558 



Fishery Bulletin 88(3), 1990 



Sosinski 1981, Kochkin 1982, Burchett et al. 1984). 

 These patterns were interpreted to be annuli; however, 

 no vahdation data were available for the deposition 

 rates of these increments. These presumed annuli, 

 which are often difficult to discern and interpret, may 

 not represent yearly growth increments (Scherbich 

 1975, Frey tag 1 980b, Radtke and Targett 1984). 



Townsend (1980) described the presence of micro- 

 structural growth increments in Antarctic fish otoliths. 

 These increments appeared to be analogous to the daily 

 increments found in a host of fish species (see review 

 by Campana and Neilson 1985). Such microincrements 

 were used by Radtke and Targett (1984) to age Noto- 

 thenia larseni, and are the foundation for the present 

 research. However, the daily or annual nature of rhyth- 

 mic patterns in otoliths has not been experimentally 

 validated for any Antarctic fish species. Microincre- 

 ments may generally form in response to changes in 

 daily periods of light and dark (Radtke 1984), sug- 

 gesting that special problems may exist in the forma- 

 tion of daily increments during times of short daylight 

 in the winter months. 



Management programs for Antarctic fisheries re- 

 quire knowledge of population parameters of both com- 

 mercial and sympatric, non-commercial, species. Noto- 

 theniops nudifrons ( =Notothenia nudifrons) is among 

 the most abundant demersal fishes in many habitats 

 of the Antarctic Peninsula (DeWitt 1971). As such, this 

 species may play a major role in the trophic structure 

 of the Antarctic marine community, both as competitor 

 with and prey for commercial species. 



We recently reported on the reproductive biology of 

 A'', nudifrons (Hourigan and Radtke 1989). In the pres- 

 ent study, the age and growth of A'^. nudifrons were 

 determined by the examination of microincrements in 

 sagittal otoliths. Microincrement deposition rates dur- 

 ing different seasons were tested under experimental 

 conditions. Correlations of age to otolith morpho- 

 metries allowed ageing of a large sample of fish and 

 an estimation of their natural mortality rates. 



Methods 



Collection of fish and initial measurements 



A total of 32 Nototheniops nudifrons were collected on 

 28 March and 17 April 1985 by otter trawl in 54-110 

 m depths off Low Island (63°24'S to 63°27'S; 62°07'W 

 to 62°17'W). An additional 30 fish were collected in 

 the same area in February 1984, and used for prelim- 

 inary otolith validation studies. All individuals were 

 transported live to Palmer Station and placed in tanks 

 with flow-through seawater. In addition to these fish, 

 scuba divers collected three small juveniles near Palmer 



Station in Arthur Harbor (64°46'S; 64°04'W) in May 

 and July 1985. 



The fish from the 1985 trawls were analyzed for 

 length-weight relationships and growth. Standard 

 length (SL) and total length (TL) were measured to the 

 nearest millimeter. Whole body weights were measured 

 to the nearest 0.001 g. Otoliths were removed from all 

 fish, cleaned, dried at 60°C, and stored in a desiccator. 

 The gonads were examined to determine sex and re- 

 productive condition (Hourigan and Radtke 1989). 



The remaining fish were injected intramuscularly 

 with acetazolamide (samples from 1985 only) or tetra- 

 cycline (samples from 1984 and 1985) and kept in 5()0-L 

 tanks with flow-through seawater, at ambient temper- 

 atures and natural photoperiods. Ten fish were kept 

 in each tank, along with 10 Notothenin gibherfrons. 

 Rocks were added to the tanks to provide shelter and 

 nesting sites. Fish in tanks were fed ad libitum with 

 kril! every 2 days. 



Fish were kept alive for a maximum of 158 days after 

 injection, from April to October, at which time they 

 were sacrificed, weighed, and measured. Otoliths of 

 these fish were removed, cleaned, and stored dry in 

 vials. 



Otolith structure and age determination 

 from otolith microincrements 



To determine the relationship of age to otolith size and 

 shape, the length, width, and weight of sagittal otoliths 

 were measured. Otoliths were segi'egated according to 

 their lateral position in the cranium. Left and right 

 sagittae from all fish were scanned using a computer- 

 aided video digitizer, which produced a measure of 

 maximum length (from the rostrum to antirostrum; 

 nomenclature of Hecht 1978) and width (the widest 

 distance in the dorsal-ventral plane). Otoliths were 

 weighed on a microbalance to the nearest 0.01 mg. 



A sample of 32 fish (from fish collected in 1985) was 

 chosen for age determinations. This sample was com- 

 posed of males and females of a representative size 

 distribution. Left sagittae from these fish were at- 

 tached to scanning electron microscope (SEM) viewing 

 stubs with epoxy, carefully ground down to the central 

 area by hand using a fine sharpening stone, and then 

 polished with 0.3-micron alumina paste. The polished 

 surfaces were etched for 1-20 minutes with 6% ethy- 

 lene diamine tetraacetate (EDTA) with pH adjusted to 

 8 with NaOH. After etching, the sections were gently 

 washed with water, dried, coated with gold, and viewed 

 by SEM at various magnifications (50-10 000 x). 



Otoliths were examined for microincrements. A 

 microincrement was defined as an unbroken incre- 

 mental zone with discontinuous zones as boundaries 

 (Radtke and Dean 1982). Sequential etching made it 



