210 



Fishery Bulletin 92(2). 1994 



Waiwood, 1985, in part). Eggs preserved in 

 Stockard's solution were photographed with a Nikon 

 F2 camera fitted with a PB6 200-mm bellows exten- 

 sion and a 24-mm 1:2.8 reverse-mounted lens. This 

 configuration produced a 47x magnification. Re- 

 flected light was supplied by two synchronized flash 

 units. Other photographs (stages 5 and 6) were 

 taken with a single-lens reflex adapter (0.32x) on a 

 Wild M-8 dissecting microscope with transmitted 

 light. At 50x, the phototube and adapter increased 

 magnification to 66x. 



Analysis 



Endpoint, midpoint, and duration of stage (in hours) 

 were estimated for eggs incubated at each tempera- 

 ture. For stages 1-20, stage endpoint was deter- 

 mined by the presence of two stages during a sam- 

 pling time; if stages n and n + \ were present, the 

 time at which the eggs were sampled was consid- 

 ered a transition and therefore the endpoint for 

 stage n. If there was no transition, the endpoint for 

 stage n was the midpoint between the last sampling 

 time during which stage-rc eggs were present and 

 the first time stage-« + l eggs were observed. Dura- 

 tion and midpoint of stage n were determined as 



Duration Stage n = Endpoint Stage (n) - 

 Endpoint Stage in - 1); 



Midpoint Stage n = Endpoint Stage (n 

 Duration Stage n 



li 



Endpoint of stage 21 was the sampling interval 

 when the last embryo had hatched. With the mid- 

 points and time of 50% hatch, a piece-wise least- 

 squares linear regression model (SAS, 1985) was 

 derived to estimate age (hours) of eggs at a specific 

 stage incubated at any temperature within the lim- 

 its of this experiment. 



Differences in mean lengths of larvae hatched 

 from the three temperature groups were analyzed 

 by a Student-Newman-Keuls test. Lengths of larvae 

 hatching at stages 20 and 21 were analyzed by a 

 two-way analysis of variance (ANOVA) by using 

 stage and temperature. 



We chose five representative developmental stages 

 and compared time to midpoint of each stage among 

 incubation studies. Comparison with Hamai et al. 

 (1971, 1974) was possible for only three stages. We 

 grouped data on time to 509f hatch into western and 

 eastern North Pacific studies and performed a log- 

 transformed analysis of covariance to test for differ- 



ences in time to 50% hatch between these two ar- 

 eas with incubation temperature as the covariate. 



Results 



Incubation rates 



Temperatures of the three water baths increased at 

 the beginning of sampling (Fig. 1). Temperature 

 spikes that occurred after 288 hours in the 5.7°C jars 

 and after 396 hours in the 3.8°C jars, were associ- 

 ated with the appearance of large numbers of lar- 

 vae; water baths may have warmed when refrigera- 

 tors were opened frequently to measure larvae. 



Eggs developed at similar rates among incubation 

 temperatures for the first 36 hours through stage 6 

 (Fig. 2). After stage 6, at about 36 hours, when tem- 

 peratures had stabilized, development rates began 

 to diverge. Duration of stages 7-21 was variable 

 (Table 3). Usually the duration of a stage was longer 

 at cooler temperatures. However, this was not al- 

 ways the case, and stages 12 and 20 required simi- 

 lar amounts of time regardless of temperature. At 

 all temperatures, hatching began during stage 20; 

 the percentage of eggs hatched by the beginning of 

 stage 21 was 35% at 3.8°, 40% at 5.7°, and 8.1% at 

 1.1'C Four larvae from the 7.7°C group hatched 

 after 192 hours; another 18 hours elapsed before 

 other larvae hatched at this temperature. These 

 early larvae were not included in this analysis be- 

 cause we assumed that the hiatus in hatching times 

 indicated that early hatching was anomalous, i.e. 

 hatching may have been mechanically induced. Af- 

 ter hatching began, time required for 50% hatch 

 decreased as temperature increased: 48, 36, and 24 

 hours at 3.8°, 5.7 , and 7.7°C. The elapsed time be- 

 tween hatching of the first and last larvae was 72 

 hours at 3.8°C and 60 hours at both 5.7 and 7.7°C. 



Eggs developed normally at 5.7° and 7.7°C; how- 

 ever, curvature of the spine was observed in some 

 late-stage embryos incubated at 3.8C These abnor- 

 mal eggs hatched, but most larvae were not mea- 

 sured because of curvature. Mean length at hatch 

 of all larvae increased with incubation temperature: 

 4.15 (SD 0.380, n = 100), 4.29 (SD 0.272, rc=192), and 

 4.55 mm (SD 0.303, rc=84> at 3.8°, 5.7°, and 7.7'C 

 (Fig. 3). Mean lengths of larvae from the three tem- 

 perature groups were significantly different 

 (P<0.05). In addition, larval lengths increased as the 

 hatching period progressed at all temperatures. 

 Length of larvae hatching at stages 20 and 21 was 

 significantly different at all temperatures (P<0.01); 

 larvae that hatched at stage 21 were 9-13% longer 

 than larvae that hatched at stage 20. 



