Populations of the Sternoptychid Fish Maurolicus muelleri on Seamounts in the Central North Pacific

The lightfish, Maurolicus muelleri (Gmelin), is a cosmopolitan sternoptychid fish most abundant near continental shelf-slope breaks and rare in the open ocean. Recent studies have documented dense populations on seamounts of the South Atlantic and North Pacific. At Southeast Hancock Seamount, a small guyot in the central North Pacific, M. muelleri populations are mainly composed of juveniles. Seasonal length frequencies suggest that recruitment at sizes greater than 20 mm standard length (SL) occurs principally in spring and summer months, with growth over summer and fall coincident with decreasing abundances. Mature fish in reproductive condition occur in winter months but do not survive to the following spring; they also may be too few at this small seamount to support annual recruitment. Potential sources of additional recruitment include populations at several larger seamounts in the southern Emperor group and also the large populations around Japan. Advection of eggs and larvae in the Kuroshio Extension may provide recruits for dependent populations at the seamounts. Mean current flow and satellitetracked drifters suggest a transit time of 100-200 days from the coast of Japan to the region of the seamounts; estimates of age at length suggest that smaller fish have similar ages at recruitment. Gill-raker counts, however, differ between Japanese and seamount populations. We suggest that the southern Emperor Seamount populations serve as the source for replenished annual recruitment to the small population at Southeast Hancock Seamount. THE LIGHTFISH, Maurolicus muelleri (Gmelin), occurs in high abundance near continental slopes in the Southeast Pacific (Robertson 1976, Clarke 1982), the Northwest Pacific near Japan (Okiyama 1971), the South Atlantic (de Ciechomski 1971, Hulley and Prosch 1987), and the Northeast Atlantic (Gjosaeter 1981). Its high biomass and shoaling behavior have suggested fishery potential in some areas I Manuscript accepted 20 May 1993. 2 Southwest Fisheries Science Center Honolulu Laboratory, NOAA-NMFS, 2570 Dole St., Honolulu, Hawaii 96822-2396 and Joint Institute of Marine and Atmospheric Research (JIMAR), University of Hawaii, Honolulu, Hawaii 96822. 3Current address: NOAA-NMFS Alaska Fisheries Science Center, 7600 Sand Point Way NE, Seattle, Washington 98115. 4 National Research Institute of Far Seas Fisheries, 5-7-1 Orido, Shimizu, Shizuoka, 424 Japan. (Clarke 1982, Prosch and Shelton 1983), and it serves as an important prey source for many predatory fishes (Okiyama 1971). Although a member of a principally oceanic, mesopelagic family, the species seems to be associated with land masses, spending daylight hours near the bottom at depths from 100 to 500 m and rising into shallow water in large shoals at night where it apparently feeds (Okiyama 1971, Robertson 1976, Kawaguchi and Mauchline 1987). This species is rarely reported in open ocean areas, but the characteristic eggs may be common over oceanic depths adjacent to population centers (Williams and Hart 1974). In large-scale midwater trawl surveys away from land, however, M. muelleri is rarely encountered (Barnett 1984), but some records of small juveniles exist (Mukhacheva 1981, Clarke 1984). Drift in ocean currents can