CLARKE; DIEL FEEDING PATTERNS OF MESOPELAGIC FISHES 



trend. Except for SS, the percentage of individuals 

 with empty stomachs was low, and there were 

 some fish with very full stomachs ( >2'^ ). Because 

 of the latter, ranges and percentile limits were 

 broad, and means were much higher than me- 

 dians. 



Ceratoscopelus warmingi fed on a wider variety 

 of taxa and sizes of prey than did the other species 

 covered here. The most frequent items were 

 copepods, ostracods, and small euphausiids, but 

 heteropods, siphonophores, and other zooplankton 

 also occurred. Intact items of such relatively small 

 prey were recorded mostly from specimens col- 

 lected at night; remains from day-collected speci- 

 mens were usually well digested. Ceratoscopelus 

 warmingi also took items up to 10'7( of bodily 

 weight; squid, other fishes, and large euphausiids 

 or decapods occurred in specimens >35-40 mm. 

 Such single large items accounted for nearly all 

 the fish with high values of stomach fullness, and 

 intact prey of this size occurred at all times of the 

 day. Most such items were vertically migrating 

 species that could have been taken at night, but 

 remains of nonmigrating Cyclothone spp., which 

 could have only been encountered between dawn 

 and dusk, were found in 1 1 specimens. Thus, while 

 the overall trend of the data indicates that C. 

 warmingi feeds principally on small zooplankton 

 in the upper layers at night, it probably takes 

 large prey whenever encountered. 



Diaphus schmidti (Figure 1) 



Diel differences in stomach fullness for D. 

 schmidti were highly significant (P<0.005), and 

 the trend was similar to that of the preceding 

 myctophids except for timing; the maximum value 

 occurred at Dl instead of SR. Empty stomachs 

 occurred only in a few specimens from D3, SS, and 

 Nl. Diaphus schmidti took a large variety of prey 

 items; the dominant taxa were small crustaceans 

 (ca. 0.5-3.0 mm PL or TL): ostracods, copepods, and 

 larval and juvenile malacostracans. Heteropods, 

 pteropods, polychaetes, and chaetognaths were 

 also noted. Excepting chaetognaths, few items 

 were >4-5 mm. Frequency of intact items was 

 highest at SR, and lowest at D3 and SS. 



Hygophum proximum (Figure 2) 



Diel differences in stomach fullness for//, prox- 

 imum were highly significant (P<0.005), and the 

 trend quite different from those of the other 



species examined here. Most stomachs were 

 empty, and even 75th percentile values were zero 

 or nearly so between SR and SS; the peak value 

 occurred at N2. Hygophum proximum fed princi- 

 pally on medium-sized copepods (1-3 mm PL) and 

 occasionally other crustaceans. Less than 109f of 

 the stomachs were empty for any of the night 

 periods, but intact items were found frequently 

 only in stomachs from Nl. By N2 most of the prey 

 were unrecognizable, and only six items were rec- 

 ognizable to even general category in all the other 

 samples. 



Larnpanyctus niger (Figure 2) 



This species, one of three forms of the L. niger- 

 complex which occur near Hawaii, has minute 

 pectoral fins and lower AO counts than the others; 

 it was designated as "Form B" in Clarke (1973). 

 Zahuranec^ has recently identified the form as L. 

 niger (sensu stricto). There was evidence from 

 deep night tows taken during the same sampling 

 period that a fraction of the population ofL. niger 

 did not vertically migrate; consequently, some of 

 the day-caught specimens may not have ascended 

 to the upper layers the previous night. (Such 

 "non-migration" was also recorded in previous 

 studies, see Clarke 1973.) 



The //-test indicated no significant diel differ- 

 ences in stomach fullness (P>0.10), and none of 

 the adjacent pairs differed significantly. The me- 

 dians from nighttime show a trend similar to that 

 of other myctophids, but the means were highest 

 during the day. No specimens were available from 

 SR. Values of stomach fullness were overall much 

 lower than observed in other species. Stomach 

 fullness exceeded 19c in only 21 of the 160 speci- 

 mens, and over 509c of the stomachs were empty at 

 all periods except N2, N3, and N4. 



The most frequent food items were large 

 copepods of the familes Metridiidae, Euchaetidae, 

 and Aetideidae and small (<10-15 mm TL) 

 euphausiids. Occasionally small fishes were 

 found. Intact prey items were found in stomachs 

 from all periods. Deep-living copepods such as 

 Metridia and Pseudochirella were noted in day- 

 caught specimens indicating that at least some 

 feeding occurs during the day. 



^B. J. Zahuranec. Oceanic Biology Program, Office of Naval 

 Research, Arlington, VA 22217. Personal communications, June 

 1977. 



501 



