LEONG: MATURATION AND SPAWNING OF SCOMBER JAPONICUS 



Test of a 16°C-14L10D Environment 



for Maintaining Mackerel 



in Spawning Condition 



After the Normal Spawning Season 



The group of 50 fish that was held outdoors 

 under ambient conditions began to spawn 

 spontaneously at the end of April 1974. On 7 July, 

 while some spawning was still in progress, 25 fish 

 were transferred indoors to a tank with ambient 

 temperature (19°C) seawater and photoperiod of 

 14L10D. On 8 July, the temperature was lowered 

 to 16°C and the fish were kept at that temperature 

 for 9 mo. Biopsies were taken at the time of trans- 

 fer and in each succeeding month to determine if 

 at least one female was in spawning condition. 

 During each sampling, fish were catheterized 

 until a female with 0.7-mm diameter eggs was 

 found. Females with eggs of this size are func- 

 tionally mature, i.e., can be spawned with hor- 

 mone injections. 



Testing of Hormones for Induction 

 of Spawning 



The agents tested for the induction of spawning 

 were gonadotropin from ground chinook salmon, 

 Oncorhynchus tshawytscha, pituitary (SP), 

 human chorionic gonadotropin (HCG), and gonad- 



otropin from pregnant mare serum (PMS). The 

 agents were applied individually and in combina- 

 tion, as indicated in Table 1. 



The salmon pituitaries were collected, pre- 

 served, and prepared as described by Haydock 

 ( 197 1). The carrier for all injections was saline and 

 the injection volume 0.1 ml. The injections were 

 applied intramuscularly near the base of the dor- 

 sal fin with a 24-gauge needle on a 0.5-ml syringe. 



The mean weight of the fish was 0.9 kg (range 

 0.8 to 1.1 kg). Dosages were not adjusted for dif- 

 ferences in fish weight, and one male and one 

 female were injected for each treatment. The fish 

 had become sexually mature in the laboratory and 

 were among those used in the photoperiodic ex- 

 periment. The injection trials were carried out 

 during June through August which is also the 

 time of spawning in nature. 



Fish were biopsied beforehand and only males 

 with generous amounts of milt and females with 

 0.7-mm diameter eggs were injected (preliminary 

 testing indicated that the eggs had to be close to 

 0.7 mm in diameter before the hormones would 

 stimulate a noticeable response). The injected pair 

 was isolated in a small swimming pool (3 m in 

 diameter, 0.5 m water depth) with water tempera- 

 ture at 17°C and a flow rate of 2.5 liters/min. An 

 egg strainer was placed at the outflow to detect 

 spawning. Biopsies and general observation were 



TABLE 1 — Results of tests with gonadotropin from ground salmon pituitary (SP), human chorionic gonadotropin (HCG), 

 and gonadotropin from pregnant mare serum (PMS) for induction of spawning in Scomber japonicus . 



'Second injection given 24 h after first injection. 



2 Time measured after first injection. 



3 Egg diameter was 0.7 mm before first injection. 



"Stripping was attempted on live fish with ovulated eggs. Stripping was attempted even if a fish spawned because the eggs were unfertilized. 



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