VERTICAL DISTRIBUTION, DIEL VERTICAL MIGRATION, 



AND ABUNDANCE OF SOME MESOPELAGIC FISHES IN 

 THE EASTERN SUBARCTIC PACIFIC OCEAN IN SUMMER* 



Bruce W. Frost and Lawrence E. McCrone^ 



ABSTRACT 



Vertical distributions of myctophid fishes and other components of the mesopelagic micronekton were 

 determined during the summers of 1973-75 at two stations in the eastern subarctic Pacific Ocean. 

 Stratified samples were collected with a multiple net Tucker trawl so that the entire water column 

 extendmg to between 385 and 460 m could be sampled during a daytime or nighttime period; two to four 

 day and night vertical series of samples were obtained each summer. Four species of myctophids made 

 up 87^^ of the total fish catch: Stenobrachius leucopsarus and Diaphus theta, which performed diel 

 vertical migrations of 300 m vertical extent: andProtomyctophum thompsoni andS. nannochir, which 

 exhibited only slight diel variation in vertical distribution. Populations of each myctophid species 

 tended to be vertically stratified by age or size with larger individuals occurring in samples taken 

 progressively deeper. Two other major components of the micronekton were euphausiids and decapod 

 shrimps, chiefly Euphausia pacifica and Sergestes stmilis; both species were conspicuous diel vertical 

 migrators. Samples collected in horizontal hauls immediately following sunset showed that three 

 migratory species, the two migratory myctophids and E. pacifica. were closely associated with the 

 single migratory sound-scattering layer (12 kHz); S. similis lagged the ascent of the migratory 

 scattering layer. A single, deep, nonmigratory sound-scattering layer corresponded closely to the 

 distribution of P. thompsoni during both day and night. As in other subpolar oceanic waters, abun- 

 dance and standing stock of myctophids were high — 0.9 fish/m^ and 0.37 g dry weight/m^. 



In 1973 we began a field study of some small 

 mesopelagic fishes of the family Myctophidae, 

 commonly known as lanternfishes or myctophids, 

 in the eastern subarctic Pacific Ocean. The objec- 

 tives of the study were to determine the vertical 

 distribution and migration characteristics of the 

 numerically dominant species, to document their 

 feeding behavior, and to ascertain if the distribu- 

 tions of fish were in any way influenced by the 

 distribution of their preferred prey. Myctophids 

 are major components of the mesopelagic fauna 

 throughout the world ocean, and in most areas 

 they are sufficiently abundant and stratified in the 

 water column to cause deep sound-scattering 

 layers (Baird et al. 1974; McCartney 1976). In- 

 deed, study of these fishes has been heavily 

 oriented toward aspects of their distribution in 

 relation to sound-scattering layers (e.g.. Tucker 

 1951; Barham 1966; Taylor 1968; Holton 1969; 

 Farquhar 1971; Baird et al. 1974), although some 

 investigations emphasized aspects of biological 



'Contribution No. 1039 from the Department of Oceanog- 

 raphy, University of Washington. Seattle, WA 98195. 



^Department of Oceanography, University of Washington, 

 Seattle, WA 98195. 



Manuscript accepted .Mav 1978. 



FISHERY BULLETIN: VOL. 76, NO. 4. 1979. 



and ecological significance, such as individual 

 growth rates, seasonal changes in abundance, and 

 association among species (e.g., Pearcy and Laurs 

 1966; Harrisson 1967; Lavenberg and Ebeling 

 1967; Smoker and Pearcy 1970; Badcock 1970; 

 Clarke 1973; Pearcy et al. 1977). Much of the 

 research on myctophids has, in addition, stressed 

 description of the prominent diel vertical migra- 

 tions which are apparently undertaken by almost 

 all species. 



In the few species studied in detail, both the 

 occurrence and pattern of vertical migration vary 

 with age or ontogeny. Larval myctophids are 

 nonmigratory, spending day and night in near- 

 surface waters (Ahlstrom 1959). Diel vertical 

 migration is first evident at or shortly after 

 metamorphosis and usually persists throughout 

 the remaining life of the fish, although in very old 

 fish, migrations may differ substantially in char- 

 acter from those of younger fish and may even be 

 supressed (Nafpaktitis 1968). Apart from this 

 variation with age, diel vertical migrations of 

 myctophids seem to be relatively regular, on a 

 day-to-day basis, and exhibit little or no seasonal 

 variation (Pearcy and Laurs 1966; Halliday 1970; 

 Pearcy et al. 1977). Among some species, however, 



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