LANGTON: FOOD HABITS OF YELLOWTAIL FLOUNDER 



empty stomachs showed an increase from 18 to 

 56% over depth ranges 0-37 m to 111-146 m. At 

 the deepest depth, 147-220 m, the sample sizes 

 were too small (<8 fish) to evaluate this factor. 

 The stomach content weight values were highest 

 at the shallowest depth, 0.28% body weight, but 

 did not systematically decrease with depth. 



Certain prey were more prevalent in the stom- 

 achs of the yellowtail flounder at different 

 depths. The polychaete S. bombyx accounted for 

 26.6% of the diet of fish in the 74-1 10 m range and 

 only 9.3 and 2.9% of the stomach content weights 

 for fish in the next lowest and highest depth 

 range, respectively. Spiophanes did not occur in 

 stomachs of yellowtail flounder collected outside 

 38 and 146 m. Crangon septemspinosa also pre- 

 dominated in only one depth group, 39.6% of the 

 diet at 147-183 m, although it did occur in stom- 

 achs at all depths <147 m. There was no pattern 

 to the occurrence of other prey species in stom- 

 achs of yellowtail flounder which could be related 

 to depth. 



DISCUSSION 



Predator size was of little importance to the 

 diet composition of yellowtail flounder when con- 

 sidering the entire study area. For all sizes of 

 fish, polychaete worms and amphipod crusta- 

 ceans were the primary prey, although amphi- 

 pods were somewhat more important for small 

 fish and polychaetes more important for larger 

 fish. Pitt (1976) also observed relatively few 

 changes in diet composition for different-sized 

 yellowtail flounder on the Grand Bank. Size 

 does, however, have an obvious influence on the 

 absolute amount of food in the stomachs with 

 an increase in mean weight of stomach contents 

 increasing with fish size (Table 1). 



Predator size also had an observable effect on 

 several other factors which were reflected by dif- 

 ferences in the stomach contents. Perhaps the 

 most interesting size (diet?)-related observation 

 is that the majority (66%) of the smaller fish, <15 

 cm long, were caught at night while the larger 

 fish (21 to 49 cm) were caught both day and night 

 in much more equal proportions. Since the fish 

 were taken at random from the catch, this is 

 likely to be indicative of a behavioral difference 

 between the small and large yellowtail flounder. 

 This same catch pattern has been observed be- 

 fore. Beamish ( 1966) found a significantly larger 

 catch of small yellowtail flounder «22.5 cm) at 

 night than during the day, while studying verti- 



cal migration by demersal fishes. This difference 

 in catch was not found for the larger fish. Beam- 

 ish (1966) attributed his results to visual condi- 

 tions where small yellowtail flounder could 

 escape through the net more easily during the 

 day than at night. Larval yellowtail flounder, 

 however, show strong diel movements and rise 

 towards the surface at night (Smith et al. 1978). 

 It may be that the juvenile yellowtail flounder 

 continue to demonstrate some nocturnal activity, 

 with a resultant increase in vulnerability to the 

 trawl, and this behavior pattern decreases slowly 

 with an increase in fish size. In any event, the lar- 

 val fish do not appear to be migrating solely for 

 the purpose of feeding ( Smith et al. 1978), and the 

 data presented here are inconclusive about any 

 feeding periodicity for these smaller yellowtail 

 flounder. A complete understanding of the fac- 

 tors controlling this size-related difference in 

 catchability, and any relationship that this has to 

 feeding, will have to await further study. 



Data on feeding periodicity may be interpreted 

 to suggest that yellowtail flounder are daytime 

 feeders with a peak in food consumption in the 

 afternoon-early evening hours (Fig. 1). However, 

 it is quite likely that the fish are feeding through- 

 out the day, and the stomach contents accumu- 

 late at a faster rate than they are digested, result- 

 ing in the dusk period stomach content weight 

 maximum followed by the peak in the percent- 

 age of empty stomachs at night. In European 

 waters the yellowtail flounder's congener, Li- 

 manda limanda, has been observed to feed dur- 

 ing the day (Arntz 1971). This daytime feeding 

 pattern is consistent throughout the year with 

 the only difference being that there is more food 

 in the stomachs during the spring than in the 

 autumn (compare with Figure 1). Changes in 

 diet composition were also observed which may 

 support the argument that yellowtail flounders 

 are daytime feeders. Polychaetes are less and 

 amphipods more important as prey at night. 

 This shift may simply be the result of a differen- 

 tial digestion rate for these two prey types. The 

 soft-bodied polychaetes would presumably digest 

 more quickly than the crustacean body parts. 

 Studies on digestion and prey selection are need- 

 ed to understand fully these observed changes in 

 stomach content weight. 



Seasonal effects on diet include the difference 

 in the absolute amount of food in stomachs be- 

 tween spring and fall (Fig. 1) and differences re- 

 lated to the reproductive cycle. Yellowtail floun- 

 der spawn from March to July with the peak 



21 



