FISHERY BULLETIN: VOL. 86, NO. 3 



DISCUSSION 



Results of this study demonstrate that sex segre- 

 gation by English sole in nearshore areas of Puget 

 Sound is strongly associated with the grain-size 

 distribution of bottom sediments. In addition to the 

 results of statistical analyses, the strength of this 

 association is demonstrated by its persistence across 

 a variety of sampling conditions. For example, the 

 association was found in two independent surveys 

 conducted 2-3 years apart. Furthermore, the 1981- 

 82 survey represented an analysis of large-scale 

 patterns across three embayments, whereas the 

 1984 survey focused on small-scale patterns within 

 a single embayment. The association between sex 

 segregation and sediment grain size also persisted 

 across seasons and across a range of depth. 



Although a significant association was found be- 

 tween sediment grain size and male proportion of 

 English sole, a variety of factors that covary with 

 sediment grain size may have been partly or totally 

 responsible for this association (Lagler et al. 1977; 

 Moyle and Cech 1982). Examples of physical/chem- 

 ical variables that often covary with grain size and 

 may influence fish distributions include depth, cur- 

 rent speed, turbidity, and dissolved oxygen. Ex- 

 amples of biological factors include the composition 

 of prey assemblages and the distribution of pred- 

 ators. Although results of this study demonstrated 

 that depth did not influence the observed associa- 

 tion between sediment grain size and male propor- 

 tion, further experimentation is required to identify 

 the factor or suite of factors that directly influences 

 sex segregation in this species. 



Despite the fact that depth did not influence the 

 patterns of sex segregation identified for English 

 sole, the range of depth considered in this study 

 (5-36 m) is a small fraction of the total range 

 occupied by this species (0-550 m; Hart 1973). It 

 therefore is possible that depth-related sex segrega- 

 tion in this species may be observed if a wider range 

 of depths were examined. For example, considerable 

 sex segregation by depth was found for Dover sole, 

 Microstomus pacificus, off the west coast of the 

 United States when a large depth range (110-440 

 m) was evaluated (Hagerman 1952; Alton 1972). 



Based on results of the 1984 survey, no relation- 

 ship between age and male proportion in English 

 sole was evident. It therefore is unlikely that age 

 influenced the observed relationship between male 

 proportion and sediment grain size. A decline in 

 male proportion with increasing age has been found 

 for several pleuronectids (Rae 1965; Kovtsova 1982). 

 Because most of the fish examined in this study were 



relatively young (<8 years old) compared with the 

 maximum age of 15-17 years sometimes reached by 

 individuals of this species (Ketchen 1956; Holland 

 1969; Van Cleve and El Sayed 1969), consideration 

 of older fish may alter the patterns observed in this 

 study. Although sample sizes for fish aged 9-10 

 years were relatively small in this study, male pro- 

 portion in those age groups (0.50) was lower than 

 that observed in all younger age groups (0.57-0.66). 



The season of sampling did not alter the observed 

 relationship between male proportion and sediment 

 grain size. This relationship remained intact despite 

 the fact that the mean abundance of English sole 

 changed substantially throughout the seasonal cycle. 

 The observed seasonal changes in mean abundance 

 are consistent with the seasonal pattern of migra- 

 tion exhibited by adults of this species and most 

 other pleuronectids. That pattern includes a migra- 

 tion to deeper water in the fall for overwintering 

 and spawning, and a return migration to shallower 

 water in spring for feeding throughout the summer 

 (Ketchen 1956; Alverson et al. 1964; Roff 1982). It 

 is unlikely that spawning substantially influenced 

 the observed seasonal patterns of sex segregation 

 in English sole, because the largest percentage of 

 fish in spawning condition during any season was 

 <15%. However, sex segregation in relation to 

 spawning in pleuronectids has been documented for 

 several species (Moiseev 1953; Alverson and Chat- 

 win 1957), and may influence sex segregation in 

 English sole when a substantial percentage of the 

 population is in spawning condition. 



Results of this study suggest that the influence 

 of sex segregation should be considered when 

 characteristics of English sole populations are com- 

 pared among habitats that differ with respect to 

 sediment character. Patterns based on population 

 characteristics capable of exhibiting a dependence 

 on sex (growth, condition, disease prevalence, tissue 

 contamination) could be strongly biased if differ- 

 ences in the sex ratio among stations are not con- 

 sidered. This concern should probably be extended 

 to studies of other pleuronectids, because the rela- 

 tionship between sex segregation and sediment 

 character is unknown for most species. 



Although the reasons for the observed patterns 

 of sex segregation in English sole are unknown, a 

 potential explanation is that different energetic 

 needs induce male and female fish to utilize differ- 

 ent habitats. Female English sole grow to a larger 

 size than males and mature at an older age (Ketchen 

 1956; Holland 1969; Van Cleve and El-Sayed 1969; 

 Hart 1973). In addition, fecundity in females is pro- 

 portionate to individual size (Ketchen 1956; Hart 



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