568 



Fishery Bulletin 88 13), 1990 



and Squires 1956, Boehlert 1985, Radtke et al. 1985). 

 Otolith-age equations are species-specific and possibly 

 population-specific. Using otolith morphometries for 

 ageing will, therefore, require developing the equations 

 for individual species. Nevertheless, in conjunction with 

 ageing techniques using microincrement counts, otolith 

 morphometries offer a simpler method of ageing a large 

 sample of individuals than is possible using more labor- 

 ious techniques. 



The external features of A^. nudifrons otoliths were 

 distinctive compared with those from eight other Ant- 

 arctic fishes examined (Chaenocephahis aceratus, 

 Champsocephalus gunnari, Harpagifer hispinis antarc- 

 ticus, Notothenia angustifrons, N. gibberifrons, N. 

 larseni, N. corriceps, and Trematomus newnesi; Rad- 

 tke, pers. observ.). Species-specific otolith features of 

 Antarctic fishes have been described by Hecht (1987) 

 and these may be useful in identifying Antarctic fishes 

 from cetacean, pinniped, bird, or fish stomachs (Hecht 

 1987, North et al. 1984). Otolith morphological char- 

 acteristics may also provide information on taxonomic 

 relationships among Antarctic fishes (Hecht 1987), as 

 they have for other fish groups (Hecht 1978, Hecht and 

 Hecht 1978, Morrow 1979). 



Life history of Nototheniops nudifrons 



Nototheniops nudifrons is a slow-growing, relatively 

 long-lived fish. Subadults and adults of both sexes 

 were found in shallow-water (< 100 m) benthic habitats 

 along the Antarctic Peninsula, as reported by other re- 

 searchers (Targett 1981, Daniels and Lipps 1978, 

 Kellermann 1986). The absence of individuals smaller 

 than 75 mm SL from trawls probably represented 

 selection by the fishing gear for larger individuals. The 

 growth curves determined by body length vs. microin- 

 crement count, as well as by body length vs. age cal- 

 culated from the multivariate relations of otolith length 

 and weight, were similar and linear in shape between 

 45 and 149 mm SL. Linear growth was observed even 

 though the data included a large size-range of fish, with 

 individuals near the largest sizes reported for this spe- 

 cies. This type of linear growth, with no marked slow- 

 ing toward an upper asymptote, has not been reported 

 for other Antarctic fishes, although the growth curve 

 oi Harpagifer bispinis antarcticus, another small Ant- 

 arctic fish, is nearly linear (Daniels 1983). 



Males and females had similar growth rates. This was 

 surprising, since mature males and females follow 

 different strategies. Males and females reach sexual 

 maturity at the same age (4-5 years); however, there- 

 after females invest much more energy into gonadal 

 growth than do males (Hourigan and Radtke 1989). All 

 parental care is provided by the males, which may 

 spend at least 4 months in nest defense of a single 



clutch of eggs (Hourigan and Radtke 1989). This 

 probably requires increased energy costs, and may 

 limit food intake by constraining the male to foraging 

 near the nest. These two strategies may entail similar 

 energy expenditures, resulting in similar somatic 

 growth rates. 



Estimation of mortality rates is central to the deter- 

 mination of demographic parameters (Gulland 1955). 

 It is possible to estimate natural mortality (Z) from the 

 data on trawl catches and age for this population of 

 A^. nudifroyis. The most widely used method to estimate 

 mortality is the Beverton and Holt mortality estimator 

 which uses the von Bertalanffy growth curve (Bever- 

 ton and Holt 1956). The age-length data for the 32 fish 

 were fitted to the von Bertalanffy growth curve, and 

 the parameters and their confidence limits are given 

 in Table 4. These values are primarily useful for com- 

 parative purposes, since the actual shape of the growth 

 curve approximated a linear relationship. Natural mor- 

 tality, calculated from the von Bertalanffy parameters 

 and derived from the sample of 32 fish of known ages, 

 yielded a value of Z = 0.68 (Table 4). When Z was 

 calculated using the von Bertalanffy growth param- 

 eters for the larger sample of 212 fish with predicted 

 ages, Z = 0.66. These estimates assume that the fish 

 grow according to a deterministic von Bertalanffy 

 growth curve. Since the present sample was more ac- 

 curately represented by a linear growth curve, this 

 assumption was violated. An independent mortality 

 rate was calculated using the actual age estimates from 

 the otolith morphometries. In this estimate, the mor- 

 tality rate is the slope of the log-survivorship curve 

 (Ricker 1975) and was calculated to be 0.89. 



Kock et al. (1985) provide a summary of mortality 

 estimates for larger Antarctic fishes of commercial in- 

 terest. Both estimates of instantaneous mortality of A'^. 

 nudifrons were higher than these estimates, which 

 ranged from 0.20 to 0.35. Predation rates may be 

 higher on A'', nudifrons than on larger species. How- 

 ever, Daniels (1983) estimated the mortality of the still 

 smaller Harpagifer bispinis antarcticus to be 0.22 and 

 0.25 for males and females, respectively (a recalcula- 

 tion of his data for both sexes combined, using the same 

 methods as for A'^. nudifrons, resulted in a value of 

 0.223). 



Although the high mortality rates for A^. nudfrons 

 accurately describe our sample, they may be subject 

 to sampling errors. Our estimates were based on a 

 single sample captured on two occasions 1 month apart. 

 Both mortality calculations assume that the sampled 

 population is age-stationary (i.e., the age distribution 

 remains the same from year to year), an assumption 

 which is frequently not met. Mortality estimates will 

 also be biased if certain age classes larger than Lc 

 (length at first capture) are less subject to capture. Our 



