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Fishery Bulletin 99(4) 
of year, or body size. At the population level, only date was 
significantly correlated with mean gut fullness (Kendall’s 
rank correlation: P- 0.032; Fig. 4). Addition of temperature 
and average lipid level did not provide significant improve- 
ments in the fit of the model (P>0.20 for both). 
Discussion 
Evacuation rates 
Laboratory-measured evacuation rates of YOY striped bass 
at representative winter temperatures were among the 
lowest reported for any fish species. None of the estimates 
compiled from multiple studies by He and Wurtsbaugh 
(1993) included rates as low as the 0.027 we measured at 
11°C. The reason for this observation is not that striped 
bass have exceedingly low digestion rates compared with 
other fishes, but rather that evacuation rate is strongly 
related to temperature and is rarely measured at the lower 
end of temperatures encountered by each species. For exam- 
ple, the lowest temperature at which evacuation rates in 
white perch (M or one americana) have been measured is 
12.6°C (Parrish and Margraf 1990), despite the fact that 
this fish often occupies waters near 0°C. The only evacua- 
tion rates we found in the literature comparable to those we 
measured were those for largemouth bass at 4°C (f? e =0.006; 
Cochran and Adelman, 1982, from data in Markus, 1932) 
and for brown trout at 1.8°C (7^=0.026; Jensen and Berg, 
1993), both measured near the species’ thermal minima. 
Factors other than temperature have been found to in- 
fluence evacuation rates such as prey type, method of feed- 
ing (voluntary vs. force-feeding), and body size. All of our 
experiments were conducted with a single prey type (C. 
septemspinosa) at one ration level (2% of body weight). 
Meal size has been found to affect evacuation rate in lab- 
oratory studies (Smith et al., 1989; Andersen, 1998), but 
this effect is substantially smaller than that of tempera- 
ture (He and Wurtsbaugh, 1993). Our meal size was slight- 
ly higher than the mean, but well within the range ob- 
served among individual fish. We found that body size 
did not affect evacuation rate over the size range of over- 
wintering YOY striped bass (88-150 mm). In several stud- 
ies comparing evacuation rates of various prey, large bod- 
ied shrimp, including C. septemspinosa, were found to be 
evacuated at lower rates than soft bodied prey or smaller 
shrimp species (Nelson and Ross, 1995; Singh-Renton and 
Bromley, 1996; Lankford and Targett, 1997), whereas oth- 
er studies found no differences among prey types (Juanes 
and Conover, 1994; dos Santos and Jobling, 1995). Al- 
though C. septemspinosa and other shrimp species are 
common diet items of YOY striped bass, they were not 
the dominant item. If other prey items, such as amphi- 
pods, are digested and evacuated more rapidly, consump- 
tion rates of wild fish will be underestimated when non- 
shrimp prey dominate the diet. 
We allowed a lag in our description of meal evacuation 
based on laboratory observations. Such a lag has been ob- 
served in several other studies, including both those where 
fish fed voluntarily (Gerald, 1973; Grove et al., 1985) and 
were force-fed (Vondracek, 1987). The length of the lag ob- 
served in juvenile striped bass decreased as temperature 
increased, as seen in juvenile turbot ( Seopthalmus maxi- 
mus\ Grove et al., 1985). The lag prior to beginning of evac- 
uation could be an artifact of experimental conditions or 
a natural delay in the passing of food from the stomach to 
the intestine following ingestion of shelled prey. Our mod- 
el estimated the evacuation rate parameter ( R e ) after the 
lag because this parameter represents the rate of passage 
of food from the stomach. Consumption rates based on gut 
fullness levels may be overestimated if there is a consid- 
erable lag prior to the beginning of digestion. However, 
the digestive lag we observed was short compared with 
