New Zealand Journal of Marine and Freshwater Research abstracts
Age and growth of New Zealand school shark, Galeorhinus galeus
MALCOLM P. FRANCIS
KEVIN P. MULLIGAN
National Institute of Water & Atmospheric
Research Ltd
P. O. Box 14 901
Wellington, New Zealand
email: m.francis@niwa.cri.nz
Abstract School shark,
Galeorhinus galeus (family
Triakidae), are fished commercially throughout New Zealand, and estimates of
their growth rate, age at maturity, and longevity are required for fishery
management. We aged school shark from X-rays of thin vertebral sections, but
many sections were unclear and ageing precision was low. Nevertheless, there
was no between-reader bias, and growth curves derived from length-at-age data
appear robust. A growth curve was derived independently from length-frequency
data for juvenile school shark up to 9 years old and 120 cm long, and it
agreed well with the curve derived from length-at-age data. For older sharks,
the growth rate is not certain because of small sample sizes and lack of
validation of the ages. Growth rate estimates from tag-recapture data suggested
faster growth for small sharks and slower growth for large sharks. Based on
length-at-age data, males and females grew at about the same rates. Males
matured at about 12-17 years and females at about 13-15 years. The oldest shark
in our samples was estimated to be 25 years old, but few large sharks were
available for ageing so the longevity of New Zealand school shark remains
uncertain.
Keywords age; growth; Galeorhinus galeus; school
shark; tagging; length-frequency; GROTAG; MULTIFAN
M97068
Received 17 December 1997; accepted 16 March 1998
PDF file of entire paper: medium quality (1706K); (scanned from paper original: notes about this process)
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