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