Pilling et al : Validation of annual growth increments in the otoliths of Lethnnus mahsena and Aprion virescens 



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Figure 3 



Transverse 0.5-mm section through the sagittal otoUth of L. mahsena from Mauritius (Nazareth bank), 

 stained with acidified neutral red (see text), and viewed with reflected light. Magnification: 23x. Black 

 dots mark are suspected annual increments. 



O, = the otolith radius at age ;; and 

 b = the intercept of the GMR calculated with the 

 Ricker procedure. 



For each species, a von Bertalanffy growth curve was fitted 

 to the back-calculated mean length at age through least 

 squares estimation. 



Marginal increment analysis 



The thickness of the outer zone in the otohths was meas- 

 ured by using the image analysis system (see Millner and 

 Whiting, 1996). Measurements were taken from the inside 

 edge (start) of the last opaque band and the edge of the 

 otolith. In stained L. mahsena otohths (indeed in all three 

 study species where otoliths were stained; see "Edge anal- 

 ysis," below), a band was stained on the border between 

 the end of the "translucent zone," and the start of the next 

 opaque zone. Measurements from the last stained band 

 were therefore equivalent to those made for the snapper 

 species. Because zones at the edge of sections taken from 

 the unstained otoliths of both snapper species were not 

 distinct, an average of four measurements was taken in 

 the region of the sulcus (where zones were more easily dis- 

 tinguished). Zones at the edge of the stained L. mahsena 

 otolith sections were clearly visible and, therefore, it was 

 possible to obtain a single measurement along the same 

 growth axis as that used for back-calculation. 



The pattern expected in the marginal increment resulting 

 from the development of an annual increment would be a 

 minimal marginal increment at the start of the growth period, 

 increasing with time until the measurement fell to a mini- 

 mum again at the formation of the next period of growth. 



The size of the growth zone varies both with time of 

 sampling during the year and the age of the fish. Because 



younger fish grow faster than older individuals, a larger 

 marginal increment is expected. For this reason, quantita- 

 tive marginal increment analyses should be standardized for 

 age. Our study was therefore carried out by age class. Owing 

 to the wide range of ages encountered, however, there were 

 insufficient samples to accomplish this standardization fully. 

 It was necessary to combine the ages into two or three groups 

 representing fast, moderate, and slow-growing individuals 

 (Table 1). These classes were based on the growth curves 

 derived from back-calculation, and hence on the assumption 

 that the increments represented annual growth. 



For P. filamentosiis. because sample sizes were either 

 small or because samples could not be obtained in specific 

 months, data were grouped into bimonthly time periods. 

 Sufficient L. mahsena and A. virescens samples were avail- 

 able to allow the analysis to occur on a monthly basis. For 

 all three species, a mean marginal increment for each time 

 period and growth category was calculated. 



Edge analysis 



The type of zone at the growing edge of the otolith was 

 identified in each month of the year Annuli were traceable 

 as a developing single translucent and opaque zone on the 

 edge of the otolith during that growth year 



Problems were encountered in identifying the edge type 

 in unstained P. filamentosiis and A. virescens from Sey- 

 chelles because of poor resolution of the banding in otoliths, 

 and what appeared to have been the seepage of resin either 

 under or through the edge of the otolith. Staining trials 

 were performed in an attempt to clarify the growth type at 

 the edge of the otolith in these species, and stained otoliths 

 were subsequently examined during edge analysis. 



Thin sections of the stained otoliths were assessed for 

 the presence of the band of stain on the growing tip of the 



