HUNTER and KIMBRELL: EARLY LIFE HISTORY OF PACIFIC MACKEREL 



ters per milligram per hour. Metabolic require- 

 ments were compared with daily ration by con- 

 verting oxygen consumption and daily ration to 

 calories (1 /i.1 Oj = 0.005 cal: one Brachionus 



plicatilis 

 ter 1971). 



0.00085 cal, Theilacker and McMas- 



Culture of Larvae 



Seven groups of Pacific mackerel were reared to 

 metamorphosis to determine growth rates and ef- 

 fects of temperature on growth. The rearing con- 

 tainers were black fiber glass, cylindrical tanks 

 (122 cm in diameter x 36 cm deep). Culture vol- 

 ume increased during the rearing period from 200 

 to 400 1 because of the addition of seawater con- 

 taining food and algae. Tank temperature was 

 controlled by a regulated water bath, and groups 

 were reared at temperatures ranging from 16.8° to 

 22.1° C. Illumination at the water surface during 

 the 12-h day was about 2,000 Ix. Tanks were 

 started with 3,000 eggs/group. Initially, larvae 

 were fed laboratory-cultured Brachionus 

 plicatilis. At age 5 d, laboratory-cultured 

 copepodids and adult copepods (Tishe sp.) were 

 added; 200,000 copepods were added daily until 

 metamorphosis. Initially 30 or more rotifers/ml 

 were added for the first few days of feeding; there- 

 after the density of rotifers was allowed to decline. 

 On a diet of rotifers alone, growth slowed after 

 larvae reached 5 mm SL and few larvae survived 

 longer than 15 d. Newly metamorphosed juveniles 

 were fed live and frozen adult A. salina and 

 minced squid {Loligo opalescens ) and northern an- 

 chovy. From 1 to 6 1 of algal culture, Tetraselmis 

 sp. (300,000-500,000 cells/ml), were added daily to 

 provide food for rotifers and copepods. Samples of 

 10 or more larvae were taken on alternate days for 

 length measurements. Some samples were 

 washed in distilled water, dried, and subsequently 

 weighed to obtain a relation between length and 

 dry weight. 



RESULTS 



Hatching, Onset of Feeding and Starvation 



Eggs of Pacific mackerel are transparent 

 spheres, ranging in diameter from 1.06 to 1.14 mm 

 (Kramer 1960). Incubation times ranged from 33 h 

 at 23° C to 117 h at 14° C (Figure 1); eggs did not 

 hatch below 14° C. The curve for hatching time as 

 a function of temperature for the western Pacific 



population (data from Watanabe 1970) appears to 

 be the same as the one for the eastern Pacific 

 population. 



I20|- 



110- 



100- 



o 90 



< 



X 



^ 80 



O 

 in 



o 70 



^ 60 



O 



X 



50 



40 



H = 3580e-6.50(l-e-00527T) 



30^ 



oV' ' I 



_L 



J \ I I I I I I I 



12 14 16 18 20 



TEMPERATURE °C 



22 



24 



FIGURE 1. — Incubation time (fertilization to 50% hatch) of 

 Scomber japonicus eggs. Solid line is for present data, points are 

 estimated time to 50% hatch of eggs in five test tubes per tem- 

 perature, dashed line is for data of Watanabe ( 1970). The general 

 equation was developed by Zweifel and Lasker (1976) and fit to 

 the 50% values. 



At hatching, larvae averaged 3.1 mm SL (Fig- 

 ure 2B) and weighed 0.040 mg dry weight, of 

 which 50% was yolk. At 19° C, first feeding oc- 

 curred 46 h after hatching; by 60 h after hatching, 

 all larvae had ingested one or more rotifers in 4 h 

 (Figure 2D). Thus the 50% threshold for onset of 

 feeding at 19° C occurred at about 50 h (2 d) after 

 hatching. At this time larvae were 3.6 mm SL, the 

 eyes were fully pigmented and 10% of the yolk 

 remained, principally the remnants of the oil drop- 

 let (Figure 2A, B). Over the threshold for the onset 

 of feeding, the mean number of rotifers in Pacific 

 mackerel stomachs increased from 2 at 46 h to 14 

 at 68 h (Figure 2E). The larvae in each group had 

 no previous feeding exposure, hence the increase 

 in feeding activity with time could not be attrib- 

 uted to experience. 



91 



