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Fishery Bulletin 93(2), 1995 



net-treated them for 7.5 min to simulate field collec- 

 tion, preserved them in Z-Fix, and remeasured them 

 several months later. The mean net-treated and pre- 

 served SL (4.58 mm ±0.25) was estimated to be 5.39 

 mm live size from Equations 3 and 7b in Table 3. 

 The observed mean live standard length of the same 

 20 larvae was 5.46 mm (± 0.21), a difference of 1%. 



Field distribution of larvae 



Yolk-sac and first-feeding walleye pollock larvae were 

 collected in the eastern Shelikof Strait in April 1991 

 (Fig. 2; stations 136-141). The abundance of first- 

 feeding larvae increased from April to May. In May, 

 larvae ranging in size from first-feeding to 7 mm were 

 fairly evenly distributed along the coast, within the 

 Strait, and out the sea valley. 3 Some yolk-sac larvae 

 were also found near the Alaska Peninsula and at 

 the exit of Shelikof Strait. 



Condition of field-captured walleye pollock 



Larvae were sampled from six stations in late April 

 and from 15 stations in early May (Table 4). The level 

 of starvation ranged from 10 to 17% at 3 of the 6 

 stations in April and at 7 of the 15 stations in May. 

 However, in May, at 3 stations along the sea valley 

 (018, 059, and 068), the percent starvation ranged 

 from 30 to 40% (Fig. 2; Table 4). 



Thirty-two percent (/z=17/54) of larvae <5.00 mm 

 SL were classified as starving in the combined April 

 and May samples (Table 5). This category included 

 larvae that were smaller than average at first feed- 

 ing and those that had shrunk from starvation. The 

 percent starving in the first-feeding size category (5.0 

 to <5.5 mm SL) was 27% (n=12/45), and the number 

 decreased to 12% («=5/41 ) for 5.5 to <6.0 mm SL group 

 and finally to zero for larvae >6.0 mm (Table 5). 



Discussion 



The histological analysis showed that the height of 

 the midgut mucosal cells of laboratory-reared wall- 

 eye pollock corresponded to their feeding condition 

 and that significant changes in cell height could be 

 detected after food was withheld for one day. Thus, a 

 simple measurement of midgut cell height discerned 

 larval nutritional condition. To confirm the useful- 



3 Bailey, K. M., M. F. Canino, J. M. Napp, S. M. Spring, and A. L. 

 Brown. Two contrasting years of prey levels, feeding condition 

 and mortality of larval walleye pollock (Theragra chalco- 

 gramma) in the western Gulf of Alaska. U.S. Dep. Commer., 

 Natl. Mar. Fish. Serv., Alaska Fisheries Science Center, 7600 

 Sand Point Way NE, Seattle, WA 98115. Manuscr. in review. 



ness of the measurement for assessing the nutritional 

 condition of field-collected larvae, it was necessary 

 to determine the effects of field collection procedures 

 on the midgut measurement. Although one might 

 expect some compression of the fish body and per- 

 haps a change in gut morphology as a larva shrinks, 

 our experiments with walleye pollock showed no 

 change in size of the midgut cells during the net 

 shrinkage experiments. Thus, we felt confident in 

 applying the laboratory calibration to assess the con- 

 dition of sea-caught larvae. Theilacker and Watanabe 

 (1989) also demonstrated that there was no change 

 in the midgut cell height for northern anchovy, 

 Engraulis mordax, held for periods up to 25 min in 

 the net. Additionally, although preservative type af- 

 fected the final size of pollock larvae, the midgut cell 

 height did not differ between two preservative types 

 tested, Bouin's and Z-Fix. Therefore field-collected 

 larval pollock that are to be analyzed for condition 

 may be preserved in either Bouin's or Z-Fix. 



Since Farris ( 1963) first noted that larval sardine, 

 Sardinops sagax, shrink when preserved in forma- 

 lin, and Ryland (1966) observed that larval plaice, 

 Pleuronectes platessa, collected in the field were 

 smaller at comparable stages of development than 

 were their laboratory-raised counterparts, informa- 

 tion has accumulated on shrinkage of larval fish 

 placed directly into preservatives and during the 

 process of field collection and preservation (Blaxter, 

 1971; Theilacker, 1980, 1986; Hay, 1981; Tucker and 

 Chester, 1984; Fowler and Smith, 1983; McGurk, 

 1985; Radke, 1989). The cause of larvae shrinkage is 

 related to the loss of osmoregulatory ability when 

 they die (Parker, 1963). Fish larvae also shrink while 

 they are alive, and this shrinkage may be due to dam- 

 age by the net to the integument, the main osmo- 

 regulatory system in larvae before the gills develop 

 (Holliday and Blaxter, 1960). The amount of shrink- 

 age is directly related to the interval that larvae re- 

 main in a collecting net. Shrinkage differs among 

 fixatives within species because of the ionic strengths 

 of the preserving fluids (Parker, 1963; Hay, 1982; 

 Tucker and Chester, 1984). Why shrinkage is spe- 

 cies specific may be related to differences in osmo- 

 larity (Theilacker, 1980; Jennings, 1991) or to thick- 

 ness of the integument and mucous layer. 



The shrinkage values found for walleye pollock 

 larvae placed directly into preservative are similar 

 to those in previous studies (reviewed by Jennings, 

 1991). Shrinkage of walleye pollock in 5% formalin 

 was greater (as a percentage of length) for smaller 

 larvae (9% for 5 mm SL) than for larger larvae (7% 

 for 7 mm SL). These values are within the range of 

 shrinkage values (3-15%) for other fish larvae sum- 

 marized by Hjorleifsson and Klein-MacPhee (1992). 



