Australia: Implications for management
Introduction
Successful fisheries management relies on understanding the regenerative ability of fish populations and having an accurate assessment of biological parameters, including reproductive traits, such as: size and age at maturity, fecundity and duration of spawning season, along with growth and mortality estimates (Quinn and Deriso, 1999). These factors are used in stock assessment models to ensure that yield is maintained near the maximum attainable level whilst ensuring that the stock remains above an undesirable threshold, such as 20% unfished spawning stock biomass (Mace, 1994). In the absence of a dynamic quantitative stock assessment, exploring yield and spawning biomass-per-recruit simulations at different levels of exploitation can provide a basis for management strategies (Sissenwine and Shepherd, 1987, Deriso, 1987, Mace, 1994).
Striped trumpeter (Latris lineata [Bloch and Schneider]) is a large temperate reef species that is relatively long lived, attaining a maximum age of over 40 years (Tracey and Lyle, 2005) and growing to 25 kg in weight (Gomon et al., 1994). The species is widely distributed around the southern hemisphere, found in all the major oceanic basins (Tracey et al., 2006). The species has had a long history of commercial and recreational exploitation in Tasmania, targeted for its high quality large white fillets. Striped trumpeter display life-stage specific habitat preferences which have effectively created two fisheries for the species; juvenile fish are taken predominantly by gillnets (recreational and commercial) in coastal reefs, usually in depths <50 m, whereas sub-adult and adult fish are taken over deeper offshore reefs (up to 350 m) by hook methods (dropline, handline, longline, trotline) and as a by-catch in large mesh gillnets (shark nets) (Tracey and Lyle, 2005).
In Tasmania, management of the striped trumpeter fishery is based on minimum size and possession limits. Since 2001 commercial operators have been restricted to a 250 kg trip limit while recreational fishers have a possession limit of eight fish per person. In 2004 the minimum legal size limit was increased from 350 to 450 mm total length (TL), this change having greatest impact on inshore gillnet catches which are based almost exclusively on fish smaller than this size (Tracey and Lyle, 2005).
Since 2000, commercial landings of striped trumpeter in Tasmania have averaged about 40 tonnes per annum, down from over 100 tonnes per annum in the late 1990s. Catch reductions have been evident for both inshore gillnet and offshore hook methods. Inshore gillnet catches, of smaller fish, taken prior to the implementation of the new size limit in 2004, had fallen from almost 20 tonnes to around 5 tonnes per annum, with catch per unit of effort (CPUE) trends providing evidence of low recruitment over the past few years (Lyle et al., 2005).
A survey of the recreational fishery during 2000/2001 provided an estimated annual recreational harvest of 38 tonnes, which was only slightly less than the commercial catch for the same period (Lyle, 2005). Striped trumpeter are currently the focus of aquaculture trials in Tasmania (Bransden et al., 2005, Trotter et al., 2005). These trials have established that females are multiple or batch spawners and that oocyte development is asynchronous (Morehead et al., 1998). The rearing of striped trumpeter larvae has indicated a complex and extended larval phase of up to 9 months prior to settlement, including a post-larval ‘paperfish’ stage (Morehead, personal communication). Despite an understanding of some of the reproductive capabilities of cultured and captive reared striped trumpeter under various hormonal and photo-thermal regimes (Morehead and Hart, 2003), very little is known about the size or age at maturity, reproductive strategies or fecundity of wild striped trumpeter stocks. The absence of key biological information, coupled with recent catch declines, compound concerns for the sustainability of the stock under current levels of fishing pressure in Tasmania.
This study describes the reproductive characteristics of striped trumpeter, with a focus on quantifiable traits necessary for stock assessment modelling: spawning season, batch fecundity and size at maturity. Secondly, this information is incorporated into yield and spawning biomass-per-recruit analyses to assess the impacts of current and alternative minimum size limits on yield and potential egg production.
Section snippets
Sampling regime
Striped trumpeter were collected opportunistically between 1991 and 2005 from the northeast, east, south and southwest coasts of Tasmania (Fig. 1). Available samples ranged from fish frames collected from fishers and fish processors, where only length and sex and, potentially, gonad stage information could be obtained, to whole fish caught for scientific research, where length, weight, sex, gonad weight and maturity stage were assessed. Otoliths were also collected when possible. Most samples
Biological data
Males included in the reproductive assessment ranged between 269 and 950 mm FL and females ranged from 269 to 845 mm FL. Length frequency distributions were not significantly different between the two sexes (Kolmogorov–Smirnov; Z = 1.16, P = 0.134). Analysis of residual sums of squares (Haddon, 2001) suggested no significant difference between the sex specific length–weight relationships (F = 0.02, df = 1, P = 0.10); consequently a power regression was applied to the length–weight data of all individuals
Discussion
Striped trumpeter exhibit a discrete annual spawning cycle that extends over 3 months in the late austral winter–early spring period, with peak spawning activity occurring in September and October. Spawning is widespread throughout the distribution range around Tasmania, with females in spawning condition collected from all offshore sampling locations. Furlani and Ruwald (1999) suggested that spawning commenced and finished earlier at higher latitudes off Tasmania, although their study did not
Acknowledgments
The authors would like to thank Ray Murphy, Alan Jordan, Stuart Nichols, and the crew of FRV Challenger for assistance and contribution to sample collection, and to Wayne Hutchinson for kindly making available biological data collected between 1991 and 1992. We also extend thanks to Philippe Ziegler, Angela Williamson and Alistair Hobday for helpful suggestions on earlier versions of the manuscript.
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