Fishery bulletin of the Fish and Wildlife Service - NOAA

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BIOLOGY OF THE ATLANTIC MACKBBEL 215 nets, excepting that no current meter was employed to measure the flow of water through the 2-meter net. In lieu of this measure, the speed of towing was measured by timing the travel of the ship past a chip cast alongside. It was later found from a statistical analysis of the relation between chip speed and flow through meter nets as measured by the current meter, that the force of the wind modified the chip speed materially. From the relationship established, a schedule of adjustments was applied to the apparent chip speed, to convert it to an approximation of true towing speed. This apparent flow was used instead of a current meter reading. Because of the substitution of a deduced value based in part on average performance instead of on actually measured value, the two members of individual pairs of hauls are not strictly comparable, but the average, or sum, of the 19 hauls with each type of net is not subject to this fault. From the distribution of sizes of larvae caught by the respective nets (table 16), it is obvious that the smallest sizes of mackerel larvae were almost entirely lost through the coarse meshes of the 2-meter net; that the 6- to 9-mm. sizes were incompletely retained; and that sizes from 10 mm. upward were fully retained by the larger net. Two conclusions may be drawn from the comparison: (1) the catches of the two nets, per unit volume of water strained, are virtually identical for larvae 10 mm. and upward, and nearly so for the 7- to 9-mm. sizes, hence no material distortion can have resulted from the pooling of data from the two types of nets, according to the methods employed in this report. . (2) Both types of net must have taken essentially all the larvae of sizes 10 to 22 cm. in length that chanced to be in their path, for if any larvae tended to dodge the nets they would surely have been relatively more successful in eluding the 1-meter net, and thus lowered its catch of the elusive sizes hi relation to that of the 2-meter net. The closeness of the paired values for the size range specified is eloquent evidence this did not take place. It is to be regretted that no such paired hauls are available for the later cruises, when catches of still larger larvae might have indicated the upper size limit for effective catching of larvae by plankton nets. TABLE 16.—Comparison of numbers of larvae caught' by 1-meter nets and by 8-meter nets at identical stations of cruises VI and VII [Catches of both nets were converted to the basis of straining 17.07 cubic meters per meter of depth fished] Length of larvae (millimeters) 3 ... . 4 s и 7 .. 8 . . 9 10 11 . 2-meter net Number 0.39 .61 1.76 7.40 17.33 28.10 20.28 13. 76 13.17 1-meter net Number 6 214 230 143 56 33 25 37 24 14 12 13 14 16 16 17 18 20 22 Length of larvae (millimeters) 2-meter net Number 12.84 9 50 5 86 3.14 .48 .48 .09 .09 1-meter net Number 13 10j4 1 1
216 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE TABLE 17.—Record of oblique hauls made by 1-meter nets during cruises I to VII, inclusive, in 1938 [For explanation, see Items of table 15 designated by the figures enclosed in parentheses in the column headings of this table] Cruise, locality, and haul CRUISE I Martha's Vineyard: II Ш IV ... New York: II Ill IV V VI Bainegat: I Atlantic City: I II in IV Cape May: II Ill IV V VI Fenwlck: I Wlnterquarter: II m Chesapeake: П III.... - CEWSE -II Martha's Vineyard: II ..... 1П :... Montauk: I II Ill Shlnnecock: I П New York: П III IV Bamegat: I Atlantic City: II Ill IV Cape May: II _ Ill IV V Wlnterquarter: II Ill Chesapeake: II Ill Station 21327 21328 21329 21330 21335 21334 21333 21332 21331 21336 21337 21338 21339 21340 21345 21344 21343 21342 21341 21346 21347 21348 21349 21352 21351 21350 21381 21380 21379 21375 21376 2137.7 21374 21373 21369 21370 21371 21372 21368 21367 21366 21365 21364 21359 21360 21361 21362 21363 21358 21357 21356 Date May 2 ...do May 3 ...do May 4 May 3 ...do ...do ...do May 4 ...do ...do ...do..... ...do May 6 ...do do do do ...do— . ...do do ...do May 6 ...do ...do May 16 do „.do May 15 . do dó May 15 ...do May 14 ...do ...do May 15 May 14 May 14 ...do ...do do May 13 ...do ...do do do May 10 ...do do Hour 20 2327 3 24 21 18 17 7 10 13 15 18 9 7 521 13 16 18 21 8 52 9 6 ' 3 15 18 21 11 8 18 21 24 3 14 10 853 11 12 15 18 20 11 96 Depth (4) 39 25 44 54 14 1,140 13 1,320 15 1,260 1* 1,320 17 1,200 19 900 21 16 19 19 19 14 18 18 18 19 21 16 16 22 20 16 15 19 17 22 IS 15 13 14 19 12 16 17 17 19 20 16 18 17 13 22 19 16 22 13 19 Time (5) 1,200 960 780 1,380 900 660 960 900 950 805 880 820 860 860 900 845 820 765 900 805 710 870 940 910 915 895 635 875 895 725 840 920 700 925 840 1)10 920 780 595 760 960 860 845 720 895 Upper net Flow (10) 1392 '406 1203 1538 1492 1470 1521 1465 1442 1292- 1391 1266 1377 1367 1416 1309 1368 1306 1350 1330 1394 1386 1295 1322 1394 •363 283 235 256 364 389 283 1250 481 421 276 1298 389 267 343 311 369 488 294 281 284 332 423 212 292 318 S factor (12) 2.16 1.34 4.71 2.18 .62 .60 .62 .70 .84 1.41 1.17 1.22 1.10 1.12 .99 .98 1.06 1.28 1.12 1.25 1.16 .90 1.18 1.48 1.10 .96 1.15 1.76 1.44 1.31 1.01 1.15 1.13 .63 .98 .95 1.17 .95 1.38 1.20 1.40 .94 .80 1.26 1.01 1.68 1.24 .82 2.26 .97 1.30 Clogft 2 0 22 0 20102 0 0 0 0 1 1 0 1 0 2 0 2 0 0 0 0 0 23 fl 02 1 0 0 0 1 0 0 1 1 0 0 0 0 0 0 0 2 11 Depth (4) 30 48 72 17 17 18 18 15 18 22 22 17 22 22 22 20 20 20 22 22 " 21 22 20 17 19 16 20 22 20 22 22 21 21 22 Time (6) 1,324 1,444 1,444 1,444 664 1,059 999 920 900 900 940 920 900 860 765 965 1,020 960 965 695 925 796 900 965 945 975 1,030 910 785 960 Flow (10) Lower net 392 507 601 578 243 419 410 360 292 380 385 385 407 390 331 254 213 441 298 297 603 329 344 442 376 550 392 479 269 280 S factor (12) 0.94 .73 .65 .68 1.61 1.14 1.17 1.03 1.64 1.26 1.24 1.13 1.07 1.11 1.44 1.88 2.14 1.08 1.46 1.24 .82 1.06 1.25 1.08 1.16 .87 1.22 .95 1.70 1.71 Clogft 0 22 3 2 0 0 0 1 0 0 1 2 0 0 1 0 0 0 12 0 .-1 0 1 0 0 0 1 0 0 0 2 2 Time (13) »116 »116 »116 »116 »116 »141 »141 100 139 154 150 145 140 150 130 140 130 145 с factor (16) 21353 21354 21355 May 9 ...do ...do 16 20 24 9 21 19 480 860 865 190 406 326 1.03 1.12 1.27 0 1 0 22 Ï.OÏO 415 1.15 0 155 .12 1 The flow was deduced from angle of stray of towing wire and degree of clogging by means of correlation diagrams based on the relation between these and flow through the net as measured by current meter at all other stations of this series. 1 Deduced from average data on subsequent hauls. 0.06 .09 .12 .12 .11 .12 .12 .10 .07 .13 .17 .12 .15 .12 .09 .15 .08 .07
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UNITED STATES DEPARTMENT OF THE INT
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