Size limits for coral trout

By Samantha Adams

Cods, trout and groupers, members of the epinepheline serranid group, are important targets of tropical and subtropical fisheries worldwide. Many members of this group change sex to male after reproducing initially as a female. This reproductive strategy needs to be considered in managing these fishes or their reproductive success could be compromised and the sustainability of their stocks affected.

CRC Reef researchers from the Effects of Line Fishing Project have been investigating the reproductive strategies of three of the most abundant and prized coral trout species in the Queensland reef line fishery: common coral trout Plectropomus leopardus; bar-cheek trout P. maculatus; and blue-spot trout P. laevis. All three species have the same minimum size regulations. However, prior to the current research, it was unclear what proportion of mature fish would be protected by current size limits, and if this was consistent across species.

Appropriate minimum size limits help to ensure adequate protection of spawning individuals. However, if the size limit is set below the size at which sex change occurs, one sex may not be protected. For coral trout, this could reduce reproductive output if there are not enough males to fertilise eggs during spawning.

Results show that for females of both common and bar-cheek trout, the current size limit of 38 cm (total length) protects individuals until they have spawned at least once, and some possibly twice. However, for blue-spot females, less than 5% are likely to have spawned before reaching 38 cm.

For the males, there is even more variation. The size at which common coral trout and bar-cheek trout change sex to male is very flexible; many have already changed sex to male and spawned (as a male) before reaching 38 cm. Although most of the fishable stock in bar-cheek trout are male. Furthermore, for blue-spot trout, very few if any have changed sex to male before reaching legal size.

This research indicates that for common and bar-cheek coral trout current legal size limits provide adequate protection for both males and females. However, for blue-spot trout, a minimum size limit of 38 cm may not be adequate protection, especially in heavily fished populations. A size limit of 60 cm for blue-spot trout as proposed in the Draft Management Plan for the Queensland reef line fishery would protect the females of this species, allowing them to spawn at least once prior to reaching legal size. However, only small numbers would have made the sex change to male before reaching this size.