Abstract
Though whole otoliths are commonly adopted in age assignment of black scabbardfish, this study showed that sectioned otoliths are more appropriate because growth increments are more evident and ageing of larger specimens is easier. Vertebrae are not the most appropriate structure for ageing but, in the absence of otoliths, this structure may be useful in age assignment of this species. To verify possible differences between age and growth among specimens from the southern NE Atlantic, 1075 sectioned otoliths from specimens from mainland Portugal, 436 from Madeira and 107 from the Azores were analysed and the distribution of length-at-age obtained for each sex and region was determined. Significant differences were obtained in the comparison of the distribution of length-at-age between Madeira, the mainland and the Azores. The von Bertalanffy growth model was applied to back-calculated mean length-at-age data from Madeira (Lt = 1586 [1 – e –0.119(t+2.282) ] females; Lt = 1461 [1 – e –0.146(t+1.441) ] males) and mainland (Lt = 1354 [1 – e –0.170(t+2.040) ] females; Lt = 1240 [1 – e –0.208(t+1.654) ] males), and significant differences in the growth equations were obtained. Furthermore, a regression tree model was used to investigate how growth is conditioned by reproduction. The results showed a clear separation between individuals from the two areas, both females and males from the mainland (non-reproductive individuals) being characterised by a lower gonadosomatic index and a lower age.
Highlights
Age determination provides essential input data for the assessment of marine fish stocks (Hilborn and Walters, 1992)
Vertebrae are not the most appropriate structure for ageing but, in the absence of otoliths, this structure may be useful in age assignment of this species
NE Atlantic, 1075 sectioned otoliths from specimens from mainland Portugal, 436 from Madeira and 107 from the Azores were analysed and the distribution of length-at-age obtained for each sex and region was determined
Summary
Age determination provides essential input data for the assessment of marine fish stocks (Hilborn and Walters, 1992). Using the periodicity in the formation of growth increments of calcified hard structures, such as scales, otoliths, fin rays and vertebrae, the age of fish can usually be estimated (Campana, 2001). Otoliths have been largely used in age and growth studies, since it is assumed that they contain information on the entire growth history of the individual fish (Bergstad, 1995). The mechanism regulating cyclic deposition of growth increments in the form of opaque and translucent increments in otoliths is not well understood (Beckman and Wilson, 1995) but it is commonly assumed to be related to seasonal variation in somatic growth and environmental factors (Campana, 1999). In deep water fish this formation is linked to spawning and seasonal variation in food availability (Morales-Nin and Panfili, 2005). Juveniles (here considered as specimens with total length smaller than 60 cm) are not commonly caught
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