Pilling et aL: Validation of annual growth increments in the otoliths of Lethnnus mahsena and Apnon virescens 



601 



"Indeterminate increments" (increments not related to 

 the annual time scale) are found in all hard parts. These 

 need to be distinguished from increments formed at reg- 

 ular intervals (Fowler and Doherty, 1992). Before incre- 

 ments can realistically be used for aging, they need to be 

 "validated" (Beamish and McFarlane, 1983) to ensure that 

 the structures, such as the otoliths used in our study, can 

 be used as accurate chronometers. They must therefore 

 fulfill a number of criteria (Fowler and Doherty, 1992): 1) 

 the otolith must gi-ow throughout the lifespan of the indi- 

 vidual fish; 2) the otolith must show an internal incremen- 

 tal structure; and 3) this structure must correspond to a 

 regular time scale. 



Our study concentrated on macro-increments. Aging 

 through micro-(daily)increment counts (e.g. Ralston and 

 Miyamoto, 1983) requires considerable investment in time 

 and equipment and is generally constrained to younger 

 ages, where daily increments remain distinct. These con- 

 traints render such techniques unfeasible for stock assess- 

 ment purposes, particularly in developing countries. 



The local fisheries institutions in the study locations 

 of the central Indian Ocean are, to date, limited to the 

 use of length-based methods for aging fish. The results 

 of these methods are felt to be uncertain when applied 

 to long-lived, slow-growing species such as snappers and 

 emperors, owing to modal overlap and a general absence 

 of modal progression (e.g. Langi, 1990). Our study aimed 

 to validate as annual the macro-increments seen in the 

 sagittal otoliths of two species of snapper, Aprion vires- 

 cens (Valenciennes 1830) and Pristipomoides filamentosus 

 (Valenciennes 1830), and one species of emperor, Le^/inVu/s 

 mahsena (Forsskal, 1775). Validation of the annual nature 

 of increments seen in the otoliths of these species would 

 allow age-based methods of stock assessment to be applied 

 to these commercially important demersal bank and deep- 

 slope reef-fish species from the central Indian Ocean. 



Compliance with the three criteria for validation was 

 examined by using two methods: back-calculation (e.g. 

 Manooch, 1987; Van Leewen and RijnsdorpM and a com- 

 bination of marginal increment and edge analysis (e.g. 

 Mason and Manooch, 1985; Manooch and Drennon, 1987; 

 Manickchand-Heileman and Philipp, 1996). Back-calcula- 

 tion was used to test the validity of the first two criteria 

 for aging. Marginal increment analysis and edge analysis 

 were then used to validate mark periodicity, fulfilling the 

 third criterion. 



Materials and methods 



Otoliths of the study species were obtained from commer- 

 cial fish catches. In waters off both Mauritius and the Sey- 

 chelles, fishing is frequently limited by rough seas during 

 the SE trade wind period from May to October. Samples 

 of Lethrinus mahsena were obtained during excursions of 

 the Mauritian mothership-dory vessels on Nazareth Bank 



of the Mascarene Ridge (15°S, 6rE). On the Mauritian 

 banks, this species composes about 80-90^7^ of the total 

 catch (Ardil, 1986; Bertrand'^). In the Seychelles, fishing 

 occurs from smaller vessels, which generally remain on 

 the Mahe Plateau (05°S, SS^E). Otoliths from both P. fila- 

 mentosus and A. virescens were obtained from this area. 



Two methods were used to examine whether the otoliths 

 fulfilled the requirements for aging: 1) back-calculation 

 of growth from otolith increments; and 2) marginal incre- 

 ment and edge analysis. A general description of these 

 methods is provided in Blacker (1974) and Williams and 

 Bedford (1974). 



Sagittal otoliths were selected for examination because 

 a number of studies have used these structures to age 

 snapper and emperor species successfully elsewhere (e.g. 

 Manooch, 1982; Mason and Manooch, 1985; Morales-Nin, 

 1989; McPherson and Squire, 1992; Newman et al., 1996), 

 including Aprion virescens and Pristipomoides filamento- 

 sus (Loubens, 1980; Ralston and Miyamoto, 1981, 1983). 



One otolith from each fish was embedded in black 

 polyester resin, and a 0.5-mm transverse section taken 

 through the center ("nucleus") of the otolith by using a 

 diamond cutting blade ( Bedford, 1983 ). Sections were then 

 either mounted in clear resin on a microscope slide or 

 stained with acidified neutral red (Richter and McDer- 

 mot, 1990) and left unmounted. Otoliths were examined 

 with a Zeiss compound microscope equipped with zoom 

 lens and magnification up to 60x. Reflected or transmit- 

 ted light was used as necessary to identify and count the 

 increments. 



In our study, terminology was based on that recom- 

 mended in Secor et al. (1995). The term "opaque zone" 

 refers to the area that appeared milky under reflected light 

 or dark under transmitted light in unstained samples. 



Back-calculation of growth by means of otolith 

 increments 



For back-calculation, snapper otoliths were most easily 

 assessed unstained, whereas L. mahsena otoliths were 

 most easily assessed stained. Only otoliths that showed 

 clear increment patterns were used for measurements; as 

 a result, seven A. virescens otoliths, in which it was dif- 

 ficult to distinguish the first or second increment, were 

 omitted from the samples. Measurements of the total oto- 

 lith radius, from the nucleus to outside edge, and the dis- 

 tance along the selected growth axis between suspected 

 annual increments were made with the tools described 

 in Millner and Whiting (1996). Unstained P. filamentosus 

 and A. virescens otoliths were assessed along a growth 

 axis close to the sulcus acousticus (Figs. 1 and 2). This 

 area provides the most consistent and easily interpretable 



' Van Leeuwen, P. I., and A. D. Rijnsdorp. 1986. The analysis 

 of the growth of turbot by back-calculation of otoliths. ICES 

 Council Meeting 1986/G:50 



- Bertrand. J. 1986. Pour une evaluation des resources en 

 capitaine/dame berri (Lethrinus mahsena) des bancs de Saya 

 de Malha. In Rapport du Groupe Travaile, French Institute of 

 Research and Exploitation of the Sea (IFREMER)/Albion Fish- 



 eries Research Centre (Mauritius). 23-25 Juillet 1985, 39 p. 

 [Available from Albion Fisheries Research Centre, Albion, Petite 

 Riviere, Mauritius.] 



