ECOLOGY OF HAWAIIAN SERGESTID SHRIMPS 

 (PENAEIDEA: SERGESTIDAE) 



John F. Walters' 



ABSTRACT 



This paper describes the vertical distribution and migration, population size, seasonal size-frequency 

 distribution, and diet of 20 species of sergestid shrimps collected between 1970 and 1973 in the vicinity 

 of Oahu, Hawaii. 



During the daytime, half-red sergestids live between 450 and 725 m, while all-red sergestids range 

 from 650 to at least 1,200 m. At night all but two species migrate into the 0- to 300-m region, half-red and 

 all-red groups mixing together. One nighttime group lives above 100 m, another lives between 125 and 

 300 m. Moonlight depresses the shallow group below 150 m; it has little effect on the deep group. In 

 addition, some species stop migrating around full moon, remaining at their daytime depths. 



All -species examined eat zooplanktonic Crustacea in the 1- to 3-mm size range. Some species can also 

 utilize smaller zooplankton around 0.4-0.6 mm. This ability is unrelated to the enlarged maxillipeds 

 found in some species. 



Most species appear to spawn mostly during the spring, although ovigerous females can be found at 

 any time of the year. Life span appears to be 1 yr for all species except Sergia bisulcata, which lives 2 yr. 

 One species does not reproduce in Hawaiian waters. 



Hawaiian sergestids are specialized by size, morphology, and vertical distribution. The most closely 

 related species pairs are always separated by size. The Hawaiian sergestid assemblage is very similar to 

 assemblages reported from two areas of the tropical Atlantic. 



Shrimps of the family Sergestidae (Decapoda, 

 Penaeidea) are one of the most characteristic 

 groups of micronekton over much of the open 

 ocean. They dominate the crustacean micronekton 

 over large areas of the North Pacific, where they 

 form sound-scattering layers (Barham 1957) and 

 feed baleen whales (Omori et al. 1972). Two 

 speciose sergestid assemblages have been de- 

 scribed from the subtropical Atlantic by Foxton 

 (1970) and Donaldson (1973, 1975). This paper 

 examines the sergestid assemblage from the 

 central Pacific near the Hawaiian Islands, report- 

 ing vertical distribution and migration, abun- 

 dance, growth and reproduction, and diet. 



MATERIALS AND METHODS 



Sampling Area 



All the sergestids examined in this study were 

 collected off the leeward (west) coast of Oahu, 

 Hawaii at about lat. 21°30'N, long. 158°20'W. Most 

 trawling was done 10-25 km offshore in water 

 1,500-4,(X)0 m deep. Physical and chemical data for 



'Department of Oceanography, University of Hawaii, 2525 

 Correa Road, Honolulu, HI 96822. 



Manuscript accepted April 1976. 



FISHERY BULLETIN: VOL. 74, NO. 4, 1976. 



this area, as well as the nearby Gollum Station (lat. 

 22°10'N, long. 158°00'W), have been reported by 

 Gundersen et al. (1972) and Gordon (1970). The 

 mixed layer is 50-80 m thick with a temperature of 

 23°-26°C. The annual variation in temperature of 

 the mixed layer is only about 3°C (Gordon 1970). A 

 broad thermocline extends to approximately 500 

 m, where the temperature is 5°-7°C. Salinity 

 varies from 34.0 "/oo at 400-500 m to 35.2 '7m at 100 m; 

 oxygen varies from 7 mg/liter at 100 m to 1 

 mg/liter at 700-900 m. The water is very clear. In 

 situ measurements of irradiance to 500 m at lat. 

 28°29'N, long. 155°14'W in August 1972 gave an 

 extinction coeflRcient of 0.029 m" ' at a wavelength 

 of 471 nm for depths below 200 m; surface ir- 

 radiance at 471 nm was 7 x 10^ jnW/cm'^ per nm, 

 decreasing to 1 x l(^^ /nW/cm^ per nm at 500 m 

 (E. M. Kampa, pers. commun.). Annual net 

 primary productivity has been estimated at 50 g 

 C/m2 (S. A. Cattell in T. A. Clarke 1973:431). 

 Nakamura (1967) found an annual mean standing 

 crop of zooplankton of 2.6 g/m^ in the upper 200 m. 

 The sampling area was chosen as the deep water 

 nearest to Honolulu. It has the further advantage 

 of being in the lee of Oahu under normal 

 tradewind conditions, an important practical 

 consideration when working from RV Teritu. In 



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