CRC REEF RESEARCH CENTRE TECHNICAL REPORT No. 3

A Long-Term Study On Population Structure Of The Coral Trout Plectropomus Leopardus On Reefs Open And Closed To Fishing In The Central Great Barrier Reef.

Garry R. Russ, Department of Marine Biology, James Cook University
Dong C. Lou, Department of Marine Biology, James Cook University
Beatrice P. Ferreira, Department of Marine Biology, James Cook University

EXECUTIVE SUMMARY

The objectives of this study were to measure the effects of line fishing on the average size, age and sex ratio of coral trout, Plectropomus leopardus in the central Great Barrier Reef (GBR). From 1990 to 1993 coral trout were collected by line fishing at two coral reefs, Glow and Yankee, closed to fishing since 1987, and at two other reefs, Grub and Hopkinson, that have always been open to fishing. There was no significant difference in average size or age of coral trout on reefs closed and open to fishing, despite the reefs having been closed for 6 years by 1993. This may have resulted from illegal fishing on the closed reefs and/or fishing pressure on the open reefs being low. There were significant differences in average size and age of trout between the open reefs (Grub, Hopkinson) and the closed reefs (Glow and Yankee), reflecting natural variability in populations between reefs or variations in accessibility of open reefs, with Grub being a much better anchorage than Hopkinson and thus being more heavily fished.

On the two reefs closed to fishing, the population structure was dominated by a strong cohort (year class) which settled onto the reefs in early 1984 (i.e. 6, 7, 8, 9 years old in 1990, 1991, 1992 and 1993 respectively). The strong cohort accounted for 54%, 45%, 42% and 36% of the experimental hook and line catches on reefs closed to fishing in 1990 to 1993 respectively. Note that maximum age for coral trout is around fourteen years on Townsville reefs. This is the first time a dominant age class has been tracked over time for any coral reef fish by examination of age structures. The results demonstrate that strong interannual fluctuations in recruitment can influence the abundance of coral trout populations substantially, and that the effect of strong recruitment events can be retained in the age structure of coral trout populations. On the closed reefs an event that occurred in 1984 was still dominating the fishery 9 years later in 1993. This finding has important implications for the management of the fishery and zoning (reef closure) strategy of the Great Barrier Reef Marine Park. It suggests the possibility that abundance of coral trout in the fishery may be predicted two to four years in advance using yearly surveys of newly settled juveniles, in a similar way to the use of settlement intensities of larval lobsters to predict catches of western rock lobsters four years in advance. Furthermore GBRMPA may, in future, be able to arrange timing and location of reef closures, based on counts of newly settled juveniles, to best maintain regional populations of trout.

A similar strong cohort was not observed on the fished reefs close by, although there was a suggestion of such a cohort on Hopkinson reef. If such a strong cohort did settle onto the open reefs fishing may have reduced its abundance relative to other age classes in the population. However, this would imply the unlikely scenario of the strong age class somehow being more vulnerable to the line fishing gear than the other age classes. The size structure of populations at all four reefs remained relatively consistent, with no dominant size group in the population over time at both closed and open reefs. Large variability in size at a given age meant that age but not size structure provided a good indicator of population dynamics.

By examining the stages of development of the gonads throughout the year the annual spawning season was confirmed to be from early spring to early summer off Townsville. Age at first reproduction was around two to three years old (30 to 35cm, total length). Coral trout change sex from female to male. Age and size at sex change was very variable. Sex change was observed in coral trout from ages three to twelve years and from lengths 30 to 55cm total length. Thus it was difficult to detect a clear effect of fishing on the size and age at sex change. Since line fishing tends to be selective for larger animals, fishing may affect the sex ratio by reducing the proportion of males (which on average are larger than females). The sex ratio varied among reefs and showed a slightly higher proportion of males on the closed reefs. However, there were no significant differences in size or age of males between reefs closed and open to fishing. The main conclusion of this part of the work was that closure to fishing did not have a strong effect on the sex structure of coral trout off Townsville. Again, this may have resulted from illegal fishing on closed reefs and/or fishing pressure on the open reefs being low.

In summary, the effects of fishing on the average size, age and sex ratio of coral trout were inconclusive. However, the presence of a dominant year class in the population of trout on the two closed reefs showed that strong interannual fluctuations in recruitment can influence the abundance of coral trout populations substantially. This effect of strong recruitment events can be retained in the age structure of coral trout populations. On the closed reefs a recruitment event that occurred in 1984 was still dominating the fishery nine years later in 1993. This suggests the possibility that abundance of coral trout in the fishery may be predicted two to four years in advance using yearly surveys of newly settled juveniles.

ISBN 1 876054 02 6.

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