ZOOPLANKTON ABUNDANCE IN HAWAIIAN WATERS, 1953-54 



By 



Joseph E. King 

 Fishery Research Biologist 



and 



Thomas S. Hida 



Fishery Aid 



Pacific Oceanic Fishery Investigations 



Honolulu, T. H. 



In conjunction with researches on the tuna 

 resources of the Hawaiian offshore waters, the 

 Pacific Oceanic Fishery Investigations (POFI) 

 has studied the geographical and seasonal var- 

 iations in zooplankton abundance. Tuna presence 

 and abundance may be dependent to a great ex- 

 tent upon food supply, and zooplankton is direct- 

 ly or indirectly an important food source for 

 these large, oceanic fishes (Reintjes and King 

 1953, King and Ikehara 1956). 



During the years 1950 to 1954 plankton sam- 

 pling was conducted on 17 POFI cruises in Ha- 

 waiian waters. An earlier report (King and 

 Hida 1954) presented an analysis of collections 

 obtained on 7 cruises in 1950-52. The present 

 report is concerned with collections from 10 

 cruises in 1953-54. In addition to zooplankton 

 sampling, on all cruises a number of observa- 

 tions and measurements were made of the chem- 

 ical and physical environment. For example, 

 two of the cruises included oceanographic ob- 

 servations to 500 - 1,200 meters depth, with 

 temperature, salinity, and phosphate nneasured 

 at each depth sampled. Bathythermograph ob- 

 servations were made routinely on all cruises. 

 The results of the oceanographic observations 

 have appeared in previous publications (McGary 

 1955. Seckel 1955), 



The primary purpose of this report is to 

 present the results of the plankton sampling, 

 information which is applicable to the study of 

 the abundance and distribution of skipjack, 

 Katsuwonus pelamis (Linnaeus), in Hawaiian 

 waters. The skipjack catch is characterized by 

 a marked seasonal variation and is also subject 

 to annual fluctuations which are as yet 

 unexplained. In the years 1950 and 1951, zoo- 

 plankton samples collected in midsummer dur- 

 ing the height of the skipjack season were about 

 20 percent larger in volume than those tciken in 

 late summer and fall after the close of the sea- 

 son (King and Hida 1954). This difference in 

 relative food abundance may have had no direct 



influence, however, on skipjack presence and 

 distribution. 



One hypothesis seeming worthy of further 

 testing was that areas to the lee of the island 

 chain nnight be higher in zooplankton abundance 

 than areas to windward because of the possible 

 enrichment from the littoral waters of the islands 

 and from the upwelling which may theoretically 

 occur in the lee of oceanic islands. As the Ha- 

 waiian archipelago extends generally in a 

 southeast-northwest direction, and since the 

 prevailing tradewinds are from the northeast 

 and the major ocean currents fronn the east and 

 northeast, a line was drawn (King and Hida 1954) 

 connecting the various islands of the group and 

 thus dividing the island waters into windward and 

 leeward areas. In order to exannine statistically 

 differences between, and within, these major 

 areas, they were subdivided into 6 subareas, 3 

 leeward and 3 windward of the islands. 



Although the abundance of zooplankton was 

 again found to be remarkably uniform, 

 geographically, throughout the Hawaiian area, 

 the easterly region adjacent to, and upstream 

 fronn, the island of Hawaii consistently yielded 

 the lowest volumes. With the benefit of recent 

 analyses of the physical environment (McGary 

 1955, Seckel 1955) we can now better understand 

 the general uniformity in the plankton and also 

 the causes for small but possibly significant 

 variations. 



SOURCE OF MATERIAL 



This study is based primarily on 215 

 quantitative zooplankton collections obtained on 

 6 cruises of M/V Hugh M, Snnith and 4 cruises 

 of M/V Charles H. Gilbert , Several of these 

 expeditions were of short duration and consisted 

 of repeated visits to three set locations, or 

 stations, which were selected as being repre- 

 sentative of plankton and hydrographic condi- 

 tions on the skipjack fishing grounds, Snnith 



