BIOLOGY OF THE ATLANTIC MACKEREL 171 



strongly, laterally compressed body; the bluefish (Pomatomus saltatrii), which differs 

 by its fewer myomeres (24) ; and the rosefish (Sebastes marinus) , which has the same 

 number of myomeres (30) and in the 4- to 5-mra. stage could be confused with mackerel. 

 With both the rosefish and mackerel available for comparison, the former is readily 

 distinguished by the closeness of the spots in the dorsal and ventral rows, those in 

 the rosefish forming almost a continuous black streak, whereas those of the mackerel 

 are discrete. Other differences, less useful, are the more slender shape and the greater 

 relative length of the post-anal region in the rosefish larva. After passing the 5-mm. 

 stage, the rosefish larva is readily separated from the mackerel larva by its prominent 

 preopercular and cranial spines. An additional character of use in separating the 

 mackerel larva from the others is its strong teeth, which are readily visible in speci- 

 mens of the 7-mm. size but less so in smaller individuals. 



Inability to keep larvae alive in the laboratory or hatchery during this stage 

 precluded direct observation on their activity, but, as is shown in a later section, their 

 movements are sufficiently well-directed for performance of diurnal vertical migrations 

 of 20 to 30 meters but not sufficiently sustainable for migrations of miles in extent. 



Transition phase. — Intervening between larval and post-larval stages is a transi- 

 tion phase including individuals 9 and 10 mm. long whose fins are in various states of 

 completion. 10 Fin formation is a gradual process, neither beginning sharply at 9 mm. 

 nor ending sharply at 10 mm. At the former length, the caudal fin already shows a 

 number of rays, and at the latter length, the laggard first dorsal fin does not 3"et show 

 any of its spines. But the tail fin makes its greatest changes, the second dorsal fin 

 and finlets and the anal fin and finlets are all developed within this size range, hence 

 it is most appropriately designated as a transition phase. 



Post-larml stage. — This stage includes the latter part of planktonic existence 

 beginning at about completion of fin formation and lasting until the young (ish arc 

 nimble enough to evade the plankton nets. It is comprised of individuals 11 to 

 50 mm. long. 



Since all the vertical fins except the first dorsal are complete, identification by 

 adult characters is simple. The larvae enter this stage somewhat laterally com- 

 pressed, and by its end fill out to the trim fusiform shape of the adult. At the begin- 

 ning of this stage the color pattern is typically larval, but by its end the dark pigment 

 has spread over the dorsal portions, and in live specimens the silvery line is apparent, 

 though the black wavy bands characteristic of the adult are yet to form. The appear- 

 ance is in general like a miniature adult with somewhat oversized head and fins. 



As appears in a later section, the post-larvae are capable of extensive swimming. 

 Furthermore, as they near the end of this stage the schooling instinct asserts itself. 

 The transition from a primarily planktonic habit to a primarily swimming and 

 schooling habit probably is gradual, in the sense that all individuals may not expe- 

 rience the change at the same size. The available evidence is that it involves indi- 

 viduals between about 30 and 50 mm. in length. This evidence is from two sources. 

 First, the survival curve (fig. 17) has a substantially uniform trend from 11 to 30 mm., 

 from which it may be inferred that there was no change of trend within this size range 

 sufficient to indicate a loss of larvae such as could be expected if some had begun to 



"The present description ot lengths at which Cns appear differs from published figures (Ehrenbaum, 1921, figs. 1 to 7, and 

 Bigelow and Welsh, 1925, fig. 92) probably because the latter give lengths inclusive of finfold or caudal fin, though this is not definitely 

 stated; whereas our measurements were taken to the end of the notochord, i. e., exclusive of the finfold in early stages; and to the 

 base of the caudal fin rays, i. e., exclusive of the caudal fin in later sts'ees. This was necessary on account of fre uienl ''' tortion or 

 injury to the caudal appendage. 



