Journal of Fish Biology (1998) 52, 796–816 Genetic evidence for depth- and spatially separated stocks of the deep-water spikey oreo in Australasian waters N. G. E *, P. S. L , P. M. G , B. H. I , G. K. Y  R. D. W CSIRO Division of M arine Research, G.P.O. Box 1538, Hobart, Tasmania 7001, A ustralia (Received 25 July 1997, Accepted 25 November 1997) Samples of Neocyttus rhomboidalis from eight areas off southern Australia and eastern New Zealand were examined to assess the stock structure of the species. No spatial heterogeneity was detected for mitochondrial DNA variation after restriction analysis by two enzymes detecting polymorphic cut sites. Twenty mtDNA haplotypes were recorded, with an overall haplotype diversity of 0·683. Twenty-eight allozyme loci were examined; the mean heterozygos- ity per locus was 13·0%. Significant sample heterogeneity was detected at three of twelve polymorphic loci: MPI *, PGM -1* and, most strikingly (G ST = 0·43), sSOD*. Most of the sSOD* differentiation was associated with depth rather than geographical separation; sSOD* allele frequencies of shallow and deep samples being very different. It is suggested that the correlation between the sSOD* allele frequencies and depth is more likely to reflect natural selection than reproductive isolation. The spatial patterning of the MPI * and PGM -1* data suggest there are at least three geographic stocks of N . rhomboidalis in Australasian waters: (1) New Zealand, (2) Western Australia and South Australia, (3) west Tasmania to New South Wales and Lord Howe Rise.  1998 The Fisheries Society of the British Isles Key words: allozymes; mitochondrial DNA; superoxide dismutase; selection; genetic variation. INTRODUCTION Oreos, or oreo dories, are found in deep water (400–1500 m) over the continental slopes and seamounts of most temperate, and some tropical and sub-tropical, regions world-wide. They are laterally compressed, deep-bodied fish with a large head and large eyes. The family has 11 recognized species, four [ Allocyttus niger (James, Inada & Nakamura), A . verrucosus (Gilchrist), Neocyttus rhomboidalis (Gilchrist) and Pseudocyttus maculatus (Gilchrist)] with commerical significance o ff southern Australia and New Zealand (James et al., 1988; Ward et al., 1996). The oreos (six species in all) account for about 22% of the demersal fish biomass on the mid-continental slope o ff south-eastern Australia (Koslow et al., 1994). Landed catches average around 1000–2000 t year 1 (Australian Fisheries Management Authority, unpublished statistics), although because of non- reporting and discarding, actual catches are likely to be higher. Catches are taken predominantly over hard bottom and seamounts o ff southern Tasmania, where aggregations of smooth (P. maculatus) and black (A. niger) oreos are targeted (Lyle et al., 1992). The spikey (N . rhomboidalis) and warty (A . verrucosus) oreos are the more common oreos in areas with soft bottom (Lyle & Smith, 1997). New Zealand catches, at over 20 000 t annually (Annala, unpublished), greatly exceed Australian catches. *Author to whom correspondence should be addressed. Tel.: 03 6232 5222; fax: 03 6232 5000; email: nick.elliott@marine.csiro.au 796 0022–1112/98/040796+ 21 $25.00/0/jb970626  1998 The Fisheries Society of the British Isles