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Marine and Freshwater Research, 2006, 57, 395–401
Validating ecological risk assessments for fisheries: assessing the impacts of turtle excluder devices on elasmobranch bycatch populations in an Australian trawl fishery Shane P. GriffithsA,C , David T. BrewerA , Don S. HealesA , David A. MiltonA and Ilona C. StobutzkiB A CSIRO
Marine and Atmospheric Research, PO Box 120, Cleveland, Qld 4163, Australia. of Rural Sciences, GPO Box 858, Canberra ACT 2601, Australia. C Corresponding author. Email:
[email protected]
B Bureau
Abstract. Demonstrating ecological sustainability is a challenge for fisheries worldwide, and few methods can quantify fishing impacts on diverse, low value or rare species. The current study employed a widely used ecological risk assessment method and incorporated new data to assess the change in sustainability of species following the introduction of Turtle Excluder Devices (TEDs) in Australia’s Northern Prawn Fishery (NPF). Population recovery ranks changed for 19 of the 56 elasmobranch species after the introduction of TEDs, with nine species showing an increase in sustainability. Unexpectedly, ten species showed a decrease in sustainability. This was due to TEDs successfully excluding large animals from the catch, resulting in a lower mean length at capture, which reduced the recovery ranks for two criteria relying on length data. This falsely indicates that TEDs increase the impact on pre-breeding animals, thus reducing the recovery potential of these species. The results demonstrate that existing attribute-based risk assessment methods may be inadequate for reflecting even the most obvious changes in fishing impacts on bycatch species. Industry and management can benefit greatly from an approach that more accurately estimates absolute risk. The development and requirements of a new quantitative risk assessment method to be developed for the NPF, and applicable to fisheries worldwide, are discussed. Extra keywords: extinction, fisheries management, prawn, rays, sharks, sustainability, tropical.
Introduction The requirement for long-term ecological sustainability is increasingly influencing the management strategies of many world fisheries. It is now well documented that fishing activities can significantly impact the populations of not only target species, but also those caught incidentally as bycatch, and have the potential to disrupt the functionality of an ecosystem (Hall 1996; Pauly et al. 2001). Although globally recognised for decades, only recently has ecosystem-based fishery management provided practical solutions through the development of guidelines and more tangible performance measures (FAO 2003). As is the case in many other countries, in recent years Australian environmental legislation has become more stringent to help ensure that all Australian export fisheries operate in an ecologically sustainable manner, in particular the Environment Protection and Biodiversity Conservation (EPBC) Act 1999. Public pressure and market drivers are also requiring Australian fisheries to demonstrate ecological sustainability (e.g. Aslin and Byron 2003). Consequently, an increasing number of fisheries are aiming to adopt ecosystem-based management strategies. © CSIRO 2006
Demersal trawling is a relatively non-selective fishing method, and the bycatch often comprises a significantly higher proportion of the catch than the target species (Saila 1983; Andrew and Pepperell 1992). In Australia’s Northern Prawn Fishery (NPF) the bycatch has averaged around 30 000 tonnes per year (Pender et al. 1992), five times the retained catch, and comprises more than 600 species (Stobutzki et al. 2001a). The NPF catches 56 species of elasmobranchs, which comprise around 4% of the NPF bycatch by weight (Stobutzki et al. 2001b); many of these may be particularly vulnerable to overfishing, due to their slow growth, low natural mortality rates and low reproductive potential (Walker 1998; Stevens 1999; Prince 2002; Baum et al. 2003). Caution needs to be exercised in managing these species, as the populations of some elasmobranchs in Australia have been projected to go extinct in as little as six years, despite very low fishing mortality (Otway et al. 2004). In 2000 the NPF introduced the mandatory use of Turtle Excluder Devices (TEDs) and Bycatch Reduction Devices (BRDs), which has resulted in the exclusion of >90% of individuals for nine species of elasmobranchs and smaller exclusion rates for six other species (Brewer et al. 2004). 10.1071/MF05190
1323-1650/06/040395
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Marine and Freshwater Research
These mainly large animals usually include the large breeding females, and their survival would be expected to improve the chances of long-term sustainability for these species. Although the impact of the fishery on elasmobranch populations has been dramatically decreased since 2000, their long-term sustainability in the fishery is still unknown. The juveniles of most species and adults of smaller species can pass through the bars of a standard TED and many suffer damage or death in trawl catches. Demonstrating that populations of elasmobranchs caught by the fishery are sustainable is difficult and expensive using traditional monitoring techniques, due to the high number of species involved, and the relative rarity of many of these species. Additionally, sampling the true population and size composition of larger species, which are almost entirely excluded by TEDs, is difficult now that TEDs are compulsory in the NPF. For this reason, an alternative strategy is needed to demonstrate the long-term sustainability of elasmobranchs with which the NPF interacts. Dulvy et al. (2000) reviewed the methods by which extinction risks can be quantified for populations of species impacted by fishing, and showed that few methods are useful for assessing large numbers of species for which biological data are few. An exception is a method developed concurrently by Milton (2001) for seasnakes and Stobutzki et al. (2001b, 2001c) for teleosts and elasmobranchs, which is herein referred to as a Susceptibility-Recovery Analysis (SRA). The method is a simple qualitative risk assessment technique that ranks each species using several criteria describing (i) their susceptibility to capture by a specific fishing method and (ii) their capacity to recovery once populations are depleted. The overall susceptibility and recovery ranks for each species can be plotted to determine the species that might be most at risk to being overfished. Because of the method’s simplicity and capability of handling hundreds of species with limited data, it has been adopted by several Australian fisheries to assess ecological sustainability, including the Queensland offshore and inshore gill-net fisheries (Gribble et al. 2004), the NPF (Stobutzki et al. 2001b, 2001c), the Western Australian Shark Bay Prawn Fishery (Environment Australia 2002), and the Queensland East Coast Trawl Fishery (QDPI 2004). The aim of the current paper was to (i) examine the change in sustainability of individual species after the introduction of TEDs in the NPF by incorporating new data on elasmobranch exclusion and (ii) validate the SRA method and assess its sensitivity to changes in catchability resulting from the introduction of TEDs in the NPF. Materials and methods The SRA method used in the current paper uses information for each bycatch species on 11 criteria describing: (i) the susceptibility of the species to capture and mortality by fishing, and (ii) the capacity of the population to recover following depletion (Milton 2001; Stobutzki et al.
S. P. Griffiths et al.
2001b, 2001c). Each criterion was weighted according to its relative importance in contributing to population sustainability. Each species was given a rank on a scale of 1–3 for each criterion. A rank of 1 was assigned to a criterion that would contribute to the species being highly vulnerable to capture, or having a low capacity to recover. A rank of 3 was assigned to a criterion that would contribute to the species having a low vulnerability to capture, or having a high capacity to recover. As a precautionary approach, a rank of 1 was assigned to a particular criterion in cases where no species-specific information was available, nor information on closely related species. A description of the ranking criteria and weighting for each criterion is shown in Table 1. The ranks were summed for all susceptibility and recovery criteria for each species, and the values plotted on a two-dimensional graph, with recovery values on the x-axis and susceptibility scores on the y-axis. The species having the lowest ranks on both axes were considered the highest risk species. Below we briefly describe the susceptibility and recovery criteria used in the SRA (after Stobutzki et al. 2001b). Susceptibility criteria Water column position – Prawn trawling in the NPF occurs on the seafloor. As a result, benthic or demersal species are more likely to be captured than benthopelagic or pelagic species. Survival – An estimate was made of the within-net survival, or the mortality of animals before they were landed on the deck. Survival ranges between 0 and 100% and was divided into thirds for the ranks. Range – The geographic distribution of a species within the NPF was determined from the presence or absence of a species in samples collected by fishery dependent and fishery independent surveys in nine fished regions of the NPF. It was assumed that species having a broad geographic range were less at risk of depletion than species with a restricted range. Day and night catchability – Fishing predominantly takes place at night in the NPF tiger prawn fishery.As a result, species more susceptible to capture at night (i.e. nocturnal vertical migrations) are more likely to be impacted by trawling. Diet – The diet of a species may attract them to trawl areas and make them vulnerable to capture. We assumed that species that fed upon commercially important prawns or fed demersally were more susceptible to capture than species that do not feed on prawns or feed in the pelagic zone. Depth range – Trawling in the NPF occurs between 15 and 40 m. We assumed that species that occur within this depth range are more susceptible than species found in deeper or shallower water. Recovery criteria Probability of breeding – This criterion is an indicator of the potential reproductive capacity of a species’ population. We assumed that where the mean length of a species in the catch is greater than the length at sexual maturity, the majority of individuals have had the opportunity to breed before capture, and the population is likely to be sustainable. In contrast, where the mean length of a species in the catch is significantly less than the length at sexual maturity, this may be seen as an indicator that large mature fish have been fished down. As a result, the reproductive capacity of the population is reduced and the population has a lower capacity to recover after depletion. A t-test was used to determine whether the mean length at capture was significantly different from the size at maturity for each species. Maximum size – Maximum size of a species was used as an indicator of relative recovery rate. Larger species generally live longer, thus their populations would recover more slowly after depletion than species with short life spans (Roberts and Hawkins 1999). Removal rate – Removal rate is the percentage of the average annual biomass removed by the fishery as bycatch. Species having a higher proportion of their biomass removed as bycatch were assumed to have
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Table 1. Susceptibility and recovery criteria, definition of ranks and relative weighting used to determine the relative sustainability of 56 elasmobranch species caught as bycatch in the Northern Prawn Fishery after the introduction of Turtle Excluder Devices Table modified from Stobutzki et al. (2001b) Criteria
Weight
Rank 1
Susceptibility Water column position Survival Range Day and night catchability Diet
Depth range Recovery Probability of breedingA
3
3
Demersal or benthic
N/A
Benthopelagic or pelagic
3 2 2
Probability of survival 66% Species range >6 fishery regions Higher catch rate during the day Feed on pelagic organisms
N/A
Deeper than 60 m
1 3
Maximum size
3
Removal rateA Annual fecundity
3 1
Mortality indexA
1
A Criteria
2
Probability of breeding before capture 1755 mm Maximum disc width 853–1755 mm Maximum total length >4781 mm Maximum total length 1861–4781 mm Removal rate >66% Removal rate 33–66% Annual fecundity ≤5 young Annual fecundity 5–19 young per year per year Mortality index >3.47 Mortality index 0.92–3.47
Probability of breeding before capture >50% Maximum disc width ≤853 mm Maximum total length ≤1861 mm Removal rate ≤33% Annual fecundity >19 young per year Mortality index
