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Why does Australia have Legal Fish Sizes and Bag Limits

We have found this great explanation from CRC Reef Research Centre - Explaining The science behind size limits.

A major tool in fisheries management is the setting of minimum size limits to protect fish from being caught until they have spawned at least once. Fishers can only legally keep fish that are longer than the minimum size limit. This ensures that fish are protected from harvest long enough to reproduce, ensuring that there will be enough new recruits to replenish the fishery each year.

The life history of tropical reef fishes can complicate the setting of minimum size limits. For example, many tropical reef fishes such as coral trout and red throat emperor usually change sex during their lives. Sex change can be triggered by genetic factors such as age or size, or environmental factors such as the numbers of other males or females present at spawning sites. These responses can mean that sex change occurs to maintain the ratios of reproductive males and females in a population.

Fishers usually catch the biggest fish, and therefore the oldest fish, which are more likely to have changed sex. For coral trout, these will generally be males. Therefore, fishing could change the ratio of males to females (or sex structure) in a coral trout population. Removal of too many males could mean that there are insufficient males to fertilise eggs during spawning, especially if sex change is determined genetically and not flexible in response to environmental factors. This may influence the success of reproduction and, therefore, the sustainability of the fishery in the future. To overcome this problem, implementation of maximum size limits may also be considered for some species.

Because tropical fish life histories can be so complex, managers need an intimate knowledge of the biological characteristics of fish species to decide if size limits are necessary and what minimum legal size should be set. CRC Reef researchers from the Fishing and Fisheries Project have been studying the breeding and growth of about a dozen target and non-target fish species to provide managers with this information.

The researchers have been working on three of the most abundant and prized coral trout species in the reef line fishery: common coral trout Plectropomus leopardus; bar-cheek trout P. maculatus; and blue-spot trout P. laevis. Currently, all three species have the same minimum size regulations. However, prior to the research, it was unclear what proportion of mature fish in each species would be protected by current size limits.

Results from the research show that for females of both common and bar-cheek trout, the current size limit of 38 cm (total length) is comparatively conservative and protects fish until they have spawned in at least one year, and possibly two. However, fewer than 5% of blue-spot females 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 varies; meaning that many already have changed sex to male and spawned (as a male) before reaching 38 cm. Most of the fishable stock of bar-cheek trout are male. For blue-spot trout, few if any fish have changed sex to male before reaching legal size.

The research shows that for common and barcheek coral trout current legal size limits adequately protect both males and mature females. However, for blue-spot trout, a minimum size limit of 38 cm may not be adequate protection for either males or females, 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 Coral Reef Fin Fish Fishery, would protect the females of this species, allowing them to spawn at least once before reaching legal size. However, few fish would have changed sex to male before reaching this size.

Research has also led to a shift in thinking about other reef fish. Many reef fish were thought to live fast and die young and so be reasonably resilient to fishing pressure. But CRC Reef researchers have found that some reef fish live much longer than was once thought. For example, the bommie cod Cephalopholis cyanostigma lives for up to 45 years and only reaches about 30 cm long. Although bommie cod are not usually kept by fishers in Australia, they may become targets for fishing in the future if other species become less abundant or market trends change. Importantly, many of the other groupers that are popular in the Hong Kong markets are being found to have similar life histories to bommie cod and may be more susceptible to overfishing than previously thought.





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