CLARKE: DIEL FEEDING PATTERNS OF MESOPELAGIC FISHES 



Table L— Towing times (Hawaiian Standard Time) for three 24-h series of oblique tows to sample vertically migrating mesopelagic 

 fishes and three all night series for deep-living nonmigratory fishes. Times given for dusk (SS), dawn (SR), and shallow night (Nl-4) are 

 for the entire tow; those for day tows (Dl-3) and deep night tows (dNl-3) are for the time the trawl fished below ca. 350-400 m. 



'The D3 tow for 28 August was fouled; time given is for tow made on 13 September. 



and its contents, if any, placed on a clean glass 

 slide. The fish including the empty stomach was 

 placed in a preweighed aluminum pan. After 

 examination, the stomach contents were rinsed 

 into a second preweighed pan using distilled wa- 

 ter. 



The stomach contents were examined only 

 casually. A rough estimate of fullness was made 

 and degree of digestion noted. Prosome length 

 (PL) of copepods and total length (TL) of other prey 

 were recorded from intact items. Intact prey items 

 could usually be identified to genus, but no serious 

 attempt was made to determine composition of the 

 diet from these samples. The remarks below on 

 types of prey include only the most frequently 

 encountered items and are not meant to be taken 

 as detailed analyses of diets. 



Both fish and stomach contents were dried at 

 60°C for 24 h ( somewhat longer for a few large fish) 

 and allowed to cool under partial vacuum before 

 weighing. The pans with stomach contents were 

 weighed to the nearest 0.01 mg on a microbalance, 

 and the content weight determined by subtrac- 

 tion. Both control pans and reweighing of several 

 pans with dried stomach contents after a second 

 period in the drying oven or desiccator indicated 

 that the weighing and handling error was of the 

 order of ±0.02 mg. There was no indication that 

 error was proportional to the amount of material 

 in the pan. Pans with fish were weighed on a 

 semimicro balance; the reading was recorded to 

 0.01 mg on small fish and to 0.1 mg on those over 

 ca. 100 mg. Based on changes in weight of control 

 pans and reweighing offish after a second period of 

 desiccation, the error was <1% of the fish weight. 



While the weighing and handling error was 

 such that estimates of stomach fullness were af- 

 fected only to the fourth or possibly third decimal 

 place, other errors or biases inherent in the mate- 

 rial should be mentioned. As noted above, an un- 

 known fraction of the material was lost due to 

 leaching. Damage to the fish positively biased the 



ratios since there was some loss of skin, scales, or 

 fin rays in almost all specimens. Such errors were 

 unrelated to the time of collection and were more 

 likely to increase variability and thus to obscure 

 rather than cause diel trends in the data. The 

 intestinal contents, which were dried and weighed 

 with the fish, may have varied with time and thus 

 introduced a systematic error in fish weights. 

 Based on visual examination, however, largest 

 amounts of materials in the intestine were almost 

 certainly <1% of the total fish weight, and, con- 

 sequently, affected the stomach fullness index by 

 <0.1%. 



The 2-3 h durations of the tows were a possible 

 source of bias and high variability. Bias in 

 stomach fullness could result from evacuation of 

 stomach contents between capture and death 

 (Eggers 1977). It is likely that this was negligible 

 since the fishes considered here were probably 

 dead soon after capture by the net. The 2-3 h possi- 

 ble differences in capture time for fishes from the 

 "same" period of the diel cycle almost certainly 

 contributed to the variability in stomach 

 fullness — particularly during periods when the 

 latter was changing rapidly. 



Stomach fullness could possibly be biased nega- 

 tively by regurgitation after capture or positively 

 by feeding in the net. (Either type of bias would 

 tend to obscure rather than cause diel differences 

 in stomach fullness.) Regurgitation apparently 

 occurred infrequently in all species considered ex- 

 cept Lampanyctus nobilis. Except for the latter 

 (see below), specimens with partially digested food 

 remains in the mouth or everted stomachs were 

 not used. Hopkins and Baird (1975) showed that 

 feeding in the net is an unimportant source of 

 error even when a fine-mesh cod end is used, and 

 there was little indication of net feeding in the 

 present study. Zooplankton in good condition, 

 usually crustaceans with appendages erect and 

 extended, were infrequently found in the mouth. 

 These were assumed to have lodged there during 



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