Changes in size distributions of commercially exploited sharks over 25 years in northern Australia using a Bayesian approach
Highlights
► We model size changes for two commercially important shark species. ► Sharks were assessed post over-exploitation and after a period of low fishing pressure. ► Shark size generally increased over this period. ► Current shark sizes indicates some recovery from overfishing. ► Monitoring shark sizes is a useful monitoring tool for indicating the stock status.
Introduction
The effects of over-fishing reach far beyond the relatively simple reduction of yields (FAO, 2006); over-exploitation of certain species within complex marine ecosystems can disrupt important biological processes, thereby exacerbating population declines of the target and collateral species (Dulvy et al., 2003, Jennings and Kaiser, 1998, Myers et al., 1995, Reynolds et al., 2005). Over-exploitation, leading to a reduction in population size from harvesting, is often only identified after it has already occurred by measuring declining catches and/or increasing effort (Hilborn and Walters, 2001). Further, the subtle biological implications of over-exploitation are also typically overlooked, with managers focusing more on the fishery than the status of the remaining fishes themselves. For example, high fishing effort can disproportionally remove particular size classes via gear selectivity (Walker et al., 1998), thus altering age structure, age at maturity, or growth patterns (Jennings and Kaiser, 1998). Indeed, such changes in size distributions are some of the most important determinants driving extinction risk of harvested species (Jennings et al., 1998, Jennings et al., 1999, Reynolds et al., 2005). Changes in these attributes have been suggested as indicators of over-exploitation (Greenstreet and Rogers, 2006, Rochet et al., 2000).
Over-fishing has led to population declines and in some cases, the commercial collapse of many economically important fish species. In the last few decades there has been much controversy regarding the underlying causes of fishery collapses (Hutchings and Reynolds, 2004, Reynolds et al., 2005), but there is now a substantial body of evidence demonstrating that shark species are highly susceptible due to their slow replacement rates relative to teleost (bony) fishes (Dulvy et al., 2008, Field et al., 2009b). For most fisheries that catch sharks, not only are the causes uncertain, so too are the magnitudes of the population declines (Baum et al., 2005, Burgess et al., 2005, Robbins et al., 2006, Stevens et al., 2000a, Walker, 1998). The implications of removing these top-order predators and the potential effects on marine ecosystem function are poorly understood and as yet unresolved (Coll et al., 2006, Jackson et al., 2001, Myers et al., 2007, Stevens et al., 2000a, Walker, 1998, Ward and Myers, 2005, Worm et al., 2006). This uncertainty is particularly acute in tropical systems such as those in northern Australia.
As with many fisheries, the history of shark exploitation in northern Australia is complex. From the early 1970s until the mid-1980s, a Taiwanese pelagic gill-net fleet operated in the waters around northern Australia targeting shark, longtail tuna (Thunnus tonggol) and Spanish mackerels (Scomberomorus spp.) (Stevens and Davenport, 1991). The size and largely unmanaged nature of the gill-net fleet caused concern over potential over-exploitation of shark (and other aquatic animal) populations in the region (Stevens and Davenport, 1991). This motivated the Australian Fisheries Service at the time to reduce substantially the maximum allowable length of surface-set drift nets, causing the industry to abandon its endeavours largely for economic reasons (Stevens and Davenport, 1991).
The areas around northern Australia accessible to the Taiwanese fleets changed over time. During its initial years of operation, the area fished by the Taiwanese gill-net fleet was unclear, but the advent of mandatory reporting following the implementation of the Australian Fishing Zone (AFZ) in 1979, shows that foreign fishing (gill-net and long-line) in north Australian waters was mainly offshore from approximately 22 km off the coast from the North West Shelf to north of the Gulf of Carpentaria. Prior to 1979, there were no quotas for catches; the average total annual catch was 17,000 tonnes. Once restrictions came into force in 1979 with the implementation of the AFZ, an annual quota for the gill-net fleets was set to 7000 tonnes (Fig. 1). Taiwanese long-line catch data are poor, but between February 1990 and September 1991, 1700 tonnes of shark were landed by eight Taiwanese long-liners. Before 1980, reporting of catch and effort was limited (Walter, 1981). However afterwards, basic catch composition, catch and effort data were collected by both Taiwanese and independent logbook programmes. These records indicated that total catch composition by weight was approximately 80% shark species, with blacktip (primarily Carcharinus tilstoni, with an unknown proportion of C. limbatus) and spot-tail (C. sorrah) sharks accounting for 60% of the total catch (Stevens and Davenport, 1991). During the early 1980s, the fishing effort almost doubled, while catch per unit effort (CPUE) decreased from 16 to 7 kg/km/h (Stevens and Davenport, 1991). Further restrictions were imposed in 1986, leading to the fishery's abandonment. However, Taiwanese long-lining and gill-netting continued outside the Australian Fishing Zone until 1991, albeit at reduced rates.
In the early 1980s, a small (order of magnitude smaller than the Taiwanese take) domestic Australian shark fishery was developed within inshore waters (Fig. 1). The fishery was concentrated around the Northern Territory, harvesting between 100 and 485 tonnes annually from 1984 to 1988 (Stevens and Davenport, 1991), but subsequently extended to Western Australia and Queensland. Shark resources in the Australian Fishing Zone were only moderately exploited until 1979 when Taiwanese long-lining vessels started fishing, taking around 3500 tonnes in the first year (Fig. 1; Stevens and Davenport, 1991). This sudden increase in the northern shark fishery generated considerable research directed toward improving management capacity (Davenport and Stevens, 1988, Lyle, 1984, Lyle, 1987, Lyle and Griffin, 1987, Lyle et al., 1984, Lyle and Timms, 1984, Stevens and Church, 1984, Stevens and Wiley, 1986). Despite the demonstration that a domestic pelagic fishery could be economically viable and sustainable (Lyle and Timms, 1984), data from the Taiwanese gill-net fleet suggested some signs of over-exploitation (Stevens and Davenport, 1991). Some signs included a decrease in the proportion of mature C. tilstoni caught from 1981 to 1986, although no such trend was observed for C. sorrah (Stevens and Davenport, 1991). Furthermore, the median size of the sharks caught decreased for both C. tilstoni and female C. sorrah.
Currently, a small tropical shark fishery, the Offshore Net and Line (NTONL) fishery, operates in the Northern Territory (DEH, 2005). In this fishery, which targets various sharks and grey mackerel (Scomberomorus semifasciatus), catches have increased slowly from 1984 to the present, so that there are now 17 licences held by 7–9 vessels that currently catch approximately 1089 tonnes of fish annually (NTDPIFM, 2005). The fishery's primary target species is grey mackerel, followed by blacktip and spot-tail sharks, and a variety of secondary shark species including tiger (Galeocerdo cuvier), pigeye (C. amboinensis) and spinner sharks (C. brevipinna). An increase in catch per unit effort and in the proportional catch of non-primary target shark species from 2000 to 2003 prompted questions regarding the industry's future sustainability (NTDPIFM, 2005). In response, management changes were put in place to reduce fishing effort and halt the industry's growth. Currently, grey mackerel dominates in terms of single-species catch (NTDPIFM, 2005). Research projects to address concerns about sustainability were implemented in 2004. These included fisheries observation (Field et al., 2008, NTDPIFM, 2005), risk assessments to determine the sustainability shark and rays around northern Australia (Salini, 2007), and tagging studies. The influence of illegal fishing in the region is unclear; however, most is thought to occur farther offshore from the coastal fisheries, and it is unlikely to have affected shark populations greatly in the Gulf of Carpentaria (Field et al., 2009a).
Since the studies of the 1980s, there have been no subsequent analyses of catch composition and size data of these shark populations to assess the status of the fishery. Specifically, we aimed to (1) compare catch compositions between the 1980s and recently (2002–2007), and (2) determine if there have been any changes in overall length of sharks from an over-exploited population during Taiwanese fishing operations, compared to the relatively small fishery of the present. We used a Bayesian model-averaging approach to account for the effects of sex, season and water depth. Recent molecular evidence suggests that shark populations of western Northern Territory and the Gulf of Carpentaria are genetically distinct (Ovenden, 2007), so we also examined the data for differences between geographic regions.
Section snippets
Shark catches
From 1983 to 1985, approximately 10,500 sharks were tagged and released from locations all around northern Australia (Stevens et al., 2000b). Species and sex of each shark were recorded and fork length measured (±5 mm). More recently (October 2002 to May 2007), we collected species composition and length data as part of a number of observer and tagging studies, including the NTONL fishery observer and tagging programme and the FRDC project (Salini, 2007) to determine the sustainability of sharks
Overall catch composition
A total of 4119 sharks were caught and measured (Table 1) from 25 species in the two sampling periods in the west coast of the Northern Territory and the western Gulf of Carpentaria combined (n1980s = 1654; ncurrent = 2465). Two species dominated the catch: Australian blacktip and spot-tail sharks and accounted for ∼75% of the total number of sharks caught (Fig. 3). However, there was a decrease in the proportions of these two species from the 1980s to the 2000s. There was evidence for a change in
Discussion
Temporal changes in the size of individuals and catch composition provide indices of fisheries over-exploitation and/or recovery (Jennings et al., 1999, Jennings and Kaiser, 1998, Rochet et al., 2000), and changes in catch composition give an indication of community change and ecosystem status (Greenstreet and Rogers, 2006, Jennings and Kaiser, 1998, Shepherd and Myers, 2005). Therefore, monitoring size changes over time can assist in identifying the forces driving observed population trends (
Conclusions
The plausible recovery we report via size changes are contrary to current global trends in many elasmobranch populations, especially in South-East Asian fisheries (Field et al., 2009b, Sodhi et al., 2007). This indication and the relatively pristine marine environment (Halpern et al., 2008) around northern Australia might provide something of a conservation haven for shark diversity in the region, although little is known about other target and by-catch species caught within the fishery.
Acknowledgements
We thank all the fishers and research staff involved in the CSIRO Northern Pelagics tagging study, Fisheries Research Development Corporation project 2002/064, and the Northern Territory Shark Tagging study for their assistance during tagging and data collection, in particular C. Tarca. The Northern Territory Tagging Study is funded by an Australian Research Council Linkage Grant (LP0667702) to CJAB and MGM, and permitted by the Charles Darwin University Animal Ethics Committee.
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