spawning occurs primarily during the northern 

 winter. 



Gonad indices .--Orange (1961) considered 

 skipjack gonad indices for the several statistical 

 areas defined by lATTC. In the northernmost of 

 these areas, off Baja California, skipjack up to 

 73 cm. were examined; all of these fish had 

 gonads in latent or resting states. Interestingly, 

 the most nearly mature females in all areas of 

 the eastern Pacific covered by that study were 

 found just a short distance from Baja California, 

 in the Revillagigedo Islands. Another area that 

 yielded fish in a comparatively high state of 

 maturity (although not as nearly "mature" as 

 the fish from the Revillagigedo region) was the 

 Cocos Island region. This region and the Revilla- 

 gigedos are the only places in the eastern Pacific 

 where relatively high proportions of maturing 

 skipjack were found. None of the coastal areas, 

 from Baja California to Peru, have yielded skip- 

 jack with gonads in as advanced a state of matu- 

 rity, even though many fish taken in these locali- 

 ties are of a size (usually over 40 cm.) at which 

 full maturity and spawning are possible. 

 Orange's data are summarized in figure 3, which 

 shows the percentage of females classified as 

 "maturing" for the various areas that contained 

 maturing fish. Orange shows that the offshore 

 insular areas, the Revillagigedos and Cocos 

 Island, have higher percentages of mature 

 females than areas nearer to the coast. A much 

 larger percentage of fish in the Revillagigedos 

 mature above 68 cm. than at smaller lengths, 

 but fish of this size are unusual. 



The Revillagigedo area also is unique in that 

 it exhibits a peak incidence of maturity during 

 the second, third, and fourth quarters, whereas 

 all the other areas considered by Orange (coasts 

 of Mexico, Central America, Colombia. Ecuador. 

 Galapagos, and Cocos Island) show tendencies 

 toward winter maturity. 



As Orange (1961) points out, the minimum size 

 at which "spawning" occurs in the Revillagigedos 

 is 55 cm., which is smaller than some of the 

 fishfoundin a condition of latent maturity in the 

 Baja California fishery. Orange's use of the 

 word "spawning'' is unfortunate, because the 

 developing gonads described by him were not in 

 such an advanced state as to be considered per se 

 indicators of imminent spawning; the amount of 

 time required for these fish to become running 

 ripe is not known. 



The concept that skipjack in imminent spawn- 

 ing condition are unavailable to the usual meth- 

 ods of capture in any of the fisheries operating 

 in the spawning areas is based on the difference 

 in ova diameters: between ova of running ripe 

 skipjack which are rarely taken, and those ot 

 fish with mature but not running ripe gonads. 

 Brock (1954: p. 100) , for example, points out that 

 the larger ova from maturing skipjack taken in 

 the Hawaiian fishery range between 0.4 and 0.9 

 mm. in diameter with an average maximum modal 

 size of 0,7 mm. He indicates, however, that the 

 single running ripe skipjack that was available 

 for his examination had ova with an average 

 diameter of 1.125 mm. Brock's observations on 

 mature ova diameter concur with those of Raju 

 (1963) and Bunag (1956). Brock's observations 

 on ova diameter from running ripe skipjack con- 

 cur with Yoshida^. The time interval required 

 for a 0.7 mm. skipjack ovum--which is 60 per- 

 cent of the attainable spawning diameter or 20 

 percent of the attainable spawning volume--to 

 reach extrusion size is unknown. The possibility 

 that small fractions of the total skipjack egg com - 

 plement become ripe and are extruded over long 

 periods of time seems contrary to Brock's ob- 

 servation of a ripe female where "...a gelatinous 

 mass of eggs came foaming out of the visceral 

 cavity in sufficient quantity to make a fair double 

 handful." It thus is evident that skipjack are 

 unavailable to the usual methods of capture for 

 some interval prior to their spawning. 



QUAKTKR OF VF.VR 



Figure 3. --Percentage of females classified as 

 maturing during each quarter of the year 

 from various areas of the eastern Pacific. 

 (Data are from Orange, 1961.) 



_3/ Yoshida, Howard 0. Skipjack tuna spawning 

 in the Marquesas Islands and Tuamotu Archipelago. 

 Bureau of Commercial Fisheries Biological Labora- 

 tory, Honolulu. (In press.) 



