722 



The development of the digestive tract 

 and eye in larval walleye pollock, 

 Theragra chalcogramma* 



Steven M. Porter 



Gail H. Theilacker 



Alaska Fisheries Science Center 



National Marine Fisheries Service, NOAA 



7600 Sand Point Way NE, Seattle, Washington 981 15 



E-mail address (for S M Porter): sportertSafscnoaaaov 



First feeding ( FF) is a time at which 

 a fish larva must initiate feeding 

 or face starvation that will weaken 

 it and eventually lead to its death. 

 In most oviparous fish with pelagic 

 eggs, organ systems develop that 

 allow larvae to switch from an in- 

 ternal nutrient source (yolk) to an 

 external source (prey) during the 

 time between hatching and FF. Star- 

 vation may be a major cause of the 

 high mortalities that occur during 

 the larval period and thus may af- 

 fect recnaitment ( Hjort, 1914; O'Con- 

 nell, 1980; Theilacker, 1986). Indeed, 

 some studies suggest that reci-uit- 

 ment is determined in this period 

 (Houde, 1987;Freebergetal., 1990). 

 Laboratory experiments show that 

 at FF, the gi'owth rate of walleye pol- 

 lock, Theragra chalcogramma, lar- 

 vae decreases (Yamashita and Bailey, 

 1989), or even ceases for a period 

 (Theilacker and Shen, 1993). Fur- 

 thermore, delaying the introduc- 

 tion of food at FF causes a reduc- 

 tion of the gi'owth rate and size-at- 

 age (Theilacker and Shen, 19931. 

 These studies indicate that FF 

 walleye pollock lack energy re- 

 serves, and thus the availability of 

 nutritious prey at this time is criti- 

 cal. Theilacker and Porter (1995) 

 found that the largest proportion of 

 starving walleye pollock larvae in 

 Shelikof Strait, Gulf of Alaska, are 

 in the size class that includes FF 

 larvae. Thus, the FF period appears 

 to be the time when walleye pollock 

 larvae are most vulnerable to star- 



vation. Studies involving larvae of 

 other marine fish have found simi- 

 lar results (Atlantic mackerel. 

 Scomber ,sco;?;6/7/.s. Ware and Lam- 

 bert, 1985; jack mackerel, Tixichurus 

 sytiimetricus, Theilacker, 1986). 



Larval walleye pollock survival is 

 dependent upon timely develop- 

 ment of organs required for feed- 

 ing. Of the many developmental 

 changes occurring during the early 

 larval period, two of the most im- 

 portant for walleye pollock may be 

 vision and the digestive tract. Eye- 

 sight is probably the most impor- 

 tant sense walleye pollock larvae 

 use in finding prey because they are 

 visual predators (Paul, 1983) and do 

 not use chemosensory cues (Davis 

 and 011a, 1995). Digestive tract de- 

 velopment is important for efficient 

 assimilation of nutrients needed for 

 growth. In this study, histological 

 sections were used to describe the 

 development of the mouth, diges- 

 tive tract, and eyes in laboratory- 

 reared walleye pollock, from hatch- 

 ing to 31 days after hatching 

 (DAH), to examine how organs nec- 

 essary for feeding develop. This is 

 essential to an understanding of 

 why walleye pollock larvae are 

 most vulnerable to starvation dur- 

 ing the first week of feeding. 



Materials and methods 



In 1991 and 1992, adult walleye 

 pollock were collected from Shelikof 



Strait, Gulf of Alaska, and spawned 

 aboard ship. Fertilized eggs were 

 maintained aboard ship at 3°C for 

 a few days, then transported to the 

 laboratory. In 1991, walleye pollock 

 larvae were reared at Friday Har- 

 bor Laboratories, University of 

 Washington, San Juan Island, 

 Washington, and in 1992, at the 

 Alaska Fisheries Science Center, 

 Seattle, Washington. Between 500 

 and 1000 larvae were reared in a 

 black, circular, 120-L fiberglass 

 tank (62 cm diameter, 43 cm deep) 

 filled with 90 L filtered seawater 

 (salinity=28.0 ±0.5 ppt) and main- 

 tained at 6° ±0.5"C, the typical sea- 

 water temperature in Shelikof 

 Strait when walleye pollock larvae 

 initiate feeding (Kendall et al., 

 1987). Larval rearing procedures 

 followed Porter and Theilacker 

 1996.1 



Fluorescent fixtures were used 

 with a 16-hour light cycle; the 

 amount of light at the surface of the 

 water in the rearing tank was 17 |i 

 mol photon/m-/s. Pi-ey consisted of 

 the rotifer Brachionus plicatilis, 

 raised on an algal diet oflsochrysis 

 galbana and Pavlova lutheri high 

 in unsaturated fatty acids (Nichols 

 et al., 1989), as well as of Acartia 

 sp. copepod nauplii and copepodites 

 collected from a local lagoon. At 3 

 DAH, four to five days before FF, 

 rotifers were added at 10/mL and 

 Acartia were added at 3/mL to the 

 rearing tank and maintained at 

 this level throughout the rearing 



■' Contribution 0285-RAO-O to Fisheries 

 Oceanogi'aphy Coordinated Investigations 

 (FOCI I, NOAA, 7600 Sand Point Way NE, 

 Seattle, WA 98115. 



' Porter, S. M., and G. H. Theilacker 1996. 

 Larval walleye pollock, Theragra chalco- 

 gramma. rearing techniques used at the 

 Alaska Fisheries Science Center. Seattle 

 Washington. AFSC processed report 96- 

 06, 26 p. U.S. Dep, Commerce, NOAA, 

 NMFS, Alaska Fisheries Science Center, 

 7600 Sand Point Way NE, Seattle, WA., 

 98115. 



Manuscript accepted 17 July 1998. 

 Fi.sh. Bull. 97:722-729 (1999). 



