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Fishery Bulletin 99(3) 
5 i 
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WF WF YT YT AC AC 
Early Late Early Late Early Late 
Witch flounder, this experiment 
Yellowtail flounder, Rabe and Brown, 2000 
Atlantic cod, Puvanendran and Brown, 1999 
Figure 5 
Average lunge frequency per minute for early (<2 weeks) and late stage (>6 weeks) 
witch flounder (WF), yellowtail flounder ( YT), and Atlantic cod larvae (AT). See text 
for details. 
Behavioral ecology 
The main finding of our study was that witch flounder are 
not affected by changes in prey availability in the same 
manner as other species of larvae observed under similar 
laboratory conditions. The typical pattern among fish lar- 
vae — that they increase their foraging behavior and prey 
consumption rate with increased prey density (Houde and 
Schekter, 1980) — was supported by our study. However, 
the results are unusual in that the effects of prey density 
on foraging behavior were not as strong as results that 
have been reported for other species. 
The ecological implications of these results can be il- 
lustrated by a comparison of the growth and behavior 
of witch flounder to other northwest Atlantic species ob- 
served under similar laboratory conditions. In rearing 
experiments on Atlantic cod larvae, Puvanendran and 
Brown (1999) found that cod have specific requirements 
for high prey densities. Larval survival, growth rate (0.20 
mm/d), and condition were highest when larvae were 
reared at a prey density of 4000 prey/L. Furthermore, in 
the same experiment, the lunge frequency (an indicator 
of consumption rate) of cod larvae increased from nearly 
1 to 3.5 prey items per minute over the six-week study 
period (Fig. 5; Puvanendran and Brown, 1999). A marked 
increase in lunge frequency with age is also seen in yel- 
lowtail flounder, another north Atlantic pleuronectiform, 
raised under similar conditions (Fig. 5; Rabe and Brown, 
2000). In that study, the growth rate of yellowtail flounder 
was 0.34 mm/d. 
Not only is the foraging behavior of witch flounder rela- 
tively unaffected by variation in prey density, but its lunge 
frequency is much lower, suggesting that it may have a 
lower consumption rate and therefore lower prey require- 
ments compared with those of other species. The lack of a 
significant effect of prey density on the foraging of witch 
flounder is not solely a result of the larger larval size of 
this species. Redfish ( Sebastes ) are relatively large at ex- 
trusion (6-8.9 mm; Penny and Evans, 1985) and their for- 
aging behavior is affected by variations in prey availabil- 
ity (Laurel et al., in press). 
It is remarkable that the growth rate of witch flounder 
is faster than that of cod and yellowtail flounder, especial- 
ly given the possible lower prey consumption rate of witch 
flounder. Two potential mechanisms that may explain this 
phenomenon are high assimilation efficiency and low met- 
abolic requirements. Witch flounder are relatively large at 
hatching and for most of the study period were larger than 
other species of similar age. This size difference alone im- 
plies that its digestive system is larger, more developed, 
and more efficient (Govoni et al., 1986; Klumpp and von 
Westernhagen, 1986). Both yellowtail flounder and cod are 
more active and swim faster than witch flounder. Higher 
activity imparts a greater need for prey, which results in 
