300 



Abstract. -Vertical distribution of 

 euphausiids on opposite sides of Baja 

 California, off Point Eugenia to the 

 west (June 1961 1 and in the central sec- 

 tor of the Gulf of California to the east 

 (May 1965), are described from day and 

 night sets of samples. Off Point Eugenia, 

 the thermocline was shallow (20 m) to- 

 ward the coast, characteristic of up- 

 welling in late spring. In the Gulf, the 

 Salsipuedes Channel showed well- 

 mixed water whereas the Guaymas 

 Basin had a stratified profile of tem- 

 perature and oxygen. Lower abun- 

 dances of the larger euphausiids dur- 

 ing day throughout the water column, 

 compared with night, are attributed to 

 net avoidance. When a thermocline was 

 present, two basic migration patterns 

 were observed: 1) species crossing the 

 thermocline (Nyctiphanes simplex and 

 most Euphausia spp.) and 2) species re- 

 maining at or beneath the thermocline 

 l Euphausia gibboides and Nernatoscelis 

 difficilis). On the basis of distribution 

 at night, the youngest larvae and adults 

 of /V. simplex were in the mixed layer 

 at the coastal station off Point Eugenia; 

 more advanced stages of development 

 were at mid-depth, between and 50 m, 

 suggesting an upwelling-downwelling 

 cell of circulation. Inside the Gulf, 

 abundant metanauplii and ovigerous 

 females of N. difficilis occurred only in 

 the upper layers of Salsipuedes Chan- 

 nel, whereas in Guaymas Basin and 

 Point Eugenia, the youngest larvae 

 were within the thermocline. The de- 

 crease of oxygen with depth did not 

 reach the critical values observed in the 

 more tropical eastern Pacific but, in gen- 

 eral, where values of [0,1 were <1 mL/L, 

 the abundance of euphausiids was low. 



Vertical distribution of euphausiid life 

 stages in waters adjacent to 

 Baja California 



Bertha E. Lavaniegos 



Centra de Investigation Cientifica y Educaci6n Superior de Ensenada 

 Apartado Postal 2732, C.R 22800, Ensenada, Baja California, Mexico 

 Present address Centra de Investigaciones Biologicas del Noroeste 



Apartado Postal 1 28. C P 23000, La Paz. Baha California Sur 



Mexico 



Manuscript accepted 11 December 1995. 

 Fishery Bulletin 94:300-312 I 19961 



Early studies of euphausiids in the 

 California Current showed a coin- 

 cidence between their vertical dis- 

 tribution and the vertical move- 

 ment of a mid-depth layer of organ- 

 isms from which sonic scattering 

 was reflected (the deep scattering 

 layer in Boden, 1950; Barham, 

 1957 ). Interaction of this layer with 

 shoaling of the sea floor was de- 

 scribed by Isaacs and Schwartzlose 

 (1965). Vertical migration, or lack 

 of it in certain species, has been re- 

 lated to daytime avoidance of nets 

 by euphausiids off California and 

 northern Baja California (Brinton, 

 1967 ). For a given species, variation 

 in the extent of vertical migration, 

 and range of nonmigrators, has 

 been related to the depth of the ther- 

 mocline and distributions of dis- 

 solved oxygen (Brinton, 1967, 1979: 

 Youngbluth, 1976). In the margins 

 of the eastern tropical Pacific (ca. 

 20-22"N), a shoaling oxygen mini- 

 mum layer constrains the vertical 

 ranges of warm-temperate eu- 

 phausiid species with a replacement 

 by tropical species tolerant of oxy- 

 gen deficiency (Longhurst, 1967; 

 Brinton, 1979). Seasonal variability 

 in vertical migratory behavior has 

 been also observed. In La Jolla 

 Bight near San Diego, differences 

 in patterns of vertical migration of 

 Euphausia pacifica and the copepod 

 Calanus pacificus were observed 

 between upwelling and downwelling 

 periods (Koslow and Ota, 1981). 



These and subsequent studies 

 dealing with vertical migration in 

 different taxa have shown variable 

 responses to environmental stimuli. 

 For example, in euphausiids, the 

 influence of light ( Boden and Kampa, 

 1967; Bright et al., 1976), proxim- 

 ity of the bottom (Cochrane et al., 

 1994), and temperature and salin- 

 ity (Wiebe and Boyd, 1978) have 

 been described. The selective ad- 

 vantage of remaining at depth dur- 

 ing the day is usually considered an 

 adaptation for escape from preda- 

 tors (Zaret and Suffern, 1976; Oil- 

 man et al., 1983; Bollens et al., 

 1992) or an energetic benefit (Mc- 

 Laren, 1963, 1974; Enright, 1977). 



The present study describes for 

 the first time the vertical distribu- 

 tion of euphausiids in the central 

 sector of the Gulf of California and 

 off Point Eugenia (28"N) during 

 spring. In the Gulf of California, 

 deep basins, island channels, and, 

 in particular, low [0 2 ] within sub- 

 tropical latitudes all suggest possi- 

 bilities for interesting variability in 

 vertical distribution of fauna. How- 

 ever, information on the subject is 

 restricted to the vertical distribu- 

 tion of euphausiids and pelagic red 

 crab, Pleuroncod.es planipes, at the 

 entrance of the Gulf, as part of an 

 eastern Pacific transect (Brinton, 

 1979). Inside the Gulf, only the sub- 

 mergence of the copepod Calanus 

 pacificus californicus to depths of 

 200-300 m during summer has 



