900 



Abstract— The sectioned otoliths of 

 four fish species from a tropical demer- 

 sal trawl fishery in Western Australia 

 revealed a series of alternating trans- 

 lucent and opaque zones in reflected 

 light. The translucent zones, referred 

 to as growth rings, were counted to 

 determine fish ages. The width of the 

 opaque zone on the periphery of the 

 otolith section as a proportion of the 

 width of the previous opaque zone 

 (index of completion) was used to de- 

 termine the periodicity of growth-ring 

 formation. 



This article describes a method for 

 modeling changes in the index of ring 

 completion over time, from which a 

 parameter for the most probable time 

 of growth-ring formation (with confi- 

 dence intervals) can be determined. 

 The parameter estimate for the timing 

 of new growth-ring formation for Leth- 

 rinus sp. 3 was from mid July to mid 

 September, for Lutjanus vitta from 

 early July to the end of August, for 

 Nemipterus furcosus from mid July to 

 late September, and for Lutjanus sebae 

 from mid July to mid November. The 

 confidence intervals for the timing of 

 formation of growth rings was variable 

 between species, being smallest for L. 

 vitta, and variable between fish of the 

 same species with different numbers of 

 gi'owth rings. 



The stock assessments of these 

 commercially important species relies 

 on aging information for all the age 

 classes used in the assessment. This 

 study demonstrated that growth rings 

 on sectioned otoliths were laid down 

 annually, irrespective of the number 

 of growth rings, and also demonstrated 

 that the timing of ring formation for 

 these tropical species can be deter- 

 mined quantitatively (with confidence 

 intervals). 



Quantitative determination of the timing of 

 otolith ring formation from marginal increments 

 in four marine teleost species from northwestern 

 Australia 



Peter C. Stephenson 



Western Australian Marine Research Laboratories 

 West Coast Drive (off Elvire St) 

 Waterman, Western Australia, 6020, Australia 

 E-mail address: pstephensongifish.wa.govau 



Norm G. Hall 



School of Biological and Environmental Sciences 



Murdoch University 



Murdoch, Western Australia, 6150, Australia 



Manuscript approved for publication 

 3 June 2003 by Scientific Editor. 



Manuscript received 26 June 2003 at 

 NMFS Scientific Publications Office. 



Fish. Bull 101:900-909 (2003). 



The Pilbara fish trawl fishery, operat- 

 ing on the North West Shelf of Western 

 Australia, has (developed rapi(ily in 

 the last ten years and is now the most 

 valuable commercial scalefish fishery in 

 Western Australia. Catch from this fish- 

 ery was valued at $7 million (wholesale 

 value) in 2001. In this multispecies fish- 

 ery, Lutjanus vitta (Quoy and Gaimard, 

 1824) (brownstripe red snapper), 

 Nemipterus furcosus (Valenciennes, 

 1830) (fork-tailed threadfin bream, also 

 known as rosy threadfin bream), Leth- 

 rinus sp. 3 (Carpenter and Niem, 2001) 

 (lesser spangled emperor, known locally 

 as blue-spot emperor) made up 8%, 10%, 

 and 20% respectively of the total sca- 

 lefish trawl catch in 2000. The highly 

 prized species, Lutjanus sebae (Cuvier, 

 1828) (red emperor), although compris- 

 ing only 4% of the catch, is important 

 because of its high market value. 



In 1993 a research project was com- 

 menced to determine the fishing effort 

 required for optimal level of catches in 

 the Pilbara trawl fishery (Stephenson 

 and DunkM. The project relied on vali- 

 dated age composition data for L. vitta, 

 L. sp. 3, N. furcosus, and L. sebae. The 

 growth rings on otoliths have been 

 shown to be formed annually for only 

 one to three growth rings for N. furcosus 

 (Sainsbury and Whitelaw, 1984), and for 

 two to three growth rings for L. sebae 

 (McPherson and Squire, 1992). After 

 pooling of all age classes, Davis and 

 West (1992) showed that growth rings 

 of L. vitta were formed annually. 



Determining age composition in- 

 volves counting growth rings on hard 

 parts of fish (otoliths, scales, spines, 

 bones) and determining the timing of 

 growth-ring formation. Sagittal otoliths 

 are commonly used for aging teleost 

 fishes and recent studies (Hyndes et al., 

 1992; Milton et al., 1995; Newman et 

 al., 1996) have indicated that for some 

 species sectioned otoliths give more 

 reliable age estimates than whole, or 

 broken-and-burnt otoliths. The peri- 

 odicity of ring formation is commonly 

 determined by the mark-recapture 

 method in which fish are injected with 

 chemical markers and the number of 

 rings created between injection and 

 recapture are compared (Ferreira and 

 Russ, 1992; Francis et al., 1992; New- 

 man et al., 1996). 



An alternative to mark-recapture is 

 marginal increment analysis in which 

 the distance from the growth ring to the 

 edge of the otolith, for a sample offish, 

 is tracked over time (Campana, 2001) 

 and a sharp drop in this marginal 

 increment, once a year, is taken as an 

 indication of annual ring formation. 

 The analysis is often performed on 



Stephenson, P. C, and I. Dunk. 1996. Re- 

 lating fishing mortality to fish trawl 

 effort on the North West Slope of Western 

 Australia. Final report of project 93/25 to 

 the Fisheries Research and Development 

 Corporation, 1995, 44 p. Western Austra- 

 lia Marine Research Laboratories, PO Box 

 20, North Beach, Western Australia 6092, 

 Australia. 



