Do size differences of juvenile snapper (Pagrus auratus) in two regions of Shark Bay, Western Australia, reflect different environmental conditions

Aspects or the population biology of juvenile Pagn,s aura/1/s in the western gulf of Shark day, Western Australia, were investigated to explore the causes of an ob~ervcd size difference between o+ 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 200 I, to collect juvenile P. aura/1/s. The o+ fish from the northern region were found to be consistently greater in length by between IO 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 bet ween fish in the northern and southern regions of the western gulf. Size differences could be explained by (I) different growth rates; (2) age differences associa1ed with different spawning times; and/or (3) fish migrating from ,ioutlt 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 l C'· · ~ 357 days to be tested using ANCOVA, while back-calculating from these ages allowed spawning times to be estilltllted. Examination of the third hypothesis was attempted by tagging 3485 o+ P. aura/1/s in the scuthem region, followed by a trawling program to recapture tagged fish Length-at-age data derived from validated age estimates indicateu that while growth rates . of o+ P. ar,ra/1/s, \ O 0-3 57 days o Id, were similar between regions, the o+ fish in the northern region were \ 5-l 6mm greater in length than fish in the southern region, at n similar age. Brick-calculated birtlt dates ofo+ P. aura/r,s showed . that spawning had · predominantly occurred between June and mid-August in both regions. While no tagged fish were recaptured during the study, indep ~ndent studies examining the spatial movement of P. auratu.1 in the western gulf suggest thnt O+ P. aura/us are unlikely to migrate between regions. Hence, the difference in length of fish between regions almost certainly resulted from differences in growth rates within the ftrSt few months of life (<100 days of age). A laboratory-based aquarium experiment was carried out at water temperatures of 18, 22 and 26'(' and salinities of 36, 39 and 42%o to determine whetlter tempernturo and/or salinity iii influences growth ofo+ P. auratus. ANOVA showed that growth, in terms oflength, of . . . o+ P. auratus increased significantly nt temperatures of 22 and 26"C compared to l 8°C. Furthennore, in tenns of weight, growth increased significantly with each 4°C increase in temperature-.Jn comparison, gr01vth was higher nt salinities of39%o than at 36 or 42%. but only in temJS of length. Sen surface temperatures (S STs) derived from remote:sensing satellite data demmistrated that P. auratus experienced SSTs 2-3°C cooler in the southern region c·ompared to the northern region of the western gulf. In light of this, and the results . of the aquarium experiment; depressed growth of wild O+ P. aura/us in the southern region · was most likely due to the cooler water temperatures occurring during their first few months of life . ·. · . ·. · . The growthffite differences between the o+ P. aura/us of the northern and southern regions are discussed wit1mgard to their 1ack orsignmcance tothe appucation of . minimum size limiB to adult P. aura/US in Shark Bay: Further implications of the know ledge gained from this study to management practices app Ii ed to the snapper fig hery . in Shark Bay are highlighted anil\iiscussed: In particuiar, a recommendation wns made to modify the dates of the closed snapper-fishing season aro:ind spawning t~ July-August of. each year, rather than mid~Angust to September, as is currently the cnse. Knowledge gained from the present study is also highly applicable to any futllreP. aura/us i:esto~king programs in Shark Bay and may enhance the effectiveness of such projecB by identifying: optimal temperature and snlinity. conditio~s for ari:ific ial re~ing of snapper; favourable . locations, times of year and optimal fish size for releases; and providing valuable advice regarding tagging methods. ·