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Fishery Bulletin 94(2). 1996 



Discussion 



The vertical distribution of the six most abundant 

 species of euphausiids off Point Eugenia, June 1961, 

 showed differences between day and night. Three of 

 them (Nyctiphanes simplex, Nematoscelis difficilis, 

 and Euphausia eximia) were important species in 

 the central Gulf of California in May 1965. In both 

 regions, when a thermocline was present, two basic 

 kinds of distribution were apparent: 1 ) species found 

 above and below the thermocline (Nyctiphanes sim- 

 plex and most Euphausia species); and 2) species that 

 remain in or below the thermocline (E. gibboides and 

 Nematoscelis difficilis). In the absence of confirming 

 replicated tows, the differences could also be inter- 



preted as local patchiness. Similar patterns, however, 

 were found during January-February of 1964 

 (Brinton, 1967 1 off Pt. Banda (32°N), DanaPt. (34"N), 

 and Pt. Reyes (38"N). During winter, upwelling is 

 weak and the difference in depth of the thermocline 

 between offshore and onshore stations is less than 

 in summer. Though transects of winter 1964 extended 

 farther offshore than in the present study, the trend 

 of migrating species to be concentrated into depth 

 ranges that were narrower nearshore was observed 

 in both studies. A more northern study, off Cape 

 Mendocino (40"N) and Cape Blanco (43"N) during 

 July-August 1970, also showed this trend (Young- 

 bluth, 1976). 



In the Gulf of California, wind-driven upwelling 

 occurs along the mainland coast in winter and on 

 the peninsular coast in summer (Roden and Groves, 

 1959; Badan-Dangon et al., 1985). However, tides 

 dominate the circulation of water near the midriff 

 islands (Angel de la Guarda and Tiburon), including 

 the Salsipuedes Channel, where currents up to six 

 knots (2.76 m/s) have been measured (Alvarez et al., 

 1984; Bray and Robles, 1991). In this well-mixed 

 area, the water column is cold, relatively rich in oxy- 

 gen, and the vertical distribution of Nematoscelis 

 difficilis was unique in that many adults were 

 present in the upper 100 m at night. A migratory 

 behavior to mate or shed the hatching metanauplii 

 could be occurring, but an absence of a thermocline, 

 acting as a barrier, could promote this behavior. In 

 the earlier ( 1964 ) California Current study, near the 

 coast the juveniles and adults of A 7 , difficilis showed 

 a bimodal vertical distribution (a peak near the sur- 

 face and another in or below the thermocline), 

 whereas offshore, the nocturnal distribution showed 

 only the deep mode (Brinton, 1967). N. difficilis was 

 therefore considered a vertical migrant, although a 

 capability of daytime net avoidance was noted. In 

 the congener N. megalops, from the northwestern 

 Atlantic, differences between day and night were 

 explained as due to daytime net avoidance (Wiebe 

 and Boyd, 1978; Wiebe et al., 1982). Unfortunately, 

 no replicate data for the Salsipuedes Channel sta- 

 tion are available. This appeared to be an important 

 area for reproduction of this species in May 1965; many 

 metanauplii and first larval calyptopes were found in 

 the upper layer, coincident with ovigerous females. 



Though larvae of many species were found off Point 

 Eugenia, most were late furcilias. Only the transi- 

 tion-zone group N. simplex, N. difficilis, and E. 

 gibboides; the more tropical E. eximia; and the cen- 

 tral Pacific E. recurva showed evidence of local re- 

 productive activity, as suggested by the abundance 

 of younger larval stages. Of these, differences in ver- 

 tical distribution were between species with stage 



