Fishery bulletin of the Fish and Wildlife Service - NOAA

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BIOLOGY OF THE ATLANTIC MACKEREL 153 between the Chesapeake capes and Cape Cod; second in importance, with perhaps one-tenth as much spawning, is the southern half of the Gulf of St. Lawrence. Other stretches of the coastal waters may at times receive negligible amounts of spawn, but it is safe to say that the entire Gulf of Maine (excepting Cape Cod Bay), and the entire outer coast of Nova Scotia, the northern two-thirds of the Gulf of St. Lawrence and the waters around Newfoundland are not regular spawning grounds of any importance. so 1 45' I. BAY OF ISLANDS £. CAPE ANGUILLE 3. LAURENTIAN CHANNEL 4. MAGDALEN ISLAND 3. STRAITS OF CANSO С. PRINCE EDWARD ISLAND 7.CAPE BRETON B CAPE CANSO 9. HALIFAX 10. CAPE SABLE 11. BROWNS BANK 12. BAY OF FUNOY U. CASCO ВАГ 14. CAPE ELIZABETH 15. CAPE ANN 16 BOSTON 17. MASSACHUSETTS BAY 18. CAPE COD BAY 19. WOOD END 20. CAPE COD 2 I. CHATHAM eZ.NANTUCKET SOUND 23. WOODS HOLE 24.eUZZAROS ВАГ 29. NO MANS LAND asê-L. 70' Í5* MARTHAS VINEYARD NANTUCKET SHOALS GEORGES BANK BLOCK ISLAND MONTAUK POINT SH1NNECOCK LOHG ISLAND SOUND LONG ISLAND FIRE ISLAND NEW YORK CITY BARNEGAT ATLANTIC С1ТГ DELAWARE BAY CAPE MAY WINTEROUARTER LIGHTSHIP OHESAPEAKECAreS CAPE HATTERAS 75' TO' Si' SO* FIOUBE 1.—Geographical features and landmarks mentioned In the text. Spawning takes place in open waters in some places close to shore, in others as far as 80 miles to sea, but mostly 10 to 30 miles from shore. Open bays, such as Cape Cod Bay and Casco Bay, are spawning sites of minor importance while wellenclosed bays and sounds, especially those receiving considerable river water, such as Chesapeake Bay, Delaware Bay, and Long Island Sound, are neglected by the spawning mackerel. Spawning occurs at any time of day or night, and probably near surface.
154 FISHERY BULLETIN OP THE FISH AND WILDLIFE SERVICE Embryological development is similar to that of other teleost fishes. It progresses more rapidly in warm water than in cold, eggs hatching in 2 days at 21° C. (70° P.) and in 8K days at 10° 0. (50° F.). The prevailing temperatures on the spawning grounds at the height of the spawning season are between 9° and 12° C., so that in nature the incubation period usually occupies about a week. During incubation the eggs are suspended in the sea water between its surface and the thermocline, which is usually 15 to 25 meters (8 to 13 fathoms) deep in the area studied. They have a tendency to shut gradually as development proceeds, so that the late stages are found at deeper levels than the early ones, but even so, not below the thermocline. After hatching, the young mackerel passes through three phases of development, conveniently designated as yolk sac, larval, and post-larval stages. During the yolk sac stage—a matter of about 5 days—the fish is about 3 mm. (%-inch) long and subsists on the yolk. During this period, the mouth and digestive organs develop into usefulness and the yolk sac is absorbed. During the period occupied by the larval stage, that is, between yolk sac absorption and development of fins, which lasts about 26 days, the fish grows from a length of 4 mm. (%-mch) to 10 mm. (%inch) in length. Then, when the fins have appeared, the post-larval stage begins. It continues about 40 days and during this time the fish grows to a length of about 50 mm. Toward the end of this stage, while growing from 30 to 50 mm., the body assumes the trim fusiform shape of the adult. At that time, the fins, relative to the body, are even larger than in the adult, and the coloration includes shiny, silvery iridescence, though still lacking the characteristic wavy black bands of the adult. During the yolk sac stage, movements are feeble, not even serving to keep the fish right side up. Swimming faculties increase during the larval stage and are exercised in performing vertical diurnal migrations, the larvae ascending toward the surface at night and descending toward the thermocline at day. But they do not swim any considerable distances during this stage ; instead they drift with the water masses in which they are suspended. In post-larval stages, true swimming takes place, the young fish at times moving in a direction opposite to the prevailing drift of water. The schooling habit probably begins to assert itself toward the end of this stage and thereafter is followed in much the same fashion as by the adults. In 1932, the larvae were drifted initially in a southwesterly direction, and the main body was transported about 80 miles down the coast, one subgroup drifting from the offing of northern New Jersey to the offing of Delaware Bay; another, from the offing of southern New Jersey nearly to the Chesapeake capes. Then, a reversal of drift returned both groups to the offing of northern New Jersey by the time they had reached the end of the larval stage, and were 9 mm. long. The southwesterly drift coincided with the predominance of northeasterly winds, and the northeasterly return with a reversal of dominant winds. Compared with other seasons,1932 had an abnormally large northeasterly wind component, which left the 9-mm. larvae farther to the southwest and farther offshore than in other seasons. After the ;post-larval stage of active swimming commenced, the direction of travel was toward southern New England, and by the latter part of July, some of the largest of the post-larvae had even passed Nantucket Shoale and were taken off Cape Cod. In 1932 the mortality over most of the developmental period was 10 to 14 percent per day. There was a notably higher mortality of 30 to 45 percent per day during the 8- to 10-mm. period, when fin development was rapid. Other departures from
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