Tuna Commission, unpublished data) and else- 

 where, show that tuna feed much less on 

 zooplankton than they do on micronekton, i.e., 

 active animals ranging m length from about 

 0.5 to 10. cm. 



Because of this, and the fact that attempts 

 to correlate standing crops of tunas with 

 contemporaneous standing crops of zooplankton 

 were mainly unsuccessful, we decided that 

 routine measurements of micronekton standing 

 crop should be made on STOR cruises for 

 analysis m relation to contemporaneous tuna 

 standing crops (see below). It was alsothought 

 that the results could be used as part of a 

 more general study of interrelationships of 

 biota in the eastern tropical Pacific (see 

 below). 



Since STOR schedules of station work were 

 already very long, it was not possible to de- 

 vote more than 1 hour per diel (24-hour 

 period) to collecting micronekton. This meant 

 that the work had to be done at night when 

 nets do not have to be lowered so far to 

 capture the diurnally migrating organisms; 

 it was also thought that fewer aninnals would 

 escape the net at night. 



A net was built for TO- 58-1 (SCOT) and 

 used on that and subsequent STOR cruises 

 in essentially the same way throughout, so 

 that all results (except on TO-58-2, when 

 the work was mainly experimental) are com- 

 parable. 



This net is shown in figure 10. It is made 

 of Marion Textiles 467 pattern nylon netting 

 throughout, the meshes being approximately 

 rectangular and about 5.5 mm. x 2.5 mm., and 

 it measures 19 ft. along the vertical axis from 

 the mouth to (but not including) the detachable 

 cod end. The cod end, made of 56xxx nylon 

 grit gauze, is the one routinely used with 

 zooplankton nets. The mouth of the net is 

 square (for simpler attachment of bridles 

 and depressors, and simpler handling and 

 stowing aboard ship), 5 ft. x 5 ft. 



The arrangement of towing bridles and 

 45-lb. depressors is shown in figure 10. The 

 ratio, filtering area/aperture area, is ap- 

 proximately 7.6. A BT is attached to the 

 frame to show the maximum depth of the net 

 during the haul. 



As now used the net is paid out on 350 m. 

 of 3/8-in. wire at 25 m./min. and hauled 

 back at 10 m./min., all with the ship steaming 

 at 5 kn. This oblique haul takes 49 min., and 

 the maximum depth averages 90 m. 



The equipment is not ordinarily used with 

 a flowmeter, since this might cause some 

 organisnns to be lost. An experiment was 



nnade in which the complete net was towed 

 with a flowmeter and a tow was then made 

 with the net removed (i.e., second tow with 

 frame, flowmeter, and depressors only). It 

 indicated a filtration coefficient of 0.757, 

 which has been used to convert all actual 

 micronekton volumes into volumes per esti- 

 mated 1,000 m.3 (see figs. 12, p. 36, and 13, 

 p. 37). 



After preservation of the catch in buffered 

 formalin all nonnektonic watery organisms 

 over I cm. in length (salps, medusae, siphono- 

 phores, and heteropods) are removed and 

 total displacement volume is measured. The 

 catch is then sorted into the following com- 

 ponents whose volumes are separately meas- 

 ured: fishes, cephalopods, adult Crustacea 

 longer than 3 cm. (galatheid and portunid 

 crabs, adult stomatopods, adult shrimps), and 

 remainder (mainly euphausiids, stomatopod 

 larvae, amphipods, phyllosomas, pteropods, 

 and very small watery organisnr\s). 



Experimentally throughout the cruises, and 

 routinely commencing with TO-59-2, a high- 

 speed micronekton net was used when the ship 

 was steaming at ordinary cruising speed 

 (9-11 kt.). This net, which is also shown in 

 figure 10, is of the same material throughout 

 as the other and used with the same detachable 

 cod end, but it is only 8.5 ft. long and mounted 

 on a circular frame of 70 cm. diameter. The 

 ratio, filtering area/aperture area, is 7.5. 

 The net is paid out on 50 m. of 3/8-in. wire, 

 with one 45-lb. depressor attached so as to 

 swim 10 ft. below it, and is towed horizontally 

 on that length of wire for as long as necessary 

 (normally from 2 to 3 hours). It can be brought 

 aboard for change of cod end and restreamed 

 within 5 min., without slowing the ship. Rec- 

 ords from attached BTs show that the normal 

 depth of towing under these conditions is about 

 10 m. There is no routine use of a flowmeter, 

 but a test similar to the above-mentioned one 

 indicated a filtration coefficient of 0.938, 

 which is used to standardize all catch vol- 

 umes in terms of an estimated 1,000 m.3 of 

 water strained. The catches are sorted and 

 measured as described above. 



Such nets eventually wear out, but have 

 been found to be good for about 1,000-1,500 

 miles of trouble-free towing and represent a 

 valuable source of information. 



Experiments with Cultures of Red Crabs 



(I'ieuroncodes planipes) (C. M . Boyd) 



Predominant in the micronekton of the Baja 

 California region, and in stomachs of yellowfin 

 and skipjack caught in that region, is the 

 galatheid crab Pleuroncodes planipes (red crab). 



31 



