Bergenius et a\ Use of otolith morphology to indicate stock structure of Pleaiopomus leopaidus 



509 



shape and chemistry indicated that the movements of 

 aduhs of P. leopardus may be limited and are consis- 

 tent with the presumed sedentary (reef-specific) habit 

 of most serranids after settlement (e.g.. Chapman and 

 Kramer, 2000; Stewart and Jones, 2001). Our results 

 confirm those of several tagging studies of P. leopardus 

 on the GBR which showed that individuals are unlikely 

 to move among reefs after settlement (Davies, 1995; 

 Zeller. 1998; Zeller and Russ, 1998). Such limited in- 

 ter-stock movement of adults means that the recovery 

 rate of stocks after significant harvest is largely (if not 

 solely) dependent on some unknown level of larval dis- 

 persal. Further investigations, therefore, are required 

 to clarify the implications of this potential stock struc- 

 ture of P. leopardus to ensure sustainable harvests at 

 the appropriate scale for both management and the 

 stock structure of the fish. 



Reasons for temporal and spatial variation 

 In otolith structure 



Little is known about what processes influence the 

 expression of traits responsible for otolith shape (Ihssen 

 et al., 1981). Differences in growth rate has been the 

 main reason given for differences in linear morphological 

 variables of otoliths (e.g., Reznick et al., 1989; Secor and 

 Dean, 1989; Begg et al., 2001), as well as outline shape 

 variables (e.g., Castonguay et al., 1991; Smith, 1992; 

 Campana and Casselman, 1993; Begg and Brown, 2000). 

 In our study, P. leopardus subsamples were restricted 

 to a certain size range to minimize the potential of a 

 growth rate effect that could confound spatial or cohort 

 related differences in otolith shape. Significant differ- 

 ences in growth rates (indicated by mean FL of four- 

 year-olds), however, were apparent among reefs within 

 regions in the complete sample from the ELF experiment 

 (Bergenius et al.**), whereas there was no consistency in 

 the patterns of variation in FL and any of the otolith 

 morphological variables. It is possible that the large 

 variation in FL among reefs could have masked potential 



differences in broad-scale regional differences in growth 

 rates, or that some other developmental rate, such as 

 maturation or reproductive output, were more important 

 in influencing the otolith shape of P. leopardus. 



Spatial and temporal variability in otolith shape 

 would be expected in P. leopardus populations along 

 the GBR given the natural variability in biological and 

 hydrodynamic factors between reefs and regions (Wolan- 

 ski, 1994), which in turn could affect various fish de- 

 velopmental rates. There was no monotonic latitudinal 

 trend apparent in any of the otolith morphological vari- 

 ables related to a temperature gradient. The maximum 

 difference in monthly average sea surface temperature 

 among the latitudes examined in our study is typically 

 less than 2°C (Lough, 1994). In contrast, there is re- 

 gional (broad spatial scale) variability in the upwelling 

 and inflow of cool nutrient rich oceanic water across the 

 continental shelf of the GBR (e.g., Andrews and Gen- 

 tien, 1982; Andrews, 1983; Wolanski, 1994; Middleton 

 et al., 1995) which could potentially influence fish de- 

 velopmental rates through changes in food availability 

 or "step-wise" patterns in water temperature. 



There is also a possibility that the temporal variabil- 

 ity in otolith shape of P. leopardus collected before and 

 after Cyclone Justin in 1997 was related to associated 

 temperature or other climatic changes. Cyclone Justin 

 was an unusually large cyclone that remained in the 

 Coral Sea adjacent to the GBR for over three weeks 

 during March 1997. It caused a large and rapid drop in 

 water temperature over the southern half of the GBR 

 (south of ~17-18°S) during this time, and bottom water 

 temperatures dropped in just one month (March) to be- 

 low the average winter minimum not usually reached 

 until July (AIMS''). Our study revealed a significant dif- 

 ference in otolith shape between nonoverlapping cohorts 

 sampled in 1995 and 1999 in the Mackay region, which 

 is one of the regions where the temperature changes 

 would have been greatest. 



Bergenius M. A. J.. B. D. Mapstone, G. R. Russ, and G. A. 

 Begg. 2005. Unpubl. data from the Effects of line fishing 

 experiment (Mapstone et al., 2004). CRC Reef Research 

 Centre, Townsville, Australia. [Data are on files at the CRC 

 Reef Research Centre, James Cook University, Townsville, 

 Queensland 4810, Australia.] 



Management and future directions 



Although differences in otolith shape of P. leopardus may 

 give an indication of stock structure, an examination of 

 the relevance of these patterns for fisheries management 

 should be the next step. A recent increase in the total 



