NOTE Rooper et al Partitioning and competition among luvenile flatfish in nursery estuaries 



619 



negative density anomalies — the most notable exceptions 

 being Willapa Bay and Grays Harbor in 1998 (Fig. 3). 

 This pattern produced significant interactions between 

 habitat and both year and estuary. 



The lengths of sand sole captured by trawling were 

 comparable to those for Pacific sanddab, ranging from 20 

 to 234 mm TL. and only 12 specimens longer than 150 

 mm TL. Sand sole catch was highly variable across all 

 systems, and there were 318 zero catches over the total 

 431 trawls. Sand sole density anomalies followed a pat- 

 tern similar to those of Pacific sanddab. Density anoma- 

 lies were significantly higher at lower main channel sites 

 than at other sites within the estuary (Fig. 2). Density 

 anomalies were similar between upper estuary sites 

 and lower side channel sites. There were no significant 

 interaction terms between estuary or year and habitat 

 type; thus the pattern in spatial distribution of sand sole 

 appeared to be consistent across estuaries and years. 



Starry flounder were the least common species of flat- 

 fish captured in the study estuaries. The starry flounder 

 captured were larger than the other species of flatfish, 

 ranging from 49 to 376 mm TL, and 44 individuals were 

 >150 mm TL. Density anomalies were highest at upper 

 main channel sites (Fig. 2). As with sand sole, there 

 were no significant interaction terms between habitat 

 type and either estuary or year. The effect of habitat 

 type on starry flounder densities was thus a consistent 

 effect observed across all estuaries and years. 



English sole lengths ranged from 14 to 200 mm TL 

 and only 12 individuals were greater than 150 mm TL. 

 English sole density anomalies were significantly higher 

 at lower side channel sites than at other estuarine loca- 

 tions (Fig. 2). There was also a significant interaction 

 term between habitat type and month. In August there 

 was no significant difference between density anomalies 

 in lower side channel and lower main channel sites, 

 while in June density anomalies were much larger in 

 lower side channels than at all other locations. The 

 significant month and habitat type interaction in the 

 analysis of English sole density anomalies was most 

 likely due to habitat changes observed in juvenile Eng- 

 lish sole as they grow (Rooper et al., 2003). A carry- 

 ing capacity for large (>50 mm TL) English sole was 

 observed in the study estuaries, and large English sole 

 distribution expanded when densities were high (Rooper 

 et al., 2003). The expansion of English sole distribution 

 when densities were large corresponds to an increase 

 in the density anomalies of Pacific sanddab in lower 

 main channel areas (Fig. 4). Conversely, as large (> 

 50-mm-TL) English sole densities decreased. Pacific 

 sanddab appeared to increase their distribution in the 

 estuaries, as observed in higher density anomalies in 

 lower side channels. 



Discussion 



We observed evidence for resource portioning in four 

 species of co-occurring juvenile flatfish in west coast 

 nursery estuaries. The analysis of density anomalies 



Figure 3 



Pacific sanddab iCitharnhtliys sordidus> density 

 anomalies (and standard error bars) by estuary 

 and habitat type from 1998 (upper panel) through 

 2000 (lower panel). 



indicated that the four flatfish species exploit different 

 areas of estuaries. The spatial segregation among juve- 

 nile flatfish species observed in our study is consistent 

 with results of other studies and indicates the potential 

 for interspecific competition among the four species. Sur- 

 veys of flatfish assemblages in Puget Sound, Washington 

 have revealed spatial segregation of juvenile flatfish by 

 depth, although the pattern was not consistent among 

 sites (Thornburgh, 1980). In Thornburgh's study, spa- 

 tial segregation may have been driven by differences in 

 sediment type at the sites and by differences in settling 

 times among the dominant species: English sole, sand 

 sole. Pacific sanddab, and rock sole (Lepidopsetta bilin- 

 ecita). Distinct patterns in spatial distribution have also 

 been observed for juvenile flatfish assemblages beyond 

 the Pacific Northwest. In estuaries along the California 



