ROBISON and CRADDOCK: MESOPELAGIC FISHES EATEN BY FRASER'S DOLPHIN 



not generally known to be regular diel vertical 

 migrators, although smaller individuals are usually 

 found at shoaler depths, and there is evidence that 

 the juveniles of at least one species do migrate ver- 

 tically (Keene 1973). Scopelogadus mizolepis bi- 

 spinosus is the only member of its genus known to 

 inhabit the area where the Fraser's dolphins were 

 captured. Adults are usually found below 400 to 500 

 m (Ebeling and Weed 1963). Based on the otolith 

 height vs. standard length relationships of S. beani 

 and S. m. mizolepis from the Atlantic and S. m. bi- 

 spinosus from the eastern Pacific, more than half of 

 the ingested Scopelogadus were between 40 and 65 

 mm long. 



Diretmidae 



Diretmus argenteus, the most abundant fish in the 

 dolphin stomachs, is another poorly understood 

 mesopelagic species. The sketchily known details of 

 its natural history suggest that it is a deep-dwelling 

 (ca. 400 to 800 m), nonmigrating fish which inhabits 

 broad temperate and tropical areas of the Atlantic, 

 Pacific, and Indian Oceans (Woods and Sonoda 

 1973). We have found euphausiid shrimp and lan- 

 ternfish remains in the few Diretmus stomachs we 

 have examined. Except for the absence of bio- 

 luminescent organs, they resemble the hatchetfishes 

 in external appearance. While many smaller in- 

 dividuals were also present, the characteristic oper- 

 cular bones and otoliths indicate that the majority of 

 the ingested fishes were between 180 and 285 mm 

 SL. 



Crustaceans 



In contrast to the relatively large size of many of the 

 ingested fishes, the crustacean remains (Table 2) in 

 the three dolphin stomachs were generally at the up- 



Table 2.— Crustaceans identified from the stomach contents of 

 three specimens of Lagenodelphis hosei captured in the eastern 

 tropical Pacific. 



per limit of the size range, which is collected by trawl- 

 ing gear. This suggests that midwater trawls are 

 capable of sampling the full size range of the crusta- 

 cean species involved but probably not that of the 

 fishes. Like the fishes, however, these shrimps are 

 relatively deep-living species, occupying depths of at 

 least 200 m at night and 400 to 700 m by day. 



Nutrition 



In order to estimate the approximate nutritional 

 value of the fish portion of the stomach contents we 

 relied upon caloric values calculated for midwater 

 fishes reported by Childress and Nygaard (1973). 

 They found a range of 57.7 to 165.8 kcal/100 g wet 

 weight for fishes which were morphologically and 

 ecologically similar to the ones eaten by the dolphins. 

 We multiplied these figures by length/weight re- 

 lationships of from 0.067 to 0.189 g/mm (our data 

 from the same kinds of fishes), the estimated length 

 ranges of the species (based on otolith length vs. 

 standard length relationships), and half the number 

 of otoliths in each stomach. The rough estimates of 

 nutritional value were not less than 5 X 10" calforthe 

 first stomach, 6X10" cal for the second, and 4X10" 

 cal for the third. We do not know if the otoliths we 

 found are the result of a single day's feeding or that of 

 some other time interval. However, the lack of large 

 numbers of heavily eroded otoliths in the stomachs 

 suggests that their residence time was relatively 

 short. 



Similar logic and calculations based on Childress 

 and Nygaard (1974) lead to estimated values of 6 X 

 10 3 cal, 2 X 10 4 cal, and 8 X 10 2 cal for the crustacean 

 portions of the stomach contents of the three Fraser's 

 dolphins. 



The total estimated caloric values for fish and crus- 

 taceans ingested by L. hosei are within the same or- 

 der of magnitude as the value that can be calculated 

 from the daily feeding rates of similar cetaceans 

 which have been kept in captivity (Sergeant 1969). 

 While we have not included the nutritional value of 

 the ingested squids in these estimates, the very small 

 proportion of the diet represented by squids sug- 

 gests that their contribution was negligible. 



DISCUSSION 



The presence of otoliths in the stomach of a pelagic 

 top carnivore does not necessarily mean that their 

 original owners were ingested directly. Rather it is 

 quite likely that some of these persistent remnants 

 were first consumed by predatory fishes or squids 

 and transferred via their stomachs as they, in turn, 



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