336 



Fishery Bulletin 93(2), 1995 



066 



066 

 ,C=S '*} BUOY 



'063 



Q 109 



059 * K 



4 



154°* 



Figure 2 



Stations in the Shelikof Strait, Alaska, where walleye pol- 

 lock, Theragra chalcogramma, were collected for histologi- 

 cal analysis in April and May 1991. 



The standard length of all field larvae was mea- 

 sured before processing for histology. We processed 

 all larvae from each field sample that were preserved 

 within 12 minutes. Larvae were processed in the 

 same manner as the laboratory-raised pollock, and 

 midgut cell height was measured in the same area 

 to determine their past feeding history. 



The vertical depth inhabited by most walleye pol- 

 lock larvae less than 10 mm is between 25 and 37 m 

 (Kendall et al., 1994). Thus we estimated that the 

 elapsed handling time from capture to placement into 

 fixative for the average larva was from 6 to 9 min (3 

 to 4 min in the net retrieved at 10 m/min plus 3 to 5 

 min to sort and preserve), or an average of 7.5 min- 

 utes. We used 7.5 min as the average handling time 

 to calculate field-collected size. 



Results 



Laboratory rearing 1 99 1 



Walleye pollock hatched at a mean SL of 3.5 mm 

 (laboratory-preserved size), grew at an average rate 

 of 0.12 mm/d, and at 17 d after hatching averaged 

 5.4 mm SL (Table 1). Larvae started feeding on av- 

 erage ( 3e +SD) 8 d after hatching (range 7-9 d) at 



5.01 ±0.15 mm SL (range 4.56-5.20 mm SL, re=15), 

 and the yolk was completely absorbed about 5 d later. 

 The growth rate from first-feeding on day 8 to 13 d 

 after hatching averaged 0.078 mm/day. 



Starved larvae decreased in size as food was with- 

 held for 11 d after first feeding (Table 1). The length 

 of starved larvae began to decrease slowly on day 9 

 after food was withheld for only 1 day. After 8 d of 

 starvation (16 d after hatching), larvae rapidly de- 

 creased in size. 



Calibration of midgut cell height 



The height of the midgut cells of larvae sampled from 

 the starved and fed tanks and preserved in Bouin's 

 solution ranged from 5 to 34 |im (Figs. 3 and 4). Cells 

 were largest in prefeeding yolk-sac larvae before mid- 

 gut differentiation was complete and the lumen fully 

 formed. Starving the larvae caused the midgut cells to 

 decrease slowly in height from about 13 urn at first 

 feeding to about 9 urn after starving for 4 days; the 

 average height remained at about 8 urn as food was 

 withheld for an additional 5 days (Fig. 3). We arbitrarily 

 set 11 um to delimit the fed and starved groups be- 

 cause it gave the best division; this cutoff separated 

 87.2% of the fed larvae and 80.4% of the starved larvae. 



The height of the mucosal cells of the anterior dor- 

 sal portion of the midgut increased slightly as fed 



