Results

Project Objectives:

  1. Collate available information on the biology, ecology, movements and commercial, recreational and artisanal catches of longtail tuna, particularly in Australia
  2. Identify key knowledge gaps in relation to biology and catch data, especially in an Australian context, and recommend future data requirements to support management of longtail tuna in Australia
  3. Develop and recommend methods for cost-effective collection of long-term recreational catch and effort data for longtail tuna in Australia

Non-technical Summary:

Available information and knowledge gaps on the biology, ecology and fisheries for longtail tuna, Thunnus tonggol, were identified in order to assess the status of this ‘recreational-only’ species in Australia.

Specialist sport fishers who catch longtail tuna can be considered a ‘hard-to-reach’ population, which cannot be cost-effectively sampled using traditional recreational fishing survey methods (e.g. creel or telephone surveys). We compared online diaries and ‘time-location sampling’ (TLS) at tackle stores with a traditional access point survey as potential cost-effective survey methods.

The online diary method was inexpensive but unsuitable for collecting representative data due to avidity, volunteerism, and differential recruitment bias. Access point surveys collected high resolution data on catch, effort and size composition of fish caught. Unfortunately, the method was very expensive, longtail tuna were recorded in only 0.3% of trips surveyed, and the results only represented the trailer boat catch. In contrast, TLS was a cost-effective method for obtaining catch and effort estimates from boat-based and land-based fishers who represented both fishing club and non-club members from a range of avidity levels.

Club membership comparison for Time-location sampling versus Online diary

Collectively, 211 days of sampling was conducted using the three methods, where catch and effort data were obtained from a total of 1182 sport fishers, who undertook 4596 fishing trips and expended 25,138 hours of effort. Most sport fishers did not specifically target longtail tuna, but caught them while targeting other inshore pelagic sport fish. On average, sport fishers expended 52 hours per year, and individual fishing trips lasted 5.4 hours. Surveyed fishers caught (retained + released) a total of 892 longtail tuna that ranged between 30 cm to 150 cm TL (average 95 cm or 8.85 kg).

Size distribution of caught fish

The overall catch rate was one longtail tuna for every 12.5 hours fished. The total population size of sport fishers who target or catch longtail tuna is unknown, and therefore our results cannot be expanded to estimate the total recreational catch of longtail tuna. However, our results do show the minimum annual catch from our small sample of sport fishers is ~80 t.

The survey data and biological information from the literature were brought together in a preliminary stock assessment of longtail tuna and to examine the possible effects of introducing size limits as a management strategy. The assessment suggested the stock is currently fished at biologically sustainable levels. However, any increase in fishing mortality may result in recruitment overfishing due to the slow growth rate of the species. Imposing a minimum legal length of 80 cm, 90 cm or 100 cm total length was found to be ineffective for reducing fishing mortality. There was large uncertainty in the assessment due to several key knowledge gaps that require urgent attention including:

  1. potential stock linkages with southeastern Asia where annual commercial catches exceed 100,000 t,
  2. age-at-maturity,
  3. post-capture mortality in recreational and commercial fisheries, and
  4. a time series of representative annual catch and effort for the recreational sector.


Download the Final Report:

The results of this survey have been included in this report to the Australian Government Fisheries Research and Development Corporation. Click on the image below to download the complete report.

Biology, fisheries and status of longtail tuna (Thunnus tonggol) with special reference to recreational fisheries in Australian Waters