154 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



Embryological development is similar to that of other teleost fishes. It pro- 

 gresses more rapidly in warm water than in cold, eggs hatching in 2 clays at 21° C. 

 (70° F.) and in 8^ days at 10° C. (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 sur- 

 face and the thermocline, which is usually 15 to 25 meters (8 to 13 fathoms) deep in 

 the area studied. They have a tendency to sink 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. (K-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. (%-inch) to 10 mm. OS- 

 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 exer- 

 cised 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, time 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 off- 

 shore than in other seasons. After the post-larval stage of active swimming com- 

 menced, 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 Shoals 

 and were taken off Cape Cod. 



In 1932 the mortality over most of the developmental period was 10 to 14 per- 

 cent 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 



