Uchikawa et al.: Diet of Berryteuthis anonychus in the northeast Pacific during spring 



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0, KM > i — i~i —  i  — r -1 — i  i —  i  — r -1 — i 



20 30 4(1 50 60 70 80 90 100 110 



Mantle length (mm) 



Figure 4 



Relationship between mandible width of 

 copepod prey and mantle length of Ber- 

 ryteuthis anonychus. 



Discussion 



The diet of Berryteuthis anonychus collected in the north- 

 east Pacific during May was dominated by calanoid 

 copepods and chaetognaths. During early July in this 

 area, B. anonychus larger than those examined in the 

 present study (ML: 75-127 mm vs. 10-102 mm) fed on 

 a wider variety of prey, including primarily calanoid 

 copepods, hyperiid amphipods, pteropods, and euphau- 

 siids (Lapshina, 1988). Possible causes for this change 

 in diet include seasonal change in prey availability and 

 an ontogenetic change in the squid's ability to capture 

 prey. 



The zooplankton composition in the upper 150 m of 

 the subarctic North Pacific is highly seasonal. Neocala- 

 nus copepods, the major prey of B. anonychus, dominate 

 the epipelagic zooplankton community during spring 

 and early summer (Mackas and Tsuda. 1999). They 

 then descend from the upper layer to spend the late 

 summer, autumn, and early winter at 400-2000 m, well 

 below the depth range of B. anonychus (0-200 m; Nesis, 

 1997). As a result of this ontogenetic descent, the upper 

 ocean zooplankton biomass decreases greatly, and the 

 community is then dominated by a different group of 

 species. This group includes euphausiids (Mackas and 

 Tsuda, 1999), which are consumed by more B. anony- 

 chus in July (28%; Lapshina, 1988) than in May (5%; 

 present study). Other prey that show a large increase 

 in frequency of occurrence between May and July are 

 amphipods (19% in May, 52% in July) and pteropods 

 (0% in May, 40% in July). 



Oceanic squids such as B. anonychus generally feed 

 on small crustaceans as juveniles and then shift their 

 diet to larger fish and other cephalopods as they grow 

 (Rodhouse and Nigmatullin, 1996). We observed no such 

 ontogenetic shift within the size range examined, but 

 copepod prey size was found to increase with growth. 



These data are consistent with those for other squids 

 in that prey size increases during development (Nixon, 

 1987; Hanlon and Messenger, 1996). Most gonatids 

 undergo ontogenetic vertical descent (Roper and Young, 

 1975; Nesis, 1997), and a clear shift in the diet can ac- 

 company this habitat shift (e.g., as seen in Berryteuthis 

 magister; Nesis, 1997). Nesis (1997), however, suggested 

 that B. anonychus does not undergo ontogenetic descent; 

 therefore no such habitat-change-related shift in diet 

 would be expected to occur in this species. 



Highest feeding intensities were recorded in the after- 

 noon and just after sunset, which would indicate that 

 B. anonychus feeds both day and night. Such a feed- 

 ing scenario is supported by the high overlap in depth 

 distributions of B. anonychus (day: 50-200 m, night: 

 0-150 m; Nesis, 1997) and its main prey, Neocalanus 

 cristatus; during spring, N. cristatus occurs mainly at 

 50-150 m, and like the other Neocalanus species, shows 

 no evidence of diel vertical migration (Mackas et al., 

 1993). Therefore B. anonychus and N. cristatus occupy 

 nearly the same depth range both day and night. 



The chaetognath Parasagitta elegans was the third 

 most abundant prey taxon and was consumed by more 

 squid than any other taxon. Parasagitta elegans forms 

 an important fraction of the springtime macrozooplank- 

 ton community in the North Pacific (Brodeur and Ter- 

 azaki, 1999) and inhabits mainly the epipelagic layer 

 (0-200 m) (Kotori, 1976; Terazaki and Miller, 1986); 

 therefore predation on P. elegans could also occur both 

 day and night. Another gonatid squid. Gonatus mado- 

 kai, has also been found to prey on Parasagitta sp. 

 (Kubodera and Okutani, 1977). 



There was no evidence of cannibalism, which com- 

 monly occurs in many gonatids, particularly Berryteu- 

 this magister and Gonatopsis borealis (Lapko, 1996; 

 Nesis, 1997). Cannibalism in squids appears to occur 

 less frequently when prey are abundant (Shchetinnikov, 

 1992; Santos and Haimovici, 1997), as is the case in 

 the North Pacific during spring. In addition, at nearly 

 every station sampled, squid of a small size range were 

 collected (Bower et al., 2002); therefore it seems that op- 

 portunities for intercohort cannibalism were limited. 



The large stock size of B. anonychus in the North 

 Pacific (Nesis, 1997) and its importance in the diet of 

 higher predators may indicate that the food chain from 

 copepods through squids and these higher predators is 

 an important trophic pathway in the pelagic food web of 

 the Subarctic Pacific during spring. The large seasonal- 

 ity in zooplankton composition in the upper 150 m may 

 indicate that these trophic pathways will show similar 

 seasonal variations. 



Acknowledgments 



We thank the late H. Richard Carlson for providing 

 us with squids collected during the May 1999 NMFS 

 salmon survey aboard the FV Great Pacific. We also 

 thank Chingis Nigmatullin and the late Kir Nesis for 

 translating two Russian abstracts into English, H. Sugi- 



