Preliminary age validation of the blue shark (Prionace glauca) in the eastern Pacific Ocean

Accurate age and growth models are some of the most important biological parameters
needed for stock assessment and fishery management. The blue shark (Prionace glauca) is subjected to one of the highest levels of fishery bycatch in the world and is the shark species caught in the greatest number in the California/Oregon drift gillnet fishery where most are discarded at sea due to a lack of market value. Despite their numerical importance, the stock status of blue shark in the North Pacific is uncertain. Assumptions regarding band pair deposition rates used for age and growth models are being made without validation studies in the Pacific Ocean. As such, the purpose of this study is to validate vertebral band counts of blue sharks tagged and recaptured in the eastern Pacific Ocean. Oxytetracycline (OTC) labeled vertebrae of 13 blue sharks have been obtained from tag-recapture activities and processed to determine timing of centrum growth band deposition. Several methodologies were used to
examine blue shark vertebrae and digital images of the whole vertebrae centrum were
determined to be the best. OTC tagging of the recaptured sharks occurred off southern
California from 2007 to 2009, with time at liberty ranging from 22 to 473 days. For vertebrae samples used in this study, shark size at release ranged from 90 to 276 cm total length (TL). OTC marked vertebrae from at least 20 more sharks have been returned and will be processed to build upon this study. Results from band counts of vertebrae distal to OTC marks thus far indicate a single band pair (1 translucent and 1 opaque) is formed per year for blue sharks of the size range examined. These preliminary results corroborate annual deposition rates found in the only other OTC validation study for blue sharks and will aid in future blue shark age and growth studies in the Pacific Ocean.