therfe may be a portion of the population which 

 does not migrate, while other members of similar 

 size and age do migrate (Clarke 1973; Badcockand 

 Merrett 1976; Pearcy et al. 1977). 



Virtually nothing is known about the biological 

 causes or consequences of these diel vertical mi- 

 grations, either with respect to the myctophids or 

 their environment. Marshall (1954) suggested 

 that myctophids migrate into the surface layer 

 each night in order to feed on zooplankton, which 

 is usually most abundant in surface waters (Vino- 

 gradov 1968). As pointed out above, larval myc- 

 tophids spend both day and night in the 

 zooplankton-rich surface layer, but as the larvae 

 grow they perhaps become more conspicuous to 

 visual predators and, after metamorphosis, they 

 descend to greater depths, returning to the surface 

 layer only at night, if at all. Vertical migrations 

 may indeed have evolved as a means of avoiding or 

 minimizing predation, but it is unlikely that this 

 hypothesis can be tested in the ocean. 



On the other hand, it is practicable to investi- 

 gate the feeding ecology of myctophid fish in rela- 

 tion to their migrations; for example, what types of 

 prey the fish utilize, when and where in the water 

 column the fish feed, and whether the vertical 

 distributions of the fish are affected by the vertical 

 distribution and abundance of their preferred 

 prey. As necessary background for such a study, in 

 this paper we present details of the vertical dis- 

 tributions of the numerically dominant species of 

 myctophids in the eastern subarctic Pacific Ocean. 



METHODS 



Study Area 



We conducted the investigation during three 

 summer cruises in areas centered at lat. 50°N, 

 long. 145°W (July-August 1973 and July-August 

 1975; Station P in Figure 1 ) and at lat. 51°N, long. 

 137°W (July 1974; Station Q in Figure 1). These 

 stations lie within the hydrographic province des- 

 ignated the Central Subarctic Domain by 

 Dodimead et al. (1963). We chose the subarctic 

 region for ease of sampling and identifying the fish 

 and zooplankton. For example, in an earlier 

 meridional cruise from Kodiak, Alaska, to Hon- 

 olulu, Hawaii (August-September 1972), we found 

 that deep sound-scattering layers are fewer in 

 number, shallower, and more intense in the sub- 

 arctic region than in transition and subtropical 

 waters (Frost unpubl. data). Apparently related to 



752 



FISHERY BULLETIN; VOL. 76, NO. 4 



1 





M'^.c^^^ 



TRANSITIONAL 



_j I I ; ; I L 



Figure l. — Sampling stations in the eastern subarctic Pacific 

 Ocean. Representative hydrographic domains for summer condi- 

 tions after Dodimead et al. ( 1963). 



this, the subarctic myctophid fauna is a simple 

 one; only a few species are abundant, and they are 

 relatively shallowly distributed in the daytime 

 (Taylor 1968). Further, the study area is an open 

 ocean environment, outside the potentially com- 

 plicating influences of coastal and transitional 

 waters (cf. McGowan 1971) and is roughly in the 

 middle of the latitudinal range of several species of 

 myctophids. Finally, the zooplankton assemblage 

 in subarctic waters is also less diverse than in 

 lower latitudes, it is well known taxonomically, 

 and relatively few species are abundant. 



Sampling Gear 



Nekton samples were collected with a modified 

 Tucker trawl ( Tucker 1951) described by Frost and 

 McCrone (1974). Briefly, the trawl had a rigid 

 rectangular mouth with a A-vcr area when inclined 

 forward at a 45° angle from vertical, and carried 

 five separate nets ( 6. 35-mm stretch mesh, knotless 

 nylon ace netting) stacked one on top of another 

 (much like fig. 4 in Harding et al. 1971). The net 

 shape followed the design of Clarke (1969). The 

 trawl carried an electronics package containing a 

 strain gage pressure transducer (range 0-1,500 

 Ib/in^) for determination of depth and a precision 

 pendulum-type tilt transducer (range 0°-90° from 

 vertical) for determination of angle of inclination 

 of the trawl mouth. A TSK (Tsurumi-Seiki 

 Kosakusho)-' flowmeter fitted with a magnetic 



^Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



