Francis et al.: Age and growth estimates for Polyphon oxygeneios 



241 



years old in our data; 2) underestimation of the ages 

 of hapuku because of failure to recognize and count 

 all the annual bands near the core; or 3) different 

 growth rates of pelagic juveniles, and demersal ju- 

 veniles and adults. Interestingly, Pavez and Oyar- 

 ziin's (1985) scaled-based growth curves had t^ val- 

 ues close to zero, and estimated ages at the length of 

 settlement similar to ours (i.e. 38 cm at age 3 and 49 

 cm at age 4). Scale bands may therefore help to re- 

 solve problems experienced in interpreting otolith 

 cores when ageing small hapuku. 



Small juvenile hapuku pass through a pelagic stage 

 in surface waters, usually associated with flotsam, 

 and are rarely caught (Cormack, 1986; Michael, 1988; 

 Roberts, 1996). At the end of the pelagic stage, they 

 become demersal in depths of 50-600 m. Bottom 

 trawl length-frequency data (Fig. 9; NIWA") and 

 bottom set-net and line records (Johnston, 1983; 

 Roberts, 1989) indicate that most hapuku switch 

 from a pelagic to demersal life style at around 50 

 cm, although hapuku as small as 40 cm have been 

 caught by bottom trawl. A 35-cm hapuku reported 

 caught in Wellington Harbour by Hector ( 1888) may 

 have been a pelagic juvenile, but no details were 

 given of habitat or fishing method. A small number 

 of pelagic juveniles up to 67 cm long have been caught 

 on surface longlines over 2000 m or more of water 

 (Roberts, 1996; Scientific Observer Database'^). 

 These large pelagic juveniles may have become 

 "stranded" in the pelagic environment after being 

 carried over deep water by oceanic currents. Presum- 

 ably they remain in surface waters until they en- 

 counter shallower depths, at which time they settle 

 to the seabed. 



Our best estimate of the age at which hapuku settle 

 to a demersal habitat is 3-4 years, but our interpre- 

 tation of the banding pattern near the otolith core 

 was uncertain and somewhat subjective. Counts of 

 possible daily increments in otolith sections also sug- 

 gest settlement at about 3 years (Roberts, 1996), and 

 a similar age range has been suggested by other stud- 

 ies (McDougall, 1975; Johnston, 1983). Polyprion 

 americanus apparently settles at lengths greater 

 than 45-55 cm and ages greater than 1-7 years 

 (Sedberry et al., 1996). 



The age at recruitment of hapuku to commercial 

 trawl catches is likely to be the same as the age at 

 settlement to the seabed. Although our research ves- 

 sel samples were caught with a small mesh (60-mm) 

 codend, hapuku longer than 50 cm are also retained 



by the larger mesh sizes used by commercial vessels 

 (typically 100-125 mm) (Hurst and Bagley 1997). 



Previous studies have suggested that female 

 hapuku grow larger and faster than male hapuku 

 (McDougall, 1975; Roberts, 1986; 1989). Our results 

 support those conclusions but also indicate that the 

 differences in both maximum length and growth rate 

 are small and, for most purposes, trivial. However, 

 our ability to detect growth rate differences between 

 the two sexes, and between sample sites, was lim- 

 ited by the low precision of our age estimates and 

 the small sample sizes available for the GROTAG 

 analysis. 



Hapuku mature at about 85 and 88 cm, males and 

 females, respectively (Johnston, 1983). Based on the 

 length-at-age growth curves given in Table 2, age at 

 maturity is estimated to be 10-13 years for both 

 sexes. 



The largest hapuku in our study was 147 cm TL, 

 but they are known to reach 178 cm TL and 76 kg in 

 weight (Roberts^). The longevity of hapuku cannot 

 be precisely determined because of difficulties we 

 experienced in estimating ages from otoliths, and 

 because our samples mostly came from populations 

 of hapuku that had been exploited for many years. 

 Our study does suggest that hapuku live at least 50 

 years, possibly in excess of 60 years. 



Acknowledgments 



We thank Alex Johnston (formerly Ministry of Agri- 

 culture and Fisheries) and Clive Roberts (Museum 

 of New Zealand-Te Papa Tongarewa, formerly 

 Victoria University of Wellington) for providing 

 hapuku otoliths. We are also grateful to Alan Coakley, 

 John Holdsworth, Alex Johnston, and Peter Saul ( all 

 formerly Ministry of Agriculture and Fisheries) for 

 conducting the tagging programs. Their combined 

 efforts provided the basis for much of this study. C. 

 D. Roberts and J. A. Colman made useful comments 

 on an earlier draft. This research was carried out by 

 NIWA under contract to the New Zealand Ministry 

 of Fisheries (contract no. PIAGOl). 



Literature cited 



Annala, J. H., and K. J. Sullivan (eds). 



1997. Report from the Fishery Assessment Plenary, May 

 1997: stock assessments and yield estimates. Ministry of 

 Fisheries, Wellington, 381 p. 



^ NIWA (National Institute of Water and Atmospheric Research I. 

 ^ P. O. Box 14901. Welhngton, New Zealand. Unpubl. data. 

 "^ Scientific Observer Database, NIWA, Wellington, New Zealand. 

 Unpubl. data. 



^ Roberts, C. D. 1997. Museum of New Zealand-Te Papa 

 Tongarewa, P. O. Box 467, Wellington, New Zealand. Personal 

 commun. 



