HOGUE and CAREY: FEEDING ECOLOGY OF 0-AGE FLATFISHES 



DISCUSSION 



The diet of recently settled English sole is a 

 function of size, location of capture, and season 

 (Tables 1, 3, 4). Both the within-year and be- 

 tween-year differences in diet noted for P. vetu- 

 lus are similar to changes documented for other 

 pleuronectid species (Macer 1967; Edwards and 

 Steele 1968) and are probably related to tem- 

 poral changes in density of prey organisms. 

 Steele et al. (1970) concluded that variations in 

 predation on Tellina siphons and polychaetes by 

 young plaice, Pleuronectes platessa, were a result 

 of changes in both the absolute and relative 

 abundances of these prey over time. The observed 

 relationship between seasonal changes in har- 

 pacticoid copepod abundance and the utilization 

 of these prey as food by English sole is the only 

 direct evidence we have to support this conten- 

 tion. However, the juvenile bivalves {Tellina and 

 Siliqua) consumed by Parophrys vetulus were all 

 young of the year which are known to have tem- 

 porally variable recruitment (Jones 5 ), suggest- 

 ing that seasonal availability of this food item is 

 also not constant. Moreover, Oliver et al. (1980) 

 seasonally sampled the nearshore macrobenthos 

 in a region of Monterey Bay, Calif., which was 

 very similar to Moolach Beach in terms of physi- 

 cal environment and fauna present. Their results 

 indicate that the abundance of amphipods and 

 such polychaetes as Magelona sacculata vary 

 both within and between years. English sole at 

 Moolach Beach probably alter their diet over 

 time in accordance with similar temporal 

 changes in the density of these larger prey spe- 

 cies, but additional benthic data obtained con- 

 currently with fish collections are necessary to 

 substantiate this conclusion. 



The marked differences between summer and 

 winter spatial variability in English sole diets 

 (Table 4) are thought to be related to changes in 

 both the abundance and spatial distribution of 

 prey. During the winter, intense storm activity 

 along the Oregon coast produces large waves 

 which continually disturb and mix the sediments 

 of the inner continental shelf (Komar et al. 1972). 

 The meiobenthos has been shown to become ran- 

 domly distributed during these periods within 

 small areas (1 m 2 ) and only slightly aggregated 

 on larger scales (Hogue 1982). Small benthic 

 prey fed upon by 0-age English sole would most 



5 H. R. Jones, School of Oceanography, Oregon State Univ., 

 Corvallis, OR 97331, pers. commun. 



likely be affected by this vigorous physical mix- 

 ing in much the same way as the meiofauna. As a 

 result, English sole feeding at either the same 

 depth or different depths may consume similar 

 prey in the winter (Table 4, January) because 

 prey organisms are more evenly distributed 

 within the study area compared to other times of 

 the year. Such a distribution, when coupled with 

 the numerical dominance of one food item, would 

 increase the similarity of food items available for 

 consumption throughout the region. During the 

 late spring and summer the physical disruption 

 of sediment is minimized and the spatial distri- 

 bution of the meiofauna becomes increasingly 

 aggregated (Hogue 1982). Distinct differences in 

 the species composition and abundance of nema- 

 todes and harpacticoids have been found at loca- 

 tions only 250 m apart. During this period there 

 is little similarity in diets of English sole from 

 replicate trawls (Table 4, May). In the spring and 

 summer, P. vetulus may be opportunistically ex- 

 ploiting different prey which are densely aggre- 

 gated in different sectors of the Moolach Beach 

 site. 



Seasonal changes in the spatial distribution of 

 prey items may also alter the rate at which prey 

 are consumed. Experiments with fish feeding in 

 aquaria (Ivlev 1961) have shown that an increase 

 in the degree of aggregation of food sources has 

 the same effect on the rate of food consumption as 

 an increase in the concentration of food. Results 

 of Tinbergen et al. (1967) suggest similar rela- 

 tionships between the spatial distribution of prey 

 and predation. Fish commencing their benthic 

 feeding in the late spring and summer at Moo- 

 lach Beach may benefit energetically from the 

 increased aggregation of benthic organisms dur- 

 ing this period relative to that found during the 

 winter. 



The consumption of parts of macrobenthic 

 organisms, e.g., Magelona palps and tellinid 

 clam siphons, rather than whole individuals by 

 English sole <35 mm in length is probably re- 

 lated to the maximum size of food items capable 

 of being captured and ingested by these fish. We 

 measured the mouth size of 30 mm P. vetulus and 

 found that prey greater than about 2 mm in their 

 largest dimension are too large to be consumed 

 by such small fish. Siphons and palps are appar- 

 ently the only portion of larger prey which are 

 available for ingestion by fish <35 mm SL. As 

 fish grow larger than 35 mm, small food items 

 are neglected in favor of polychaetes, amphi- 

 pods, and cumaceans which yield far more en- 



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