670 



Fishery Bulletin 98(4) 



growth may result from a differential use of deep-water 

 habitat, where adult males spend more time, on average, 

 in the oxygen minimum zone, or may result from the 

 effects of egg production and spawning on growth and 

 deposition of rings in the annuli of females. In summary, 

 Dover sole exhibit a moderate degree of sexual dimor- 

 phism, and differences between sexes become apparent, on 

 average, at about 5 years of age. 



In addition to differences between sexes, moderate levels 

 of geographic variation in gi'owth of Dover sole off the 

 west coast were apparent. Growth cui-ves for the Columbia 

 area and for the combined Vancouver, Eureka, and Mon- 

 terey areas differed for both male and female Dover sole. 

 Asymptotic lengths of males and females from the Colum- 

 bia area were 3'^/i: and 2% greater than those for the com- 

 bined Vancouver, Eureka, and Monterey areas. Male and 



female Dover sole from the Columbia area also had lower 

 Brody growth coefficients and larger asymptotic lengths 

 than other areas. These differences, however, were rela- 

 tively minor and would not be expected to have a substan- 

 tial impact on target harvest rates if the Dover sole fishery 

 were managed by INPFC areas rather than as a coast- 

 wide unit. 



The reason for geographic variation in growth is un- 

 known but probably stems from regional differences in 

 productivity within the California Current. In particular, 

 oceanographic properties of the California Current differ 

 north and south of Cape Blanco (U.S. GLOBEC, 1994), 

 the southern boundary of the Columbia area, and physi- 

 cal differences in the strength and duration of upwelling 

 may affect the growth potential of Dover sole inhabiting 

 these regions. The fact that adult Dover sole are benthic 

 feeders (Pearcy and Hancock, 1978) and are 

 relatively sedentary, having negligible north- 

 south movements (Westrheim et al., 1992), 

 indicates that geographic variation in phys- 

 ical habitat and benthic community struc- 

 ture are probably important determinants 

 of Dover sole growth. Indeed, Jacobson and 

 Hunter ( 1993) showed that there is relatively 

 more Dover sole habitat off Oregon than off 

 central California. and that depth preferences 

 of Dover sole appear to differ between these 

 regions. 



Alternatively, the cumulative effects of long- 

 term harvest may have also affected the geo- 

 graphic pattern of Dover sole growth because 

 commercial harvests by INPFC area have dif- 

 fered through time. Landings of Dover sole in 

 the Eureka area peaked in the early 1970s, 

 whereas landings in the Columbia, Mon- 

 terey, and U.S. Vancouver areas peaked in 

 the 1980s (Brodziak, et al. 1997). Although 

 historic size at age of Dover sole has not 

 been documented, some otolith aging data 

 collected off California in the 1940s have 

 provided evidence that mean size at age 

 of males and females may have changed 

 through time (Hagerman, 1952). In particu- 

 lar, Hagerman reported mean sizes at age-5 

 and age-9 of male Dover sole were 32.3 and 

 35.5 cm, whereas mean sizes of age-5 and 

 age-9 females were 33.6 and 41.5 cm. In com- 

 parison with our estimates, these data sug- 

 gest that growth may have declined since the 

 1940s and are consistent with reductions in 

 mean size at age due to fishery-size selection 

 (see Ricker, 1968). Thus, geographic patterns 

 in Dover sole growth may also have been 

 affected by differences in harvest pressure 

 among regions. 



Temporal variability of the California Cur- 

 rent (Ware, 1995) may have also influenced 

 observed growth, as well as maturation pat- 

 terns. Research survey samples collected 

 during the 1980s had a higher percentage of 



