THE DISTRIBUTION OF MYCTOPHID FISHES ACROSS 

 THE CENTRAL EQUATORIAL PACIFIC 



A. RucKER Hartmann' and Thomas A. Clarke^ 



ABSTRACT 



Analysis of myctophid fishes collected in the upper 50-75 m at night along long. 145°W between lat. 

 12°N and 3°30'S indicated three faunal groups. Warmwater species ocurred across the entire transect 

 and were most abundant at or just north of the equator. A second group of species occurred only in the 

 North Equatorial Current or the Counter Current, but most are known to be more widely distributed. A 

 third group occurred only at or just north of the equator; all are apparently found within a small 

 latitudinal range between about long. 130° and 170°W. Certain species which are abundant in the 

 central water mass were absent from the present samples. The faunal change within the equatorial 

 water mass is likely a response to the increased primary production and food supply resulting from 

 upwelling near the equator and northward transport of enriched surface waters. Some of the faunal 

 changes observed appear to be replacements of one species by a congener or morphologically similar 

 species. 



The geographic ranges of oceanic organisms 

 frequently conform with the boundaries of the 

 major water masses (Johnson and Brinton 1963). 

 The water masses are, however, large-scale, sub- 

 surface features defined by the temperature- 

 salinity profiles of the deeper water (Sverdrup et 

 al. 1942). In the surface layers of a given water 

 mass there are variations in temperature and 

 salinity as well as in other biologically relevant 

 factors. Thus it is not surprising that, within 

 major water masses, subpatterns of abundance 

 and distribution have been reported for species 

 which might likely respond to such variations in 

 the upper layers. There are numerous reports of 

 subpatterns in epipelagic zooplankton, e.g., Fager 

 and McGowan (1963) and McGowan (1971); and 

 Ebeling (1962) and Backus et al. (1969) have noted 

 similar changes within water masses for 

 mesopelagic fishes. Backus et al. related these to 

 shallow thermal fronts, and Ebeling suggested 

 that, where species' ranges deviated from water- 

 mass boundaries, they were related to variation in 

 primary productivity in the surface layers. 



Within the Pacific equatorial water mass, major 

 and presumably biologically relevant changes are 

 observed in the upper layers. Grandperrin and 



'University of Hawaii, Hawaii Institute of Marine Biology, 

 P.O. Box 1346, Kaneohe, HI 96744; present address: P.O. Box 422, 

 Bodega Bay, CA 94923. 



^University of Hawaii. Hawaii Institute of Marine Biology, 

 P.O. Box 1346, Kaneohe, HI 96744. 



Manuscript accepted October 1974. 

 FISHERY BULLETIN: VOL. 73, NO. 3, 1975. 



Rivaton (1966), studying the mesopelagic fishes 

 along the equator, found four longitudinal faunal 

 zones which appeared related to the depth of the 

 Cromwell Current or Equatorial Undercurrent, 

 variations in primary productivity, and concen- 

 trations of dissolved oxygen and nutrients in the 

 upper layers. Latitudinally, one encounters four 

 separate current regimes in the upper 

 layers (Cromwell 1951) with associated changes in 

 thermocline depths. Marked changes in primary 

 productivity (Koblentz-Mishke et al. 1970), 

 zooplankton volume (King and Hida 1957), and 

 tuna abundance (Murphy and Shomura 1972) also 

 occur across the water mass. 



This study considers the distribution and abun- 

 dance of myctophid fishes collected by shallow 

 night trawl tows across a latitudinal transect of 

 the Pacific equatorial water mass. It was expected 

 that any patterns observed might be related to 

 available data on differences in surface layer fea- 

 tures. 



MATERIALS AND METHODS 



Most data were collected during cruise 43 of the 

 U.S. Fish and Wildlife Service RV Townsend 

 Cromwell (29 April- 11 June 1969). Pelagic trawl 

 collections were made along long. 145°W at five 

 latitudes: approximately 12°N, 7°30'N, 3°30'N, 0°, 

 and 3°30'S. Five tows were made with a modified 

 Cobb pelagic trawl (CT) described by Higgins 



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