344 



Abstract— The narrow-barred Span- 

 ish mackerel (Seomberomorus com- 

 merson) is widespread throughout the 

 Indo-West Pacific region. This study 

 describes the reproductive biology of 

 S. commerson along the west coast 

 of Australia, where it is targeted for 

 food consumption and sports fishing. 

 Development of testes occurred at a 

 smaller body size than for ovaries, 

 and more than 90^ of males were 

 sexually mature by the minimum 

 legal length of 900 mm TL compared 

 to 50 f7 f of females. Females dominated 

 overall catches although sex ratios 

 within daily catches vary consider- 

 ably and females were rarely caught 

 when spawning. Seomberomorus 

 commerson are seasonally abundant 

 in coastal waters and most of the 

 commercial catch is taken prior to 

 the reproductive season. Spawning 

 occurs between about August and 

 November in the Kimberley region 

 and between October and January 

 in the Pilbara region. No spawning 

 activity was recorded in the more 

 southerly West Coast region, and only 

 in the north Kimberley region were 

 large numbers offish with spawning 

 gonads collected. Catches dropped to 

 a minimum when spawning began in 

 the Pilbara region, when fish became 

 less abundant in inshore waters and 

 inclement weather conditions limited 

 fishing on still productive offshore 

 reefs. Final maturation and ovulation 

 of oocytes took place within a 24-hour 

 period, and females spawned in the 

 afternoon-evening every three days. 

 A third of these spawning females 

 released batches of eggs on consecu- 

 tive days. Relationships between 

 length, weight, and batch fecundity 

 are presented. 



Variability in spawning frequency and 

 reproductive development of the narrow-barred 

 Spanish mackerel (Seomberomorus commerson) 

 along the west coast of Australia 



Michael C. Mackie 



Paul D. Lewis 



Daniel J. Gaughan 



Stephen J. Newman 



Western Australian Marine Research Laboratories 



Department of Fisheries 



Government of Western Australia 



West Coast Drive 



Waterman, Western Australia 6020. Australia 



E-mail address (for M C Mackie) mmackietg'fish.wa. gov.au 



Manuscript submitted 1 October 2002 

 to the Scientific Editor's Office. 



Manuscript approved for publication 



14 December 2004 by the Scientific Editor. 



Fish. Bull. 103:344-354 (2005). 



The narrow-barred Spanish mack- 

 erel (Seomberomorus commerson) is a 

 prized food fish targeted by fishermen 

 throughout its range in the Indo-West 

 Pacific region (Collette and Nauen, 

 1983). Reaching over 2.4 m in length 

 and 45 kg in weight, this pelagic spe- 

 cies is seasonally abundant in coastal 

 waters where it often schools in large 

 numbers. In Australian waters, the 

 commercial mackerel fishery targets 

 these schools using trolling methods, 

 and 2362 metric tons were caught in 

 2001-02 for domestic and overseas 

 markets (ABARE. 2003). 



Seomberomorus commerson is also 

 a premier sport fishing species, tar- 

 geted by an increasing number of rec- 

 reational anglers throughout its broad 

 Australian distribution. The combined 

 commercial and recreation take of S. 

 commerson has put significant pres- 

 sure on stocks in Queensland (QLD) 

 waters, leading to a possible decline 

 in the spawning stock abundance 

 (McPherson and Williams, 2002). 

 The biology of S. commerson in these 

 waters has been well studied (e.g., 

 Munro, 1942; McPherson, 1981, 1992, 

 1993). Biological information is also 

 available for S. commerson in wa- 

 ters of the Northern Territory (NT; 

 Buckworth 1 ), where stocks are still 

 recovering from a prolonged period of 

 exploitation by foreign gill-net opera- 

 tors that ended in 1986. In contrast, 

 little is known about the stock status 



and biology of S. commerson in West- 

 ern Australian (WA) waters, despite 

 the fact that catches are similar to 

 those taken in QLD and the NT, and 

 commercial fishermen have expressed 

 concern about increasing fishing pres- 

 sure on this species in WA. Recent 

 moves to overhaul management of the 

 mackerel fishery in WA (in which S. 

 commerson is the dominant species) 

 have further highlighted the need for 

 more information on the biology and 

 stock status of S. commerson along 

 the WA coast. 



Research to enable a stock assess- 

 ment of S. co?7imerson in WA waters 

 was therefore commenced in 1999. 

 Description of reproductive biology 

 was a key focus of this study, since 

 this information is required for stock 

 assessment models and for manage- 

 ment controls such as minimum legal 

 lengths, which were previously set 

 with little knowledge of the biology of 

 S. commerson in WA. Information on 

 other reproductive parameters, such 

 as batch fecundity and spawning be- 

 havior, which are also required for 



1 Buckworth, R. C. 1999. Age structure 

 of the commercial catch of Northern Ter- 

 ritory narrow-barred Spanish mackerel. 

 Final Report to the Fisheries Research 

 and Development Corporation (FRDC) on 

 project no. 1998/159. Fishery report 42, 

 27 p. Department of Business Indus- 

 try and Resource Development, Darwin, 

 Northern Territory, 0800, Australia. 



