862 



Fishery Bulletin 92(4). 1994 



In diet studies of the related temperate species S. 

 acanthias (Bonham, 1954; Holden, 1966; Jones and 

 Geen, 1977), consumed prey diversity was often lower 

 than that of S. mitsukurii at SE Hancock with a large 

 variation in the dominant prey species by season and 

 location. These studies typically found that S. 

 acanthias feed on the most abundant prey items 

 available (normally teleost fishes), although other 

 pelagic organisms (primarily euphausiids) were im- 

 portant, particularly for smaller sharks (Jones and 

 Geen, 1977). Squalus acanthias often inhabit shelf 

 areas that may maintain large influxes of a few dif- 

 ferent pelagic species that opportunistic predators 

 such as squalids can exploit (Brodeur and Pearcy, 

 1992). Squalus mitsukurii at SE Hancock Seamount 

 fed on a great diversity of mesopelagic prey typical 

 of oceanic environments (Boehlert and Genin, 1987; 

 Reid et al., 1991). 



Because S. mitsukurii specimens in our feeding 

 study were taken from gear set on the bottom, find- 

 ing benthic prey, such as pleuronectiforms, crabs, or 

 octopods, in the diet was not surprising. Based on 

 results from our vertical longline sets and midwater 

 trawling, however, S. mitsukurii did not appear to 

 move high enough above the summit to feed on me- 

 sopelagic prey. Rather, these prey species (e.g. 

 stomiiform fishes, lophogastrid mysids) were likely 

 consumed by S. mitsukurii as the fauna were ad- 

 vected over or around the seamount. A similar mecha- 

 nism may exist for other resident fish predators at 

 SE Hancock (Seki and Somerton, 1994). Various stud- 

 ies have documented the importance of current-to- 

 pographical interactions to biological processes in 

 these environments (reviewed in Boehlert and Genin, 

 1987), as well as the exploitation of oceanic prey spe- 

 cies by demersal predators as the former are advected 

 over banks (Isaacs and Schwartzlose, 1965; Genin 

 et al., 1988) or concentrate in submarine canyons 

 (Pereyra et al., 1969). Over other seamounts, meso- 

 pelagic micronekton prey have been found to be an 

 important, if not principal, forage base for resident 

 fish populations (Parin and Prut'ko, 1985). 



A high proportion of S. mitsukurii stomachs were 

 classified as empty in our study (65%). The occur- 

 rence of numerous empty stomachs for squalid spe- 

 cies has often been attributed to a combination of 

 intermittent feeding behavior and partial regurgita- 

 tion of food items (Bonham, 1954; Holden, 1966; Bow- 

 man, 1986). Whether the use of hook-and-line gear 

 is biased towards actively feeding fish with empty 

 stomachs is unknown. Holden ( 1966) discounted this 

 hypothesis after he found a greater proportion of 

 empty guts from trawl-caught rather than line- 

 caught S. acanthias. However, the greater frequency 

 of empty stomachs from longline-caught rather than 



gillnet-caught fish in our study suggest further con- 

 sideration of this hypothesis. It is possible that the 

 rapid recovery of longlines to the surface may have 

 increased the likelihood of food regurgitation or stom- 

 ach flushing. However, we did not observe direct evi- 

 dence of regurgitation, although the high frequency 

 of empty stomachs filled with water suggests that 

 undetected regurgitation may have occurred (see 

 Bowman, 1986). Whether the preponderance of 

 empty guts in this study actually reflects some as- 

 pect of feeding behavior or is simply an artifact of 

 the sampling process requires further work. 



Acknowledgments 



We thank G. W. Boehlert, G. A. McFarlane, J. A. 

 Musick, M. Saunders, D. A. Somerton, and two 

 anonymous reviewers for their helpful comments on 

 an earlier draft of the manuscript. 



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