FISHERY BULLETIN: VOL. 75. NO. 1 



between 1 February and 7 March 1973. The dates 

 of capture were 1 to 2 mo in advance of the major 

 spawning season. The fish ranged from 325 to 340 

 mm fork length; most fish of this size are capable of 

 spawning (Knaggs and Parrish 1973). During the 

 period of collection, the fish were held under 

 continuous incandescent lighting and a tempera- 

 ture of 19°C. These were arbitrary holding condi- 

 tions. 



On 14 March, 1 wk after the last fish was 

 captured, the mackerel were divided into four 

 groups and placed in three indoor plastic swim- 

 ming pools (4.6 m in diameter, 1 m water depth) 

 and one outdoor pool ( 7.3 m in diameter, 1 m water 

 depth). Each of the three indoor pools was enclosed 

 in a separate room lined with black opaque 

 polyethylene film. A 200-W incandescent bulb, 1.2 

 m above the water surface, illuminated each in- 

 door pool during the artificial day. A timer- 

 controlled rheostat gradually lit and dimmed the 

 bulb over 30 min to avoid startling the fish. The 

 length of day was considered as the time of full 

 illumination. Two 3-W lamps, 1 m above the water 

 surface, burned continuously and provided low- 

 level illumination during the dark period. The 

 light intensity was about 215 lx at the brightest 

 spot on the surface during the day and less than 

 5.4 lx at night. The outdoor pool was shielded from 

 direct sunlight by an opaque plastic canopy 1.2 m 

 above the water surface but the sides were open 

 and the fish received a natural photoperiod. 



Temperature control was achieved with a 

 commercial temperature regulator and mixing 

 valve unit which automatically adjusted the 

 inflow of chilled (10°C) and heated (20°C) seawater 

 to maintain a desired pool temperature. For this 

 series of observations the temperature was set at 

 19° ± 0.5°C for all tanks. I chose this temperature 

 because captive mackerel had spawned at this 

 temperature during a preliminary study. The flow 

 rates were 32 liters/min for the indoor tanks and 

 50 liters/min for the outdoor tank. Each tank also 

 had a recirculating pump of 250 liters/min 

 capacity. 



Each of the experimental groups contained 50 

 fish. Commencing on 17 March, the three groups of 

 fish in the indoor tanks were maintained on 

 photoperiods of 4L20D, 8L16D, and 16L8D, re- 

 spectively. The group of fish in the outdoor tank 

 remained under the natural photoperiod where 

 the time between sunrise and sunset was 12 h. 

 Biopsy samples of the gonads were taken prior to 

 the photoperiodic change and again a month later 



to note the change in maturation. The biopsies 

 were taken by inserting the tip of a glass pipette 

 (1.2 mm in diameter) through the genital pore of a 

 fish anesthetized in 7 ppm quinaldine and remov- 

 ing a small piece of gonad by mild suction. The 

 technique, a modification of that used by Stevens 

 ( 1966), did not appear to cause permanent damage 

 to the fish. All ovarian samples were examined 

 with a dissecting microscope and the diameter of 

 the largest eggs measured to the nearest 0.1 mm. 

 No effort was made to categorize the males except 

 to note if milt was obtained. Six females were 

 biopsied at the start of the trial and two from each 

 treatment at the end. 



An egg strainer was positioned at the outflow of 

 each tank and inspected daily to detect spon- 

 taneous spawning. The strainer, a 1 x 1 x 0.2 m 

 wooden frame with 202-^tm mesh netting 

 stretched across the bottom, was partially im- 

 mersed in a water bath to prevent desiccation of 

 eggs. The mackerel were fed daily with either 

 freshly thawed frozen anchovies or ground squid. 

 Occasionally, Oregon moist chow was mixed in 

 with the ground squid as a supplement. The 

 estimated daily ration was 49c of body weight. 



Maturation of Mackerel Under Ambient, 

 15°C, and 18°C Temperatures 



Mackerel judged to be in or near postspawning 

 condition were collected between 23 August and 

 28 September 1973. The fish ranged from 330 to 

 370 mm fork length and were kept indoors at 

 18°C-14L10D during the period of collection. The 

 mackerel were subsequently divided into three 

 groups of 50 fish each and placed in two of the 

 indoor pools and in the outdoor pool already de- 

 scribed. Beginning on 3 October, the two groups of 

 indoor fish were kept at 15°C and 18°C, respec- 

 tively. The fish in the outdoor tank received 

 seawater at ambient temperature (19°C at the 

 outset) which fluctuated with ocean conditions at 

 the intake. The intake was located at the end of the 

 pier at the Scripps Institution of Oceanography, 

 La Jolla. The photoperiods were 14L10D for both 

 indoor groups and natural for the outdoor group. 

 Six females were biopsied at the start of the trial 

 for ova measurements. Several fish from each 

 group were biopsied at various intervals af- 

 terwards until March 1974 to observe changes in 

 ovarian development. I attempted to obtain eggs 

 from at least two females per group with every 

 round of sampling. 



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