FISHERY BULLETIN: VOL. 76, NO. 2 



began to separate from each other. It appeared 

 that the midgut was extremely vulnerable to a 

 deficiency of food and usually after 1 day of starva- 

 tion, single mucosal cells could be seen sloughed 

 (9) into the lumen. The margin of the lumen con- 

 tinued to lose its integrity as starvation advanced. 

 Cells of the hindgut of feeding larvae exhibited an 

 accumulation of eosin staining inclusions. The in- 

 clusion bodies, which may be the sites of intracel- 

 lular digestion, have been observed in other 

 marine and freshwater fish (Kostomarova 1962; 

 Iwai and Tanaka 1968a, b; Iwai 1968; Umeda and 

 Ochiai 1975; O'Connell 1976). The amount, size, 

 and intensity of the staining (10) of these inclu- 

 sions varied in feeding larvae. They were not pres- 

 ent in starving larvae. 



Musculature (Figures 9, 10, 12) 



In feeding jack mackerel larvae, individual 

 muscle fibers were close together (11); they were 

 composed of closely packed, striated, and parallel 

 myofibrils. After a period of starvation, the fibers 

 separated, the fibrils were not distinct, and occa- 

 sionally they lost their parallel structure. Be- 

 tween some fibers there was a granular, 

 basophilic, nucleated substance called "intermus- 

 cular tissue" (12) by O'Connell (1976). In starved 

 larvae, this tissue was usually absent. 



General Histological Characteristics 



After jack mackerel larvae had starved for 3 

 days, signs of depletion were widespread. In addi- 

 tion to changes in major tissues and organs there 

 was a general atrophy and disintegration of all 

 cells and tissues including those of cartilage, kid- 

 ney, endocrine pancreas, and swim bladder. The 

 number of pyknotic nuclei (i.e., darkening and 

 shrinking nuclei, which give the first indication 

 that a cell is dying) increased in all tissues (see eye 

 and brain, Figure 4). Epithelial cells of the in- 

 tegument were hypertrophic, twice as large as 

 normal in 3-day starved larvae (Figure 10), and 

 kidney tubules were swollen (Figure 6). There was 

 always a larger yolk reserve retained by starving 

 larvae (Figure 6). A decrease in yolk absorption in 

 starving larvae was also reported by Kostomarova 

 (1962) for pike and carp and by Umeda and Ochiai 

 (1975) for yellowtail. 



Histological Grading 



To determine whether the classification of a jack 

 mackerel larva required the grading of all his- 

 tological features or a lesser number, a group of 27 



408 



larvae, 14 feeding and 13 starving, was examined 

 and the resulting grades for each criterion were 

 submitted to a SWDA. The experimental treat- 

 ment (fed or starved) was unknown until after all 

 larvae were microscopically examined. The larvae 

 were 7 days old and had been feeding or starving 

 for 2 days. The grading system classified all fed 

 larvae (n = 14) into the healthy group (individual 

 average grade of the 12 histological features 

 ranged between 2.42 and 2.92). The average 

 grades for the 2-day starved larvae were more 

 variable. The larvae were classified, about 

 equally, into each of the three nutritional groups: 

 four had a grade range between 2.35 and 2.54, 

 ranking in the healthy group; four were classified 

 as intermediate, grade range 2.08 to 2.31; and five 

 larvae were ranked as starved with the average 

 grades ranging between 1.15 and 1.54. 



Results of the SWDA on the above data disclosed 

 that grading only two histological characteristics, 

 the arrangement of the cells in the pancreas (vari- 

 able 7 ) and the sloughing of mucosal cells from the 

 midgut (variable 9), gave the same conclusions as 

 using all 12 features. Therefore, in all subsequent 

 histological assessments, the average grade of 

 these two criteria, variables 7 and 9, was used as 

 the index of larval condition. 



Morphological 



The jack mackerel larvae were 2.45 mm SL (pre- 

 served) at hatching and initiated feeding at 3.35 

 mm, 5 or 6 days after hatching (hatching = day 0, 

 Figure 13A). Atthetimeof first feeding, some yolk 

 and oil were present but the yolk sac was not 

 discernible. The relationship between the five 

 morphological characteristics, measured to de- 

 termine the effects of starvation, and days of star- 

 vation is illustrated in Figure 13B-F. Since no 

 data have been published on the daily growth rate 

 of field-caught jack mackerel larvae, I used length 

 as an estimate of age. When the morphometric 

 measurements were plotted against length, no 

 single measurement was a reliable index of star- 

 vation, as illustrated by pectoral body depth plot- 

 ted for fed and starved larvae (Figure 14). How- 

 ever, some limits can be set from this graph: 1) all 

 larvae <3.30 mm SL that do not have a yolk sac 

 probably are starving (feeding is initiated at 3.35 

 mm); and 2) larvae with a body depth >0.47 mm 

 are feeding. This leaves the size class between 3.30 

 and 3.55 mm where the cases cannot be separated. 

 Most individuals in this class (29 fed and 24 



