TRAWLING FOR FORAGE ORGANISMS IN CENTRAL PACIFIC 



293 



and distribution of tunas. Studies have been pur- 

 sued on the ocean cun-ents, chemical nutrients, 

 photosynthetic activity, zoophinkton, and forage 

 fish simultaneously with studies on the tunas. The 

 trawling program has been conducted on the 

 premise that highly mobile fishes, such as the tunas, 

 are most likely to occur in areas with the most 

 favorable concentrations of food. Other environ- 

 mental factors, of course, such as temperature, can 

 also be of a limiting nature and influence fish dis- 

 tribution. In general, however, when broad areas 

 of the sea are being compared, it is our belief that 

 a positive relation must exist among the various 

 levels of the food chain. This does not mean that 

 we expect to find a high positive correlation at all 

 times and places between the volume of food and 

 the abundance of tunas; in fact, it is possible that 

 an inverse relation may exist locally after a period 

 of heavy predation. 



STANDING CROP AND PRODUCTIVITY 

 MEASUREMENTS 



Correlation analyses, made to investigate the 

 association between trawl-catch volumes and en- 

 vironmental variables that might be expected to 

 have some direct or indirect influence on the trawl 

 catches, are summarized in table 16. For these 

 analyses only data from the major cruises which 

 were sufficient for meaningful statistical tests were 

 employed. 



On two of three cruises examined {Smith cruises 

 27 and 35), we found a significant (P<0.01) posi- 

 tive correlation of trawl volumes and surface in- 

 organic phosphate concentrations; on the third 

 cruise {Smith cruise 31) the relation was again 

 positive but not significant (F>0.05). 



Only two cruises {Smith cruises 31 and 35) pro- 

 vided estimates of photosynthetic activity based 

 on the uptake of O* by phytoplankton. Since the 

 tr;nvl hauls on these cruises were all made at night, 

 at which time the photosynthetic activity was neg- 

 ligible, we also determined the correlation of trawl 

 catch with the C'^ uptake recorded at a station on 

 the morning of the same day, and also with the O* 

 uptake measured at the station occupied on the 

 morning following the night trawl haul. For both 

 cruises the resulting correlation coefficients have 

 positive values, but only those for Smith cruise 35, 

 relating trawl catch to the rate of C" uptake at 

 the morning stations, were significant {P<0.01). 



Table 16. — Correlations of traicl catch (millHiterg per 

 hour of hauling), as the Xi variate, icith Xi variate the 

 surface inorganic phosphate, C" uptake, or zooplankton 

 volumes at the same station or from adjacent stations 

 of the Hugh M. Smith 



Trawl and 

 cruise 



6-foot Isaacs-Kldd: 

 Cruise 27 



Do --. 



10-foot Isaacs-Kidd 

 Cruise 30 



Cruise 31 



Do 



Do 



Cruise 35 



Do 



Do 



Xt variate 



Degrees 

 of 

 free- 

 dom 



Surface inorganic phos- 

 phate, iiS- at./l- 



tZooplankton volumes, ml./ 

 l,000m.i; oblique 0-100 m. 

 hauls. 



tZooplankton volumes,- ml./ 

 l,000m.>; oblique 0-140 m. 

 hauls. 

 {Surface Inorganic phos- 

 phate, )jg. at./l. 

 {Surface Ci< uptake; mg. C/ 

 hr./m.s 



a. Same station (1900-2000) 



b. Adjoining station (0800- 

 0900, same day). 



c. Adjoining station (0800- 

 0900, following day). 



{Zooplankton volumes, ml./ 

 l.OOOm.J; oblique 0-200 m. 

 hauls. 

 Surface Inorganic phos- 

 phate, /ig. at./l. 

 Surface C'< uptake, mg. C/ 

 hr./m.' 



a. Same station (1900-2000). 



b. Adjoining station 

 (0800-9000, same day). 



c. Adjoining station (0800- 

 0900, following day). 



llZooplankton volumes, ml./ 

 l.OOOm.s; oblique 0-140 m. 

 hauls. 



Corre- 

 lation 

 coefli- 

 clent (r) 



0.665 

 0.617 



0.701 

 0.166 



0.214 

 0.198 



0.221 



0.371 



0.450 



0.237 

 0.571 



0.581 

 0.473 



<0.01 

 <0.01 



<0.01 



>0.05 



>0.05 

 >0.05 



>0.05 



<0.01 



<0.01 



>0.05 

 <0. 01 



<0.01 

 <0.01 



• From McQary and Strcup (1958). .,-,,.. .„. 



t Unpublished data in the files of tlie Bureau of Commercial Fisheries Bio- 

 logical Laboratory, Honolulu. 

 } From McQary, Jones, and Austin (1956). 

 i From King, Austin, and Doty (1957). 

 t From Austin (1957). 



In all four cruises examined {Smith cruises 27, 

 30, 31, and 35) , the correlation between trawl-catch 

 volume and zooplankton volume, as sampled on 

 0-100 m., 0-140 m., and 0-200 m. oblique hauls 

 with 1-meter nets of silk or nylon grit gauze 

 (aperture widths approximately 0.65 mm.), was 

 found to be highly significant (P<0.01). It was 

 certainly to be expected that the larger forage 

 organisms sampled by the trawl would be found 

 more closely related to zooplankton — their food 

 for the greater part— than to inorganic phosphate 

 or the metabolic activity of phytoplankton. 



The distribution of the number and size of the 

 organisms making up the different trophic levels, 

 whether on land or in the sea, takes the form of 

 a pyramid, with the plants or primary producei-s 

 forming the Ijroad base and the larger carnivores 

 the peak (Odum, 1953: p. 73). Actually three 

 ecological pyramids result from food and energy 

 relationships: the pyramid of numbers, the pyra- 

 mid of bioniass, and the pyramid of energy or food 



