CHAETOGNATHS AND PTEROPODS AS BIOLOGICAL INDICATORS 

 IN THE NORTH PACIFIC 



By 



Thomas S. Hida 



Fishery Aid 



Pacific Oceanic Fishery Investigations 



Honolulu, T. H. 



This study is part of a general survey by 

 the Fish and Wildlife Service of potential fishing 

 grounds for albacore tuna in the North Pacific. 

 The distribution and occurrence of the Chaetog- 

 natha and Pteropoda were examined to deter- 

 mine whether any representatives of these 

 groups could serve as biological indicators, 

 that is, supplement oceanographic data and help 

 distinguish types of water in the North Pacific, 

 thus assisting in locating and defining areas 

 where albacore are likely to occur. 



Indicator organisms have been used to 

 identify water types in various parts of the 

 world where the physical and chemical proper- 

 ties were not sufficiently definitive. The or- 

 ganisms must be readily identifiable and suffi- 

 ciently abundant to be sampled in fair numbers. 

 The chaetognaths and pteropods in the North 

 Pacific fulfill these requirements. 



The distribution and occurrence of 

 chaetognaths and pteropods in the central North 

 Pacific are not well known, and in this region 

 have not been previously examined for possible 

 use as indicators of different oceanographic 

 environments. In other parts of the world, they 

 have been employed for this purpose by several 

 workers: Bigelow (1928), Russell (1939). 

 Thomson (1947), Moore (1949), Fraser (1952). 

 and others. As an example of the results, 

 Fraser (1952) in his study of the Scottish area 

 related chaetognaths, pteropods, and other 

 organisms to changes in oceanographic condi- 

 tions and cited Professor Meek's 1928 dennon- 

 stration that Sagitta setosa donninated the waters 

 along the Northumbrian coast when the southern 

 North Sea water flooded the area and that Sagitta 

 elegans was dominant when the northern North 

 Sea water plus an Atlantic component moved 

 into the region. 



On the basis of species composition, the 

 chaetognath and pteropod faunas of the central 

 North Pacific were found to be naturally segre- 

 gated into three regions, or "zones", which we 

 have termed the Subtropic (a region of low zoo- 

 plankton abundance). Transition (a region of 



variable abundance), and Subarctic (a region of 

 high abundance) Zones. On the basis of 

 temperature-salinity relations we can demon- 

 strate the presence of three types of water, the 

 Pacific Central Water mass, the North Pacific 

 Current, and the Subarctic Water mass, with 

 boundaries that coincide generally with those of 

 the faunal zones established fronn the distribu- 

 tion of chaetognaths and pteropods. 



The author is indebted to Thomas Austin 

 and James McGary for their suggestions on in- 

 terpretation of the oceanographic data. 



SOURCE OF MATERIAL 



The material for this study was collected 

 in 1954-55 in an area extending latitudinally from 

 22°N. to 50*N. and longitudinally from 170°E. 

 to 145°W. The sannples were obtained in most 

 cases with 1 -meter nets o£ 30XXX-grit gauze in 

 30-minute oblique tows. A detailed description 

 of the gear used and the method of hauling waa 

 given by King and Demond (1953). The cruise 

 period, sampling depths, and numbers of sam- 

 ples examined for the various cruises were as 

 follows: Hugh M. Smith cruise 25 (February 

 1955, 0-200 m., 8 samples); Charles H. 

 Gilbert cruise 17 (September - November 1954, 

 - 100 m. , 40 samples ) ; Charles H. Gilbert 

 cruise 18 (December 1954, - 100 m. , 21 sam- 

 ples); Hugh M. Smith cruise 27 (January 1955, 

 - 100 m. , 32 samples); Charles H. Gilbert 

 cruise 23 (September 1955, - 140 m. , 14 

 sannples); and HughM. Smith cruise 30 (August - 

 September 1955, - 140 m. , 106 samples). On 

 Hugh M. Snnith cruise 30, 20-nninute 0- to 20-nn. 

 or 0- to 40-m. tows were taken in addition to 

 the deeper tows. 



Data on albacore caught by trolling are 

 available from the above cruises of POFI vessels. 

 W e are indebted to the North Pacific Salmon 

 Investigations of the Service, Seattle, Wash, for 

 data on albacore caught in gill nets by the char- 

 tered vessels Mitkof and Paragon during the 

 summer of 1955. 



