Bower: Paralarval drift of and inferred hatching sites for Ommastrephes bartramu 



401 



surface drifters and by dynamic height measure- 

 ments from conductivity-temperature-depth (CTD) 

 probes. Four ARGOS-tracked drifters released by the 

 National Marine Fisheries Service were used as 

 Lagrangian tracers of the surface currents during 

 the 1992 survey periods to assess "typical" current 

 speeds and directions near the Hawaiian Archi- 

 pelago. Drifter positions were recorded every six 

 hours over the survey period. Each drifter's 

 Lagrangian net path was plotted. Geostrophic flow 

 measurements were calculated between each pair of 

 successive sampling stations during the three 

 Hokusei Maru surveys on the basis of dynamic height 

 measurements taken at each sampling station from 

 CTD casts to 1,200 m. Sampling stations averaged 

 approximately 110 km and 24 hours apart. 



Results 



A total of 1,720 O. bartramii paralarvae was collected 

 at 41 of the 59 sampling stations, making up 16.5% 

 of the cephalopod paralarvae caught. Sizes of O. 

 bartramii ranged from 0.9 mm to 8.3 mm ML. 



Age-mantle-length relationship 



Statolith growth increment counts ranged from 3 to 

 22.7 (Table 1). A well-defined relationship existed 

 between the number of statolith growth increments 

 and the dorsal mantle length of the paralarvae, with 

 the exponential function yielding a good fit for the 

 regression (r 2 =0.88) of the number of increments on 

 the dorsal mantle length (Fig. 2). The following ex- 

 ponential model was fitted to the data: 



ML = 0.95e'° 067 *', 



where ML is the dorsal mantle length in millimeters 

 and .r equals the number of increments. This equation 

 was used to estimate the ages (x ) of the 1 ,635 paralarvae 

 not examined during statolith increment analysis. The 

 y-intercept of the exponential function (a=0.95 mm) is 

 close to the actual hatching length (ca. 1 mm). 



Size structure, ages, and projected hatch 

 dates 



The dorsal mantle length ranges of O. bartramii 

 paralarvae varied slightly among surveys (Fig. 3). 

 The largest range of size occurred during the TC-91 

 survey (0.9-8.3 mm), and the smallest range of size 

 occurred during the HM-91 survey (1.0—5.6 mm). 



Estimates of paralarval ages for total samples, 

 predicted from the exponential mantle length on age 



10 15 20 



Increments 



Figure 2 



Relationship between paralarval mantle length and num- 

 ber of daily growth increments in statoliths of Omma- 

 strephes bartramii, and proposed growth curve. 



model, ranged from 1 to 30 days. Projected paralarvae 

 hatch dates for the five surveys are summarized in 

 Figure 4. During the HM-91 and rC-91 surveys, cap- 

 tured paralarvae were estimated to have hatched 

 between 13 January and 8 February. During the HM- 

 92 and TC-91 surveys, estimated hatch dates of cap- 

 tured paralarvae fell between 22 January and 1 April. 

 During the HM-93 survey, estimated hatch dates of 

 captured paralarvae fell between 11 January and 6 

 February. 



Paralarval size with distance offshore 



Mean mantle lengths at each station were compared 

 with distance from the archipelago (Fig. 5). The mean 

 mantle lengths at each station were not significantly 

 correlated (P>0.05; Student's <-test) with distance 

 offshore for any of the five surveys. 



Hydrographic observations 



No clear advective current patterns could be esti- 

 mated from the ARGOS-tracked drifters' tracks (Fig. 

 6). Average net displacement of the four drifters near 

 the Hawaiian Archipelago was 2.5 km per day. Dur- 

 ing the HM-91 survey, CTD data revealed surface to 

 50-m flows ranging from 4.1 to 16.0 cm per second 

 relative to 1,000 m in reversing bands relative to the 

 archipelago north of the island chain. During the HM- 

 92 survey, CTD data revealed flows ranging from 0.0 

 to 13.8 cm per second relative to 1,000 m in revers- 

 ing bands relative to the archipelago both north and 

 south of the island chain; HM-93 CTD data revealed 



