FISHERY BULLETIN: VOL. 75, NO. 1 



taken at 24 and 40 h after injection to note the 

 effects of the hormones. If ovulation or spawning 

 had occurred, stripping was attempted and the 

 eggs fertilized by the dry method (Davis 1961). 



RESULTS 



Maturation of Mackerel Under 

 Four Photoperiods 



The female mackerel caught before the spawn- 

 ing season became mature in the laboratory under 

 the three constant photoperiods (4L20D, 8L16D, 

 and 16L8D) and under ambient light conditions. 

 At the start of the experiment (17 March) the 

 diameter of the largest eggs sampled from the six 

 females ranged from 0.4 to 0.6 mm. Thus, the 

 females were not fully mature but two of the males 

 sampled already had milt and may have been 

 capable of spawning. Recently spawned eggs 

 appeared in the egg strainer of the 16L8D tank on 

 17 April, 1 mo after the beginning of the ex- 

 periment. Catheterization of two females from 

 each treatment showed that all treatments 

 contained females with 0.7-mm diameter eggs 

 indicating sexual maturity. None of the treat- 

 ments appeared to inhibit maturation. The results 

 indicated that female mackerel in prespawning 

 condition will become sexually mature in the 

 laboratory under a wide range of photoperiods at 

 19°C. 



The dates of initial spawning showed no relation 

 to the length of day. Spawning was detected in the 

 4L20D tank on 25 April and in the outdoor tank on 

 1 May 1973. In the outdoor tank, the time between 

 sunrise and sunset had lengthened from 12 h at 

 the start of the trial to 13V2 h on 1 May. Spawning 

 was never detected in the 8L16D tank although it 

 contained functionally mature males and females. 



The mackerel spawned during the dark period 

 but the exact time is not known. Watanabe (1970) 

 stated that mackerel spawn between 2000 and 

 2400 h in nature. Spawning occurred three or four 

 times a week in the outdoor pool and two or three 

 times a week in the indoor pools from May to 

 mid-June. The frequency of spawning then de- 

 creased and was rare by mid-July when observa- 

 tions ended. 



Although the fish spawned spontaneously, the 

 predictability of spawning and the viability of 

 eggs were not satisfactory. The number of eggs 

 collected per day was usually less than 3,000, 

 although one collection was over 50,000. The 



percentage of viable eggs seldom exceeded 10% 

 and was often zero. The spontaneously spawned 

 eggs were translucent and of the proper size, 1.1 

 mm in diameter, but most were not fertilized. 



Observations ended in mid-July because the fish 

 began to feed poorly and started to die. An ac- 

 companying symptom of failing health was the 

 malformation of jaws in about half of the fish. 

 Afflicted fish swam with their jaws constantly 

 agape and were unable to bite on food items. The 

 condition may have been partly due to the high 

 water temperature as some fish recovered when 

 transferred to a tank with 15°C seawater. Thus, 

 while the mackerel became fully mature at 19°C a 

 prolonged exposure may be detrimental. 



Maturation of Mackerel at Ambient, 

 15°C, and 18°C Temperatures 



Mackerel captured near the end of the spawning 

 season redeveloped their ovaries more rapidly at 

 18°C than at 15°C or ambient temperature (Figure 

 1). Three of the females sampled at the start of the 

 trial, 3 October, had eggs 0.7 mm in diameter 

 while three others had eggs 0.2 mm in diameter. 

 This difference in egg size can be expected near the 

 end of the spawning season as some females stop 

 spawning and begin resorption of ovaries earlier 

 than others. In November, one female from the 

 18°C treatment still had eggs 0.7 mm in diameter 

 but four other fish from that treatment and five 

 from each of the other two treatments could not be 

 sexed because of immaturity of the gonads. Below 

 a certain stage of maturity gonads are too small to 

 remove tissue for biopsy. Biopsies were still 

 difficult to perform in January and samples were 

 obtained from less than half of the fish. The 

 females that did provide samples had eggs 

 measuring 0.3 to 0.4 mm in diameter. Biopsies 

 were more successful in February; the females 

 from the 15°C and ambient temperature groups 

 still had eggs measuring 0.3 to 0.4 mm in diameter 

 but two females from the 18°C group had eggs of 

 0.5 and 0.6 mm in diameter, respectively. Two of 

 the three females sampled from the 18°C group on 

 20 March had eggs of 0.7 mm in diameter and one 

 was spawned with hormone injections. The 

 spawning date was about 5V2 mo after the start of 

 the trial. On 20 March, the females from the 15°C 

 and ambient temperature groups did not as yet 

 have eggs exceeding 0.5 mm in diameter. Ob- 

 servations ended shortly after for the 15° and 18°C 



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