FROST and McCRONE: MKSOPEI.AGIC FISHES IN THE EASTERN SUBARCTIC PACIFIC 



sunset, and only from fine temporal spacing of 

 samples did it become apparent that some species 

 were more closely associated with the scattering 

 layer than others (Table 11). Similarly, in the day- 

 time some of these migratory species cooccurred at 

 the depth of the sound-scattering layer together 

 with the nonmigratory P. thonipsoni, and any or 

 all could have contributed to the daytime sound- 

 scattering layer. Despite extensive cooccurrence 

 of several types of potential sound-scattering or- 

 ganisms, the most reasonable hypothesis is that 

 myctophids were primarily responsible for both 

 the migratory and nonmigratory sound-scattering 

 layers in the eastern subarctic Pacific. 



Taylor (1968), also working in the subarctic 

 Pacific, found the best correlation between deep 

 sound-scattering layers and those mesopelagic 

 fish which possessed gas-filled swim bladders. Al- 

 though Taylor grouped Stenuhrach ius leucopsarus 

 and D. theta among fish with fat-invested swim 

 bladders, gas is present in the swim bladders of 

 immature individuals ( <30 mm SL) of both 

 species (Capen 19671. Taylor made no mention of 

 the size of the fish caught in his study; however, in 

 view of the very coarse-meshed nets he used, it is 

 probable that he did not quantitatively sample 

 immature fish. At Stations P and Q, some indi- 

 viduals of S. leucopsarus and D. theta were 

 theoretically the right size to resonate at 12 kHz 

 while at their daytime depths (Capen 1967), and 

 the abundance of either species was probably 

 sufficient to produce deep sound-scattering in the 

 daytime (Hershey and Backus 1962). This pre- 

 sumably holds also for P. thompsoni, which has a 

 gas-filled swim bladder throughout life (Taylor 

 1968: Butler and Pearcy 1972). Indeed, concentra- 

 tions of either D. theta or P. thompsoni alone in 

 Figure 12 were comparable with the concentration 

 of D. taaningi, which Baird et al. (1974) believe 

 was responsible for the migratory sound- 

 scattering layer over the Cariaco Trench. 



As pointed out above, Sergestes si m His may be 

 excluded as a potential sound scatterer; it was 

 distributed too broadly and deeply in the daytime 

 and lagged the ascent of the migratory sound- 

 scattering layer at sunset (Figure 11, Table 11). 

 Although E. pacifica (Figure 10) was about five 

 times more abundant in the depth of the daytime 

 sound-scattering layer than all myctophid fishes 

 combined, it did not approach concentrations 

 necessary for it to be an effective scatterer of 

 12-kHz sound (Hersey and Backus 1962; Bary 

 1966; Beamish 1971). 



In conclusion, we suggest that the nonmigratory 

 deep sound-scattering layer (Figure 2B) in the vi- 

 cinities of Stations P and Q in the eastern subarc- 

 tic North Pacific was caused by P. thompsoni, and 

 that the migratory sound-scattering layer (Figure 

 2B) recorded the migrations of smaller size classes 

 of Stenohrachius leucopsarus and D. theta. Pro- 

 tomyctophum thompsoni may have been largely 

 responsible for the deep scattering layer observed 

 in the daytime, with possible lesser contributions 

 from the two migratory myctophid species. Pearcy 

 (1977) found similar general correspondence be- 

 tween vertical distributions of the same three 

 species of myctophids and deep sound-scattering 

 layers off Oregon, but he pointed out that quan- 

 titative correlation between abundance of poten- 

 tial sound-scatterers and distribution of volume 

 scattering was not always strong. A more defini- 

 tive analysis, similar to that of Baird etal. (1974), 

 is required; that is, simultaneous observations 

 should be obtained on distribution of volume scat- 

 tering and abundance and acoustical properties of 

 suspected sound-scatterers. 



In single hauls, we observed concentrations of 

 myctophids, all species combined, which regularly 

 exceeded 100 fish/10'* m'^ in the region of the deep 

 sound-scattering layer in the daytime and in the 

 surface layer at night. Similar concentrations of 

 myctophids are found in other oceans (e.g., Kash- 

 kin 1967). Further, the maximum concentrations 

 of myctophids observed by us in the surface layer 

 at night (365 fish/ 10-* m^ Table 8) and at depth in 

 the daytime (874 fish/ 10"* m'K horizontal haul at 

 327-333 m. Station Q) equal or exceed maximum 

 concentrations inferred from the apparently high 

 catch rates of single hauls reported by Halliday 

 (1970) and Backus et al. (1971) for the western 

 Boreal Atlantic, where one species of myctophid, 

 Benthosema glaciale , predominates. The very low 

 concentrations of myctophids found by Pearcy et 

 al. (1977), using a 2.4 m Isaacs-Kidd midwater 

 trawl, are puzzling and seem to indicate that myc- 

 tophids are about Vio as abundant off the Oregon 

 coast as in the open subarctic Pacific. However, the 

 data of Pearcy et al. ( 1977) differ from the earlier 

 results of Pearcy and Laurs (1966), in which re- 

 ported concentrations of myctophids were much 

 higher and similar to concentrations observed by 

 us; the difference could be due to year-to-year var- 

 iability (Pearcy 1977). 



There is relatively little variability between 

 years in our estimates of abundance of myctophid 

 fishes (the three most abundant species. Table 2) 



767 



