Walter and Austin: Diet composition of Morone saxalilis in Chesapeake Bay 



415 



and Grant, 1970; Setzler et al., 1980; 

 Boynton et al., 1981; Limburg et al., 

 1997; Hartman and Brandt, 1995a) 

 and in other locations (Schaefer, 

 1970; Manooch, 1973; Rulifson and 

 McKenna, 1987), no studies have in- 

 cluded enough specimens larger than 

 600 mm total length to adequately 

 characterize the diet of migratory 

 fish. The absence of dietary informa- 

 tion for these larger striped bass may 

 have been due to the difficulty in 

 sampling larger striped bass and also 

 to the relative scarcity of large striped 

 bass in Chesapeake Bay during times 

 of severe overfishing (Koo, 1970). 

 Nevertheless, the absence of diet data 

 represents a gap in our knowledge of 

 the trophic dynamics of large striped 

 bass that form the major portion of the 

 spawning stock, are prized fisheries 

 targets and, through successful fish- 

 eries management, have emerged as 

 a significant seasonal predatory force 

 within Chesapeake Bay. We specifi- 

 cally address the diet composition of 

 large (458-1151 mm) striped bass in 

 Chesapeake Bay to determine the im- 

 portant species in their diet during the 

 spring and fall periods of abundance. 





James River '-.j.^ yi^' ^' 



ii^V-V ir, AflanBc Ocean 



'4- 



100 



7S'0O- 



100 



74'.S0- 



200 Kilometers 



Figure 1 



Map of Chesapeake Bay showing spatial distribution of striped bass samples 

 from March 1997 to May 1998. 



Methods 



From March 1997 to May 1998, 1225 striped bass were 

 collected from various localities in Chesapeake Bay, its 

 Virginia tributaries, and the Chesapeake Bay mouth 

 (Fig. 1). Fish were collected from recreational fishermen, 

 charterboat captains, and seafood dealers, as well as from 

 scientific monitoring programs in the spring (48.5%) and 

 fall (51.5%), corresponding to seasonal migration patterns 

 and fishing seasons. Fish ranged in size from 458 to 1151 

 mm TL (mean 653.7mm) and were 0.91-17.6 kg in weight 

 (mean 3.69 kg). Hook-and-line gear, gill nets, fyke nets, 

 and otter trawls were used to capture fish. Fish captured 

 in pound nets were excluded from this analysis because 

 of complications introduced by the confinement of the 

 fish in pound nets. Fish captured by hook and line were 

 recorded as such in order that the bait and chum used with 

 this gear could be excluded from the diet analyses. Total 

 length (±1.0 mm), sex, and weight (±0.001 kg wet weight) 

 were recorded for each fish, as well as location, date, and 

 method of capture. Stomachs were removed by cutting the 

 alimentary canal anterior to the stomach and posterior to 

 the pylorus and the contents were frozen until processed. 

 In some cases, stomachs of fish donated by charterboat 

 captains and recreational fishermen were removed by the 

 fisherman. 



Fish stomachs were thawed and emptied, and their 

 contents were blotted dry and weighed. Contents were 



sorted and identified to the lowest possible taxon, weighed, 

 counted, and measured. Diet composition was analyzed by 

 using three measures described in Hyslop (1980): percent 

 frequency of occurrence, percent weight, and percent 

 number These values were combined to give an index 

 of relative importance (Pinkas et al., 1971). The index of 

 relative importance for a particular prey category i (IRI,) 

 is expressed as 



IRl^ = (%N+ %W) X %F, 



where %N = the percentage of a prey species by number; 

 %W - the percentage of a prey species by weight; 



and 

 %FO = the percent frequency of occurrence of a prey 

 species. 



IRI values were calculated as percent IRI values (Cortes, 

 1997 ). In calculating IRI values, we excluded several items 

 appearing in the stomachs, such as chum (ground men- 

 haden), bait, trash and plant material because they were 

 deemed to be non-naturally occurring food items. Several 

 prey species were combined either because of difficulties 

 in identification of partially digested prey to species or 

 because of ecological or taxonomic similarity. Both bay 

 anchovy iAnchoa mitchilU) and striped anchovy (Anchoa 



