Vinciguerria nimbaria fed upon a wide variety 

 of sizes and taxa of prey. Small ( < ca. 2 mm PL) 

 copepods and ostracods were most frequent, but 

 larger copepods and small euphausiids occurred 

 regularly. Both the number of prey items and ab- 

 solute values of stomach fullness for the peak 

 period were higher than for most of the the other 

 species examined here; in several instances the 

 remains of 20-40 prey items were found in a single 

 stomach. Intact items were most frequent at SS 

 and common in specimens from day samples. Some 

 intact items were noted from Nl and a few from 

 N2, but stomachs from N3, N4, and SR contained 

 practically nothing but well-digested remains. 



Sternoptychidae 



Datictphos oculatus (Figure 3) 



Few D. oculatus were available for any period 

 except Dl, and numbers were particularly low for 

 D2. Nine of the specimens used came from the May 

 1974 series. In spite of this, there was an evident 

 and highly significant (P<0.005) diel trend in 

 stomach fullness. Median values rose steadily 

 from a minimum at SR to a maximum at SS and 

 declined nearly constantly throughout the night. 

 There were a few empty stomachs at SR and Dl 

 and none at other periods. Danaphos oculatus fed 

 almost exclusively on Pleuromamma xiphias, 

 Euchaeta media, and similar-sized juveniles and 

 adults of several aetideid species. Intact items 

 were most frequently noted in D3 and SS speci- 

 mens; some were found in those from Dl and D2. 

 Almost none of the night specimens contained any 

 but well-digested remains. 



Valenciennellus tripumtulatus (Figure 3) 



Few V. tripunctulatus were available from any 

 period except Nl; 31 of the total examined came 

 from the May 1974 collections. Still, like D. 

 oculatus, V. tripunctulatus showed a clear and 

 highly significant iP <0.005) diel trend in stomach 

 fullness. Medians rose from zero at SR to a 

 maximum at SS and declined throughout the 

 night. The principal prey items were P. xiphias, P. 

 abdominalis, E. media, and similar-sized 

 aetideids. The stomachs from D2 to SS were nearly 

 uniformly packed with intact prey while those 

 from late night and SR were either empty or con- 

 tained only traces of well-digested remains. 



FISHERY BULLETIN: VOL. 76, NO. 3 



DISCUSSION 



Feeding Chronology 



Interpretation of data on stomach fullness is 

 limited because observed fullness is a function of 

 two rate processes — feeding rate and stomach 

 evacuation rate. Diel changes in stomach fullness 

 indicate that one or both rates vary over the diel 

 cycle, but without independent estimates of one or 

 the other, the only certain statements that can be 

 made are that feeding exceeds evacuation during 

 periods when fullness increases, the opposite 

 when fullness decreases, and that both rates are 

 zero when the stomach is empty. Notes on state of 

 digestion of stomach contents are helpful, but 

 must be interpreted with caution. Absence of in- 

 tact items indicates that feeding rate is zero, but 

 presence of intact items does not necessarily mean 

 feeding rate was positive during a given period 

 since some items may remain intact for an un- 

 known time after feeding ceases. Still, it is possi- 

 ble within these limits to qualitatively consider 

 changes in the two rates and to relate them to 

 environmental changes which the fishes en- 

 counter over the diel cycle. 



The species considered here undergo diel 

 changes in numerous environmental factors, some 

 of which are likely to affect either feeding or 

 stomach evacuation rate in a qualitatively pre- 

 dictable manner. The migrating species encounter 

 higher temperatures at night. Diel temperature 

 changes for each species (Table 3) were deter- 

 mined using temperature-depth profiles from the 

 study area (Maynard et al. 1975 give profiles from 

 several seasons of three different years) and depth 

 ranges of the fishes (Clarke 1973, 1974; Clarke 

 and Wagner 1976). Because all species considered 

 occur below the steepest part of the thermocline 

 during the day, the magnitude of the diel tempera- 

 ture change is mostly a function of nighttime 

 depth range and not day depth or absolute range of 

 migration. For the same reason, juveniles, which 

 occur shallower than adults in most species 

 (Clarke 1973), incur greater temperature change 

 than adults of the same species. The migrating 

 species also encounter lower pressures and higher 

 oxygen concentrations at night (oxygen-depth 

 profiles for a site near the study area are given in 

 Gordon 1970). 



Unless the fishes are able to regulate metab- 

 olism over the range of diel changes, the day-night 



506 



