Zeldis et al.: Development of Hoplostethus atlanticus eggs in the water column 



38) 



£ 300 



Q. 



en 

 en 

 LU 



523 d 



MOCN 

 523 u 537 d 



tow number 



537 u 544 



Ji 



lll.li 



[aj 



545 



1 



, 374 



111 



546 



summary 



4 



J^Li 



M 



lull 



IB-L^ 



111 



4 12 20 28 4 12 20 26 4 12 20 28 4 12 20 28 4 12 20 28 4 12 20 28 4 12 20 28 4 12 20 28 



Egg stage 



Figure 7 



Orange roughy, Hoplostethus atlanticus, egg densities-at-stage (standardized to eggs- 100 m -3 ) 

 from the MOCNESS tows conducted over the North Chatham Rise during TAN9206. Tows la- 

 belled with 'd' Cu'l indicate the descent (ascent) phase of that tow. Box size around each histo- 

 gram is an approximation of the depth range sampled by that net; the actual minimum depths of 

 each net are annotated (also maximum depths for bottom nets). The "summary" column shows 

 the egg densities-at-stage (standardized to eggs- 1,000 wr 3 ) summed over all the tows within each 

 200-m stratum. 



ferent egg stages at the various depths of the 

 MOCNESS nets (Fig. 7) depended on the distance of 

 the tows from concentrations of spawning adults. Two 

 tows were conducted near an aggregation of spawn- 

 ing adults at 177°W (tows 545 and 546; Fig. 8A) in 

 which most eggs were in young and middle ages (up 

 to stage 13) and were caught from the bottom to 

 middle depths (Fig. 7). In three tows conducted to 

 the west of the aggregation (tows 544, 523, and 537; 

 Fig. 8A), most eggs were middle-aged and older and 

 were caught from middle depths to the upper water 



column. This distribution suggested that the eggs 

 were advecting horizontally away from the spawn- 

 ers as they aged and ascended the water column. 



Second, as the eggs developed, their depth changed 

 in a manner consistent with predictions of the as- 

 cent and descent rate experiments and the thermal 

 history model (Figs. 3 and 6; summary column in 

 Fig. 7). The young eggs ascended from the spawners 

 toward the surface, middle-aged eggs approached 

 neutral buoyancy in the mixed layer, and oldest eggs 

 sank. In the to 200 m MOCNESS stratum, stage-13 



