In Tasmania, both recreational and commercial gillnetting is permitted. This study, conducted by the Institute for Marine and Antarctic Studies between 2010 and 2013, represents the most comprehensive investigation into the Tasmanian gillnet fishery and its implications for by-catch and biodiversity. Gillnet catch composition (target, by-product, by-catch), post release survival, interactions with threatened, endangered and protected species, and implications of management changes on gillnetting practices were investigated. In addition, catch composition and abundances of key gillnet species over the past 20 years were examined using a range of historical data. This information was then used to inform an ecological risk assessment to identify the vulnerabilities of the species impacted by gillnetting.
Recreational gillnet fishers target a wide variety of species with the main target species being Blue Warehou, Bastard Trumpeter, Atlantic Salmon (escapees from marine farms), Australian Salmon and Yelloweye Mullet. The commercial fishery is a dynamic multi-species fishery with fishers adapting to species availability, market preferences and opportunities. Commercial fishers target similar species to recreational fishers although in the early 1990s a fishery targeting Banded Morwong for the domestic live fish trade developed rapidly and the majority of the commercial effort is now directed at this species.
Over the past decade there have been a number of management initiatives, including a prohibition on overnight recreational netting (with the exception of Macquarie Harbour), introduction of attendance requirements for commercial night gillnetting, and more recently the introduction of maximum soak times for both the recreational and commercial fishery, which have been designed to improve fishing practices and reduce wastage and impacts on non-target species. Despite this, there have been conspicuous declines in the abundance of several key gillnet species along with increasing community concern about the ecological impacts of gillnetting. There is a need, therefore, to better understand how recent management initiatives have influenced netting practices and to objectively assess the risks and impacts on target and non-target species. Ultimately such an understanding will be pivotal in informing the on-going debate over the future management of gillnetting in Tasmania.
1 Synthesise available gillnetting information, with particular reference to links between operational parameters and catch composition
2 Determine catch composition and levels of by-catch associated with the main commercial gillnet fisheries
3 Assess implications of recent management changes on recreational netting practices
4 Assess the relationships between gillnet soak times, capture condition and by-catch survival
5 Evaluate the impacts of gillnetting on the biodiversity of key inshore ecosystems and potential strategies to mitigate these impacts
In relation to Objective 1 available information based on previous research and commercial gillnet catch sampling studies were collated and assessed to examine for regional and temporal changes in target and non-target species abundance. For Objective 2 a variety of data sources were investigated, including commercial logbook data, previous recreational fishing survey data, on-board commercial catch sampling and results from research netting. Objective 3 was primarily addressed through a survey of recreational gillnet fishers and the synthesis of trends based on previous recreational fishing surveys. For Objective 4 research gillnetting trials involving post release survival experiments, along with on-board commercial catch sampling, provided information about operational relationships between soak times, catch condition and by-catch survival. Finally, Objective 5 involved the synthesis of information reported for the present study integrated with long-term biodiversity monitoring data based on underwater visual census surveys and a formal ecological risk assessment of the major Tasmanian gillnet fisheries.
Gillnet fisheries target a range of habitats, including reef and non-reef areas, and land a wide diversity of fish species, with over 90 taxa reported in commercial catch returns. The recreational gillnet fishery targets much the same species as the commercial sector and there is considerable overlap between sectors in the areas fished. For both sectors comparatively few species account for the majority of the landings. Catches in the Banded Morwong fishery are dominated by the target species (>85%), only Bastard Trumpeter and Longsnout Boarfish are of any significance amongst the other species harvested. The general graball net fisheries target a range of species with Bastard Trumpeter, Blue Warehou and Australian Salmon key components of the catch. Bastard Trumpeter, Blue Warehou and Atlantic Salmon (escapees from fish farms) comprise the main species retained by the recreational gillnet sectors. Catches in the commercial small mesh and recreational mullet net fisheries although low, are dominated by Australian Salmon, ‘Pike’ (Snook and Longfin Pike) and Yelloweye Mullet. The difference in catch composition between graball and small mesh nets is due to mesh selectivity, along with the prohibition of setting recreational mullet nets over reef.
In each of the gillnet fisheries a component of the catch is not retained (by-catch), either because of regulation (size or catch limits, closed seasons for selected species, prohibited or protected species) or because of market and/or fisher preferences. The by-catch component, as a proportion of total catch numbers was found to be relatively high; 52% for Banded Morwong fishers, 49% for the general graball fishery, 66% for the small mesh fishery and 35% for the recreational gillnet fishery, although the latter may be an underestimate as it is based on self-reported information. A wide diversity of species that included target species comprised the by-catch component, but in terms of overall contribution to by-catch numbers relatively few species accounted for the bulk of the discards. The main non-target by-catch species included Draughtboard Shark, Marblefish, Bluethroat Wrasse, Leatherjackets and Skates/Rays. Discard rates for by-catch species tended to exceed 80%, whereas discard rates for species typically targeted or retained as by-product typically ranged between 10 – 20%.
Capture condition (based on an assessment of physical damage and responsiveness) and delayed mortality rates (based on tank survival trials) of gillnet caught fish varied between species and were influenced by operational factors including soak time and in some instances season. Several species were particularly resilient, suffering minimal physical damage and low rates of initial and delayed mortality, and experienced high overall post release survival (PRS) rates (>85%) irrespective of soak duration. Species in this category included Banded Morwong, Bastard Trumpeter, Marblefish, Draughtboard Shark, Purple Wrasse, Leatherjackets, Longsnout Boarfish and Skates/Rays. Species with moderately high PRS rates (70 – 85%) included Bluethroat Wrasse, Elephantfish, Whitespotted Dogfish and Bluestriped Goatfish. Southern Sand Flathead, Gummy Shark and Jackass Morwong had lower PRS rates (50 – 70%), while survival rates for a suite of other species including Blue Warehou, Australian Salmon and Atlantic Salmon were quite poor (< 50%).
A number of interactions with threatened, endangered and protected species (TEPS) were observed in this study. Fur Seals were commonly observed in the vicinity of gillnets and the majority of direct interactions with the nets involved provisioning (removal and consumption of entangled fish); there were no instances involving entanglement of seals. Entanglement and drowning of seabirds (Cormorants and Penguins) in gillnets was observed, though such incidences were rare making it difficult to identify contributing factors. In Macquarie Harbour, the endangered Maugean Skate was regularly caught in gillnets set in depths of between about 5-15 m. Although the majority of individuals captured were in excellent condition and lively when released, a small proportion of those captured in overnight deployments were either in poor condition or had died, confirming some by-catch mortality in these longer soak times.
Analyses of historic gillnetting data and underwater visual census data revealed that there have been some changes to species abundance and species composition over the past 20 years but, on the whole, this has been dominated by the decline in Banded Morwong abundance and inter-annual variability in the abundance of Bastard Trumpeter and Blue Warehou. Marblefish abundances have declined in most regions since the mid-1990s despite being rarely retained and having high post release survival. Previous fishing and poor handling practices may have resulted in higher than expected by-catch mortality.
Overnight netting was a common practice for recreational fishers prior to its prohibition in all areas apart from Macquarie Harbour. This ban appears to have had a significant impact on netting effort, not only has it achieved a marked reduction in the proportion of overnight sets but there has been a substantial reduction in overall recreational netting effort. Virtually all recreational gillnet fishers engage in other types of recreational fishing, only a small proportion identified gillnetting as their main recreational fishing activity or that they would consider giving up fishing altogether if they could not gillnet. There was general agreement amongst recreational fishers that recent management changes had been effective in improving fishing practices and in reducing wastage and by-catch.
A formal ecological risk assessment was conducted based on four sub-fisheries that make up the Tasmanian gillnet fishery. These are the large mesh graball (Banded Morwong) sub-fishery, the general graball net fishery, comprised of reef and non-reef sub-fishery components, the latter occurs predominately within shark refuge areas, and the small mesh fishery, which includes commercial small mesh and recreational mullet net components.
Level 1, Scale, Intensity and Consequence Analysis identified that target, by-catch/by-product and TEPS components had consequence scores above moderate for several hazards (principally ‘capture by fishing’, ‘fishing without capture’ and ‘external hazards’). By contrast habitats and communities were judged to be impacted with low consequence by each of the gillnet fisheries and thus were not considered in the Level 2 Productivity Susceptibility Analysis (PSA) assessment.
The PSA identified a number of species at high risk, each specific to a sub-fishery and a result that reflects differences in mesh selectivity as well as differences in the spatial coverage of the fisheries. Bastard Trumpeter was the only species ranked as high risk in the graball (reef) sub-fishery, largely because inshore reefs represent the core habitat for juveniles and sub-adults and the species is particularly susceptible to gillnet capture. None of the species that interacted with the graball (Banded Morwong) sub-fishery were ranked as high risk, predominantly due to the high level of selectivity achieved for the target species by the large mesh size. Atlantic Salmon and Rainbow Trout were ranked as having high vulnerability in the non-reef sub-fishery but, being introduced exotics, this represents a positive ranking, with fishing pressure contributing to their removal from the environment. Maugean Skate and Whitespotted Dogfish were also identified as high vulnerability species; the former has a highly restricted distributional range, presumed low population size and key biological attributes are unknown, and the latter on the other hand is amongst the least productive chondrichthyan species known. Within the small mesh fishery, the Great Cormorant, Rock Flathead and Snook were ranked as having high vulnerability, although low catches and wide distribution outside of Tasmania waters suggest the actual vulnerabilities for the fish at least may not be as high as implied by this analysis. Of the marine mammals, other seabirds and other chondrichthyans considered in the PSA most were ranked as medium vulnerability, mainly due to low productivity levels.
Implications for relevant stakeholders
This study has identified a number of issues that have particular relevance to the future management of gillnetting in Tasmania, noting that gillnet usage has emerged as an area of particular focus in the 2014 review of the Scalefish Management Plan and while it is beyond the scope of the present study to make recommendations on whether or not recreational gillnetting should be banned, this study does provide information that will assist in informing this debate.
There is little doubt that gillnetting has had demonstrable impacts on populations of the key target species, in particular Banded Morwong, Blue Warehou and Bastard Trumpeter. There are specific management measures now in place for Banded Morwong (quota management) and Blue Warehou (Commonwealth stock rebuilding strategy) to help sustain and rebuild populations. There is also a case for management intervention to reduce fishing pressure on Bastard Trumpeter, especially given its high vulnerability ranking; such measures could include expansion of no-netting areas, increase in legal minimum size and/or reduction of bag or trip limits.
This study has established that post release survival of many of the key by-catch species is likely to be high, a situation enhanced by improvements in fishing practices over the past few years. While there would be some benefit, albeit minor, for by-catch survival in reducing the maximum soak time to less than six hours, the prohibition on night netting and introduction of the soak time regulations appear to have been quite successful in reducing wastage and impacts on non-target species.
Interactions with seabirds appear to be an inevitable consequence of gillnetting in shallow coastal waters, though in the main they do tend to occur with low frequency. However, if gillnets are deployed near rookeries, or in corridors that seabirds use to access rookeries, there is potential for interactions involving greater numbers than occurred in the present study. In order to minimise this risk, consideration should be given to establishing no-netting areas around key rookeries. The development of a code of practice for gillnet usage that includes voluntary cessation of gillnet activities while flocks of seabirds (especially Short-tailed Shearwaters) are present in high net use areas would also help reduce the risk of interactions.
This study has established that the endangered Maugean Skate is particularly susceptible to capture in gillnets and although the vast majority are expected to survive, some mortalities, especially in overnight sets, are expected. As a listed species, options to reduce such interactions need to be considered. There are a number of strategies that would help miminise Maugean Skate by-catch and mortality, these include a ban on overnight netting (bringing Macquarie Harbour into line with the remainder of the state), an expansion of the areas closed to netting and/or restricting gillnet usage within Macquarie Harbour to shallow waters (< ~5 m). Implemention of a strategy based on fishing depth may be best achieved through a code of practice and education, noting that the main target species – Atlantic Salmon and Flounder – are commonly caught in the shallows. Deployment of gillnets in shallow waters would also have the benefit of reducing the by-catch of Whitespotted Dogfish, assessed along with the Maugean Skate as having high vulnerability.