Stock assessment and efficacy of size limits on longtail tuna (Thunnus tonggol) caught in Australian waters

A stock assessment of longtail tuna (Thunnus tonggol) in Australian waters was undertaken using per-recruit analyses to assess: (i) the current stock status, and (ii) the effect of Minimum Legal Lengths (MLL) as a management tool. Exploited age compositions differed between the commercial (age classes 3-4 years) and sport fishery (4-6 years). The fishing mortality (Fcurrent) from these fisheries for 2004-2006 was estimated as 0.167-0.320 year−1. Yield-per-recruit analyses revealed that size limits and improving post-release survival had no significant effect on the population. This was due to fish becoming vulnerable to both fisheries at age 2-3 years, after fish presumably had the opportunity to spawn. Under all MLL scenarios, the current fishing mortality rate did not exceed biological reference points. However, any significant increase in fishing mortality may result in recruitment overfishing due to longtail tuna being slow-growing and the stock currently in the vicinity of the F40% reference point. Since longtail tuna are now a “recreational-only” species in Australia with minimal commercial bycatch, investigation of daily catch limits for the sport fishery requires exploration using more sophisticated age-structured stock assessment models.