474 



Fishery Bulletin 88 13). 1990 



Laboratory experiments 



Laboratory investigations were conducted using cap- 

 tive walleye pollock in March and April 1987. Adult 

 prespawning walleye pollock were caught in January 

 and February by handline in the Tacoma Narrows 

 region of Puget Sound. Fish were transported to the 

 laboratory and held in net pens under ambient light and 

 temperature conditions and fed chopped herring ad 

 libidum until they were nearly in spawning condition 

 (i.e., eggs and milt were expelled with slight pressure 

 to the abdomen). These fish were then transferred 

 to individual 1700- or 2500-L tanks. One female and 

 one or two males were placed in each tank and held 

 throughout their spawning cycle. Water temperatures 

 in the tanks were ambient and varied over the spawn- 

 ing season from 9.5 to 11.5°C (x 10.08°C). Ambient 

 water temperature increased significantly (i?^ = 20.9%) 

 over the experimental period. Salinity was constant at 

 28 ppt. A 20-cm plankton net with a hard codend was 

 suspended in each tank to catch a sample of eggs from 

 each spawning event. Plankton nets were checked each 

 morning for the presence of eggs. If eggs were pres- 

 ent in the codend, they were saved, and the water in 

 the tanks replaced. 



In 1988, pollock caught in Puget Sound were held in- 

 dividually in tanks to further examine the effect of fe- 

 male length on egg size. Eggs from the first batch from 

 each of seven females were collected and measured in 

 the same manner as described for the 1987 studies. 

 Egg diameters were measured on 15-25 unpreserved 

 eggs, and egg dry weights were determined for 5-10 

 eggs from each spawning event. The remainder of the 

 eggs were incubated separately O^y female and spawn- 

 ing event) in 4-L jars filled with filtered seawater. The 

 jars were placed in a water bath with running seawater 

 at ambient water temperatures. Light levels were am- 

 bient. Water in these jars was changed several times 

 during incubation (which lasted 6-9 days) and dead 

 eggs were removed daily. 



Six to fifteen larvae were collected on the day of 

 hatch, anaesthetized with MS222, and their standard 

 lengths measured. The rest of the larvae were held in 

 the 4-L jars until yolksac absorption (defined as the 

 point when individual larvae had used up 90-100% of 

 their yolksacs). Standard lengths were measured (on 

 anaesthetized larvae) and dry weights determined for 

 5-10 larvae at this stage. 



A sample of live eggs collected from the ichthyo- 

 plankton in Shelikof Strait was transported to Seattle 

 in April of 1987. Diameters and dry weights were 

 determined on a subsample of these eggs and the rest 

 were incubated. Larvae were measured at hatching and 

 at yolksac absorption in the same manner as described 

 above. 



Table 3 



Differences in mean walleye pollock egg diameter among years 

 in April in the Shelikof Strait region of the Gulf of Alaska. 



Egg diameter 



Year* 



Mean 



SE 



SNK 

 N grouping* 



1981 

 1986 

 1985 

 1984 

 1982 



1.360 



1.317 

 1.299 

 1.298 

 1.296 



A 

 B 



C 



c 

 c 



'Data not available from 1983. 



**Student-Newman-Keuls multiple range test(SAS Institute 

 1985). A, B, C indicate significantly different groups. 



Eggs collected from the Gulf of Alaska ichthyoplank- 

 ton surveys for examination of yearly, seasonal, and 

 regional differences in egg size were preserved in 3-5% 

 formalin. No correction factor was used to correct for 

 changes in egg size due to preservation, as all eggs used 

 in these analyses were preserved in the same manner. 

 The field samples taken from spawning adults and used 

 to examine the relationship between egg size and fish 

 size, age, and condition were also preserved in 3-5% 

 formalin. These, however, were collected before spawn- 

 ing, and the diameters may not be directly comparable 

 to sizes of spawned eggs collected from the ichthyo- 

 plankton, due to possible changes in egg size at fertiliza- 

 tion and activation (Fleming and Ng 1987, Kj0rsvik 

 and Lonning 1983). Eggs from the laboratory were 

 measured fresh and are therefore not directly com- 

 parable with eggs from the field studies. The com- 

 parison between sizes of Gulf of Alaska eggs and Puget 

 Sound eggs was made only on the fresh samples col- 

 lected in 1987 from both areas. 



Results 



Geographical, seasonal, 



and regional differences in egg size 



Examination of 1319 walleye pollock eggs from April 

 plankton samples (the month when most spawning 

 occurs; Kim 1987) collected in Shelikof Strait demon- 

 strated a significant difference in mean egg diameters 

 among years (F = 19.862, p<0.0005). Mean egg diam- 

 eter was largest (Table 3) in 1981 and smallest in 1982; 

 there was a 4.7% difference in size between these 

 years. Mean egg size was not statistically different in 

 1982, 1984, and 1985 (SNK test. Table 3). Mean egg 

 size was intermediate in 1986. 



