178 



Fishery Bulletin 101(1) 



darkblotched rockfish). The GSI estimates for both species 

 were at the lower end of the distribution of the GSI and M 

 values used to develop the original predictive relationship 

 (Fig. 7). 



Indirect estimates of the instantaneous rate of natural 

 mortality were estimated to be M = 0.08 for arrowtooth 

 flounder with the gonadosomatic index and 0.11 with the 

 growth coefficient K' (Table 1 ). Corresponding estimates for 

 darkblotched rockfish differed more substantially (M=0.11 

 with the gonadosomatic index and 0.30 with K). Precision 

 for the estimates of M was higher for the estimates based 

 on K (CV=5 to 12%) than for those based on the gonadoso- 

 matic index (CV= 17%). 



Discussion 



An estimate of Z=0.18 was previously obtained for arrow- 

 tooth flounder by using Hoenig's (1983) relationship be- 

 tween total instantaneous mortality rate (Z) and maxi- 

 mum age in 84 stocks offish, and an estimated maximum 

 age of 23 years (Turnock et al.-*). Because this stock has 

 been exploited only lightly, it was assumed that the result- 

 ing estimate of Z was equivalent to M. The resulting esti- 

 mate was somewhat higher than the value obtained from 

 reproductive effort (M=0.08) or K (M=0.11) and outside 

 the approximate 95% confidence limits (±2SE) for both 

 estimates (Table 1). 



A range of maximum ages (60 to 105 years) was previ- 

 ously used to obtain estimates of Z (assumed to be ap- 

 proximately equal to M) = 0.025-0.05 for darkblotched 

 rockfish based on Hoenig's method, due to uncertainties in 

 age determination (Rogers et al.^). An estimate of M=0.05 

 provided the best fit to a population dynamics model, al- 

 though it is somewhat lower than the estimate of M=0.11 

 obtained in this study with the gonadosomatic index and 

 substantially lower than the estimate of M=0.30 with the 

 growth coefficient K. The estimate obtained with Hoenig's 

 method was once again outside the approximate 95% con- 

 fidence limits for the other indirect estimates. 



Although none of the data on somatic and ovary weights 

 were collected with the specific aim of estimating natural 

 mortality, they sei-ved to provide a reasonable estimate of 

 M for arrowtooth flounder and a first approximation of 

 this parameter for darkblotched rockfish. More detailed 

 histological data on the ovaries used in the darkblotched 

 rockfish analysis would have improved the reliability of 

 this estimate and a larger sample size of mature ovaries 

 for both species would have been preferable. 



The estimates of M derived from the growth coefficient 

 K (Table 1) were obtained by using age-length data for 

 sexes combined because most of the original data (Pauly, 

 1980) was in this format. However, because of sexual di- 

 morphism in growth, estimates of M would have differed 

 if males and females had been treated as separate "stocks" 

 as was done for 33 of the obser\'ations in Pauly's database. 

 Sexual dimorphism was substantial for arrowtooth floun- 

 der, where K was 0.194 (corresponding to an estimated 

 M=0.31) for males and 0.065 (M=0.10) for females and was 

 less pronounced in darkblotched rockflsh, where K was 

 0.211 (A/=0.34) for males and 0.164 (M=0.26) for females. 

 Averaging these sex-specific estimates would not change 



^ Turnock. B. J.. T. K. Wilderbuer, and E. S. Brown. 1999. Ai-- 

 rowtooth flounder In Stock assessment and fishery evaluation 

 report for the gi-oundfish resources of the Gulf of Alaska, p. 

 226-253. North Pacific Fishery Management Council, Anchor- 

 age, AK. 



■'■' Rogers, J. U. H D. Methot, T. L. Buikler. K I'mer, and M 

 Wilkins. 2000. Status of the darkblotched rockli.sh iSfhcislcs 

 craineri) resource in 2000. In Stock assessment and fish- 

 ery evaluation: api)endix to the status of the Pacific Coast 

 groundfish fishery through 2000 and recommended acceptable 

 biological catches for 2001. 71 p Pacific Fishery Management 

 Council, Portland, OK. 



