210 



Fishery Bulletin 94(2). 1996 



Composition and dynamics of the spawning 

 aggregation 



Blue-eye trevalla in spawning condition were mostly 

 caught off the northeast coast of Tasmania, which 

 may be a major spawning ground. However, the oc- 

 casional occurrence of mature blue-eye trevalla on 

 nearly all other Tasmanian fishing grounds, includ- 

 ing an offshore seamount 120 miles south of Tasma- 

 nia (data not included), and the presence of spawn- 

 ing aggregations off New South Wales ( Rowling 6 ) and 

 South Australia (Jones 7 ) indicate that spawning 

 takes place on several grounds within southern Aus- 

 tralia. The fact that no genetic difference was found 

 between blue-eye trevalla collected off New South 

 Wales, Tasmania, and South Australia (Bolch et al., 

 1993) further supports the view of a widely distrib- 

 uted spawning activity. Horn and Massey ( 1989) also 

 believed that blue-eye trevalla spawned on many 

 grounds in New Zealand. 



During his work on hoki spawning aggregations, 

 Langley (1993) identified several spawning groups 

 arriving successively and then leaving the spawn- 

 ing ground. The situation is different for blue-eye 

 trevalla, where the three groups of females identi- 

 fied in the summer aggregation seemed to correspond 

 to progressive arrivals of different size (or age) groups 

 and not to commuting spawning groups, spawning 

 taking place at the end of summer. Older females 

 join the aggregation just before spawning and leave 

 shortly after; younger females (first-time spawners) 

 aggregate earlier and stay on the grounds for some 

 time after spawning. 



The presence of preadults in the aggregation (i.e. 

 at size below the average size at maturity and, thus, 

 not all capable of spawning) suggests that this sum- 

 mer aggregation off northeast Tasmania is related 

 to both feeding and spawning behaviors. Blue-eye 

 trevalla feed mainly on the pelagic tunicate Pyrosoma 

 atlanticum (Winstanley, 1978), and Cowper (1960) 

 noted a maximum incidence of Pyrosoma in the stom- 

 achs of blue-eye trevalla during the summer. As ob- 

 served for Atlantic cod, Gadus morhua (Swain, 1993), 

 it is possible that blue-eye trevalla off northeast Tas- 

 mania migrate between deeper wintering grounds 

 and shallower feeding and spawning summer 

 grounds. The presence of preadult fish on other Tas- 

 manian fishing grounds during spring and summer 



"Rowling.K. 1994 Fisheries Research Institute, P.O. Box 21, 

 Cronulla, New South Wales, Australia. Personal commun. 



' Jones, G. K. 1985. An exploratory dropline survey for deepsea 

 trevalla (Hyperoglyphe antarctica ) in continental slope waters 

 off South Australia. Department of Fisheries, South Austra- 

 lia, Fisheries Research Paper 15, 20 p. 



indicates that feeding aggregations are not restricted 

 to the northeast coast of Tasmania. Winstanley ( 1979 ) 

 also noted that blue-eye trevalla off Victoria prob- 

 ably move from deeper to shallower depths in spring. 



The spawning population observed in commercial 

 catches was young and the majority were first-time 

 spawners. Larger fish used to be caught by drop-line 

 in the early years of the fishery (Baelde, unpubl. 

 data), and it is not known whether their low propor- 

 tion in current catches reflects a lower vulnerability 

 to line fishing (owing to age- or size-specific change 

 in distribution and aggregating behavior), or reflects 

 a significant decline in abundance owing to fishing. 

 In addition, it is believed that there is one stock of 

 blue-eye trevalla in southern Australia, and, as 

 shown for other fish species with a wide geographic- 

 distribution (Schaefer, 1987; Bell et al., 1992; 

 Rijnsdorp, 1993; Clark etal., 1994), size at maturity, 

 spawning season, and fecundity could vary signifi- 

 cantly between areas. Differences in the time of 

 spawning and the size of spawners have already been 

 observed between New South Wales and Tasmania. 6 



This study points out that a proper estimate of the 

 spawning biomass of blue-eye trevalla stock off south- 

 ern Australia would require fishery-independent 

 surveys (i.e. identification of other spawning grounds 

 and analysis of the age-specific movements of fish 

 during spawning) to determine more accurately the 

 structure and abundance of the spawning population. 



Acknowledgments 



This project would not have been possible without 

 the assistance of commercial drop-line fishermen, 

 and I thank them all. I also thank L. Hick, F. Ewing, 

 and A. Brown for their technical assistance at vari- 

 ous stages of the project, R. Kennedy for helping with 

 generalized linear models techniques, and finally J. 

 Lyle, G. West, and K. Rowling for reviewing early 

 drafts of the manuscript. This study was partly 

 funded by the Fishing Industry Research Develop- 

 ment Council (Project No 91/20). 



Literature cited 



Bell, J. D., J. M. Lyle, C. M. Bulman, K. J. Graham, 

 G. M. Newton, and D. C. Smith. 



1992. Spatial variation in reproduction, and occurrence of 

 non-reproductive adults, in orange roughy, Hoplostethus 

 atlanticus Collett (Trachichthyidae), from south-eastern 

 Australia. J. Fish. Biol. 40:107-122. 



Bolch, C. J. S., N. G. Elliot, and R. D. Ward. 



1993. Enzyme variation in south-eastern Australian 

 samples of the blue-eye or blue-eye trevalla, Hyperoglyphc 



