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



621 



The temperature preferences of English sole may allow 

 them to exploit areas of the estuary not used exten- 

 sively by Pacific sanddab. Temperatures less than 17.5°C 

 have been observed to lead to high English sole growth 

 (Yoklavich, 1982). These temperatures were observed 

 in lower side channels of the study estuaries. For the 

 speckled sanddab iCitharichthys sfigmacus), a close rela- 

 tive of the Pacific sanddab. maximum growth has been 

 observed from 8° to 13°C (Ehrlich et al.. 1979). This 

 temperature range is closer to the temperatures found 

 in lower side channels, and lower than the temperature 

 range found in lower main channels in our study area. 

 Thus, the Pacific sanddab, if it has temperature toler- 

 ances similar to those of its congener, may be limited to 

 lower main channel locations in order to optimize growth 

 rates, whereas optimal English sole growth occurs at the 

 higher temperatures found in lower side channels. 



An alternative explanation for Pacific sanddab distri- 

 bution may be found in its response to high English sole 

 density. The changing distribution of Pacific sanddab, 

 when confronted by high densities of English sole, pro- 

 vides evidence that competition may occur between 

 these two species. When English sole densities are high, 

 Pacific sanddab seem to be limited in distribution to 

 lower main channel locations (sites not preferred by 

 English sole), and when English sole densities are low. 

 Pacific sanddab appear to be more evenly distributed 

 throughout the estuary. This pattern does not apply 

 to starry flounder or sand sole, whose distributions re- 

 main the same regardless of English sole density. The 

 implication of this resource partitioning is that English 

 sole year-class strength may be affecting the ability 

 of Pacific sanddab to use areas of the estuaries where 

 conditions are optimal for flatfish growth. 



Distinct patterns of habitat use by juvenile English 

 sole, starry flounder, sand sole, and Pacific sanddab 

 were observed in all four estuaries that we examined. 

 Highest densities of Pacific sanddab and sand sole were 

 found in lower main channel sites, highest densities of 

 English sole were found in lower side channels, and 

 highest densities of starry flounder were found in upper 

 estuarine sites. These patterns of spatial segregation 

 were consistent from 1998 through 2000. The patterns 

 were consistent across the four estuaries encompassing 

 400 km of coastline. Resource partitioning is probably 

 based primarily on temperature and salinity tolerances 

 and growth preferences of the individual flatfish spe- 

 cies; however, there is evidence of competition between 

 English sole and Pacific sanddab when their spatial 

 distributions are examined in response to their relative 

 abundances. 



Acknowledgments 



Assistance in the field provided by Tim Loher, Jennifer 

 Boldt, Geoff Hosack, Grace Tsai, Noble Hendrix, Neil 

 Banas and Kirsten Holsman was greatly appreciated. 

 This research was funded by the National Oceanic and 

 Atmospheric Administration's Coastal Ocean Program 



under award no. NA96OP0238 to the University of 

 Washington. 



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