192 



Fishery Bulletin 88(1), 1990 



Bristol Bay 



.oL^ 



Figure I 



Location of spawning grounds of the Togiak herring stock in Bristol 

 Bay. Alaska. 



Hatching began on 10 June 198fi and was completed 

 by 12 June. Survival to hatching was about 50%, with 

 25,200 larvae produced. The eggs were not treated dur- 

 ing incubation, and heavy fungal growth was noted on 

 the eggs. Newly hatched larvae were collected from in- 

 cubation boxes in white plastic cups in groups of 5-25, 

 counted, and transferred to 5-L cylinders placed in an 

 8.rC water bath. Each cylinder held 1000 larvae. 



On 13 June, 24,840 larvae were released into a large 

 outdoor basin (2000 nr' volume, 600 m- surface area, 

 maximum depth 4.0 m) filled with seawater pumped 

 from Fl0devigen Bay. Phytoplankton and zooplankton 

 almndance was high when the larvae were introduced 

 to the basin. A detailed description of the basin is pre- 

 sented in Moksness (1982). 



Larvae were retained in the laboratory to examine 

 starvation-induced mortality. Four batches of 100 lar- 

 vae were placed in 5-L cylinders supplied with filtered 

 seawater. Temperature was maintained at 8°C by 

 placing the cylinders in a water bath. A batch was ter- 

 minated every 7 days and the larvae recovered and 

 preserved with buffered 4% formalin. 



The larvae in the basin were sampled daily using a 

 two-chambered plankton net of 500-^m mesh and a 

 total sampling area of 0.3 m-. The net was drawn 

 diagonally across the basin at a depth of 2 m. The total 

 volume sampled was 7.5 nr^. Larvae and invertebrates 

 captured in the plankton nets were preserved in buf- 

 fered 4% formalin for later analysis. 



Weekly estimates of zooplankton density were ob- 

 tained from pump samples taken from depths of 0.5, 

 1, 2, and 3 m. Water was pumped from each depth for 



a short period prior to filtering a 30-second sample 

 through a 90-/.(m plankton net. Samples were preserved 

 in 4% formalin and examined later using a binocular 

 microscope and counting chamber. Temperature, salin- 

 ity, and oxygen saturation were measured at each 

 depth on the day pump samples were obtained. 



Preserved larvae were examined using a binocular 

 microscope vernier eyepiece; they were measured, ex- 

 amined for food contents, and weighed. Length was 

 measured from the snout to the tip of the notochord 

 in larvae, and to the hypural plate in postlarvae. To test 

 for shrinkage due to preservation, a sample of 21 newly 

 hatched larvae were anesthetized, measured, and then 

 preserved in 4% formalin. After 10 days the larvae 

 were measured. The live length, 7.70 mm (SD 0.42), 

 and the preserved length, 7.74 mm (SD 0.35), were not 

 different. Under the preservation conditions employed, 

 shrinkage of about 8% would be expected based on 

 results presented by Hay (1982). 



For food examinations, the entire gut was removed 

 from post-yolk sac larvae. After the stomach differen- 

 tiated, it was removed for food content analysis. After 

 gut or stomach removal, larvae were placed on Teflon 

 strips and dried at 60°C for 24 hours and then weighed 

 using an electronic balance. 



Results 



Environmental parameters 



The Fl0devigen Biological Station is located at 58°24'N 

 which is approximately the same latitude as the her- 

 ring spawning ground in Bristol Bay (59°00'N), so 

 herring in the basin experienced the same daylength 

 as they would in Bristol Bay. The temperature in the 

 basin averaged 11.75°C when the larvae were intro- 

 duced on day-0, 3 days after hatching (Fig. 2a). Initial- 

 ly the basin was at a nearly uniform temperature 

 varying from 11.9°C at the surface to 11.6°C at 3.5 

 m. After the larvae were introduced, temperatures in 

 the basin began to rise owing to a prolonged period of 

 sun and above-average air temperature. On day-12, sur- 

 face temperature reached 18.8°C and then declined to 

 range between 14 and 16°C for the remainder of the 

 experiment. Temperatures below 2 m increased but not 

 as sharply as surface waters (Fig. 2a). For the dura- 

 tion of the experiment, temperatures averaged 15.2, 

 15.1, and 15.8°C at 1, 2, and 3-i- m, respectively. Tem- 

 perature data recorded on the same dates offshore of 

 the Bristol Bay spawning grounds at a depth of 2 m 

 show that the temperatures in the basin averaged 

 4.5°C higher (15.26°C vs. 10.7°C). 



Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



