FISHERY BULLETIN: VOL. 84, NO. 1 



grading system usually classified these fish into the 

 recovering category. 



General Histological Observations 



In jack mackerel that were considered healthy, 

 swim bladder inflation was first noted at 3.4 mm. 

 Swim bladders were inflated in larvae taken at night 

 whereas they were deflated in those taken in the day. 

 The swim bladders of 72% of the fish were deflated 

 by 0700 (n = 81) except for fish scored in the 

 starving category where inflation was common at 

 any time of day, which was possibly a symptom of 

 starvation or an additional energy-sparing func- 

 tion of the swim bladder (Hunter and Sanchez 

 1976). 



Theilacker (1978) pointed out that the gallbladder 

 was always enlarged in jack mackerel that were 

 deprived of food in the laboratory, and this condi- 

 tion occurred in sea samples of starved larvae taken 

 in the day. On the other hand, gallbladder enlarge- 

 ment was also found in the healthy fish as well as 

 starved fish collected at night. According to Love 

 (1970), the gallbladder discharges its contents when 

 stimulated by food. Jack mackerel do not eat at 

 night, so the gallbladder of healthy fish may remain 

 distended during the night. Thus enlargement of the 

 gallbladder was not used to diagnose starvation. 

 Theilacker's (1978) samples of fed and unfed fish 

 were taken only during the day, when feeding oc- 

 curs. 



Mitotic figures in the brain of jack mackerel oc- 

 curred in fish collected at all times of day and night. 



On the other hand, mitosis of mucosal cells in the 

 midgut was restricted to the night. It seems that 

 mucosal cells of northern anchovy also divide late 

 at night, when the digestive tracts are empty (O'Con- 

 nell 1981). 



- Evidence for Starvation in the Sea 



s 



Results of the histological analysis showed that 

 starvation was a major source of mortality for the 

 smallest jack mackerel larvae (<3.5 mm) as 59% ap- 

 peared to be dying of starvation, 23% were eating 

 but had fasted previously, and only 19% were class- 

 ed as healthy. The incidence of starving larvae 

 decreased to 16% in the 3.5-4.0 mm size class and 

 was 3% in the older larvae (Table 4). The numbers 

 of fish used for the histological assessment of star- 

 vation was adequate for the smallest (<3.5 mm SL) 

 larval size class (coefficient of variation ranged be- 

 tween 0.09 and 0.15 for the four condition 

 categories), but larger samples would be needed to 

 give a reliable estimate of the fraction starving for 

 the older larvae (>3.5 mm SL) because of the low 

 incidence of starvation. 



Despite the fact that jack mackerel abundance 

 decreased from west to east and north to south (Fig. 

 4), I found no consistent differences in the incidence 

 of starvation between fish taken from areas of high 

 larval density and those taken from areas of low lar- 

 val density (Fig. 15). Therefore, to estimate mortality 

 due to starvation, I combined all samples collected 

 in the offshore area. To estimate mortality rates on 

 a daily basis, the observed number of fish belong- 



Table 4. — Histological condition of jack mackerel collected 350 km off the coast 



of California. 



<fi 



<r 



^ 



& 1 





£? 4? ^ 



^ 





'Number dying/d + starving/d 



Total 

 2 Number dying/d 

 Total 



Daily percent 

 Starving 1 Dying 2 



10 



