Bower Paralarval drift of and inferred hatching sites for Ommastrephes bartramii 



399 



Vertical distribution data suggest that O. bartramii 

 paralarvae occur within the 0-40 m depth layer dur- 

 ing both day and night (Okutani, 1968; Young and 

 Hirota, 1990). Upon hatching, paralarvae are active 

 swimmers, but owing to their small size remain 

 planktonic for an unknown period, presumably for 

 several weeks. Given the role of ocean currents in 

 the transport of cephalopod paralarvae, any analysis 

 of cephalopod paralarval distribution requires an in- 

 vestigation of the prevailing oceanographic currents to 

 determine from what region the animals have been 

 advected since hatching. Although there are few records 

 of naturally spawned eggs or egg masses from oceanic 

 cephalopods and no observations of O. bartramii egg 

 masses, O'Dor and Balch (1985) have suggested that 

 pelagically spawned egg masses may become suspended 

 in the mesopelagic zone, where reduced current speeds 

 would limit transport of egg masses away from spawn- 

 ing sites. I have assumed that each paralarva hatched 

 at the site at which it was spawned. 



Korzun et al. (1979) have suggested that spawn- 

 ing locations of O. bartramii may be related to the 

 position of islands. The present study was under- 

 taken to determine whether and how the spawning 

 locations of O. bartramii from the North Pacific are 

 related to the proximity of the Hawaiian Archipelago. 

 This objective was addressed by examining the distri- 

 bution and ages of paralarvae captured during five 

 plankton surveys near the Hawaiian Island chain from 

 1991 to 1993 with ocean current data derived from 

 Lagrangian tracers and by estimating geostrophic flow. 



Materials and methods 



Collection and ageing of paralarvae 



Five plankton surveys were conducted near the Ha- 

 waiian Archipelago to collect cephalopod paralarvae 

 (Fig. 1). Plankton samples were collected during 6- 

 15 February 1991, 5-26 February 1992, and 4-19 

 February 1993 with the Hokkaido University (Ja- 

 pan) ship FTS Hokusei Maru, and during 5-20 Feb- 

 ruary 1991 and 22 March-7 April 1992 with the RV 

 Townsend Cromwell of the National Marine Fisher- 

 ies Service (NMFS). The standard sampling proce- 

 dure during all surveys was to conduct 30-minute 

 oblique tows to 100 m depth. 



The Hokusei Maru February 1991 (HM-Feb 91) 

 survey sampled paralarvae at seven stations along 

 an inshore— offshore transect to the northeast of the 

 Hawaiian Island chain from Kauai Island (Kauai 

 transect), including a lone station in the lee of Ha- 

 waii Island. Windward stations were positioned 9, 

 16, 106, 235, 349, 460, and 583 kilometers offshore. 

 Thirty-six tows (mean depth=95 m; SD=9 m) were 

 taken. Sampling consisted of five back-to-back repli- 

 cate tows at each station. This and subsequent 

 Hokusei Maru surveys used a 4-m 2 ring net equipped 

 with 0.505-mm mesh, a General Oceanics flowme- 

 ter, and a Benthos time-depth recorder. 



The Townsend Cromwell February 1991 (TC-Feb 

 91) survey sampled at ten stations along the archi- 

 pelago. Transects orthogonal to the archipelago were 



