for the Gulf cruise in February 1956 showed 

 very heavy concentrations of larvae off the 

 mainland of Mexico not delimited by the 

 southernmost extent of that cruise. The Gulf 

 population, then (see 1.31 above) may extend 

 as far as the limit of Banderas Bay defined by 

 Fitch (see footnote 1), or farther. 



2.2 Differential distribution 



2.21 Spawn, larvae, and juveniles. 

 See 2.1 above. 



2.22 Adults 



The adults occur in the same areas shown 

 for the distributions of their larvae (also see 

 2.1), and farther north, at least to Pt. Sur, 

 Calif, (see footnote 4). 



2.3 Determinants of distribution changes 

 No data. 



2.4 Hybridization 



No evidence of hybridization. 



3 BIONOMICS AND LIFE HISTORY 

 3. 1 Reproduction 



3.11 Sexuality 



The Pacific mackerel is heterosexual with 

 no evidence of sexual dimorphism. 



3.12 Maturity 



Fry (1936>b) stated that mackerels, ". . . 

 under 11-1/2 inches total length at the begin- 

 ning of the spawning season will not spawn that 

 season; some of the 12- and 1 2- 1/2-inch fish 

 and over three -fourths of the 13 -inch fish will 

 spawn. In terms of age--the yearling fish do 

 not spawn whereas most of the Z-year-olds 

 do." He stated further that these facts are 

 true for mackerels in California waters but 

 that some mackerels from the southern part of 

 Baja California seem to mature at a smaller 

 size. 



3.13 Mating 



Not observed but believed to be promiscu- 

 ous. 



3.14 Fertilization 

 External. 



3.15 Gonads 



Few data are available on the fecundity of 

 the Pacific mackerel. MacGregor ( 1966) stated 

 that this mackerel produces 304 eggs per gram 

 of fish (based on counts for six specimens). 

 One of these was collected off Avalon, Catalina 

 Island, Calif, and another from San Hipolito 

 Bay, Baja California. The remaining four were 

 taken from a holding tank at the Scripps In- 

 stitution of Oceanography, La JoUa, Calif. For 

 want of better data, he included data from these 



four in his totals, although he was not sure of 

 their validity. His data showed 309 and 220 

 eggs per gram of fish for the Avalon and San 

 Hipolito specimens, respectively, and 141 to 

 457 eggs per gram of fish for the other four 

 specimens. 



3.16 Spawning 



Studies of spawning of Pacific mackerel have 

 been made off southern California and Baja 

 California. Fry (1936b) stated that in 1936 

 Pacific mackerel spawned off southern Cali- 

 fornia from late April or early May to August -- 

 the peak spawning was from May to early July. 

 Kramer (1960), using larvae as indicators of 

 spawning times, stated that spawning occurred 

 in the same months from 1952 through 1956 on 

 the surveys of the CalCOFI. He stated further 

 that there was some variation in peak spawning 

 times in different regions --April through July 

 off southern California and northern Baja 

 California and March through August off central 

 Baja California. Ahlstrom (1959a, text table 

 10) showed that the peak of spawning in 1957 

 off central Baja California occurred from June 

 through October and that the numbers of 

 larvae off northern Baja California indicated a 

 possible peak in April and May. Ahlstrom 

 (nnanuscript^ ) showed that numbers of larvae 

 in 1958 indicated peak spawning in March 

 through July off southern California and 

 northern Baja California and about August and 

 September off central Baja California. He 

 showed that in 1959 abundance of larvae was 

 very low but that a spawning peak was indi- 

 cated off southern California and northern Baja 

 California in June and July and off central 

 Baja California from June through September. 



3.17 Spawn 



The Pacific mackerel egg is pelagic and 

 spherical. The egg and the embryonic develop- 

 ment were described from live material by 

 Fry (1936a) and Orton (1953). Kramer (1960) 

 described the egg and embryonic development 

 in greater detail from preserved material, 

 from very early stages to hatching (figs. 3 and 

 4, table 2). Because of the various similarities 

 of Pacific mackerel eggs to those of the jack 

 mackerel, Trachurus symmetricus and the 

 Pacific hake, Merluccius productus , with which 

 they might be collected, Kramer compared the 

 three as shown in table 2. The sizes of Pacific 

 mackerel eggs shown in this table are averages 

 and may vary according to time as noted by Fry 

 (1936a) who concluded that eggs found at the 

 beginning of the spawning season were larger 

 than those found at the end of the season. 



^Ahlstrom, Elbert H., BCF Fishery-Oceanography Cen- 

 ter, La JoUa, Calif. Sardine eggs and larvae and other 

 fish larvae off the Pacific coast, 1958-60. 



