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

in capturing all peak spawning moons if rain doesn’t subsequently fall at the
right time.
The current size limit and seasonal closure on the east coast is very well
established—changing either may have resulted in difficulties with compliance.
Discussion
Summary of analysis
In order to examine the impact of shortening the seasonal closure, a monthly size
and age structured model was developed. This analysis was undertaken in
conjunction with modelling the impact of amending the minimum legal size to 58 cm.
Details of this analysis can be found in the ‘Modelling of impacts’ section of this
paper; however, it should be noted that the analysis does not provide an absolute
estimate of yield or egg production; it simply provides a comparison between closure
and size limit scenarios.
Two alternative lengths of seasonal closure and two levels of minimum legal size
were examined through the model. The two spawning lengths were four months, to
reflect the current Gulf closure, and three months to reflect the impact of aligning the
closure with the east coast closure that runs from 1 November to 1 February. There
is no quantitative information, however, to assess the relative importance to fishery
productivity of the lunar-based seasonal closure compared to one that is fixed in
time.
Under the model overall yield tended to be higher (up to 6%) with a four-month
closure compared to a three-month closure, but there was no noteworthy overall
difference between the alternative minimum legal sizes.
Minimum legal length
The model indicated that decreasing the minimum legal length limit by 2 cm has a
positive effect on recreational yield but almost no impact on commercial yield. This
was because recreational fishers tend to harvest smaller fish than the commercial
sector, due to a low selectivity of 58–60 cm fish in commercial nets. However, as the
scale of fishing pressure from recreational fishing for barramundi in the Gulf is a lot
smaller than from the commercial fishery, the impact of changing the minimum length
on overall yield would be expected to be relatively small.
The model also indicated that a size reduction was unlikely to have a detrimental
impact on the Gulf barramundi stock in terms of abundance of mature males. Davis
(1982) showed that 50% of southern Gulf barramundi reach sexual maturity at or
around 60 cm. Such a 50% maturity level is often used to set the minimum legal size
of fish, so from that perspective the current size appears appropriate.
Barramundi growth rates tend to be very high, however, particularly as the water
begins to warm. As an example a 58 cm fish from the Fitzroy River in central
Queensland can be expected to grow at a rate of up to 15 mm per month in the
period from October to December (see Figure 7). At this rate a fish just below the
minimum legal size at the start of the seasonal closure could be expected to grow to
sexual maturity before the end of the closure.
Technical report prepared for the Gulf finfish review. Not government policy. 6