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Fishery Bulletin 93(2), 1995 



salinity (35.0 ppt). These conditions emulated sea- 

 water densities that are experienced by eggs at these 

 stages in nature, because the embryos are approach- 

 ing neutral bouyancy and reside primarily within the 

 mixed layer (see Results section). 



The descent rate of hatching-stage eggs (stage 30) 

 was measured at 9.0°C. These eggs were very nega- 

 tively buoyant and were probably sinking rapidly 

 through the water column. Because surface salinity 

 water was used in the experimental cylinder, the 

 measured descent rate (366 m-day -1 ) is probably an 

 underestimate for waters below the mixed layer be- 

 cause of decreasing salinity. The measured descent 

 rate of these eggs at 9.0°C and surface salinity were 

 used to predict their density: 1.02707 gem -3 . This 

 was then used to predict their descent rates at 9.0°C 

 and 34.65 ppt (the salinity at 9°C in the water col- 

 umn) which was 381 m-day -1 . 



A thermal history model 



As orange roughy eggs ascend the oceanic water col- 

 umn, it can be assumed that they will develop at the 

 rate governed by their immediate environmental 

 temperature (Pauly and Pullin, 1988). However, their 

 actual stage at any age is determined by an accumu- 

 lating average of their previous and present tempera- 

 tures. A model which enables this "thermal history" 

 of an orange roughy egg to be considered, when the 

 egg's observed stage is used to estimate its age, was 

 created in the following way. 



First, it was observed that the temperature-at- 

 depth data for the five MOCNESS profiles (Fig. 4) 

 below the mixed layer had three segments for which 

 the depth change per 1°C change was nearly con- 

 stant. These were 870 m to 450 m, 450 to 315 m, and 

 315 to 250 m (the bottom of the mixed layer). These 

 depth changes were divided by the predicted egg as- 

 cent rates at each of these 1°C depth intervals (Table 

 1) to determine the amount of time eggs would spend 

 in each of the 1°C temperature strata. Next, a fam- 

 ily of age-at-stage curves was graphed by using Equa- 

 tion 1 from 6.0°C (the near-bottom temperature) to 

 11.0°C, each curve representing development over an 

 interval of ±0.5°C. Eggs were then "developed" 

 graphically, by moving along the 6.0°C curve until 

 the change in age equalled the amount of time eggs 

 spent in that stratum (in this case 9.4 h, since this 

 stratum was truncated by the bottom at 5.75°C). The 

 point reached was age = 9.4 h, stage = 2.8 (Fig. 5). 

 Then, these stage 2.8 eggs were developed further 

 by "dropping down" to the 7°C curve and moving 

 along it until the change in age was 11.4 h (the length 

 of time required to ascend through the 6.5-7.5°C stra- 

 tum). The point reached was cumulative age = 20.8 

 h, stage = 6.7, which was the stage the eggs had 

 reached after accounting for development at 7°C and 

 the previous development at 6°C. This procedure was 

 repeated until eggs were aged through to 11.25°C (at 

 250 m; the bottom of the mixed layer) at cumulative 

 age = 46.0 h. The eggs at this age were in stage 13. 

 Subsequent egg development to stage 26 occurred in 



