352 



Fishery Bulletin 103(2) 



and Salter, 1990; Begg, 1998). In the case of S. com- 

 merson, females usually dominate size classes above 

 the MLL because they grow faster and reach a larger 

 maximum size (McPherson, 1992; Mackie et al. 3 ). How- 

 ever, they are rarely caught when actively spawning, 

 despite observations by fishermen of leaping fish that 

 might indicate that they are still present on the fish- 

 ing grounds. Regional differences in fishing gear can 

 also affect catchability. The lighter monofilament and 

 reel outfits used in the Pilbara and West Coast regions 

 likely catch larger fish than the heavier rope and thick 

 monofilament hand-hauled rigs used by Kimberley fish- 

 ermen that do not allow the fish to be "played" (i.e. do 

 not allow the fish to swim) and may result in more 

 gear failure. 



The spring-summer spawning pattern observed in 

 our study is similar to that of S. commerson along the 

 east coast of Australia (McPherson, 1981). Water tem- 

 perature may influence spawning in fish by affecting 

 gametogenesis, gonad atresia, and spawning behav- 

 ior (Lam, 1983). In WA waters S. commerson spawn 

 as water temperatures are rising and, as found in 

 Queensland, may compensate for latitudinal differences 

 in temperatures by spawning earlier in northern waters 

 (McPherson, 1981). No evidence of spawning activity 

 was found within the West Coast region although the 

 annual range of water temperatures overlap with those 

 in which spawning occurs farther north. Restricted 

 spawning by S. commerson on the east coast occurs at 

 similar latitudes to northern parts of the West Coast 

 region, and anecdotal evidence suggests that spawning 

 may be restricted in some years in this region. 



During the spawning period the average female S. 

 commerson may spawn every three days and about one 

 third of fish spawn on consecutive days. Female fish 

 similarly spawn every 2-6 days and possibly on con- 

 secutive days in Queensland waters (McPherson, 1993). 

 Our study showed that estimates based on the fraction 

 of histologically staged prespawning (stage 5a) ova- 

 ries provided the best estimate of spawning frequency. 

 However, only samples taken during the morning can 

 be used for this analysis because of decreased catchabil- 

 ity of running, ripe females in the afternoon. In com- 

 parison, macroscopic staging of ovaries with hydrated 

 oocytes underestimated spawning frequency because 

 migratory nucleus oocytes (which comprised 54% of 

 histologically staged, prespawning [stage 5a] ovaries) 

 cannot be identified. It is also impossible to identify fish 

 that have spawned on more than one occasion with mac- 

 roscopic criteria, resulting in a further underestimate of 

 spawning activity (by 25% for S. commerson). 



Maturation, ovulation, and spawning of oocytes by 

 female S. commerson was completed within a 24-h cycle 

 in the Kimberley region compared to 24-36 hours in 

 Queensland waters (McPherson, 1993). Maturation of 

 the oocytes is underway by sunrise and probably com- 

 pleted in all spawning ovaries by mid to late morning to 

 allow for ovulation prior to spawning in the afternoon. 

 Few samples were obtained at or after dusk because 

 fish are generally not catchable, indicating that a high 



incidence of spawning at this time because only one 

 spawning fish was obtained during the study. Dusk 

 spawning is prominent among pelagic spawning species 

 that inhabit tropical reefs (Thresher, 1984). However, 

 spawning in the afternoon is less common and may be 

 linked to large tidal cycles and strong currents in the 

 north of WA, as indicated for the brown stripe snapper 

 (Lutjanus vitta) that also spawns in the afternoon in 

 the Pilbara region (Davis and West, 19931. 



Batch fecundity of S. commerson has not previously 

 been recorded and such data are rare for other Scomb- 

 eromorus species. Fecundity estimates for S. commerson 

 from the Indian Peninsula (Devaraj, 1983) were not 

 comparable because those data appeared to be obtained 

 from counts of both vitellogenic and previtellogenic oo- 

 cytes. Although the current study provided fecundity 

 estimates only for females up to 13 kg whole weight, it 

 shows that S. commersorj is highly fecund (the highest 

 estimated batch fecundity of 1.2 million eggs was ob- 

 tained from an ovary that was less than half the weight 

 of the heaviest ovary sampled). This study highlighted 

 the need to histologically check that oocytes in the 

 spawning batch are fully hydrated because fecundity 

 may otherwise be over-estimated. Similarly, fecundity 

 will be under-estimated if ovulation has commenced. 

 The best time to collect gonad samples so that these 

 biases are minimized is during the mid to late after- 

 noon for this species. 



Fishing activity is also regulated by the reproductive 

 cycle. About 3-6 months prior to the spawning sea- 

 son catches of S. commerson by commercial fishermen 

 increase as large numbers of smaller S. commerson 

 appear on offshore reefs sometime between March and 

 May, and soon after throughout the coastal waters of 

 WA (Mackie 4 ). By the time reproductive development in 

 ovaries begins (approximately August and September 

 in the Kimberley and Pilbara regions, respectively) 

 catches have peaked or are declining. In the Pilbara 

 region commercial catches have dropped to a minimum 

 when spawning begins, as fish become less abundant 

 in inshore waters and inclement weather conditions 

 limit fishing on still productive offshore reefs. Because 

 S. commerson generally do not make substantial long- 

 shore movements (Buckworth et al. 5 ), it is likely that 

 most spawning activity occurs at offshore locations in 

 this region (e.g., in mid to outer areas of the continen- 

 tal shelf), although anecdotal evidence indicates that 



4 Mackie, M. C. 2001. Spanish mackerel stock status 

 report. In State of the fisheries report 1999/2000 (J. W. 

 Penn, W. J. Fletcher, and F. Head, eds.), p. 71-75. Depart- 

 ment of Fisheries, Perth, Western Australia, 6020. http:// 

 www.fish.wa.gov.au/sof/1999/comm/nc/commnc26.html. 

 Accessed 10/2/2001. 



6 Buckworth, R. C, S. J. Newman, J. R. Ovenden, R. J. G. 

 Lester, and G. R. McPherson. 2004. In prep. The stock 

 structure of northern and western Australian Spanish mack- 

 erel. Final report to the Fisheries Research and Development 

 Corporation (FRDC) on project no. 1998/159. Department 

 of Business Industry and Resource Development, Darwin, 

 Northern Territory, 0800, Australia. 



