YOUNGBLUTH: VERTICAL DISTRIBUTION OF EUPHAUSIIDS 



somewhat deeper, 250-350 m and 400-600 m, re- 

 spectively. At night all stages of these species 

 migrated into the upper 100 m. Euphausia mutica 

 and E. recurva appeared in the upper 50 m, 

 whereas E. gibboides was more widely distributed 

 with most of the population between 50 and 250 m 

 (e.g., CalCOFI 50.140). 



The relative abundance, vertical distribution, 

 and diel migration of E. pacifica varied with 

 distance from the coast (Figure 3). Data from all 

 stations are illustrated to show- the number of 

 patterns exhibited by this species. Larvae and 

 juveniles tended to occupy a much wider vertical 

 range in nearshore waters. The bulk of the larvae 

 was usually found in the upper 150 m day and 

 night. The single exception to this pattern was 

 observed at CalCOFI 70.75 where the larvae were 

 abundant at 250 m throughout the day and in very 

 large numbers in the upper 100 m at night. 

 Juveniles were numerous in the surface waters as 

 well as at depths to 450 m. The adult phase was 

 frequently most abundant between 200 and 400 m 

 during the daytime. Offshore, during the day, 

 densities of this species were reduced, adults were 

 rarely collected, and populations occurred at 

 deeper, narrower intervals. At night, both near- 

 shore and offshore, only some members of each 

 stage migrated to the surface waters from depths 

 of 250-450 m. The general features of the geo- 

 graphical distribution of this species in the central 

 regions of the California Current agree with 

 observations by Brinton (1962b, 1967); the vertical 

 dimensions are more detailed. 



Tessa rab rachio n 



The only species of this genus, T. oculatus was 

 frequently found in small numbers, i.e., 10- 

 20/1,000 m^. Juveniles and adults of this charac- 

 teristic subarctic species were common and oc- 

 curred between 70 and 500 m. Somewhat greater 

 numbers were collected at night. The larvae tend- 

 ed to remain closer to the surface, i.e., from 70 to 

 200 m (SOE 74; CalCOFI 50:110), than juveniles 

 and adults which were usually found between 200 

 and 400 m. Thus, this species inhabits a wide depth 

 interval below the thermocline regardless of the 

 time of day. 



Thysanopoda 



Three species of Thysanopoda were collected- T. 

 aequalis, T. acutifrons, and T. egregia. Very few 



specimens of these species were taken. Larvae of 

 T. aequalis, a species typical of central water 

 masses, were found in the upper 200 m at the 

 offshore stations of the southernmost transect. 

 Adults, found only at night at the same stations, 

 were collected above 300 m. Larvae of T. acuti- 

 frons were not observed. One juvenile and one 

 adult were taken during the day between 400 and 

 500 m at different offshore stations. At night a 

 total of 11 adults and 2 juveniles were caught 

 between 200 and 500 m (SOE 16, 74; CalCOFI 

 50.110, 50.140). One to four larvae of T. egregia 

 were collected between 50 and 450 m at nearly all 

 but the most northern stations. 



Thysanoessa 



Three species of Thysanoessa were found- T". 

 spinifera, T. gregaria, and T. longipes. Thysan- 

 oessa spinifera was only collected near the coast, 

 most frequently in the upper 150 m. Small densi- 

 ties of juveniles, the most abundant stage, were 

 present in tows from 150 to 350 m (CalCOFI 50.80, 

 70.75). Adults were not collected. The preponder- 

 ance of T. spinifera in the neritic environment 

 has been noticed previously (Brinton 1962a; 

 Hebard 1966). Diel changes in the vertical dis- 

 tribution of juveniles indicate that perhaps some 

 members of this phase migrated into the upper 100 

 m at night (Figure 4a). These data support other 

 studies that have suggested this species is a diel 

 migrant (Regan 1968; Day 1971; Alton and Black- 

 burn 1972). 



Thysanoessa gregaria occurred at all but one 

 location (SOE 68). This species was found most 

 often in the upper 150 m, although it ranged to 300 

 m. Juvenile phases dominated the catches during 

 the SOE cruise. All stages were abundant among 

 the CalCOFI samples gathered 2 wk later. Densi- 

 ties were greater along the southern transects. 

 From 50 to 500 individuals/ 1,000 m"* were recorded 

 within depth intervals where the largest concen- 

 trations occurred. Larvae usually resided in the 

 upper 50 m. Juveniles and adults were numerous 

 between 50 and 200 m and often 3-10 times more 

 numerous in the night tows. These data suggest 

 that the older stages probably avoided the sam- 

 pling gear during the day. At one station 

 (CalCOFI 70.75), all stages of T. gregaria were 

 observed only in the upper 20 m during the day. At 

 night this species ranged to 400 m with the largest 

 densities occurring between 60 and 100 m and no 

 specimens were collected in the upper 30 m. These 



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