370 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



Table 1. — Average number of zooptanklers per unit of wafer strained and percentage composition of the catch obtained loith 

 SOX X X grit gauze {apertures 0.65 mm.) and 56XXX {apertures O.Sl mm.) grit gauze at three stations of Hugh M. Smith 

 cruise IB in June 1952 



Organisms 



Foraminifera ._- 



Radiolaria 



Coelcnterata _, 



Chaetognatha.- 



Annelida 



Copepoda 



Ostracoda 



Euphausiacea. 



Amphipoda. — 



Shrimp 



Crustacean larvae.— 



MoUusca 



Tunicata 



Fish. 



Eggs 



Miscellaneous 



Total (or sample 



Station 45 (7°00' S.) 



Average number 

 per 100 m.' 



30XXX 56XXX 



214 







51 



489 







1,072 



51 



274 



17 







17 



43 



17 



51 



77 



17 



1.257 



35 



70 



1,188 



70 



9,745 



244 



349 



35 







175 



140 



978 











35 



2, 390 14, 321 



Percent composition 



30XXX 56XXX 



9.0 







2.2 



20.4 

 



44.8 

 2.2 



11.5 

 0.7 

 



0.7 

 1.8 

 0.7 

 2.2 

 3.2 

 0,7 



100.1 



0.2 

 0.6 

 8.3 

 0.5 

 68.0 

 1.7 

 2.4 

 0.2 

 



1.2 

 1.0 

 fi.8 

 

 

 0.2 



99.8 



Station 62 {VW N.) 



.\verage number 

 per 100 m.' 



30XXX 66XXX 



166 



104 



139 



521 







2,811 



17 



174 



17 



17 







35 



330 



52 



156 







4,529 



1,665 







166 



1,332 



33 



17, 145 



233 



333 











67 



266 



466 



67 



932 



33 



22,738 



Percent composition 



30XXX 56XXX 



3.4 

 2.3 

 3.1 



11.6 

 



62.1 

 0.4 

 3.8 

 0.4 

 0.4 

 



0.8 

 7.3 

 1.1 

 3.4 

 



100.0 



7.3 

 



0.7 

 5.9 

 0.1 

 75,4 

 1.0 

 1.6 

 

 



0.3 

 1.2 

 2.0 

 0.3 

 4. 1 

 0. 1 



Station 60 (9°00' N.) 



Average number 

 per 100 ra.' 



30XXX 56XXX 



16 







317 



269 



32 



1,473 







48 







16 



16 



111 



1,061 











190 



3,649 



299 







220 



439 



140 



6,924 



439 



220 











60 



299 



778 



40 



120 



120 



10, 098 



Percent composition 



30XXX 66XXX 



0.6 







8.9 



7.6 



0.9 

 41.5 







1.3 







0.5 



0.5 



3.1 

 29.9 











5.4 



100.: 



3.0 

 



2.2 

 4.3 

 1.4 

 68.6 

 4.3 

 2.2 

 

 



0.6 

 3.0 

 7.7 

 0.4 

 1.2 

 1.2 



100.1 



as many plankters as the coarser meshed nets. 

 Table 1 gives for each sample the average number 

 per unit (100 m?) of water strained and the per- 

 centage composition for the major constituents. 

 It appears that the greatest difference in the catch 

 of the 2 nets is in the larger numbers of foraminifers 

 and copepods retained by the 56XXX net. 



The results show a marked difference between 

 the 2 nets in average size (volume) of individual 

 organisms in the catch (table 2) ; plankters in the 

 catch of the 30XXX net were about 3 times as 

 large as those taken by the 56XXX net, primarily 

 because of the difference in catch of small copepods 

 such as the microcalanoids and cyclopoids. As 

 the result of an increased catch of the larger zoo- 

 plankton forms (coelenterates, salps) and fewer 

 of the smaller forms (foraminifers, chaetognaths) , 

 both nets yielded larger organisms, on the average, 

 at the northernmost station (station 60). 



Table 2. — Average size {i. e., volume of catch divided by the 

 number of organisms) of zooplankters captured in 30XXX 

 and 56XXX grit gauze nets at three stations of Hugh M. 

 Smith cruise 15 in June 1952 



It is obvious that these 2 nets of different mesh 

 exercised a strong size-selection in sampling the 

 zooplankton community. The question as to 

 which net-size yielded the most reliable measure of 

 abundance of zooplankton as potential fish food 

 cannot be decided from the few data presented 

 here. It is generally known that no one net or 

 other sampling device will quantitatively sample 

 the entire zooplankton community, and therefore 

 the investigator must choose the method and gear 

 that in his opinion will contribute the most toward 

 his particular objective. Our objectives, to obtain 

 a representative sample of the larger zooplankton 

 forms and to retain all tuna eggs and larvae with a 

 minimum of mesh-clogging, were realized, we be- 

 lieve, with the use of 1-meter nets of 30XXX grit 

 gauze. 



VARIATION IN CATCH WITH SAMPLING DEPTH 



On 7 stations of Hugh M. Smith cruise 14 in 

 February 1952, horizontal hauls were made simul- 

 taneously at 3 levels with open 1-meter nets. The 

 hauls were of 1-hour duration; the nets were low- 

 ered and raised at the start and end of the haul as 

 rapidly as possible to minimize contamination in 

 tlie intermediate and deep samples. All 7 sta- 

 tions were off Canton Island in the South Equa- 

 torial Current between 2°41' S. and 2°45' S. lati- 

 tude at about 172° W. longitude, and were oc- 

 cupied consecutively between 1315 and 0338 hours 

 of February 9-10, 1952. Although the primary 



