SMITH ET AL.: ORGANIZATION OF NEKTON 



significant adaptive feature of the latter that 

 would provide better survivorship in open waters 

 (at least with regard to predation). Just how this 

 species and others minimize the effects of preda- 

 tion in open waters is an important research ques- 

 tion for the future. 



Trophic Comparisons 



The six species examined in detail are clearly 

 trophic opportunists and overlap in many food 

 categories. In addition, each goes through distinct 

 ontogenetic stages in feeding which include sig- 

 nificant shifts in the portions of the water column 

 searched. The prey taxa have been categorized by 

 Darnell (1961), Qasim (1972), and Chao and 

 Musick (1977) according to their vertical occur- 

 rence in the water column from open waters to the 

 bottom: fishes, macrozooplankton (e.g., Neomysis 

 americana), microzooplankton (e.g., calanoid 

 copepods), epibenthos (e.g., harpacticord 

 copepods), infauna, and organic matter. At sizes 

 <21 mm SL all five of the species examined appar- 

 ently spent considerable periods foraging in the 

 water column. Between 21 and 40 mm SL several 

 species continued to feed on "pelagic" prey, al- 

 though by this size the transition to benthic feed- 

 ing was nearly complete in spot and hogchoker. 



Whether resource partitioning or dietary 

 specialization (Chao and Musick 1977) occurs in 

 these taxa as a means of reducing interspecific 

 interactions is a matter of speculation. Without 

 question, there are differences in feeding localities 

 of the fishes examined — e.g., white perch and 

 white catfish are generally restricted to oligo- 

 haline habitats, while Atlantic croaker and hog- 

 choker are more abundant on the shoals. Also 

 noted are differences in seasonal abundance (Fig. 

 6), size related feeding distributions reflecting on- 

 togenetic shifts (Fig. 5), morphological differences 

 among predators (Chao and Musick 1977), etc. But 

 whether or not any of these traits reflect past or 

 present competitive pressures remains unknown. 

 Food that is generally limiting for several of these 

 species and others is currently an area of con- 

 troversy. Currin et al. (in press) have suggested 

 that predation, not resources, limits production 

 rates of spot and Atlantic croaker in shallow 

 marsh embayments in Albermarle Sound, N.C. In 

 contrast, Weisberg and Lotrich (1980) found that 

 growth rates of the mummichog, Fundulus 

 heteroclitus, could be altered by manipulating fish 

 density. Increased growth rates were also demon- 

 strated with food enrichment experiments in sub- 



tidal areas. Similar findings were reported by 

 Miklas and Reed ( in press ) for F. heteroclitus popu- 

 lations in a tidal tributary of the Rhode River, Del. 

 Our own findings of a seasonal decline in relative 

 fullness index values in several species, along with 

 a parallel decline in benthic biomass (T Fre- 

 dette 6 ), tend to support the possibility of periodic 

 food scarcity. 



Trophic opportunism has often been cited in 

 studies of estuarine fishes (Darnell 1958, 1961; 

 Livingston 1982). Several investigators have 

 pointed to the importance of omnivorous and on- 

 togenetic progressions in feeding stages (Sheridan 

 1979; Stoner 1980; Livingston 1982) as obscuring 

 distinct trophic relationships in nektonic food 

 webs. Along with these difficulties are problems 

 associated with the "snapshot" view often gained 

 of the system. Numerically abundant species are 

 likely to play the major role in conversion and 

 production of organic materials in estuaries (and 

 are, therefore, mainly responsible for the con- 

 struct of food webs and energy flow therein), yet 

 the identification of these species often comes from 

 the sampling program itself. 



Thus, although the dominant species in this 

 study, spot, is undoubtedly important in this re- 

 gard, the selective nature of our sampling effort 

 does not allow us to place this importance in 

 proper perspective. It is probable that dominance, 

 expressed in numbers and/or biomass of those 

 species captured in this study, is shared and some- 

 times surpassed by other local species not sampled 

 quantitatively by this program. These include 

 young-of-year bluefish, Pomatomus saltatrix; var- 

 ious cyprinodonts, especially F. heteroclitus; an- 

 chovies; and silversides. On an estuary-wide basis, 

 we also do not completely "track" species distribu- 

 tions in time and space (Purvis 1976) so that our 

 already distorted view of local habitats cannot 

 easily be extrapolated to system-wide consider- 

 ations. Such difficulties occur in most studies and 

 must be recognized and eventually accounted for 

 in considerations of fish community ecology in 

 estuaries. 



SUMMARY 



Tidal creeks of the York River estuary were 

 characterized by distinct nekton communities dis- 

 playing low diversity and dominated by relatively 



6 T. F. Fredette, Marine Scientist, Department of the Army 

 Corps of Engineers, P.O. Box 631, Vicksburg, MS 39189, pers. 

 commun. September 1982. 



465 



