Tailoring codend mesh size to improve the size selectivity of undifferentiated trawl species

Tiger flathead, Neoplatycephalus richardsoni (Castelnau), and sand flathead, Platycephalus bassensis Cuvier, are undifferentiated and managed with a common legal minimum length (LML). The Commonwealth Trawl Sector (CTS) and the Tasmanian Danish‐seine fishery (TDSF) use a minimum codend mesh sizes of 90 and 70 mm, respectively. The codend mesh size should be tailored to the LML, which is based on the length of first maturity of females (M₅₀). This study found the length–girth relationship of N. richardsoni and P. bassensis was not significantly different. Using the covered codend method, these two species had 50% retention lengths (L₅₀) of 294 ± 2 and 307 ± 3 mm, in 70‐mm and 90‐mm codends, respectively. L₅₀ estimates for mesh sizes from this study and others produced a curvilinear relationship: y = 120 ln(x) ? 214, with an r² of 0.8504. The size at maturity (M₅₀) for female N. richardsoni was 337 mm, which is larger than the estimate for female P. bassensis (247 mm). There is a mismatch between the estimates of L_50, the estimates of M₅₀ and the LMLs in each fishery, leading to suboptimal exploitation of female Platycephalus. The model produced in this paper recommends a codend mesh size of 98 mm for both fisheries to exploit Platycephalus species sustainably.