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FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



net, but we did hope to retain the juveniles 20 mm. 

 and larger. 



The plankton collections obtained on the other 

 four cruises cited in table 21, on which one or two 

 juvenile tunas were taken in midwater trawls, have 

 not yet been sorted for larval tunas. From our 

 knowledge of the widespread occurrence of the 

 larvae (Matsumoto, 1958), however, and of the 

 distribution of the adults, we believe that juvenile 

 tunas were present in the areas sampled. Our 

 failure to catch them in larger quantities indicates 

 the ineffectiveness of our gear for sampling this 

 portion of the fauna. 



SUMMARY 



This study is based on the quantitative analysis 

 of the catches from 274 midwater-trawl hauls 

 made on 22 cruises of Bureau of Commercial 

 Fisheries vessels in the central Pacific Ocean be- 

 tween latitudes 49° N. and 19° S., and longitudes 

 108° W. and 162° E., during the years 1951 to 

 1956. 



Chief purpose of the trawling program was to 

 obtain quantitative estimates of the abundance and 

 distribution of forage organisms, particularly 

 those utilized by tunas. 



Four types of trawls were employed: 6-foot 

 beam trawl, 1-meter ring trawl, and 6-foot and 

 10-foot Isaacs-Kidd trawls. Descriptions and 

 specifications of the four trawls are provided. 

 Hauls were usually of the double oblique type 

 with the net fisliing between the surface and depths 

 as great as 400 meters. 



In the laboratory, the catch was sorted and the 

 different kinds of organisms identified to the most 

 precise degree considered practical. The num- 

 ber and displacement volume were determined 

 for each kind or group of organisms identified. 



Volume of the catch varied generally with size 

 of the trawl. In terms of volume of catch per unit 

 of mouth area, the trawls were about equal in 

 catching efficiency within a geograpiaical area. 

 Essentially, all four trawls sampled organisms 

 from the same phyla, classes, and orders, but dif- 

 fered in the families and genera of fishes they 

 sampled — the largest and most frequently used 

 trawls cauglit the greatest variety of organisms. 



Night catches greatly exceeded day catches both 

 in volume and number of organisms, and also 

 yielded organisms of larger size. Marked diurnal 



differences were noted in the composition of the 

 trawl catches. Coelenterates, amphipods, stoma- 

 topod larvae, certain molluscs, and tunicates oc- 

 curred in about equal frequency in day and night 

 collections, whereas the faster-swimming forms, 

 such as the decapod Crustacea, squids, and most of 

 the fishes, were taken principally at night. We 

 believe that the day/night difference in catch was 

 due largely to a diurnal migration and the move- 

 ment of the organisms into the surface layer at 

 night, rather than to an ability to dodge the net 

 during the day. 



With respect to geographic variation, catches 

 of the Isaacs-Kidd trawls showed two peaks in 

 abundance: in the Aleutian Current and in the 

 region of upwelling and enrichment at the Equa- 

 tor. The poorest catches were made in tlie South 

 Equatorial Current south of 5° S. latitude, in the 

 North Equatorial Current between 10° N. and 18° 

 N. latitude, and in Hawaiian waters. Evaluation 

 by a two-way analysis of variance indicated differ- 

 ences (P<0.05) among longitudes in the volume 

 of trawl catches made with the 10-foot Isaacs-Kidd 

 trawl during one cruise to the equatorial Pacific. 



On the basis of "t" tests of the difference be- 

 tween mean catch volumes, we found no signifi- 

 cant differences between cruises to the same gen- 

 eral area (equatorial Pacific) at about the same 

 time of year, or to the same area (central North 

 Pacific) at different seasons (late summer and 

 winter) . 



In correlation analyses of the relation of trawl 

 catch volumes and various environmental factors, 

 we found a significant (P<0.01) positive correla- 

 tion of trawl volumes and surface inorganic phos- 

 phate for two cruises and a positive but not 

 significant (F>0.05) relation for a third cruise. 

 On one cruise the relation of trawl volumes to C" 

 uptake by phytoplankton was positive but non- 

 significant (/''>0.05), whereas on a second cruise 

 the correlation coefficient was significant {P< 

 0.01). On all four cruises studied, the correla- 

 tion between zooplanlcton volumes and trawl 

 catches was found to be significant {P<0.01). 

 These results were in line with our expectations: 

 that the forage organisms would be more 

 closely related to zooplankton — their food for the 

 greater part — than to inorganic phosphate or the 

 metabolic activity of phytoplankton. 



