Does fishing affect reproduction in common coral trout?

By Samantha Adams

The common coral trout, Plectropomus leopardus, are prized targets for most fishers on the Great Barrier Reef. They are protogynous hermaphrodites - females when they are small and then change sex to become males later in life. Fishers will always try and catch the biggest fish, and therefore the oldest. In the case of coral trout these will generally be the males. Therefore, fishing could cause changes to the ratio of males to females in a population (or sex structure of the population). This is an important consideration for fisheries management because it may have a strong influence on reproduction and therefore, the number of juveniles coming into the fishery in the future.

The Effects of Line Fishing (ELF) Experiment by CRC Reef researchers is studying the effects that fishing has on the sex structure of coral trout populations. Comparing populations of coral trout on open reefs with those protected by Great Barrier Reef Marine Park Authority (GBRMPA) zoning where fishing is not allowed can give information about how fishing effects the sex structure of the populations.

From prior information about coral trout biology and the fishery, it was considered that coral trout would respond to fishing in one of two ways:

• If sex change was triggered by social cues (such as the ratio of males to females), when males were removed, the female could change sex at an earlier and smaller stage to balance the sex ratio. The result would be smaller and younger fish, both male and female, in fished populations. Smaller females produce fewer eggs than larger fish and therefore, fewer young; an important consideration for the fishery in the future.
• If sex change in coral trout is fixed at a certain age or size, fishing will have no affect on this, and fished populations would presumably have less males than females. This could also affect reproduction if there were not enough males in the population to fertilise the female's eggs at spawning time.

The research has concentrated on two areas in the Great Barrier Reef - the Swains and near Townsville. There are large differences in the sexual structure of the populations of coral trout between the two regions. Indeed, the difference in the sexual structure of coral trout between Townsville and the Swains was often greater than any differences between open and closed reefs. However, there were some patterns that indicated there were fishing effects.

In both regions, female fish were consistently younger and smaller on the open reefs, indicating that females may be changing sex earlier in fished populations. However, the male fish were smaller on reefs open to fishing in the Swains region. Interestingly, male fish in the Townsville region, while not being smaller, were a lot younger on reefs open to fishing compared with closed reefs in that area. Possibly, fishing may be affecting the growth rate of fish.

The results show that the response of sexual structure of coral trout is likely to be more complex than previously thought, and is further complicated by regional differences. The research is continuing to try and overcome these problems by looking at more areas in the Great Barrier Reef. This will allow us to investigate more factors that could affect the sexual structure of coral trout populations, and identify these from regional differences.