Theilacker and Porter: Condition of Theragra chalcogramma in the Gulf of Alaska 



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MICROVILLI 



Figure 1 



The location where the midgut cell height measurement was taken in larval walleye pollock, Theragra chalcogramma. The 

 left photomicrograph shows a sagittal section of the midgut and surrounding organs (larva 8 d after hatch, 4.88 mm SL in 

 Bouin's fixative); the right photomicrograph shows the area where midgut cell height was measured (larva 12 d after hatch, 

 5.68 mm SL in Bouin's fixative). 



pling net (Theilacker, 1980). The larva was remeasured 

 after 5 minutes. The final measurement was taken at 

 10, 15, or 20 min, and then the larva was placed into 

 Bouin's, Z-Fix, or 5% formalin. For additional shrink- 

 age due to preservation, larvae were remeasured from 

 1 month to 1 year after they were preserved. 



Change in midgut cell height due to fixative 

 and net treatment 



Two fixatives used to preserve field-collected larvae 

 were compared to determine their effect on the height 

 of the midgut cells. The effect of Bouin's solution and 

 Z-Fix was compared by sampling 10 fish daily from 

 the fed tank and preserving 5 in Bouin's solution and 

 5 in Z-Fix. To determine whether the height of the 

 midgut cells changed as larval length decreased dur- 

 ing the net shrinkage experiment, one group of 20 

 larvae was measured and placed directly into Z-Fix 

 while a second group of 20 was measured, net- 

 treated, and then preserved in Z-Fix. 



Data analysis 



Data were analyzed by using SAS (SAS, Inc. 1988), 

 SYSTAT (Wilkinson, 1988), and Minitab (Ryan et al., 

 1985) computer software. Simple linear regression 

 analysis was used to derive equations to adjust lar- 

 val size for shrinkage. ANOVA was used to compare 



the midgut cell height of starved and fed larvae of 

 the same size. Midgut cell height of starved larvae 

 reared in 1991 and 1993 was compared with midgut 

 cell height of fed larvae reared in 1991 and 1992 (in 

 1992, no starved larvae were sampled for midgut 

 measurement, and in 1993, midgut was not measured 

 for fed larvae). Years were used as independent treat- 

 ments to avoid pseudoreplication (Hurlbert, 1984). 



Field collections 



Larval walleye pollock were collected for histology 

 from stations located throughout the Shelikof Strait, 

 Gulf of Alaska, during two cruises in the spring of 

 1991 (Fig. 2). The area sampled covered the entire 

 spawning area in the Strait (Kendall and Picquelle, 

 1990). Collections were made with a 60-cm bongo net 

 equipped with a 333-|am mesh and solid cod end, 

 which were retrieved vertically from 70 m in about 7 

 minutes. Because larval fish tissues deteriorate 

 quickly owing to autolysis (Theilacker, 1978), the net 

 was not washed following a tow, and larvae were quickly 

 sorted on sea water ice and preserved in Bouin's fixa- 

 tive. Earlier laboratory studies conducted with larval 

 walleye pollock indicated they must be preserved within 

 12 min at 6°C in order to retain cellular integrity. 2 



2 Theilacker, G. H., S. M. Porter. U.S. Dep. Commer., NOAA, Natl. 

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

 Point Way NE, Seattle, WA 98115. Unpubl. data. 



