Seasonal closure and minimum legal size for barramundi in the Gulf ...

Summary
The current minimum legal length of barramundi in Queensland waters of the Gulf of
Carpentaria is 60 cm. The species is also subject to an annual seasonal closure that
starts and finishes on variable dates that are determined by lunar and tidal cycles.
A monthly size and age structured model was used to predict the impact of changing
both the minimum legal length of barramundi and the seasonal closure in the Gulf to
be consistent with the east coast (58 cm and 1 November to 1 January respectively).
The modeling results indicated that reducing the minimum legal length from 60 cm
back to 58 cm was unlikely to have a detrimental impact on the Gulf barramundi
stock. However, reducing the duration of the closure by one month would be likely to
have a negative impact on the size of spawning stocks as well as overall yield.
While the analysis could not compare the use of a variable closure to one fixed in
time, it appears unlikely that such a move would have any detrimental effect on
barramundi productivity, particularly in the southern part of the Gulf, as long as the
overall length of the closure is not significantly reduced.
The potential changes to the management regime for Gulf barramundi to address the
issues that appear most suitable for consideration are:
setting the Gulf seasonal closure to start on 7 October of every year and end on
1 February of the next year
changing the minimum legal size of Gulf barramundi from 60 cm to 58 cm.
These do not represent preferred management arrangements, but are simply a
starting position for discussion of these issues through the review.
While the current harvest level of Gulf barramundi is considered sustainable, there is
a significant level of fishing pressure on these stocks. It may therefore be worth
considering, even if the risk of impacts on southern Gulf stocks from such changes is
regarded as small, those measures that could even further reduce that risk. Possible
mitigating measures that have been identified are:
Restrict fishing access (prior to 7 October) to a significant estuarine area in the
north western part of Cape York to protect aggregating barramundi and other
spawning species.
Decrease the maximum legal size of barramundi to protect a greater proportion
of female fish and increase egg production.
Amend commercial barramundi fishing net mesh sizes to select for a more
restricted size range of fish and thus increase the spawning biomass.
Introduce another mechanism to constrain the catch of barramundi below
historical levels.
Again it should be noted that these measures do not represent preferred options,
only options for consideration if any change to current management measures is
supported through the process of the review.
Technical report prepared for the Gulf finfish review. Not government policy. 3