LEONG: MATURATION AND SPAWNING OF SCOMBER JAPONICUS 



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FIGURE 1. — Development of eggs in 

 female Scomber japonicus under three 

 temperature conditions. Upper panel, 

 weekly ambient temperature ranges 

 and medians. Lower panel, diameter of 

 the largest eggs in individual females 

 under 18°C, 15°C, and ambient 

 temperatures. Shaded area, numbers of 

 individuals which could not be sexed 

 due to immaturity. Closed circles 

 represent egg diameters in initial 

 sample, open circles at 18°C, squares at 

 15°C, and triangles at ambient 

 temperature. Arrow indicates when 

 group under ambient conditions 

 spawned naturally. 



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IMMATURE 



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NATURAL 

 SPAWNING 



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3 10 

 OCT. 



groups because of a water system failure and total 

 loss of fish indoors. 



The fish in the outdoor tank survived and began 

 to spawn spontaneously on 30 April, nearly 6 wk 

 after the induced spawning. The end of April is 

 also the approximate time that the natural 

 population begins to spawn off the southern 

 California coast (Kramer 1960). The temperatures . 

 in the outdoor tank were very similar to the 

 temperatures at Scripps Pier, which can be 

 considered indicative of surface coastal conditions 

 off southern California (Radovich 1961). Thus the 

 mackerel in the outdoor tank should have received 

 temperatures which were like the temperatures 

 found in the southern California spawning 

 grounds and the similar time of initial spawning 

 may be expected. However, it should be pointed 

 out that mackerel in the wild can migrate over 

 long distances (Roedel 1952) and the average 

 temperature cycle they undergo in nature is not 

 precisely known. 



The temperature in the outdoor pool at the time 

 of initial spawning was 16°C which is a favorable 

 temperature for mackerel spawning in nature. 

 Kramer (1960), utilizing data from the California 

 Cooperative Oceanic Fisheries Investigations 

 surveys, found mackerel larvae occurring at 

 temperatures (taken at 10 m) ranging from 10.3°C 

 to 26.8°C with more than 68% of all occurrences 

 between 14.0° and 17.9°C. Watanabe (1970), using 

 Japanese data, found early stage mackerel eggs 



17 24 31 7 14 21 28 5 12 19 26 2 9 16 23 30 6 13 20 27 6 13 20 27 3 10 17 24 I 

 NOV. DEC. JAN FEB. MAR. APR. MAY 



DAY 



occurring between temperatures (taken at the 

 surface) of 13° and 23°C with the mode of positive 

 stations between 16° and 19°C. 



Test of a 16°C-14L10D Environment 



for Maintaining Mackerel 



in Spawning Condition 



After the Normal Spawning Season 



The group of 25 fish that was placed under a 

 16°C-14L10D environment contained functionally 

 mature individuals at the start of the trial, 7 July. 

 Monthly biopsies indicated that at least one 

 female in the group was sexually mature from 

 July 1974 through March 1975. The months of 

 sampling included December, January, and 

 February when the maturity indices of mackerel 

 are at the lowest levels (Knaggs and Parrish 

 1973). No more than three females were 

 catheterized in any month before one with 0.7-mm 

 diameter eggs was found. The eggs in the other 

 females ranged from 0.2 to 0.6 mm in diameter. I 

 am not certain if the ripe females remained 

 sexually mature continuously or if they resorbed 

 and subsequently redeveloped their ovaries. 



Effectiveness of Hormones for the 

 Induction of Spawning 



All injections of ground salmon pituitary (SP) 

 from 1 to 25 mg stimulated hydration and ovula- 



209 



