Date of Award

1-1-2003

Document Type

Thesis

Publisher

Edith Cowan University

Degree Name

Master of Science

School

School of Natural Science

Faculty

Faculty of Computing, Health and Science

First Supervisor

Glenn Hyndes

Second Supervisor

Mike Moran

Abstract

Aspects of the population biology of juvenile Pagrus auratus in the western gulf of Shark Bay, Western Australia, were investigated to explore the causes of an observed size difference between 0+ fish (first year of life) in the northern and southern regions of that gulf. Five trawl surveys were conducted in each region of the western gulf, from November 2000 to December 2001, to collect juvenile P. auratus. The 0+ fish from the northern region were found to be consistently greater in length by between 10 and 20mm LCF, than those in the southern region, thereby confirming those observed size differences. Three hypotheses were developed and investigated to explain the confirmed size difference between fish in the northern and southern regions of the western gulf. Size differences could be explained by (I) different growth rates; (2) age differences associated with different spawning times; and/or (3) fish migrating from south to north. The first and second hypotheses were tested by determining the daily ages of 125 fish collected by trawling in both regions between November 2000 and December 2001. Length-at-age data allowed the growth patterns of fish aged from 100 to 357 days to be tested using ANCOVA, while back-calculating from these ages allowed spawning times to be estimated. Examination of the third hypothesis was attempted by tagging 3485 0+ P. auratus in the southern region, followed by a trawling program to recapture tagged fish. Length-at-age data derived from validated age estimates indicated that while growth rates of O+ P. auratus, 100-357 days old, were similar between regions, the 0+ fish in the northern region were 15-l6mm greater in length than fish in the southern region, at a similar age. Back-calculated birth dates of O+ P. auratus showed that spawning had predominantly occurred between June and mid-August in both regions. While no tagged fish were recaptured during the study, independent studies examining the spatial movement of P. auratus in the western gulf suggest that 0+ P. auratus are unlikely to migrate between regions. Hence, the difference in length offish between regions almost certainly resulted from differences in growth rates within the first few months of life (age). A laboratory-based aquarium experiment was carried out at water temperatures of 18, 22 and 26oC and salinities of 36, 39 and 42% o to determine whether temperature and/or salinity influences growth of O+ P. auratus. ANOVA showed that growth, in terms of length, of 0+ P. auratus increased significantly at temperatures of 22 and 26oC compared to 18oC. Furthermore, in terms of weight, growth increased significantly with each 4oC increase in temperature. In comparison, growth was higher at salinities of 39%o than at 36 or 42%o but only in terms of length. Sea surface temperatures (SSTs) derived from remote-sensing satellite data demonstrated that P. auratus experienced SSTs 2-3oC cooler in the southern region compared to the northern region of the western gulf. In light of this, and the results of the aquarium experiment, depressed growth of wild 0+ P. auratus in the southern region was most likely due to the cooler water temperatures occurring during their first few months of life. The growth rate differences between the 0+ P. auratus of the northern and southern regions are discussed with regard to their lack of significance to the application of minimum size limits to adult P. auratus in Shark Bay. Further implications of the knowledge gained from this study to management practices applied to the snapper fishery in Shark Bay are highlighted and discussed. In particular, a recommendation was made to modify the dates of the closed snapper-fishing season around spawning to July-August of each year, rather than mid-August to September, as is currently the case. Knowledge gained from the present study is also highly applicable to any future P. auratus restocking programs in Shark Bay and may enhance the effectiveness of such projects by identifying: optimal temperature and salinity conditions for artificial rearing of snapper; favourable locations, times of year and optimal fish size for releases; and providing valuable advice regarding tagging methods.

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