VERTICAL DISTRIBUTION OF ZOOPLANKTON IN CENTRAL EQUATORIAL PACIFIC, JULY-AUGUST 1952 SPECIAL SCIENTIFIC REPORT-FISHERIES No. 144 UNITED SHTES DEPARTMENT OF THE INTERIOR FISH AND WILDLIFE SERVICE Explanatory Note The series embodies results of investigations, usually of restricted scope, intended to aid or direct managanent or utilization practices and as 2;uides for administrative or legislative action. It is issued in limited quantities for the official use of Federal, State or cooperating Agencies and in processed form for economy and to avoid delay in publication. S7901 United States Department of the Interior, Douglas McKay, Secretary Fish and Wildlife Service, John L. Farley. Director VERTICAL DISTRIBUTION OF ZOOPLANKTON IN THE CENTRAL EQUATORIAL PACIFIC, JULY-AUGUST 1952 By Thomas S. Hida Fishery Aid and Joseph E. King Fishery Research Biologist Pacific Oceanic Fishery Investigations Special Scientific Report: Fisheries No. 144 WASHINGTON: APRIL 1955 CONTENTS Page Area and methods Estimating sampling depth 2 Treatment of samples in laboratory 6 Diurnal variation 8 Variations with depth and latitude 13 Composition of the zooplankton 14 Comparison with other sampling methods 1 ^ Sumnnary and conclusions 21 Literature cited ^^ LIST OF ILLUSTRATIONS Figure Page Frontispiece. --Securing the Clarke-Bumpus plankton sampler to the towing cable aboard the Hugh M. Smith. 1. Locations of stations 1-30 occupied by the Hugh M. Smith, cruise 16, July- August 1952 2 2. Reconstruction of the curvature of the towing wire during four test hauls utilizing wire-angle and wire-out observations . , 5 3. Combined results of the four test hauls, plotted on a logarithmic scale, showing the relationship between estimated depths based on straight-wire and curved- wire calculations ° 4. Vertical temperature section along 150 W. longitude, showing the estimated hauling depths of the surface, intermediate, and deep samplers in relation to the temperature structure 7 5. Bathythermographs representative of the vertical temperature structure in the North Equatorial Current, the Countercurrent,and the South Equatorial Current, at the station positions indicated 7 6. Logarithms of zooplankton volumes of Hugh M. Smith cruise 16 plotted against the sine value corresponding to the hour of hauling, and showing the calculated regression line for the relationship 13 7. Variations in the horizontal and vertical distribution of zooplankton as measured on Hugh M. Smith cruise 16 15 8. Horizontal and vertical distribution of eight major groups of the zooplankton sampled on Hugh M. Smith cruise 16 16 9. Variations in average size of the organisms in the collections of cruise 16 calculated by dividing the displacement volume of the sample by the estimated number of constitutents 17 VERTICAL DISTRIBUTION OF ZOOPLANKTON IN THE CENTRAL EQUATORIAL PACIFIC, JULY-AUGUST 1952 Longline -fishing surveys by the Pacific Oceanic Fishery Investigations (POFI) have shown a concentration of deep-swimming yellowfin tuna between the Equator and 5 N. latitude from 140° to 160°W. longitude (Sette 1954). This area is also relatively high in zooplankton abundance (King and Demond 1953), and it is believed that the concentration of both zooplankton and tuna within these latitudes is influenced by the increased fertility of the surface layer result- ing from the equatorial upwelling of nutrient-rich water (Cromwell 1951, Sette 1954, and King_'). In longline fishing in this region there has been a tendency for the deeper hooks, fish- ing at depths of about 300 to possibly 500 feet, to catch more tunaL' than the shallower hooks fishing at 150 to 250 feet (Murphy and Shomura, 1953a, 1953b). In the area of best catch, the deeper hooks apparently fish at about the level of the thermocline. One hypothesis immediately suggests itself: that these deep-swimming tunas are concentrating at the level which provides the most available food. It is known that the sharp temperature gradient associated with the thermo- cline may have a concentrating effect on certain plankton forms (Sverdrup et al. 1942, p. 794) and may restrict the migration of others (Moore et al. 1953). The food of the yellowfin tuna consists of a great variety of organisms, both fish and invertebrates, varying widely in size (Reintjes and King 1953). Although zooplankton comprises on the average a very small percentage of adult tuna food, it is essential food of the forage fish, squid, and shrimp which are utilized directly by the tuna. Therefore, in sampling the zooplankton we believe we are obtaining a reliable index to the basic fish-food present in an area whether uti- lized directly or indirectly by the tunas. Since the vertical distribution of zooplankton cannot be determined by the 200-meter oblique tow which has been used by POFI in surveying the relative abundance of zooplankton, a series of horizontal closing-net hauls was made with Clarke-Bumpus samplers (Clarke and Bumpus 1940) to investigate the vertical distribution of zooplankton in relation to the thermocline. Townsend Cromwell was field party chief on cruise 16 of the Hugh M. Smith on which these col- lections were made, and Heeny Yuen, field party member, was largely responsible for making the hauls. The figures were prepared by Tamotsu Nakata. The oceanographic data resulting fronn cruise 16 have been published (Austin 1954). AREA AND METHODS o o The sampling was done at 30 stations along 150 W. longitude extending from 12 N. to 7 S. latitude in the 9-day period, July 27 to August 4, 1952, on cruise 16 of the Fish and Wildlife Service vessel Hugh M. Smith. The approximate position of each station is shown in figure 1 and given more exactly together with the date, hour, and depth of hauling in table 1, Of the 90 hauls nnade, 68 are quantitatively usable. Improper functioning of the gear vitiated the remaining 22 hauls. At each station, horizontal hauls were nnade sinnultaneously at three levels with Clarke-Bumpus aannplers equipped with nets of 56XXX grit gauze having mesh apertures averag- ing 0.31 mm. in width. These were clamped on 5/32-inch (diameter) wire cable at intervals in- tended to place one sampler at a depth of about 200 naeters during the haul, one at about the 70 F. 1/ MS. Variations in zooplankton abundance in the central equatorial Pacific, 1950-52. To be published in Proceedings of the Fifth Meeting of the Indo-Pacific Fisheries Council. 2/ Yellowfin, Neothunnus macropterus (Temnninck and Schlegel); bigeye, Parathunnus sibi (Temnninck and Schlegel); and albacore, Germo alalunga (Bonnaterre). isotherm (-.vhich occurs within the thermocline in this region), and one just below the surface. A bathythermograph (BT) cast was made at each station before the plankton haul to determine the depth of the 70 F. isotherm. A 150-pound streamlined weight was attached to the end of the tow- ing cable. The hauls were of about 1 hour's duration with a ship's speed of approximately 2 knots. The samples were preserved in 8 to 10 percent formalin neutralized with borax. The samplers were calibrated before and after the cruise, and the average of these calibrations was used to compute the cubic meters of water strained for each haul. 160° 155° 20° 0° :^CHff/srA/>s 160° 150° 145° W Figure 1. .-Locations of stations 1-30 occupied by Hugh M. Smith, cruise 16, July-August 1952. Estimating Sampling Depth Throughout the cruise-- for lack of exact information- -the spacing of samplers on the towing wire was in accordance with the as- sumption that during the tow the wire described a straight line in the water; thus the cosine of the angle of stray of the towing wire from the vertical was used in calculating the amount of wire to pay-out to reach a desired depth. For example, an- ticipating a final wire angle of 60 and intending to have the naiddle sampler operate at the 70 isothernn located at 100 meters, the proce- dure would be: (1) attach the 150- pound weight to the end of the tow- ing wire; (2) have the winch opera- tor pay-out 10 meters of wire; (3) attach sampler No. 1; (4) pay-out 200 meters of wire; (5) attach sampler No. 2; (6) pay-out an addi- tional 200 meters of wire; (7) attach sampler No. 3; (8) pay-out wire until the last sampler was just be- low the surface; (9) then begin the 1-hour tow, measuring the wire angle at 5-minute intervals and attempting to nnaintain a wire angle of about 60 by varying the vessel's speed. In operating the samplers, however, it was found that the wire angle increased steadily with the in- crease in the amount of wire out and we doubted that the assumption of a straight wire provided a good esti- mate of the depth of the samplers. In order to obtain a closer approxi-. mation of the true sampling depth, four test hauls were made following the cruise essentially duplicating the procedure outlined above except Table 1. --Estimated numbers and volumes of zooplankton collected on Hugh M. Smith cruise 16 using Clarke-Bumpus samplers (S = surface sample, I = interme- diate sample, D = deep sample) Sta- tion Position Date 1952 Timei^ Sample Depth, meters Water strained, m^ Zooplankton Latitude Longitude No. /m^ Vol. , cc / lOOOm^ Adj. sur. vol. , cc/ lOOOm^ 1 11°56'N. 150°09'W. 7/27 0506- S 11 45.4 85 31.3 30.2 0606 I 125 29.5 107 36.3 -- dLI -- -- .. -- -- 2 11°00'N. 150°00'W. 7/27 1355- S 10 38.2 221 43,5 58.3 1500 I 117 25.8 77 39. 1 -- D 260 28.5 29 7.7 -- 3 10°01'N. 150°03'W, 7/27 2245- S 6 42.7 172 74.9 52.6 2345 I 92 42.2 96 30.3 -- D 185 38.9 50 8.5 -- 4 09°05'N. .150°06'W. 7/28 0731- S 8 39. 8 167 43.2 51.8 0831 I 65 30.2 94 25.2 -- D 223 36.3 43 12.7 -- 5 08°06'N. 150°02'W. 7/28 1613- S 10 25.0 394 81,9 92.5 1714 1 83 26.6 175 45,9 -- D 257 41. 1 24 12.9 .- 6 07°08'N. 149°58'W. 7/29 0130- S 8 19.5 329 98.3 72. 3 0212 I 121 13.6 198 42,5 -- D 223 35.2 57 20.2 .- 7 06°10'N. 149°49'W. 7/29 1015- S 8 34. 1 368 62.8 87.9 1115 I 142 33.6 58 17.8 -- d2/ .- .. -- .. -. 8 05°06'N. 149°51'W. 7/29 1942- S 9 34.5 367 77.3 63.0 2042 li/ .- .- -- -- -- D 225 42.2 45 12.3 -- 9 04°34'N. 149°51'W. 7/30 0240- S 9 29.6 353 85,5 66.4 0340 I 160 41. 1 42 34,6 -. D 230 41.0 36 14,6 -- 10 04°02'N. 149°52'W. 7/30 0728- S 9 29.2 313 52,3 62.5 0828 I 156 38.9 66 15,9 -- D 233 50.4 84 25.2 .- 11 03°32'N. 149°53'W. 7/30 1315- S 10 32.7 221 28, 1 38.9 1415 li/ -- -- -- -- -- d2/ .- -- -- -- -- 12 03°02'N. 149°59'W. 7/30 1903- S 9 27.0 584 89,5 76.9 2003 I 172 34.8 94 20,4 .- Till -- .- -- _- -. 13 02°30'N. 150°02'W. 7/31 0050- S 9 25.2 644 133,8 95.5 0150 I 152 34.8 88 31.0 -. D 232 39.9 52 12.5 -. 14 01°57'N. 150°07'W. 7/31 0642- S 8 30.7 538 72.6 82.0 0742 li/ -- -- -- -- -- D 215 42.0 65 24.5 ._ 15 01°24'N. 149°57'W. 7/31 1240- S 8 27.7 344 50.9 72.0 1340 I 117 36. 1 199 39.6 -- D 215 47.2 92 25.4 -- 1/ y. Zone time for 150 west longitude, _' No samples due to mechanical failure of samplers. Table 1. --Estimated numbers and volumes of zooplankton collected on Hugh M. Smith cruise 16 using Clarke-Bumpus samplers (S = surface sample, I = interme- diate sample, D = deep sample) (continued) Sta- tion Position Date 1952 Timel/ Sample Depth, meters Water strained, m3 Zooplankton Latitude Longitude No. /m^ Vol., cc/ lOOOm^ Adj, sur vol.. ccl lOOOm^ 16 00°57'N, 150°01'W. 7/31 1753- S 9 26. 1 515 93. 1 90.2 1853 I 132 33.8 164 39.4 .. D 243 44.6 77 26.4 .- 17 00°28'N. 150°01'W. 7/31- 2332- s, , 8 28.4 403 85. 1 59.6 8/1 0032 Di/ -- -- -- -- -- -- .. -- -- -- 18 00°06'N. 149°55'W. 8/1 0519- si/ .. -- -- -- -- 0629 ill -. -- -- -- -- T>ll -. -- -- -- -- 19 00°39'S. 149°49'W. 8/1 1047- s 8 21.6 273 51.9 73.8 1147 I 127 38.8 258 44. 3 -- D 224 49.8 92 20.5 -- 20 00°59'S. 149°50'W. 8/1 1509- S 10 26.4 576 100.6 123.4 1609 ill -- -- -- -- -- Di/ _. .- -- -- -- 21 01°28'S. 149°45'W. 8/1 2003- S 9 18.7 722 121.2 96.4 2103 ill -- -- -- -- -- Di/ -- -- -. -- -- 22 01°58'S. 149°48'W. 8/2 0133- S 8 28.4 353 85.3 62.7 0233 ill -- -- -- -- -- Di/ -- -- -- -- -- 23 02°29'S. 149°53'W. 8/2 0710- S 7 30. 0 266 47.4 55.2 0810 ill -- -- -- -- -- D 191 49.8 58 27. 1 -- 24 02°59'S. 149°59'W. 8/2 1230- S 6 30.0 204 20.7 29.2 1330 I 99 41. 7 288 41.0 -- D 176 40.5 84 14.8 -- 25 03°29'S. 149°57'W. 8/2 1752- S 8 29.2 228 41. 7 40.4 1852 I 132 35.7 113 25.5 -- D 211 34.2 68 12.9 -- 26 03°59'S. 150°05'W. 8/2-3 2305- S 8 23.9 300 37. 3 26. 1 0005 I 144 41.6 100 24.5 -- Di/ -- -. -- -- -- 27 04°29'S. 150°12'W. 8/3 0432- S 6 28.2 285 36.2 33.0 0532 I 116 46.3 126 22.3 -- Di/ -. -- -- -- -- 28 05°02'S. 150°12'W. 8/3 0949- S 9 25.3 279 24.5 33.9 1053 k' 176 31.3 100 7.7 -- 29 06°00'S. 150°10'W. 8/3 1756- s 6 29. 1 239 49.9 48.3 1856 I 147 53.5 94 20.6 -- D 168 47.3 42 12.7 -- 30 06°58'S. 150°05'W. 8/4 0209- s 8 27.7 297 55.9 42.5 0309 I 188 38.0 55 10.2 -- D 225 37.4 31 7.0 -- \J Zone time for 150° west longitude. ZJ No samples due to mechanical failure of samplera. that a bathythermograph was attached just below the 150-pound weight to record depth and that the wire angle and wire out were measured at 1- to 2-minute intervals during each haul. From these measurements the approximate curvature of the towing wire on these four hauls has been recon- structed (fig, 2) by means of a series of triangles. If we assume that the wire maintains the same angle as it is lowered farther into the sea, then the sum of the vertical sides of the triangles should approximate the true depth attained under uniform current and ship speed. The pertinent data on the four test hauls are given in table 2. It is evident from this table that the method illus- trated in figure 2 produces an estimate of the depth of hauling which is much closer to the depth recorded by the bathythermograph than the estimate based on the assumption that the towing wire conformed to a straight line. Figure 2. --Reconstruction of the curvature of the towing wire during four test hauls utilizing wire-angle and wire-out observations. Depths of the samplers assuming the towing wire to describe a straight line are shown for comparison. The next step was to plot for the four test hauls the estimated depths calculated by the straight-line method against the estimated depths calculated by the curved-line method. On logarithmic graph paper the distribution appears to be rectilinear and a straight regression line was fitted as shown in figure 3. This line was then used to convert the straight-line estimated depths of cruise 16 to the equivalent curved-line estimated depths (given in table 1), using the maximum wire out and an average wire angle for the 1-hour haul. For example, if the estimated depth of the deep sampler on one of the hauls was 100 meters calculated by the straight-line method, the converted depth for that sampler would be 120 meters as derived from the graph in figure 3. When the converted depths are plotted in reference to the temperature structure, as in figure 4 and the selected bathythermographs of figure 5, it is apparent that the intermediate sampler was operating within the thermocline and usually between the 60 and 70 isotherms. The deep sampler was below 60 F.--with one exception (station 29)--and frequently below the 50 F. isotherm. This method of estimation was necessary because wire out and wire angle were not re- corded on cruise 16 while the samplers were being lowered but only during that portion of the haul when the samplers were open and fishing. This precludes direct estimation by the use of Table 2. --RcbuUb of four test hauls showing differences in the estimated depth reached, employing three methods of determination Haul 1 2 3 4 Wire angle with full amount of wire out 48° 62° 67° 72° Total wire out (meters) 370 450 427 431 Vertical height of work platform above sea surface (meters) 3 3 3 3 Length of wire between lead weight and deep sampler (meters) 10 10 10 10 Estimated depth of the deep sampler (meters): a. Assuming a straight wire 238 204 163 127 b. On curved-wire basis 294 300 257 197 c. As obtained from BT 293 301 239 204 triangles. However, it should be noted that in the waters traversed during this cruise there is marked horizontal shearing associated with differential current flows which undoubtedly exert a strong influence on the shape of the towing cable with its attached instruments. Under these con- ditions the actual depth of sampling will vary considerably from the estimated depth, especially when the latter has been derived from average relations. It is regretted that a continuous depth recorder was not available at the time of the cruise. Treatment of Samples in Laboratory 500 400 300 ^ 10 20 30 4050 100 200 400 1000 ESTIMATED DEPTHS BY STRAIGHT-LINE METHOD(M) Figure 3. --Combined results of the four test hauls, plotted on a logarithmic scale, showing the rela- tionship between estimated depths based on straight - wire and curved-wire calculations. The regression coefficient (b) = 1. 1238. In the laboratory, displace- ment volume measurements were made on the 68 acceptable samples. First the few organisms with longest dimen- sion greater than 2 cm. were removed from the sample, identified as precisely as possible, and their displacement volume measured. Then the volume of the remainder and bulk of the sample, i.e. those organisms with longest di- mension less than 2 cm. , was deter- mined. In measuring the displacement volume, the plankton was poured into a draining sock of 56XXX grit gauze to filter off the preserving liquid. When the sample stopped dripping it was transferred to a graduated cylinder of appropriate size (usually 5 or 10 ml. capacity). By means of a burette a known volume of water was added to the drained plankton. The difference between the volume of the plankton plus the added liquid and the volume of added liquid was recorded as the displaceme"-t or net wet volume of that portion of the sample. STATION 30 29 28 27 26 25 24 Z3 22 21 20 19 18 17 16 15 14 13 12 11 10 9 7 6 5 4 3 2 I Figure 4, --Vertical temperature section along 150 W. longitude, showing the estimated hauling depths of the surface (•), intermediate (■), and deep samplers (▲) in relation to the temperature structure. The isotherms are derived from 900-ft. bathythermograph observations taken on stations 1 through 30 of Hugh M. Smith cruise 16, July-August 1952. The top of the thermocline is indicated by the dashed line. TEMPERATURE (°F) 60 70 1 1 r ' -' [ ' ' ! _ 200 - - 1- / y/ lll V ^ y UJ /^ y^ Ll. 400 _ / jff^ — I / /^ 1- ^ / / lll 600 - SEC / cc / NEC - a / STATION 15 / STATION 5 / STATION 1 01 24'N-I49°57'W 08°06'N-I50°02'W / M°56'N-I50°09' W 800 " , / 1 1 1 1 1 1 1 " Figure 5. --Bathythermographs representative of the vertical temperature structure in the North Equatorial Current (NEC), the Counter- current (CC), and the South Equatorial Current (SEC), at the station positions indicated. The hauling depths of the surface, intermediate, and deep plankton samplers are shown (X). A dashed line is drawn at the top of the thermocline. Following the usual procedure at our laboratory, the volume of all organisms less thcin 2 cnn. in length plus the volunne of organisms 2 to 5 cm. in length that might be considered of significant nutritional value (such as the annelids, crustaceans, cephalopods and fish) were combined to give a single volume measurement for each sample. This figure was divided by the estimated amount of water passing through the net to give the volume of zooplankton per unit of water strained. After the volume measurement the contents of each sample were spread out in a shallow rectangular dish (10 x 15 cm. |. A Wolffhuegel counting plate was placed over the dish and all organisms greater than 0.2 mm._' in their longest dimension were counted in 10 fields, each 1 cm. square. Organisms larger than 2 cm. had been previously removed from the sample and were counted separately. The counts of 12 major groups of the zooplankton, together with a mis- cellaneous category, including such things as annelids, shrimps, and fish, are given in table 3. An identification to species level was made of most of the organisms appearing in the six samples collected at stations 3 and 13. These results are given in table 4 and discussed in a later section (p. 14). DIURNAL VARIATION The importance of diurnal variation in estimating zooplankton abundance in the central Pacific has been discussed by King and Demond (1953) and King and Hida (1954). It was evident from initial examination of the volunnetric data on which this report is based that the hour of hauling provided an important source of variation. This is demonstrated by the ratios of the average volumes of night hauls to day hauls, which were 1. 63 for the surface samples, 0. 90 for the intermediate samples, and 0. 66 for the deep samples (table 5). When the dataZ' were ex- amined by means of a "t" test, we found a highly significant (P < 0. 01) difference between the vol- umes of day and night samples taken at the surface, a slight but nonsignificant (Pi; 0. 5) difference at the intermediate level, and a considerable, and nearly significant (P- 0,08), difference at the deep level. Although the difference at the deep level may not be judged statistically significant, the fact that the observed volumes of the day hauls materially exceeded those of the night hauls at the deep level with just the reverse being true in the surface layer (table 5), suggests that there was some vertical migration taking place and that this marked day-night difference at the surface was not entirely the result of a simple dodging of the net. The lack of a strong day-night difference at the intermediate level suggests that this stratum received from layers below about as much plankton as it lost to layers above. An adjustment to remove the effect of diurnal change in zooplankton volunnes has been suggested by O, E. Sette, Director, Pacific Oceanic Fishery Investigations, and described by King and Hida (1954). The method is based upon the similarity between the diurnal variation in zooplankton abundamce and the curve of the sine function with midnight equated to the angle whose sine is +1.0. To examine the suitability of this method for correcting the day-night variation in the surface samples of cruise 16, the data were plotted as in figure 6, the abscissa being the sine function of the cingle corresponding to the time of hauling. As the values have essentially a recti- linear distribution, a straight line was fitted as shown. The regression of the zooplankton volumes on the sine function is highly significant (b = 0. 1553, P < 0,01), Following adjustment the night- day ratio is 0.98, as compared with 1.63 before adjustment, and the variance is reduced by one- third (table 6). From this evidence it appeared that the sine method of adjustment as devised for ^/ It should be noted that this differs from the lower limit of 0.5 mm. used by King and Demond (1953) on plankton from, hauls made by nets of coarser (30XXX grit gauze) mesh. 4^/ Transformed by means of logarithms since the standard deviations of the untransformed data were proportional to the means. Table 3. --Variations in numerical abundance (numberj.' of organiBms per cubic meter of water strained) of 12 major groups of the zooplankton aa sampled on cruise 16 of the Hugh M. Smith. (S = surface sample, I = intermediate sample, and D = deep sample) nl > u rt nl n! n] rt 3 u X v 0 c 01 ^1 N. -150°03'W. 02°30'N. -150°02'W. Organisms N. Equatorial Current S. Equatorial Current Surface Interm, Deep Surface Interm. Deep sample sample sample sample Scimple sample Copepoda Calanoidea Euchaeta prestandreae (Philippi) xxx XXXX XX XX XX -- Pleuronnamma abdominalis (Lubbock) -- XX -- XXXX X -- Pleurorriamma robusta (Dahl) ? -- -- -- -- X -- Pleuromamma xiphias (Giesbrecht) X -- -- X -- -- Candacia pachydactyla (Dana) X X -- XX -- -- Neocalanus gracilis (Dana) xxx X -- XXX XXX -- Rhincalanus cornutus (Dana) -- X -- -- X XXXX Calanus minor (Claus)? -- XXXX -- -- -- -- Eucalanus attenuatus (Dana) X X X xxx XXXX -- Heterorhabdus spinifrons (Claus) -- -- XX -- X -- Phyllopus bidentatus (Brady) -- -- -- -- -- XX Metridia longa (Lubbock) -- -- -- X -- X Scottocalanus securifrons (T. Scott) -- -- -- -- X -- Scolethrix danae (Lubbock) -- -- X X -- -- Undeuchaeta sp. — X -- -- -- -- Undeuchaeta major (Giesbrecht) — -- -- -- X X Bradyidius armatus (Brady)? -- -- -- -- X -- Undina vulgaris (Dana) XX -- -- -- -- -- Cyclopoidea Oncaea sp. XX xxx X xxx X xxx Oithona sp. XX XX -- -- X -- Corycaeus sp. X xxx X xxx X xx Saphirinella stylifera (Lubbock) -- -- -- X X -- Copilia mirabilis (Dana) -- -- -- X X -- Saphirina scarlata (Giesbrecht) -- -- -- X -- -- Saphirina metallina (Dana) -- X X -- -- -- Harparticoidea Aegisthus mucronatus (Giesbrecht) -- -- -- -- -- XX Microsetella rosea (Dana) -- X X -- -- -- Euphausiacea Euphausia sp. -- -- X -- -- -- Euphausia diomedeae (Ortmann) XX X -- XX X -- Stylocheiron sp. -- X X -- -- -- Amphipoda Primno latraillei (Gosse)? -- -- X -- -- -- Phronima sp. -. X -- -- -- -- Phronimella elongata (Claus) X -- -- X -- -- Cyphocaris micronyx (Stebbing) -- -- -- -- -- X Hyperia luzoni (Stebbing) -- -- -- X -- -- Letocotis ambobus (Stebbing) X -- -- -- -- -- Anchylomerablossevillii (Milne- Edwards) -- X -- -- -. -. Decapoda Lucifer reynaudii (Milne-Edwards) -- -- -- X -- -- 11 Table 4. --Partial list of organisms occurring in samples collected at stations 3 and 13 of Hugh M. Smith cruise 16, with the general estimated abundance classified as present (x), frequent (xx), very frequent (xxx), and conspicuously abun- dant (xxxx). Both stations were occupied at night (continued) Organisms p Station ^ 10 01'N.-150 03'W. N. Equatorial Current Station 1 3 02°30'N. -150°02'W. S. Equatorial Current Surface sample Interm. sample Deep sample Surface sample Interm. sample Deep sample Chaetognatha Sagitta sp. Pteropoda Cymbulliopsis sp. Heteropoda Atlanta sp. Fish Vinciguerria lucetia (Carman) -- x X X XX X X X X Table 5. --Average volumes and night/day ratios of zoo- plankton collected by Clarke-Bumpus san-iplers on cruise 16 for 3 depths of sampling (twilight hauls omitted) Tinne of No. of Ave. volume, Ratio, Depth hauling samples cc/lOOOm^ N/D Surface Night 11 85.2 1.63 Day 13 52.3 Intermediate Night 7 27.6 .90 Day 9 30. 7 Deep Night 6 12. 5 .66 Day 9 19.0 Table 6. --Certain statistics showing the mean volumes and extent of variation for the surface, intermediate, and deep hauls Surface Intermediate Deep Adjusted Unadjusted Number of samples (n) 29 29 21 18 Mean volume (i) 62.7 64.7 29.2 16.6 Variance (s^) 581.4 850.7 129.9 46.2 Standard deviation(8) 24. 1 29.2 11.4 6.8 Coefficient of variation (C), as percent 38. 5 45. 1 39.0 41.0 12 Figure b. --Logarithms of zooplankton volumes of Hugh M. Smith cruise 16 plotted against the sine value corresponding to the hour of hauling, and showing the calculated regression line for the relationship. the ZOO-meter oblique hauls provided a means for approximately removing the diurnal variation in this group of surface samples. This nnethod of adjustment was not considered applicable to the in- termediate samples where there was al- most no day-night difference nor to the deep samples where there may be a difference but in a direction opposite to that at the surface. The numerical data show essentially the same diurnal variation in zooplankton abundance as the volume data, although statistically the day-night dif- ference must be considered nonsignifi- cant (since P > 0.05) at all three levels. Table 7 gives the night-day ratios based on numbers of organisms for the total samples and for Copepoda, the major constituent. Although the differences are not significant, the ratios suggest that nnore zooplankton was captured at the surface at night than in the day time, with just the reverse being true for the intermediate and deep levels. VARIATIONS WITH DEPTH AND LATITUDE The average volumes for the three sample depths, surface (adjusted for hour of haul- ing), intermediate, and deep, were 62.7, 29.2, and 16.6 cc/lOOOm-* respectively (table 6). The corresponding variances were 581.4, 129.9 and 46.2. Since most of the variation in the surface volumes related to hour of sampling had already been removed, the chief sources of variation re- maining are those associated with latitude, depth, and sampling error. When the zooplankton volumes (transformed by means of logarithms) are subjected to an analysis of variance with 2-way classification, we find significant differences (P < 0. 05) among stations (latitudes) and highly significant differences (P < 0. 01) related to the depth of sampling. In respect to latitude, the area of best catch, particularly for the surface samples, extended from approximately 2°S. to 8°N. (fig. 7A). About the same degree of variation with latitude is evident at all three depths. Although there was no marked indication of increased abundance immediately at the Equator, the largest single volume was taken at the surface at about 1 S. latitude. Table 7. --Average numbers and night-day ratios of total zooplankton and of Copepoda as obtained in the Clarke -Bunnpus hauls of cruise 16 for 3 depths of sampling (twilight hauls omitted) Depth Time of Number of Total zoopla nkton Copepoda | Ave. number Ratio Ave. number Ratio hauling samples per m-' N/D per m^ N/D Surface Night 11 411. 1 1.28 278.3 1.43 Day 13 320.2 194.2 Intermediate Night 7 96.0 0.66 66.0 0. 74 Day 9 146.0 89. 1 Deep Night 6 45. 1 0.71 25. 3 0.68 Day 9 63.4 37.4 With only two exceptions (stations 1 and 24, at latitudes 12 N. and 3 S. ) the largest volumes were found among the surface samples. There was no evidence of a concentration of zoo- plankton in the region of the thermocline. The apparent greater abundance of deep-swimming tuna at this depth cannot, therefore, be explained on this basis. Information on the abundance of the intermediary forage organisms, which have not as yet been sampled quantitatively, and their graz- ing effect on the zooplankton are needed if we are to fully understand the complex interrelationship that exists. The plankton counts dennonstrated the same general variation with latitude and depth as was found for the volumes (fig. 7B). Figure 8 illustrates the variation in numbers of organisms with latitude and depth for eight of the major zooplankton groups. Although there is considerable station-to-station variation — partly due to differences in the hour of sampling--the figure shows that, for most groups, the largest numbers were found at the surface and in the general region of the Equator. This is particularly well demonstrated by the Copepoda, Foraminifera, eggs (mostly invertebrate), and Tunicata. The other groups shown on figure 8 were present in relatively small numbers and do not provide as definite conclusions. Another variation related to depth and hour of hauling is that of size of organism. A rough estimate of average size (volume) for the constituents of each sample was obtained by di- viding the displacement volume of the sample by the estimated nunnber of organisms. The results, summarized in figure 9, show an increase in average size with depth. Disregarding time of haul- ing, the means for the three sampling depths--surface, internnediate, and deep--were 2. 0, 2. 7, and 3.0 X 10"'*cc. , respectively. These were found to be significantly different (P < 0. 05) when examined by means of an analysis of variance. When the data are segregated into day, night, and twilight hauls, we find some sugges- tions that the mean size of the zooplankton was greater at night than in the day at the surface and intermediate levels, but the opposite was true at the deep level (fig, 9). It is possible that the re- latively large organisms captured in the day hauls at the deep level swam upward at night to be taken in the night hauls at the intermediate level, but most of this group, apparently, never reached the surface. The presence of larger organisms in the night hauls than in day hauls at the upper two levels is possibly the result of upward movement of larger organisms from deeper layers in the case of the intermediate depth, but dodging of the net in the daytime could also be involved, es- pecially at the surface level. COMPOSITION OF THE ZOOPLANKTON The great variety of organisms making up the collections is characteristic of tropical zooplankton populations as contrasted with the larger volumes but relatively few species which are typical of temperate and boreal waters. The Copepoda were the most abundant group in actual numbers in all samples (table 3). The similar innportance of copepods in 1-meter net collections from the central Pacific was previously noted by King and Demond (1953). Next in order of numer- ical abundance were the Foraminifera (mostly Globigerina and Globorotalia), eggs (mostly inverte- brate), Tunicata (mostly Appendicularia), Gastropoda (mostly Pteropoda and Heteropoda), Chaetognatha, Radiolaria, crustacean larvae, Ostracoda, Euphausiacea, Siphonophora, and Amphipoda. Average numbers per cubic meter and percentage composition, by major constituents, of the zooplankton are summarized by depth for the entire cruise in table 8. While most groups decreased in absolute number with depth of sannpling, the Ostracoda showed a small but consistent increase with depth. The Radiolaria also averaged greater in number in the deep samples than at the surface and intermediate levels. With respect to percentage composition, the Copepoda and Tunicata became relatively less important with increased depth of sampling, while Ostracoda, Euphausiacea, and Annphipoda consistently gained in relative importance with depth. The per- centages for the remainder of the groups varied in irregular manner or were approxinnately the same at all three depths. 14 "1 \ I r ^^TO^^k^^TOTOTOTO^^^^ _ FTTT^^^T^^^^^ ■ "1 1 r IVVkTOVTO £^0001/03 '3lNn"10A £iM/a3awnN 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 - ^ I ! , — ^ < i LlJ 5 1 < - Q eI < UJ 1 < I— < Q O X 5 5 < IE , < O c^ t t cn 5 Q. c Z 1= < 2 Q_ z C3 o O C3 _] < LU o < - o I E 2 < CD UJ 1- O I '~~ o o Q < , e: o o ^ C < C IT =) ! q: _ ^ I ! E e ^ 1 o s c 1 1 c [^ [Z 1 E i 1 1 EZ 1—^ C L 1 ■^ (= IZ £ f ( E e 1 EiZ 1 ■ c 1^ 1 ■ 1= 1 1= I c c 1 ■ ■■ I , '■ c: 1 1 1^ E I ■ ■ ^ c 1 c c E CI ( _ ^ c ^ 1 C i ■ s ^ I ^ I c 1 1 1 1 ^ 1 cd ! J RXiSsWW ■ 1 ■ E E ES ! ^ EZ 1 1= 1 1 1 c c C i ^ - E - ( ^s^\^^^^\^^^^^\^^\ 1 1= c 1 1 1 1 '^^ 1 cz I 1 1 ^ = = I 1= 1 — cr= 1 c : c IZ c=l c 1 ™ ' s cw\\\v^ \«\v«\v 1= c c c [Z I ■ ■ ■ ^ c i 1=: ! E I - ^ 1 c: 1 ^^ E t E cz E - c=! c cz c .^ 1 - J d c5 c e: ^ ^ J 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 cz 1 1 1 1 1 1 (Z 1 1 1 1 1 1 :)Oooooooooooooooo o om^rw^w - -CM a3Niwyis a3iVM do y3i3iN oisno a3d ysawnw 16 I I DAY NIGHT VZ2i TWILIGHT Wo bj O SURFACE INTERMEDIATE DEEP Figure 9. -- Variations in average size of the organisms in the collec- tions of cruise 16 calculated by dividing the displacement volume of the sample by the estimated number of constit- uents. (Number of samples is indicated above each block, ) A comparison of the zooplankton popu- lations in the North Equatorial and South Equato- rial currents was made by a detailed exannination of the composition of the zooplankton at two typical night stations, station 3 on latitude 10 N. in the North Equatorial Current and station 1 3 on latitude 2°30'N. in the South Equatorial Current. The list of organisms (table 4) is not complete for any of the samples but does include the domi- nant forms as well as certain of the less abundant species. Of the 28 kinds of copepods identified or partially identified from the six collections, only 13 were noted at both stations. The species occurring in greatest abundance at station 3 in the North Equatorial Current were Euchaeta prestandreae and Calanus minor while Pleuro- mamma abdominalis, Rhincalanus cornutus, and Eucalanus attenuatus were the nnost abundant species at station 1 3 in the South Equatorial Current. There are a few notable similarities in the distribution of species recorded at both stations: e.g., Pleuromamma ziphias and Phroni- mella elongata were found only in the surface samples at the two stations; Euphausia diomedeae was obtained in the intermediate and surface samples at both stations but not at the deep level. An-iong the remainder of the organisms there appeared to be no pattern of vertical stratification. According to references in the available literature, all of the 23 identified species of copepods have been collected at the surface or at shallow depths, and nnost are considered to be cosmopolitan in distribution. It would require much more extensive study to determine whether or not there exist groups of organisms with preponder- ant tendencies to inhabit certain portions of the equatorial current system or certain levels within these portions. COMPARISON WITH OTHER SAMPLING METHODS This series of collections obtained with Clarke -Bunnpus samplers, with finer meshed nets and with a smaller (5-inch) mouth opening than the 1 -meter nets which have been regularly employed in POFI's plankton surveys, might be expected to sample a sonnewhat different elenaent of the zooplankton community. Tables 9 and 10 have been compiled to provide a comparison. Two series of hauls employing 1-nneter nets of 30XXX grit gauze and oblique tows to 200 meters depth were nnade on Hugh M. Smith cruises 5 and 11 and were taken at approximately the same time of year and in about the same area as the Clarke- Bumpus hauls of cruise 16. The average zooplankton volumes of 37.9 and 36.0 cc/lOOOm^ obtained on these two sections are con- siderably less than the average volume (64. 7 cc/lOOOm^) of the Clarke-Bumpus surface hauls, and greater than the average volumes for the intermediate (29.2 cc/lOOOm ) and deep (16.6 cc/lOOOm^) levels of hauling (table 9). The standard deviations of these different lots of data vary in about the same manner. When we examine the coefficients of variation, which provide a nneasure of average variation independent of the nnean, we find the largest value (61.3 percent) for the 1 -meter net hauls of cruise 5; the coefficient of 47.8 percent for cruise 11 does not differ greatly from 45. 1, 39.0, and 41,0 percent, the coefficients obtained for the surface, intermediate, and deep hauls of the Clarke-Bun-ipus samples. Thus neither the average volume nor the variance differed appreciably between the two types of gear. The composition of the catches (table 10) obtained by these two methods indicates that the 56XXX mesh retains a n-iuch larger number of Copepoda, particularly the microcalanid and cyclopoid copepods of length less than 1 mm. , which may be important constituents of tropical Table 8. --Summary of average numbers and percentage composition of the major groups of zooplankton collected on Hugh M. Smith cruise 16 Organisms Surf ace Intermediate Deep Average Percentage Average Percentage Average Percentage number /m comp. number /m comp. number /m-' comp. Copepoda 226.0 65.3 78.0 63.2 33.6 59.0 Foraminifera 40.6 11.7 15.4 12. 5 3.2 5.6 Eggs 35. 8 10. 3 9.3 7. 5 6. 1 10.7 Tunic ata 15.0 4.3 1.5 1.2 0.5 0.9 Gastropoda 9. 5 2.7 3. 5 2.8 1. 1 1.9 Chaetognatha 7. 0 2.0 4.4 3.6 1.3 2.3 Radiolaria 2. 1 0.6 2.0 1.6 3.0 5.3 Crustacean larvae 3.3 1.0 1.9 1.5 0.7 1.2 Ostracoda 0.2 0. 1 3.2 2.6 4. 1 7.2 Euphaus iac e a 3. 1 0.9 1.2 1.0 1.5 2.6 Siphonophor a 1.6 0.5 0.6 0. 5 0.2 0.4 Amphipoda 0.5 0. 1 0.4 0. 3 0.2 0.4 Miscellaneous 1.3 0.4 2.0 1.6 1.4 2.5 Table 9. --Comparison of average volume of catch and degree of variance in results obtained with Clarke-Bumpus samplers and 1 -meter (mouth diameter) nets Hugh M, Smith 16 Hugh M. Smith 5 Hugh M. Smith 1 1 Longitude sampled 150°W. 158°W. 150°W. Range of latitude 7°S.-12°.N 5°S.-21°N. 5 S.-19 N. Month and year July- August 1952 July-August 1950 Sept. -Oct. 1951 Sampling device Clarke-Bumpus sampler 1-meter nets 1-meter nets Net material 56XXX grit gauze 30XXX grit gauze 30XXX grit gauze Mesh aperture width 0. 31 mm. 0. 65 mm. 0. 65 mm. Type of haul Horizontal Oblique Oblique Depth of haul Number of observa- tions Surface Interm. Deep 200 m. to surface 24 200 m. to surface 29 21 18 23 Average sample volume (x), cc/lOOOm^ 64.7 29.2 16.6 37.9 36.0 Variance (s ) 850.7 129.9 46.2 474. 3 296.7 Standard deviation(s) 29.2 11.4 6.8 23.2 17.2 Coefficient of variation ( /x), as percent 45.1 39,0 41.0 61. 3 47.8 Table 10. --Compariaon of differences in composition of zooplankton catches obtained with Clarke-Bumpus samplers using 56XXX grit gauze nets (Hugh M. Smith cruise 16, horizontal hauls) and those obtained with 1 -meter (mouth diam- eter) 30XXX grit gauze nets (Hugh M. Smith cruise 5. oblique hauls) Cruise 16 Cruise 5 | Sur face Intermediate Deep 200 m. oblique Average Per- Average Per- Average Per- Average Per- number cent number cent number per m^ cent number cent per m-^ comp. per m^ comp. comp. per nr* comp. Copepoda 226.0 65. 3 78. 0 63.2 33.6 59. 0 21.8 53.0 Foraminifera 40. 6 11. 7 15.4 12. 5 3.2 5.6 4. 1 9.9 Eggs (mostly in- vertebrate) 35. 8 10. 3 9.3 7. 5 6. 1 10. 7 1.7 4.0 Tunicata 15.0 4. 3 1. 5 1.2 0.5 0.9 2.3 5.6 Chaetognatha 7.0 2. 0 4.4 3.6 1. 3 2. 3 4. 1 10.0 Euphausiacea 3. 1 0.9 1.2 1. 0 1. 5 2.6 1.4 3. 5 Siphonophora 1.6 0. 5 0.6 0.5 0.2 0.4 1.4 3.4 Miscellaneous 16.9 4.9 13.0 10.4 10.5 18. 5 6. 1 10. 6 plankton. The greater retention of Foraminifera, small invertebrate eggs, and Appendicularia (Tunicata) is also evident. The other major groups of the zooplankton are apparently captured in about equal proportions by the two methods. The tendency for the capture of smaller organisms with the finer meshed, smaller mouthed Clarke-Bumpus gear, noted qualitatively above, can be expressed quantitatively by com- parison of average size of organisms taken on cruises 5 and 16, from which both volumes and counts are available (table 11). In most of the hauls of cruise 16, employing the Clarke-Bumpus gear, the average size of organisms was between 1 and 3 x 10"* cc. , while in nnost of the hauls of cruise 5 the average size was between 6 and 14 x 10" cc. The mean size of organism in the samples (each sample given equal weight) was about five times as large for the coarse-meshed meter net of cruise 5 as for the fine-meshed nets of cruise 16. It is quite obvious that the gear of the two cruises exercised a strong size selection in sampling the plankton community. The volume of catch was not greatly different however, indicating that with the larger net of coarse mesh the loss in small organisms was compensated by the less successful dodging of the larger organisms. 19 Table 11. --Average size of individuals in zooplankton hauls of cruise 16, employing Clarke -Bunnpus samplers equipped with 56XXX grit gauze nets, amd in hauls obtained on the eastern leg (158° W. longitude) of cruise 5 employing 1 -meter nets of 30XXX grit gauze Average size at center of class interval, in cc. x 10-4 Number of hauls 1 Cruise 16 Cruise 5 Surface Inter- mediate Deep Total 1 7 2 - 9 . 2 18 11 6 35 - 3 2 5 7 14 - 4 2 1 3 6 1 5 - 1 2 3 1 6 - - - - 5 7 - - - - 1 8 - 1 - 1 4 9 - - - - 2 10 - - - - 4 11 - - - - 1 12 - - - - 2 13 - - - - 2 14 - - - - 1 Mean (x lO"'* cc) 1.9 9.2 20 SUMMARY AND CONCLUSIONS At 30 stations in a series along 150 W. longitude from 12 N. to 7 S. latitude, 68 samples were obtained at three depths--the surface, the level of the 70 F. isotherm, and at approxi- mately 200 meters--by means of horizontal closing-net hauls with Clarke-Bumpus samplers. According to an analysis of variance, for surface hauls the volumes of the night samples were significantly greater (P < 0.01) than the volumes of the day sannples. For intermediate and deep levels the volumes of the day hauls exceeded those of the night hauls, but the differ- ences were not statistically significant. Within the range of latitudes sampled, the area 2 S. to 8 N. contained the greatest amount of zooplankton, with a peak in abundance near 1 S. latitude. The surface samples ranked considerably above the intermediate and deep samples in volume and number of organisnns. There was no evidence of a concentration of zooplankton in the region of the thermocline. The average size of organisnns in the collections increased with depth of sampling, and was greater in the night hauls than in the day hauls except at the deep level, where the opposite was true. The copepods were by far the nnost abundant group present in the sannples, followed by fora- minifers, eggs, tunicates, gastropods, chaetognaths , radiolarians, crustacean larvae, ostracods, euphausiids , siphonophores, and annphipods in that order. A detailed examination of collections obtained at two stations, one in the North Equatorial Current and the other in the South Equatorial Current, provided little evidence of nnajor dif- ferences in species composition between these two current sytems. Clarke-Bumpus sannplers equipped with 56XXX grit gauze nets retained large numbers of small Copepoda, Foraminifera, and Appendicularia which passed through the coarser meshes of 1 -meter nets of 30XXX grit gauze. The volunne of catch per unit of water strained was not greatly different, however, for the two types of gear, indicating that with the larger net the loss in small organisms was compensated by the less successful dodging of the larger organisnns. LITERATURE CITED AUSTIN, T. S. 1954. Mid-Pacific oceanography V; Transequatorial waters, May-June 1952, August 1952. U. S. FiBh and Wildlife Serv. , Spec. Sci. Rept. - -Fisheries No. 136. 86 p. CLARKE, G. L. , and D. F. BUMPUS 1940. The plankton sampler - an instrument for quantitative plankton investigation. Linano- logical Soc. Amer. , Pub. No. 5. 8 p. CROMWELL, T. 1951. Mid-Pacific oceanography, January-March 1950. U. S. Fish and Wildlife Serv., Spec. Sci. Rept. --Fisheries No. 54. 79 p. KING, J. E. and JOAN DEMOND 1953. Zooplankton abundance in the central Pacific. U. S. Fish and Wildlife Serv., Fish. Bull. No. 82, vol. 54, pp. 111-144. , and T. S. HIDA 1954. Variations in zooplankton abundance in Hawaiian waters, 1950-52. U. S, Fish and Wildlife Serv. , Spec. Sci. Rept. --Fisheries No. 118. 66 p. MOORE, H. B. , O. HARDING, E. JONES, and T. DOW 1953. Plankton of the Florida current. The control of the vertical distribution of zooplankton in the daytime by light and temperature. Bull. Mar. Sci. of the Gulf and Carribbean. 3(2):83-95. MURPHY, G. I. , and R. S. SHOMURA 1953a. Longline fishing for deep-swimming tunas in the central Pacific, 1950-51. U. S. Fish and Wildlife Serv., Spec. Sci. Rept. --Fisheries No. 98. 47 p. 1953b. Longline fishing for deep-swinnming tunas in the central Pacific, January-June 1952. U. S. Fish and Wildlife Serv., Spec. Sci. Rept. --Fisheries No. 108. 32 p. REINTJES, J. W., and J. E, KING 1953. Food of the yellowfin tuna in the central Pacific. U. S. Fish and Wildlife Serv., Fish. Bull., No. 81, vol. 54, pp. 91-110. SETTE, O. E., and STAFF OF POFI 1954. Progress in Pacific Oceanic Fishery Investigations, 1950-53. U. S. Fish and Wildlife Serv., Spec. Sci. Rept. --Fisheijies No. 116. 75 p. SVERDRUP, H. U. , M. W. JOHNSON, and R. H. FLEMING 1942. The oceans, their physics, chennistry, and general biology. 1087 p. New York: Prentice-Hall Inc. 22 5 wHgrs's