Mackie et al.: Variability in reproductive development of Spanish mackerel (Scomberomorus commerson) 



353 



large, more solitary individuals may spawn in inshore 

 waters. 



Catches of S. commerson peak and decline rapidly 

 along the Kimberley coast during the main spawning 

 period because of declining fish abundance and weather 

 conditions. As in the Pilbara region, few S. commerson 

 are caught at this time in southern or midsections of 

 the Kimberley coast. Fishermen must therefore under- 

 take extensive trips north to the remaining productive 

 grounds located between 12.5° and 15°S latitude where 

 the majority of S. commerson spawning activity was 

 encountered in the present study. Although it is pos- 

 sible that S. commerson in other areas of the Kimberley 

 region may move offshore to spawn, it is also possible 

 that some move northward, mixing and spawning with 

 otherwise temporally and spatially discrete northern 

 populations, in a similar manner to S. cavalla in U.S. 

 waters (Broughton et al., 2002). 



Monitoring of the WA fishery for S. commerson is 

 likely to be based on the collection of head and gonad 

 samples because limited funding and large distances 

 will restrict future research trips. Onboard storage of 

 filleted frames for research purposes is also prohibited 

 by the large body size of S. commerson. In contrast, the 

 head of this species is relatively small and easy to store, 

 and as shown in the present study, provides a general 

 measure of reproductive activity through calculation of 

 head-to-gonad ratios. These ratios can also be supple- 

 mented by staging the gonads by using the macroscopic 

 staging system developed for this species (Mackie and 

 Lewis 2 ). Head length can also be used to estimate body 

 length of S. commerson (Mackie et al. 3 ) and the otoliths 

 contained in the head can be used to determine age. 

 Although data gathered by such means is less accu- 

 rate than that obtained from whole, fresh samples, it 

 presents the best option for gathering ongoing data in 

 sufficient quantities for meaningful analyses. 



Acknowledgments 



The authors thank the numerous commercial and rec- 

 reational fishermen who assisted in the collection of 

 samples and provided invaluable advice. The assistance 

 of Department of Fisheries staff and volunteers on field 

 trips is also appreciated. We also thank Rod Lenanton, 

 Rick Fletcher, and Peter Stephenson for reviewing the 

 manuscript, and to the Fisheries Research and Develop- 

 ment Corporation for funding Project 1999/151, of which 

 this study formed a part. 



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