to the south, at the Revillagigedo Islands and 

 reportedly at Acapulaco and in the Gulf of 

 Tehauntepec, Mexico. The latter two records 

 may be fish transported south in bait tanks 

 of tuna boats. 



The record of Clemens and Nowell (1963) of 

 one specimen of T. symmetricus taken off 

 Costa Rica at lat. 10° Ol'N., long, 85° 55' W. 

 by dip net and night light in July 1957 has been 

 found to be in error. 



2.2 Differential distribution 



The distribution of adult jack mackerel ap- 

 pears to correspond closely to that of eggs 

 and larvae. The adults are present off southern 

 California throughout the year, and seasonal 

 and annual variations in landings are caused 

 by primarily economic factors acting upon the 

 fishing industry. 



2.3 Determinants of distribution changes 



Jack mackerel eggs and larvae have been 

 taken for the past 15 years at most California 

 Cooperative Oceanic Fisheries Investigation 

 (CalCOFI) stations off southern California and 

 northern Baja California except for those 

 closest to shore. The catch of adults also in- 

 dicates that the population is more stable with 

 respect to distribution and more offshore than 

 the other three small pelagic species, the 

 sardine, Sardinops caerulea (Girard), the Paci- 

 fic mackerel, Pneumatophorus diego (Ayres), 

 and the anchovy, Engraulis mordax Girard, 

 taken by the purse seine fishery. 



2.4 Hybridization 



No evidence of hybridization. 

 3 BIONOMICS AND LIFE HISTORY 

 3.1 Reproduction 



3.11 Sexuality 



Jack mackerel are heterosexual and without 

 apparent sexual dimorphism. 



3.12 Maturity 



According to Fitch (1956) 50 percent of fe- 

 males are mature at 250 mm. fork length and 

 age 2; 100 percent at 350 mm. and age 3. 



3.13 Mating 



No record of observation but probably 

 promiscuous. 



3.14 Fertilization 



External. 



3.15 Gonads 



No data are available on the relation of 

 number of eggs to age and body length and 

 weight for the jack mackerel. Within nnost 

 fish species the number of eggs produced at 

 one time is approxinnately proportional to 

 the weight and the cube of the length of the 

 fish. 



Fecundity data for a jack mackerel taken 

 off central Baja California in July 1953 are as 

 follows: 



Standard length 215 mm. 



Fork length 229 mm. 



Total length 242 mm. 



Weight 171 grams 



Gonad weight (left 3.17) , ,, 



(right 2.97) 6.14 grams 



Eggs (0.40-0.58 mm. diameter ... 52,600 



Eggs per gram of fish 308 



The pelagic eggs of marine fishes of most 

 species have diameters within a few tenths of 

 1 nnillimeter. The number of eggs per gram 

 of fish, developed as one spawning batch, tends 

 generally to be high for small species of fish 

 (i.e. about 600 eggs per gram of fish for 

 Vinciguerria lucetia (Garman) weighing less 

 than 1 gram and having ripe- egg diameters 

 of about 0.7 mm.) and low for large species 

 of fish (i.e. less than 50 eggs per gram of 

 fish for some of the large tunas weighing over 

 100 kg. and having ripe- egg diameters of about 

 I mm.). 



The 308 eggs per gram of fish and ripe-egg 

 diameter of about 1 mm. for the jack nnackerel 

 are typical for a fish of this size producing 

 a pelagic egg. A scombrid, the Pacific mack- 

 erel--which inhabits much of the same range 

 as the jack mackerel, is of similar size and 

 has many comparable habits- -spawns a pelagic 

 egg about 1.1 mm. in diameter and produces 

 304 eggs per gram of fish (based on counts 

 for six specimens). 



3.16 Spawning 



The percentage frequency distribution of 

 diameters of eggs containing yolk found within 

 the ovaries of the 215 mm. jack mackerel (see 

 preceding section) are presented in Figure 3. 

 Numerous eggs less than 0.20 mm. diameter 

 and not containing yolk are not shown. The 

 eggs that form a distinct nnode from 0.40 to 

 0.58 mm. diameter are considered to be the 

 group destined to be spawned. A bimodal 

 distribution of yolked eggs may indicate 

 two spawnings. The ratio of eggs 0.20 to 

 0.38 mm. to eggs 0.40 to 0.58 mm. diameter 



