Graduate Thesis Or Dissertation
 

The effects of different otolith ageing techniques on estimates of growth and mortality for two species of rockfishes, Sebastes pinniger and Sebastes diploproa

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  • The use of some age determination methods may result in the incorrect age structure of fish populations. This in turn may lead to incorporation of inappropriate data into models used for stock management. Otoliths are commonly used for determining the age of fishes, and ages based on the whole otoliths (surface ages) are often lower than those based on otolith sections of long-lived, slow-growing fishes such as rockfishes (Sebastes spp.). Therefore, significant overestimates in growth and mortality may occur when surface ages are used for these species. This study provides estimates of growth and total instantaneous mortality for Sebastes diploproa and S. pinniger based on ages determined from the otolith surface versus those determined from otolith sections. For both species, estimates of mortality were reduced by more than half when section rather than surface ages were used. Estimates of growth based on surface rather than section ages were nearly identical for S. diploproa but appeared higher for S. pinniger. The inclusion of posterior otolith projections in the surface counts of S. diploproa and not S. pinniger are suspected of producing these differences. Because of these revised estimates of mortality and growth the life history features and response to exploitation of these species must be re-evaluated. Maximum sustainable yields of S. ninniger and S. diploproa based on surface ages are 2 to 5 times greater for either species than yields based on section ages. Mistaken estimates such as these can lead to management errors resulting in overexploitation and may thus explain the depletion of some stocks of Sebastes spp..
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