CLARKE: DIEL FEEDING PATTERNS OF MESOPELAGIC FISHES 



Table 3. — Depth ranges, estimated diel changes in tempera- 

 ture, and probable day-night differences in prey concentration 

 for the 16 species of fishes considered. (See text for sources of 

 estimates.) 



differences in temperature and oxygen concentra- 

 tion both predict lower rates of metabolic proces- 

 ses in general and in particular lower feeding or 

 stomach evacuation rate during the day. Childress 

 (1975) and Childress and Nygaard (19731 indi- 

 cated that mesopelagic organisms can regulate 

 over a wider range of oxygen partial pressures 

 than these fishes encounter off Hawaii. Thus 

 temperature changes are more likely to affect rate 

 processes. Teal ( 1971 1 showed that increased pres- 

 sure can stimulate metabolic rates and thus 

 mediate or cancel effects of temperature; however, 

 it seems likely that temperature effects are pre- 

 dominant for the species considered here since 

 these fishes migrate through a much stronger 

 thermocline than did the shrimps studied by Teal. 

 As a consequence of vertical migration — by the 

 fishes and by many of their prey — the fishes en- 

 counter diel differences in prey concentration, 

 with which feeding rate is likely to be positively 

 correlated. As noted above, the depth distributions 



of all prey species in the study area are not known 

 in detail; however, general, qualitative features 

 were evident from the available plankton samples 

 (see above). Most of the important prey species 

 were either shallow-living nonmigrators that oc- 

 curred above ca. 200 m day and night or were 

 vertical migrators with maximal concentrations 

 at ca. 300-450 m by day. Some important genera, 

 e.g. Euphausia, Pleuromamma, and Euchaeta, oc- 

 curred as deep as 600 m during the day but not at 

 high densities. At night, most copepods and many 

 of the euphausiids occurred at highest densities 

 above ca. 150-200 m. Many prey species occurred 

 between 200 and 300 m at night, but except for a 

 few euphausiid species, concentrations were much 

 lower than in the upper 200 m. Below ca. 600 m by 

 day and below ca. 300 m at night, total zoo- 

 plankton concentration was low and that of impor- 

 tant prey species nearly zero. Based on the above 

 features and the fishes' depth ranges, qualitative 

 estimates of day-night differences in prey concen- 

 tration were made for each species (Table 3). 



Nine species of myctophids and probably 

 Melamphaes danae had similar diel patterns in 

 that median values of stomach fullness were min- 

 imal at or near dusk and increased only at night, 

 but details of the patterns were variable. Six 

 species, Benthosema suborbitale, Bolinichthys 

 longipes, Ceratoscopelus warmingi, Diaphus 

 schmidti, Lampanyctus steinbecki, and L. nobilis 

 (Figures 1, 2), had two periods of increasing 

 stomach fullness during the night separated by a 

 decline. Maximum stomach fullness occurred at or 

 near dawn, and the fish reached day depth with 

 relatively full stomachs. Stomach fullness ap- 

 peared to decrease during the day in some species 

 and showed no clear trend in others, but in most 

 there was a significant decrease at or near dusk. In 

 Notolychnus valdiviae, Triphoturus nigrescens, 

 and possibly Melamphaes danae (Figure 2), me- 

 dian stomach fullness appeared to increase stead- 

 ily throughout the night to a peak value just before 

 dawn. In the first two of these species, stomachs 

 were partially evacuated by the time they reached 

 day depth. In Hygophum proximum (Figure 2) 

 median fullness reached a peak value early in the 

 night, and stomachs were completely evacuated 

 by dawn. 



For most of the above species there was no evi- 

 dence of significant feeding at depth during the 

 day. Intact items were more frequent at night, and 

 stomach contents of day-caught fish were usually 



507 



