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

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

4 GEORGE W. BOEHLERT,2 CHRISTOPHER D. WILSON 2,3, AND KEISUKE MIZUN0

ABSTRACT: 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. Advec tion of eggs and larvae in the Kuroshio Extension may provide recruits for dependent populations at the seamounts. Mean current flow and satellite tracked drifters suggest a transit time of 100-200 days from the coast ofJapan 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), (Clarke 1982, Prosch and Shelton 1983), and occurs in high abundance near continental it serves as an important prey source for many slopes in the Southeast Pacific (Robertson predatory fishes (Okiyama 1971). Although a 1976, Clarke 1982), the Northwest Pacific member ofa principally oceanic, mesopelagic near Japan (Okiyama 1971), the South Atlan family, the species seems to be associated with tic (de Ciechomski 1971, Hulley and Prosch land masses, spending daylight hours near the 1987), and the Northeast Atlantic (Gjosaeter bottom at depths from 100 to 500 m and rising 1981). Its high biomass and shoaling behavior into shallow water in large shoals at night have suggested fishery potential in some areas where it apparently feeds (Okiyama 1971, Robertson 1976, Kawaguchi and Mauchline 1987). I Manuscript accepted 20 May 1993. This species is rarely reported in open ocean 2 Southwest Fisheries Science Center Honolulu Labo areas, but the characteristic eggs may be ratory, NOAA-NMFS, 2570 Dole St., Honolulu, Hawaii 96822-2396 and Joint Institute of Marine and Atmo common over oceanic depths adjacent to spheric Research (JIMAR), University ofHawaii, Hono population centers (Williams and Hart 1974). lulu, Hawaii 96822. In large-scale midwater trawl surveys away 3Current address: NOAA-NMFS Alaska Fisheries from land, however, M. muelleri is rarely Science Center, 7600 Sand Point Way NE, Seattle, Wash ington 98115. encountered (Barnett 1984), but some records 4 National Research Institute of Far Seas Fisheries, of small juveniles exist (Mukhacheva 1981, 5-7-1 Orido, Shimizu, Shizuoka, 424 Japan. Clarke 1984). Drift in ocean currents can 57 58 PACIFIC SCIENCE, Volume 48, January 1994

2 result in colonization of new habitats and area ca. 50 m ) with 6-mm cod end mesh. range extensions for this species (Borokin and When fish densities were estimated, only the Grigorev 1987). IKMT hauls were considered. The scattering In the North Pacific, M. muelleri is gener layer was also observed with a Simrad EQ-38 ally limited to the western half, with no echosounder operating at 38 kHz. Specimens records from the eastern North Pacific (Muk were preserved at sea in 10% buffered for hacheva 1981). Two population centers of malin. Subsamples from September 1986 and M. muelleri are present in Japan. The greatest April 1987 cruises were preserved in 70% density occurs in the Sea of Japan, where the ethanol for later examination of otoliths. population biomass may exceed 3 million Standard lengths were measured to the near metric tons (Okiyama 1981). A smaller popu est 0.1 mm using a dial caliper. Statis lation occurs along the Pacific coast of tical comparison of length frequencies southern Japan (Okiyama 1971). Other speci was accomplished using the nonparametric mens have been encountered in the central Kolmogorov-Smirnov test. North Pacific as stomach contents of whales In the laboratory, M. muelleri was sorted (Kawamura 1973). More recently, large num from samples. Sex ofspecimens with develop bers have been reported from relatively iso ing gonads was determined according to Gjo lated seamounts, both in the North Pacific saeter (1981) and Yuuki (1982), and standard (Borets and Sokolovsky 1978, Borets 1986, length (SL) was measured. Otoliths were ex Boehlert 1988) and South Atlantic (Linkow amined for age determination with techniques ski 1983). In this paper we describe certain following Gjosaeter (1981); presumptive daily aspects of the population of M. muelleri at increments on whole otoliths mounted on Southeast Hancock Seamount, an isolated microscope slides were counted under a com oceanic seamount at the northern end of the pound microscope. Otolith radii measure Hawaiian Ridge, and consider the origin and ments were made with a dissecting microscope persistence ofthis population and its potential following Yuuki (1984). Total gill rakers were biogeographic relationship to larger popula counted on the first gill arch. tions elsewhere in the North Pacific. A 150-kHz vessel-mounted acoustic Dop pler current profiler (ADCP) was used to measure currents in 1988 while the vessel was over three seamounts of the southern Em MATERIALS AND METHODS peror-northern Hawaiian Ridge. Techniques Observations were made from the NOAA for ADCP data processing generally followed vessel Townsend Cromwell during seven Wilson and Boehlert (1990). Reported current cruises (July 1984, February and July 1985, speeds were depth averaged over 40-210 m. September and November 1986, April 1987, and July-August 1988) to Southeast Hancock Seamount (29° 48' N lat., 179° 04' E long.). The 1988 cruise extended northwestward to RESULTS the southern Emperor Seamounts, including In the North Pacific, Maurolicus muelleri Colahan (31 ° 02' N lat., 175° 54' E long.), populations occur principally in the western Kammu (32° 10' N lat., 173° 00' E long.), and region; in the central North Pacific, specimens Koko (35° 00' N lat., 171 ° 30' E long.) sea have been captured at several seamounts be mounts. Collections ofM. muelleri were made tween lat. 29° 48' and 38° 40' N (Mukhacheva with a 1.8-m Isaacs-Kidd midwater trawl 1981, Borets 1986; this study, Figure 1). Open (IKMT) with a 3-mm mesh body and a cod ocean records are relatively few. Examination end of either 3-mm or 0.505-mm mesh towed of published and unpublished collection rec at 150-200 cm/sec; volumes filtered with this ords revealed few occurrences that are not in net were estimated using mechanical f1ow the vicinity of the seamounts marked in Fig meters. Other samples were taken with a ure 1. The westernmost specimens taken were larger square midwater herring trawl (mouth eggs (T. Watanabe, Tohoku Regional Fish- Populations of Maurolicus muelleri on Seamounts-BoEHLERT ET AL. 59

120'E 130'E 140'E 1S0'E 160'E 170'E 180'E 170'W 160'W 1S0'W 140'W 60'N 60'N

SO'N ~--'---=-+---"'--ft-+-JI--+----"--+---+---+---t---I----II------IoI SO'N

•• 40'N • 40'N b. • b. • b. • • b. b. • 30'N 30'N

20'N ~-+---t---I------+---+--+---+----+-' ·...:.:,.,t..--I------I'l 20'N

120'E 130'E 140'E 1S0'E 160'E 170'E 180'E 170'W 160'W 1S0'W 140'W

FIGURE I. Distribution of Maurolicus muelleri in the North Pacific. Open triangles represent the locations of seamounts where M. muelleri occurs in topography-associated populations (Borets 1986; this study). Circles represent eggs taken in midwater trawl samples (T. Watanabe, Tohoku Regional Fisheries Laboratory, pers. comm.). The diamond west of the dateline represents a juvenile from a midwater trawl collection; that east of the dateline the stomach contents of a fish taken on a longline (R. H. Rosenblatt, Scripps Institution ofOceanography, pers. comm.). Squares represent records from the stomachs ofsei and Bryde's whales (Kawamura 1973). The diamond off the main Hawaiian Islands represents data from Reid et al. (1991). eries Research Institute, pers. comm.) and one Hokkaido University Museum of Zoology juvenile from a midwater trawl survey (R. H. (K. Amaoka, pers. comm.) show specimens Rosenblatt, Scripps Institution ofOceanogra taken at Kinmei Seamount from stomach phy, pers. comm.). Additional collection rec contents ofthe mirror dory, Zenopsis nebulosa ords from Scripps Institution of Oceanogra (Temminck & Schlegel), a species known to be phy indicate another from the stomach of abundant at the seamounts (Borets 1986). a lancetfish, Alepisaurus borealis (= ferox North and east of the southern Emperor Lowe), taken on longline over deep water Seamount-associated populations (Figure I), east of the seamount chain (Figure I; large numbers were taken in stomach contents Rosenblatt, pers. comm.). Records from the of whales (Kawamura 1982). Finally, the 60 PACIFIC SCIENCE, Volume 48, January 1994 most southeasterly occurrence in the North of the seasonal data and the lack of larger Pacific is in the main Hawaiian Islands (Reid animals in spring and summer suggest an et al. 1991). annual population with no evidence for a Because of the infrequent cruises to South second year of life at Southeast Hancock east Hancock Seamount, insufficient seasonal Seamount. Fish at the seamount attain repro length data were collected from any single ductive status, however, as shown by the year to assess modal progressions. Thus, to presence of reproductively mature animals in consider changes in length frequency across winter collections (Wilson 1992) and by M. seasons, data from 1984 to 1987 were com muelleri larvae taken in ichthyoplankton col bined. Median lengths were different among lections in winter (but not summer) collec seasons of the year (Figure 2): the smallest tions (Boehlert and Mundy 1992); this is individuals, about 23 mm SL, were captured consistent with spawning in winter time when in spring; median size increased over the year, larger, reproductive animals are present. with the largest fish captured in winter. Al Otoliths were analyzed from 22 specimens though we have not comprehensively sampled collected in late summer 1986 and 21 from throughout a single year, the unimodal nature spring 1987. They ranged from 26 to 43 mm SL (mean, 35.4), with otolith radii ranging from 0.55 to 0.70 mm. Gross examination under a dissecting microscope failed to reveal an annulus on any otolith. Examination of mounted otoliths under a compound micro ,~ scope demonstrated clear increments pre ] .....,1... ., 1 sumed to represent daily growth. Counts of the increments from these fish ranged from 80 to 162, with an average of 119. Because ofthe narrow range of length and age, we fit no =0°) July I growth curve to the data, but length at age -:=::::=:::==.~,,~,,~,~dIIIb~~,~,.!:::=::~~= suggests that growth was more rapid than observed in Japan (Figure 3). Gill rakers were counted from 86 specimens from Southeast Hancock Seamount taken in :'0:) ---I 1986, from 78 specimens from the stomachs - ,. .•11IIII...... ofII sei and Bryde's whales (capture locations as in Figure I), and from 28 specimens from the Pacific coast ofJapan (including data from 12 specimens in Mead and Taylor [1953]). The ] .J&. ---I gill-raker counts of seamount fish and those from whale stomachs (taken east of the sea mounts) ranged from 26 to 32 (Figure 4). =0°) .11 rebruary I Counts from specimens from Japan were uniformly lower, ranging from 23 to 26; over -~~~--+I_~_..-+-+-JI.'lJJII_+-1 lap in gill-raker numbers was minor between _-+-I 20 30 40 50 Japanese and central North Pacific specimens. standard Length (mm) Midwater trawl samples were taken in August 1988 at three ofthe southern Emperor FIGURE 2. Seasonal length-frequency distributions of Seamounts northwest of Southeast Hancock. Maurolicus muelleri from Southeast Hancock Seamount. A total of 20 hauls captured 637 M. muelleri, Samples were taken in different years and combined by both above and southwest of the seamount month, as follows: April 1987, n = 92; July 1984, n = 352; July 1985, n = 654; September 1986, n = 105; November summits (Table I). Length frequencies mea 1986, n = 42; February 1985, n = 21. sured for 567 of these fish are different from Populations of Maurolicus muelleri on Seamounts-BoEHLERT ET AL. 61

...----- S 50 S "---'" ....,..c1 40 QlJ ~ Q) 30 ~ ¥ '"d • Seamounts. >-< 20- cd Japan - '"d ~ 10 ....,cd lf1 0 0 100 200 300 400 500 600 Age (days)

FIGURE 3. Age and growth of Maurolicus muelleri from Southeast Hancock Seamount. Age of specimens was determined using daily growth increments; age-length data points from this study are shown as circles. For comparison, the growth curve from spring-spawned fish from the Sea ofJapan (Yuuki 1982) is indicated as a solid line.

50,------, Japan·~ Seamounts [=:J Whales _ 40

24 25 26 27 28 29 30 31 32 33 Total Gill Rakers, First Arch

FIGURE 4. Gill-raker counts from Maurolicus muelleri from three capture locations. Specimens are from the Pacific coast of Japan (crosshatched bars, n = 28; M. Okiyama, pers. comm.; Mead and Taylor 1953), Southeast Hancock Seamount (open bars, n = 86), and the stomachs ofsei and Bryde's whales (solid bars, n = 78). those for 414 fish at Southeast Hancock Sea 40 km southwest of the southern Emperor mount taken on the same cruise (K-S test, Seamount summits were ca. lO-fold less than P < 0.01), with slightly larger fish at South over the summits (Table I). Also, the length east Hancock (median, 34 versus 35 mm SL). frequencies differed from those taken above Densities offish taken at distances from 10 to the summit (K-S test, P < 0.01; Figure 5), 62 PACIFIC SCIENCE, Volume 48, January 1994

TABLE I ISAACS-KIDD MID WATER TRAWL HAULS TAKEN OVER THE SOUTHERN EMPEROR SEAMOUNTS

SEAMOUNT LOCATION NO. OF TOWS NO. OF M. muelleri DENSITY (10- 5 m- 3 )

Colahan Summit 3 II 1.433 Southwest 2 0 0.000 Kammu Summit 3 543 68.933 Southwest 4 53 5.300 Koko Summit 4 27 2.725 Southwest 4 3 0.300

NOTE: "Summit" tows were taken over the seamount summits; "Southwest" tows were taken at distanoes of 5 to 40 km (mean, 3 19 km) to the southwest of the summits. Average volumes filtered per tow were 25,000 m •

20 Summit_ Off summit ~ 16

o +-+~r-t-'_.l\- 20 30 40 50 Standard Length (mm)

FIGURE 5. Length-frequency distributions of Maurolicus muelleri taken in August 1988 at the southern Emperor Seamounts. Data from fish captured over the seamount summits (n = 511) are contrasted with those taken at distances varying from 10 to 40 km southwest of the summits (n = 56). with median sizes of33 and 39 mm SL on and were very weak (6 cm/sec), to the southeast off the summit, respectively. Length frequen initially but later flowed primarily northward. cies of fish taken off the summit were also significantly larger than those of fish taken at Southeast Hancock on the same cruise. Different current patterns were observed DISCUSSION over each of the southern Emperor Sea Populations occurring on seamounts are mounts visited during the 1988 cruise. Strong often small because of very limited habitat. (31 cm/sec), generally northward currents As with island populations, local extinction were present over Colahan Seamount. At may occur periodically and reestablishment of Kammu Seamount, currents were relatively populations depends upon recruitment from weaker (14 cm/sec) and generally to the east other sources (MacArthur and Wilson 1967, southeast. Currents over Koko Seamount Choat et al. 1988). In the South Atlantic, for Populations of Maurolicus muelleri on Seamounts-BoEHLERT ET AL. 63 example, the rock lobster Jasus tristani Hol muelleri at Southeast Hancock Seamount thuis recruits in large numbers to Vema Sea (Figure 2) suggest that recruitment occurs in mount from an insular source population spring and summer months. The annual na some 2000 km away (Lutjeharms and Hey ture of the population, the low abundance in dorn 198Ia,b). Populations of M. muelleri winter months, and the absence of fish in the exist on some South Atlantic seamounts in size range between small larvae and 23 mm SL densities as high as 147 g/m2 (Kalinowski and juveniles suggest that recruitment may depend Linkowski 1983). Because specimens on most upon an upstream source. of these seamounts are immature (Linkowski We examined evidence for two possible 1983), it is likely that they are adveeted from source populations, those in Japan and those populations near Argentina and Tristan da on the southern Emperor Seamounts. Large Cunha Island. Such populations fit the de populations of M. muelleri occur in coastal scription of "dependent populations," which waters ofJapan (Okiyama 1971), and a great require separate, independent populations for deal of evidence exists for the advection of replenishment. Dependent populations have coastal fauna from Japan to waters of the been described in the North Pacific. Vasilenko central Pacific under the influence of the and Sokolovskaya (1989), for example, de Kuroshio and Kuroshio Extension. Tropical scribed marked range extensions of popula reef fish from the south of Japan have been tions of the black filefish, Navodon modestus captured at the northern end ofthe Hawaiian (Giinther), and horse mackerel, Trachurus chain (Ralston 1981), and several species of japonicus (Temminck & Schlegel), into open fish in Hawaiian waters share a close affinity ocean areas of the northwestern Pacific and to the subtropical and temperate fish fauna of suggested that the distribution extended to the southern Japan (Hobson 1984). From central region of the Emperor Seamounts. Belyayev Japan, a variety of coastal pelagic species (1990) developed this idea further and showed are adveeted to mid-Pacific waters. In many that the mackerel populations originated years, Japanese sardine, Sardinops melano from the same stock; he suggested that a sticta (Temminck & Schlegel), is a major prey minimum population size of2.7 million metric item ofalbacore captured at the Emperor Sea tons in the "independent," or source, popula mounts (Yasui 1986). Similarly, juvenile S. tion is necessary to support dependent popu melanosticta, Engraulisjaponica Temminck & lations at the seamount. Biomass estimates of Schlegel (anchovy), Scomber japonicus Hout M. muelleri in the Sea of Japan are on the tuyn (mackerel), and M. muelleri are major order of 3 million metric tons (Okiyama fish components ofthe diet ofsei whales in the 1981 ). vicinity of the seamounts (Kawamura 1973). Although the Southeast Hancock Sea With the exception of M. muelleri described mount population ofM. muelleri was a consis in Wilson (1992), no spawning populations tent feature of our sampling over several of these species have been identified in the years, it is difficult to determine whether or southern Emperor-northern Hawaiian Ridge not it is a self-sustaining population. Mid seamount region. Although S. japonicus water trawl samples and acoustic observa apparently has mechanisms for population tions suggest a strong seasonality in abun maintenance in Japanese waters (Watanabe dance. Generally, abundances are greatest in 1970), larvae and juveniles are often captured summer, lowest in winter, and intermediate in at considerable distances from shore, typically fall based upon acoustic data (Wilson 1992). in years that result in poor year class strength In late July 1984, densities exceeded four (Belyayev 1985); thus the magnitude of this 3 individuals per 1000 m , whereas in winter faunal advection may vary interannually, pos 1985 abundance was less than 0.2 individuals sibly with the strength of transport of the per 1000 m3 (Boehlert 1988). The reduction in Kuroshio. abundance could result from either natural Ocean currents could conceivably advect mortality or advection away from this small M. muelleri from populations in both the Sea seamount. Length-frequency patterns of M. of Japan and the Pacific coast of Japan to 64 PACIFIC SCIENCE, Volume 48, January 1994 the region of Southeast Hancock Seamount. and Grigorev (1987) observed a range exten From the southern coast of Japan advection sion of several hundred kilometers for M. to the region of the seamounts (near 180 0 E muelleri in the Barents Sea and attributed it to long.) would take ca. 100 days given the mean enhanced transport ofthe Norwegian Current current speed of the Kuroshio Extension (ca. combined with a strengthening of the Mur 50 em/sec). This time scale is supported by mansk Current; thus, advection may be a trajectories of satellite-tracked drifters de common pattern in the life history of this ployed in the Kuroshio near Japan, which species. typically reach the region of the seamounts in Several biological features, however, differ periods of 100 to 200 days (Cheney et al. between the seamount and Japanese popula 1980). For the population in the Sea ofJapan, tions. Length-frequency distributions of sea Okiyama (1981) suggested that the major mount populations are unimodal and only spawning was associated with flow of the reach 1 yr of age, contrasting with the multi warm Tsushima Current, which flows north modal, multiyear size and age distributions ward. Long-term mean flows of about 10 reported in Yuuki (1982,1984). Although the em/sec suggest that eggs and larvae entrained otolith radius-standard length relationship in this current would take about 70 days to does not differ, no annuli were observed on drift from Toyama Bay to the Tsugaru Strait. seamount fish. This may be related to the The Tsugaru Warm Current (TWq, which seasonality of collection of the specimens could carry the eggs, larvae, or juveniles (spring and fall), but there is little difference between Honshu and Hokkaido, enters the between summer and winter temperatures at Pacific and directs to the south (Kawai 1972). Southeast Hancock Seamount at the depths The modified TWC water could be entrained inhabited by M. muelleri (Boehlert 1988). into the Kuroshio Extension near the Joban Growth of M. muelleri at the seamounts was area; the transit in this case would take some much faster than that of spring-spawned fish 60 days. After entrainment, the path would be from the Sea of Japan (Figure 3) and more similar to that of the Pacific coast population similar to early growth rates noted by Lin and thus the total time to reach the seamount kowski (1983) in the South Atlantic. A similar from the Sea of Japan would be 230 to 330 situation exists in the North Atlantic, where days. populations in the Rockall Trough live only 1 Our biological results, however, do not yr (Kawaguchi and Mauchline 1987), con strongly support a Japanese origin for the fish trasted with Norwegian populations living 3 taken at Southeast Hancock Seamount. The yr (Gjosaeter 1981). In the Pacific example, distribution ofages (Figure 3) suggests that at this may be related to higher temperatures and least the smallest recruits are too young to enhanced foraging conditions in the region of have been advected from the Sea of Japan the seamount, where oceanic zooplankton populations, because animals at the apparent represent the major diet item (Hirota and recruitment size are on the order of 80-120 Boehlert 1985). The smaller population size at days ofage (we cannot rule out recruitment of Southeast Hancock Seamount compared with older, larger fish later in the summer, how those on continental slopes may combine with ever). Advection from the Pacific coast of abundant, advected oceanic prey to create an Japan is possible in this time frame. The optimal feeding environment (Isaacs and Sch length-frequency distributions from South wartzlose 1965) and thus more rapid growth. east Hancock Seamount, when compared Like growth, gill-raker counts differ between with those from Japan, are reminiscent of the the Southeast Hancock and Japanese popula situation in the South Atlantic, where Lin tions as well. kowski (1983) observed large, mature popula The second potential source of recruitment tions with 1- to 3-year-old animals in the at Southeast Hancock Seamount is from the westernmost area sampled but only unimodal southern Emperor Seamounts (north of32 0 in juvenile populations (to 47 mm SL) in the Figure I). The biology of these populations downstream seamount populations. Borokin has not been described in detail; biological Populations of Maurolicus muelleri on Seamounts-BoEHLERT ET AL. 65 characteristics of the Southeast Hancock and these seamounts may indicate larger, more southern Emperor Seamount populations are stable populations ofM. muelleri. In addition, more similar. The range oflengths in summer the larger size of these seamounts or banks show little difference (Figures 2, 5). Gill-raker may result in more complex hydrographic counts of M. muelleri from both Southeast regimes that include recirculation and en Hancock Seamount and whale stomachs (Fig hanced population retention as contrasted to ure 4) agree with the counts from Mukha smaller seamounts (Loder et al. 1988). The cheva (1981) for the southern Emperor Sea chain of seamounts with populations of M. mounts. They are considerably greater than muelleri evident in Figure 1 may serve as counts from Japanese populations from either stepping stones (Wilson and Kaufmann 1987) the Sea of Japan (Grey 1964, Okiyama 1971) for replenishment ofthe small seamount pop or the Pacific coast (Figure 4). Although ulations. There may be a southerly directed number of gill rakers is a flexible character in flow along the axis of the seamounts (Darnit fish (McCart and Andersen 1967, Lindsey sky 1980, Darnitsky et al. 1984) that could 1988), it is unknown whether this indicates result in transport from the southern Emperor genetic, population-level differences or varia Seamounts to the region of Southeast Han tion related to temperatures, growth rate, or cock Seamount. Although our observations other factors during development. Meristic were temporally limited, the currents noted differences within a species may be related from ADCP observations in this study were to local productivity (Johnson and Barnett highly variable, with intermittent periods of 1975) and possibly the associated growth rate; southerly flow. The presence of larger fish Mukhacheva (1981), however, noted a range 10-40 km from the seamount summits (Fig of differences in gill-raker counts, including ure 5) might indicate dispersal of larger fish. those between fish from the seamounts and Thus, advection ofM. muellerifrom these sea Japan, but failed to show any relationship mounts, with their larger populations, would with local productivity. Because it seems like be a likely source of recruitment or replen ly that M. muelleri is advected to the central ishment ofthe relatively small Southeast Han North Pacific from Japan, it would be useful cock population. to examine genetic differences between popu Recruitment of large numbers of M. muel lations from these areas to resolve the rela leri to Southeast Hancock Seamount is an tionships among the populations or the exact annual feature, as shown by abundances taxonomic status, because species-level differ evident over a 5-yr period (1984-1988). We ences have been proposed (N. Parin, pers. propose that the population is maintained comm.). by a combination of local recruitment and Although we are uncertain ofthe size ofthe periodic replenishment from larger, upstream southern Emperor populations, they may be populations, most likely those from the south related to habitat size. Ifwe consider the areas ern Emperor Seamounts. Recruitment to the of the seamount slopes and summit at depths same seamounts occurs for the pelagic armor above 500 m as an index of available habitat, head, Pseudopentaceros wheeleri Hardy; this then the southern Emperor Seamounts have species remains pelagic for ca. 2 yr before far more extensive habitat than do the small recruitment (Boehlert and Sasaki 1988), and seamounts along the northern Hawaiian densities of armorhead were high enough to Ridge. Southeast Hancock and Northwest sustain fisheries for several years on those Hancock seamounts, for example, have a seamounts (Sasaki 1986). Some physical char combined area ofless than 34.4 km 2 at depths acteristic of seamounts may facilitate recruit above 500 m. Comparable values for the ment; unique features of seamounts in the southern Emperor Seamounts are as follows: open ocean include current-topography inter 2 Colahan Seamount, 19.0 km ; Kammu Sea actions and anomalies in the earth's gravita 2 mount, 469.2 km ; Kinmei Seamount, 26.6 tional or magnetic field (Keating et al. 1987). 2 2 km ; Yuraku Seamount, 37.5 km ; Koko Such features, plus the potential for water to 2 Seamount, 3546.3 km • The large areas of remain trapped above seamounts (Shomura 66 PACIFIC SCIENCE, Volume 48, January 1994 and Barkley 1980, Dower et a!. 1992), may Atmos. Adm.) Tech. Memo. NMFS maximize the probability of encounter of SWFSC-176. these pelagic fauna at seamounts. BOEHLERT, G. W., and T. SASAKI. 1988. Pelagic biogeography of the armorhead, Pseudopentaceros wheeleri, and recruitment ACKNOWLEDGMENTS to isolated seamounts in the North Pacific Ocean. U.S. Dep. Commer. Fish. Bull. We thank several colleagues for assistance 86: 453-465. with the many samples and collections sur BORETS, L. A. 1986. Ichthyofauna of the veyed here. Muneo Okiyama provided infor Northwestern and Hawaiian submarine mation on gill-raker counts and biology of ranges. J. Ichthyo!. (Eng!. Trans!. Vopr. fish from Japan; Akito Kawamura provided Ikhtio!.) 26: 1-13. specimens and unpublished location data on BORETS, L. A., and A. S. SOKOLOVSKY. 1978. whale capture locations. For information Species composition of ichthyoplankton from museum collections we thank Kunio of the Hawaiian submarine ridge and Amaoka, Tomio Iwamoto, and Richard Ro Emperor Seamounts. Izv. Pac. Sci. Res. senblatt. H. G. Moser provided taxonomic Inst. Fish. Oceanogr. (TINRO) 102:43-50 advice. An early version of this paper was (trans!. by W. G. Van Campen). reviewed by Thomas Clarke, Muneo Oki BOROKIN, I. V., and G. V. GRIGOREV. 1987. yama, and Michael Seki. 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