220 



Fishery Bulletin 92(2). 1994 



for eastern and western stocks. However, their com- 

 parison was made with midpoints of stages calcu- 

 lated from a regression model instead of observed 

 midpoints. Also, Yusa (1954) reported a temperature 

 range of 6— 7"C instead of a mean; our interpreta- 

 tion of Haynes and Ignell's (1983) classification and 

 calculation of Yusa's (1954) data suggests incubation 

 temperatures were always above 6.5°C (see their 

 Table 6 and our Table 4). Finally, Haynes and Ignell 

 (1983) monitored midpoints of stages more closely 

 than midpoint of hatch and did not specifically re- 

 fer to 50% hatch. 6 We assumed the values reported 

 as observed midpoints of hatch (their Table 7) were 

 close to 50% hatch. Yusa's (1954) study could not be 

 compared with ours with regard to time to 50% hatch. 



Time of hatch is often a result of how eggs are 

 treated during incubation and may vary with differ- 

 ent batches. 7 However, walleye pollock eggs from 

 Japanese waters are larger than those from the Gulf 

 of Alaska (mean=1.4— 1.6 mm and 1.3—1.4 mm, re- 

 spectively; Bailey and Stehr, 1986). At similar tem- 

 peratures, larger eggs take longer to develop (Pepin, 

 1991). The difference in incubation time emphasizes 

 the need to collect data from fish specific to the area 

 of interest. This will reduce the 

 sources of variation in develop- 

 ment time for laboratory- 

 reared eggs; failure to identify 

 and improve these sources 

 would compromise the useful- 

 ness of models predicting 

 egg age based on water tem- 

 perature. 



Development is a continuous 

 process. The sampling intervals 

 and arbitrary designation of 

 stage endpoints break develop- 

 ment into subjective units. Us- 

 ing the 21-stage scheme, we did 

 not see a clear decrease in each 

 stage duration with an increase 

 in temperature. However, this 

 will not affect the usefulness of 

 our results. When stages are 

 grouped to encompass a greater 

 degree of morphological devel- 

 opment, as in Haynes and 



Ignell (1983) and Picquelle and Megrey (1993), de- 

 velopment time is inversely related to temperature. 

 A greater number of stages within a group increases 

 the accuracy of prediction of egg age. A large num- 

 ber of stages also allows others greater flexibility in 

 grouping those stages. 



Our regression model predicts temperature-spe- 

 cific development time for purposes of computing 

 rates of egg production and egg mortality. There is 

 no biological basis upon which the regression is 

 predicated because stages that are assigned to the 

 eggs are arbitrary; stages are ordinal data that are 

 based on morphological criteria without consider- 

 ation for development time. An alternative method 

 to estimate development time from temperature is 

 to fit a separate regression for each stage. The dis- 

 advantage of this alternative method is that many 

 parameters are fitted with few data points. 



Two studies describing morphological develop- 

 ment, Gorbunova (1954) and Yusa (1954), have been 

 published. Gorbunova ( 1954) was not comparable to 

 our study. We compared our descriptions of morpho- 

 logical development with Yusa (1954). We assigned 

 stages to descriptions of hourly morphological devel- 



6 Haynes, E., National Marine Fisher- 

 ies Service, Auke Bay Laboratory, 

 11305 Glacier Highway, Juneau, AK 

 99801-8626. Pers. commun. April 

 1991. 



7 Paul, A. J., University of Alaska 

 Fairbanks, Institute of Marine Sci- 

 ence, Box 730, Seward, AK 99664. 

 Pers. commun. 17 March 1992. 



O 

 -*- 

 CO 



o 



CO 

 Q 



30 



25 



20 



15 



10 



-&- Matarese 1 983 (see caption) 

 —8- Paul 1 984 (see caption) 

 — This study 

 -B- Haynes and Ignell 1983 



— Hamaietal. 1974 

 -A- Hamai et al. 1971 

 O Nakatani and Maeda 1 984 



Western North Pacific 

 ncubation Studies 



Eastern North Pacific 

 Incubation Studies 



4 6 8 



Incubation temperature (°C) 



10 



12 



Figure 10 



Days to 50% hatch for Theragra chalcogramma eggs at various temperatures 

 of incubation. (A. C. Matarese, unpubl. data, Alaska Fisheries Science Cen- 

 ter, National Marine Fisheries Service, 7600 Sand Point Way N.E., Seattle, 

 WA 98115. A. J. Paul, unpubl. data, University of Alaska Fairbanks, Insti- 

 tute of Marine Science, Seward Marine Center Lab, P.O. Box 730, Seward, 

 AK 99664.) 



