MESOPELAGIC MICRONEKTON IN HAWAIIAN WATERS: 



FAUNAE COMPOSITION, STANDING STOCK, AND 



DIEL VERTICAL MIGRATION 



Sherwood D. Maynard, Fletcher V. Riggs, and John F. Walters' 



ABSTRACT 



On one cruise off leeward Oahu, Hawaii, micronekton from 9 deep-oblique mid-water trawl tows 

 (0-1,200 m) and a 24-h series of 14 consecutive shallow-oblique tows (0-400 m) were sorted into 16 faunal 

 groups, enumerated, and weighed. Diel-related trawl avoidance was not significant in deep tows. Mean 

 total micronekton standing stocks for the deep tows were 898 organisms/ 100 m- ocean surface and 494 

 g (wet weight)/ 100 m- ocean surface. Fishes comprised half of the total numbers and biomass, while 

 crustaceans contributed one-third of the total numbers and one-fifth of the biomass. Of the micronek- 

 ton deeper than 400 m during the day, 43% of the number of organisms with 47% of the biomass 

 migrated into the upper 400 m at night. 



Micronekton play important roles in the oceanic 

 ecosystem, yet few studies have examined the 

 whole fauna in one area to measure standing 

 stock, percent standing stock involved in diel ver- 

 tical migration, or the relative contributions of 

 major groups to the standing stock. This study 

 considers such data taken on one cruise off Hawaii 

 in the fall of 1972. 



Samples were taken over 2,500-m deep waters 

 10-25 km off the leeward (west) coast of Oahu, in 

 the Hawaiian Archipelago (approximately 

 between lat. 21°15'N, long. 158°15'W and lat. 

 21°30'N, long. 158°22'W). Physical, chemical, and 

 microbiological data are available for the sampling 

 area (Schuert 1970; Gundersen et al. 1972) and 

 nearby Station Gollum (lat. 22°10'N, long. 

 158°00'W) (Gordon 1970, 1971). A broad ther- 

 mocline about 50-500 m deep is present throughout 

 the year (Figure 1). A salinity minimum of M.O"/ uo 

 occurs at 400-500 m and a maximum of about 

 35.2"/ w occurs around 100 m (Gordon 1970). The 

 oxygen concentration varies from a minimum of 

 0.7 ml 02/liter at 600-800 m to a maximum of about 

 5.3 ml O^/liter at the surface (Gordon 1970). Water 

 transparency is very high. In August 1972, noon 

 irradiance measurements at a nearby station, lat. 

 28°29'N, long. 155°14'W, dropped from a surface 

 level of 7xlOVW/cm2(at471 nm) to 1.8x10^ 



jLiW/cm2) at 471 nm) 400 m deep, k = 0.029 below 

 200 m (E. M. Kampa, pers. commun.) (cf. G. L. 

 Clarke 1971:43). Primary productivity in this area 

 has been estimated at 50 g C/m^-yr (S. A. Cattell, 

 pers. commun.). The mean seasonal standing stock 

 of zooplankton in the upper 200-m layer off Oahu is 

 about 2.6 g (wet weight)/m2 (Nakamura 1967). 



GEAR AND METHODS 



Trawl 



All samples were taken with a 10-foot (3-m) 

 Isaacs- Kidd Mid-water Trawl (IKMT) of standard 

 design (Devereaux and Winsett 1953). The 

 anterior portion of the net was lined with 6.35-mm 

 (stretch) knotless nylon mesh; the middle portion 

 was lined with 4.75-mm knotless nylon mesh; and 

 the cod end was a 1.0-m diameter plankton net of 

 0.333-mm Nitex- mesh. Water flow was measured 

 with a flowmeter (General Oceanics 2030) sus- 

 pended near the center of the mouth, about 1 m 

 inside the net. A time-depth recorder (TDR) 

 (Benthos 1170-1000 or -2500) provided data on the 

 sampling path. 



Trawling Method 



All samples were taken on cruise TEUTHIS-18 



'Department of Oceanography, University of Hawaii, 2525 

 Correa Road, Honolulu, HI 96822. 



Manuscript accepted February 197.5. 

 FISHERY BULLETIN: VOL! 73, NO. 4, 1975. 



726 T2 5.-?-^<? 



-Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



