Radtke and Houngan: Age and growth of Norotheniops nudifrons 



Sb7 



Figure 1 1 



Regression of fish standard length (SL) on estimated age for 

 Nototheniops nudij'rons. Age was estimated by a multivariate 

 mathematical model based on fish standard length, fish weight, otolith 

 length, and otolith weight. 



separate growth curves calculated for males and fe- 

 males showed no significant difference (df= 1,177; 

 F = 0.87; p>0.36), indicating that males and females 

 grow at the same rate. 



Discussion 



Otolith microincrement deposition 

 and age determination 



The accuracy of age and growth estimates depends on 

 the assumption that the microincrements viewed were 

 deposited on a daily basis. Daily increments have been 

 foimd in otoliths from many temperate and tropical fish 

 species (see review by Campana and Neilson 1985). The 

 daily nature of microincrement deposition in Notothe- 

 niops nudifrons was validated through the tetracycline 

 and acetazolamide marking experiments. Daily incre- 

 ment deposition occurred even during the shortest days 

 of the Antarctic winter. The formation of microincre- 

 ments may become less regular in some fishes after 

 sexual maturity (Pannella 1980), but our validation 

 study included both immature subadults and mature 

 males and females, which all deposited daily microin- 

 crements. Hourigan and Radtke (1989) found that daily 

 microincrement deposition also occurs in larval A'^. 

 nudifrons, beginning at or around the date of hatching. 



Together, these studies provide the first validation of 

 daily otolith microincrements for any Antarctic fish. 



In addition to the microincrements, the otoliths of 

 N. nudifrons contained larger banding patterns. In 

 temperate regions, environmental factors such as tem- 

 perature and food availability show regular and marked 

 seasonal changes, giving rise to clearly identifiable 

 growth periods in fish otoliths. The regular annual for- 

 mation of these seasonal rings in the otoliths of tem- 

 perate fishes provides a successful and widely used age- 

 ing technique. Growth increments laid down in the 

 otoliths of some Antarctic fishes may be annuli (e.g., 

 Harpagifer bispinis antarcticus; Daniels 1983); how- 

 ever, those observed in this study, as well as those in 

 the otoliths of Nototheniops larseni (Radtke and 

 Targett 1984), are not. Nevertheless, such increments 

 have been used, perhaps erroneously, for ageing pur- 

 poses. Compared with temperate zones, Antarctic 

 habitats undergo smaller fluctuations in temperature, 

 which may result in a lack of distinct annual growth 

 increments. 



Daily microincrements allow the ageing of Antarc- 

 tic species without otolith annuli. Microincrements are 

 the result of a discontinuous zone which is generally 

 formed under changing light conditions, probably at 

 daybreak (Tanaka et al. 1981). The near-constant light 

 and constant dark conditions experienced by Antarc- 

 tic fishes during the summer and winter, respectively, 

 could potentially interfere with daily deposition pat- 

 terns. However, in the present study, daily increment 

 formation continued despite great variation in photo- 

 period. Although areas in otoliths were detected in 

 which increment width decreased (Fig. 8), none of these 

 appeared to indicate a cessation of increment forma- 

 tion. Slight changes in light intensity, which occur in 

 both Antarctic summer and winter, or endogenous 

 rhythms may be responsible for the daily pattern of 

 otolith deposition. This problem is currently being in- 

 vestigated using Antarctic fishes kept in constant dark 

 or constant light, and under different light intensities 

 (Radtke and Hourigan unpubl. data). 



Use of a multivariate mathematical model relating 

 age to otolith length and weight and fish size is a 

 simpler method of age determination. The preparation 

 and counting of microincrements in fish over 1 year 

 of age is time-consuming and impractical. However, the 

 multiple regression equation calculated from the 32 in- 

 dividuals analyzed with SEM provided an alternative 

 ageing method. Otolith weight (with its consistent rela- 

 tionship to body length) was the best predictor of age, 

 demonstrating the usefulness of otoliths in such a 

 regression. This method should be applicable to other 

 fish species, serving as a quick means to provide growth 

 information. The dimensions of otoliths have been used 

 to estimate the age of temperate fishes (Templeman 



