Assoc. Professor Jeremy Lyle, Graeme Ewing
Since 2012, IMAS has conducted annual fishery-independent fishing surveys to identify trends in Sand Flathead populations off south-eastern Tasmania.
Surveys are based on fishing gear and targeting practices typical of recreational fishers.
Standardised and consistent sampling design is used to allow comparison of key population parameters – size and age composition, catch per unit effort, and mortality rates, between areas and over time.
As Sand Flathead are almost exclusively harvested by the recreational sector, this represents key information in assessing stock status and implications of management change.
Sampling is undertaken in areas of significant recreational fishing activity, including D’Entrecasteaux Channel, Frederick Henry-Norfolk Bay and Great Oyster Bay, with between 18-21 sites fished per region.
Trends in catch rates, size and age structure since the increase in minimum size limit (from 300 to 320 mm) in 2015 suggest some stock rebuilding has occurred, particularly in the most heavily exploited region of the fishery (D’Entrecasteaux Channel).
However, this trend was masked in the Great Oyster Bay region by high catches early in the time-series of dominant year classes (2007 and 2008), which have subsequently passed out of the population.
Catch curve analysis has generated plausible estimates of total and fishing mortality, with higher rates for females. This is consistent with their longer exposure to the fishery, due to faster growth rates and greater maximum sizes.
Comparison of mortality rates prior to and after the increase in minimum size limit show marked reductions in the mortality of males. This reflects the slower growth and smaller maximum sizes attained by males, that is, they are less exposed to the fishery.
The reduction in fishing mortality for females has been more modest, despite a lower bag limit introduced at the same time as the size limit increase, and is linked to the fishery which is now effectively selecting for females.
Image (above right): Sand flathead Platycephalus bassensis. Photo Rick Stuart-Smith, Reef Life Survey.
Image (below): Average catch rates (fish per line hour) by region and year for Sand Flathead: (a) raw catch rates; (b) standardised catch rates; (c) standardised catch rates for fish above 300mm, and (d) standardised catch rates for fish above 320mm. Error bars represent 95% confidence intervals.
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