Brodziak and Mikus; Variation in life history parameters of Microstomus padficus 



671 



Dover sole that had been spawned prior to a 

 regime shift in the late 1970s (see McGowan 

 et al., 1998) than samples collected during the 

 1990s. Although temporal changes in growth 

 due to ocean regime may have occurred, such 

 changes would be difficult to detect given the 

 unbalanced temporal and spatial coverage of 

 the available research sui-vey data. Regard- 

 less of the mechanism, continued sampling of 

 Dover sole for data on size at age and matu- 

 rity will be needed to understand whether 

 the observed geographic variation is stable 

 through time. 



We found that maturation rates differed 

 between sexes. On average, males matured 

 at a smaller size and at a younger age 

 than did females. This difference might be 

 expected given the sexual dimorphism in 

 Dover sole growth and is consistent with 

 conclusions drawn by Beverton (1992, Fig. 

 2), who reported that males generally have 

 smaller size at 50'"f maturity than females 

 within the Pleuronectiformes. 



In addition to differences between sexes, 

 we found geographic variation in maturity 

 rates and an apparent latitudinal cline in 

 the median length at maturity of male and 

 female Dover sole. Results were similar for 

 maturity-at-age curves, with the exception of 

 results for Columbia area males. In his review 

 of latitudinal patterns in flatfish reproduc- 

 tive life history, Castillo ( 1995) found no evi- 

 dence of a latitudinal trend in age or length 

 at first maturity of Dover sole off Oregon and 

 California. In our study, results suggested 

 that Dover sole mature at smaller size and 

 younger ages at higher latitudes. This find- 

 ing contrasts with expectations that Dover 

 sole would mature at larger size and older 

 age at higher latitudes (e.g. Castillo. 1995). 

 The apparent trend in Dover sole maturation 

 rates may reflect local adaptation to latitudi- 

 nal differences in oceanographic properties of 

 the California Current (U.S. GLOBEC, 1994). 

 Nonetheless, sample sizes from some areas 

 (e.g. Vancouver) may have been insufficient to 

 accurately characterize the pattern of matu- 

 ration in length or age. Also, effects of poten- 

 tial latitudinal differences in the timing of 

 spawning were not accounted for in our study 

 Thus, although we found some evidence of 

 geographic variation, we recommend further 

 sampling to verify our observed geographic 

 patterns in maturation. 



Maturity-at-length estimates from our study were con- 

 sistent with those from other studies of Dover sole matu- 

 ration. Although maturation criteria and sampling design 

 differed between our study and those of Hunter et al. 

 ( 1992), estimated median lengths for female maturity were 

 remarkably consistent. In particular. Hunter et al. (1992) 



estimated female L 

 female L 



5Q to be 33.2 cm, whereas in our study, 

 was estimated to be 33.4 cm. These female 



L5Q estimates contrast with earlier estimates that were 

 based on commercial fishery samples in which fish were 

 36 cm (Hagerman 11952] reported within Hunter et al. 

 [1990]) and 38 cm (Harry 119591). Although it is unknown 

 whether the earlier estimates are directly comparable to 



