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



383 



178 W 



176 



176 E 



178 



180 



178 



176 W 



174 



Figure 8 



(A) Start positions (crosses) of MOCNESS tows on North Chatham Rise for or- 

 ange roughy, Hoplostethus atlanticus, eggs, and start positions and relative adult 

 catch rates (circle areas) from trawls conducted coincidentally with this study on 

 TAN9206. MOCNESS tow numbers from west to east are 544, 523, 537, 546, and 

 545. (B) Relative catch rates (circle areas) of 0+ and 1+ (<6 cm tail length) juve- 

 nile orange roughy during juvenile surveys done in 1988 and 1989 (Mace et al., 

 1990). Crosses show that tow positions and crosses with no circle had zero catch 

 rate. The inset is the area of the survey shown in (A) and the dashed vertical line 

 indicates the longitude of the spawning aggregation in (A). 



(Pankhurst, 1988) were seen on echo sounders dur- 

 ing these surveys. 



Discussion 



In culturing experiments, orange roughy eggs 

 hatched after 235 h of development at 10°C. This was 

 close to the prediction of Pauly and Pullin ( 1988) that 

 the development period for orange roughy eggs at 

 egg diameter 2.32 mm and 10°C would be 217 h. This 

 result is in the upper part of the scatter in Pauly 

 and Pullin's plot of development period vs. tempera- 

 ture (their Fig. 1), which would be expected, because 

 orange roughy eggs are relatively large and will take 

 longer to develop than will smaller eggs when both 

 are measured at the same temperature ( Pepin, 1991 ). 

 The eggs changed from positive to neutral to nega- 

 tive buoyancy as they aged (Fig. 3). This was un- 



likely to have been caused by poor condition of the 

 cultured eggs, because many eggs were neutrally 

 buoyant when captured and were later grown to 

 hatching. Also, older eggs, all of which had been sit- 

 ting on the bottom of the culture vessels for some 

 days during late development, had virtually 100% 

 hatching success. Finally, late-stage eggs were caught 

 in MOCNESS samples deep in the water column (Fig. 

 7), albeit in low numbers. This ontogenetic pattern 

 of change in bouyancy is a common one in marine 

 fish eggs (reviewed by Page et al., 1989) and could 

 be the consequence of density changes in the egg as 

 high initial concentrations of yolk are converted to 

 embryonic tissue during ontogeny (Mangor- Jensen 

 and Huse, 1991). 



Thus, it is likely that orange roughy eggs undergo 

 extensive movement (both in ascent and descent) 

 through the water column as they develop. Young 

 eggs ascend rapidly to the mixed layer, reaching it 



