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

Discussion on analysis
The analysis showed that the fixed four-month closure from October to January
resulted in better total yield (i.e. catch) than the fixed three-month closure, particularly
when the fishing pressure was high. It is considered that an extra one-month closure
protects a proportion of barramundi from fishing activities and allows them to grow
quickly during the summer season, which results in an increase in overall yield.
Decreasing minimum legal size limit by 2 cm has a positive effect (i.e. up to 12%
increase) on recreational yield but almost no impact on commercial yield (up to 2%).
This was because recreational fishers tend to harvest smaller fish (i.e. high selectivity
around the minimum legal size) than commercial sectors, which have a low
selectivity of 58–60 cm fish in commercial nets. However, as the scale of fishing
pressure of the recreational fishing is a lot smaller than commercial fishery (1/10 of
commercial), the impact of changing the minimum legal size on overall yield was
relatively small. Consequently, changing the duration of the closure was the most
significant factor affecting overall yield.
The fixed three-month closure resulted in a decline in egg production of up to 30%.
However, the current analyses assumed constant recruitment (i.e. no spawner–
recruitment relationships). This means that even if the spawning index becomes
small (e.g. only 10% of virgin spawners remain), the same level of new recruits were
added into the population model each year. Therefore, the current model was unable
to detect the impacts of low spawning stock on the subsequent recruitment (i.e.
unable to detect recruitment overfishing). As a consequence, the current analyses
were likely to underestimate any declines in egg production (spawning fish).
In conclusion, reducing the minimum legal size from 60 cm to 58 cm was unlikely to
have a detrimental impact on the Gulf barramundi stock in terms of yield or
abundance of mature males under the model. However, reducing the duration of the
spawning closure by one month was likely to have negative impacts on the size of
spawning stocks as well as overall yield under high fishing pressure.
Technical report prepared for the Gulf finfish review. Not government policy. 17