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

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Table 5 Scenarios tested in the model. Scenario no. Closure period Minimum legal size Spawning pattern* 1 3 (Nov–Jan) 580 Early 2 3 580 Normal 3 3 600 Early 4 3 600 Normal 5 4 (Oct–Jan) 580 Early 6 4 580 Normal 7 4 600 Early 8 ** 4 600 Normal * Figure 11 in appendix ** Current management arrangement: status quo Results Yields per recruit The total yield curve is shown in Figure 1 and yields for each fishing sector are shown in Figure 2 and Figure 3 respectively. Commercial yield tended to be marginally higher (up to 8%) with a four-month closure (Scenarios 5 to 8) compared to a three-month closure (Scenarios 1 to 4). Within each closure, commercial yield was consistently slightly higher (up to 2%) with a minimum legal size of 60 cm compared to a minimum legal size of 58 cm. Recreational yield tended to be higher (up to 12%) with a minimum legal size of 58 cm (Scenarios 1, 2, 5, 6) than 60 cm (Scenarios 3, 4, 6, 7). Within each minimum legal size, recreational yield was consistently higher with a four-month closure than a three-month closure, but the difference was minimal (up to 1%). Overall yield was similar to that of the commercial sector. A higher yield of up to 6% was observed with a four-month closure compared to a three-month closure, but there is no noteworthy difference among minimum legal size and spawning patterns (
Relative proportion 1 0.8 Commercial yield 0.6 Scenario1 0.8 Scenario2 Scenario3 0.79 0.4 Scenario4 0.78 Scenario5 0.2 Scenario6 0.77 Scenario7 0.76 Scenario8 0 0 0.5 1 1.5 2 1.85 1.86 1.87 1.88 1.89 1.9 F F Figure 2 The relative commercial yield shown as proportion scaled to 1 at the maximum yield (F=0.7985, Scenario 7) Recreational yield Zoom 1 1 0.82 0.81 Zoom Relative proportion 0.8 0.6 0.4 0.2 Scenario1 Scenario2 Scenario3 Scenario4 Scenario5 Scenario6 Scenario7 Scenario8 0 0.5 1 1.5 2 F Figure 3 0.95 0.9 0.85 1.6 1.7 1.8 1.9 F The relative recreational yield shown as proportion scaled to 1 at the maximum yield (F=1.8971, Scenario 5) Spawning per recruit Total number of eggs per recruit was higher with a four-month closure than a threemonth closure and this difference became larger with greater fishing pressure (Figure 4). Figure 5 shows the ratio of the spawning index with a three-month closure to the index with a four-month closure. With the highest F applied in the model (F =1.8971 ~ 85% harvest rate), the spawning stock would be about 30% lower with a three-month closure than with a four-month closure. Under moderate to high fishing pressure (F = 0.4308 to 0.7985 ~ 35–55% harvest rate), the spawning index would be about 15% lower with a three-month closure. Reducing the minimum legal size to 58 cm resulted in up to 3% less abundance of mature males and up to 3% less egg production (under the assumption of constant recruitment though time). Technical report prepared for the Gulf finfish review. Not government policy. 15
Page 1 and 2: Seasonal closure and minimum legal
Page 3 and 4: Contents Contents 1 Purpose of the
Page 5 and 6: Summary The current minimum legal l
Page 7 and 8: Protection for spawning barramundi
Page 9 and 10: While the significance of this to m
Page 11 and 12: seasonal closure. For this reason t
Page 13 and 14: Modelling of impacts As discussed p
Page 15: Table 4 Definitions of symbols Nota
Page 19 and 20: Discussion on analysis The analysis
Page 21 and 22: Appendix Model settings Table A1 Ba
Page 23 and 24: 12 10 log(ML/month) 8 6 4 2 0 2 4 6
Page 25 and 26: 0.5 0.4 Average spawning pattern Ea

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