Age and growth studies of Gummy Shark, Mustelus antarcticus Gunther, and School Shark, Galeorhinus galeus (Linnaeus), from Souther Australian Waters

Abstract

Age-length data were derived from counting stained bands on whole vertebral centra obtained from gummy shark, Mustelus antarcticus, captured by gill-nets during 1973-76 in Bass Strait and from gummy shark and school shark, Galeorhinus galeus, captured during 1986-87 in Bass Strait and waters off South Australia. The data were fitted to the von Bertalanffy growth equation after adopting the Francis reparametrization and correcting for sampling bias caused by the selectivity effects of the gill-nets of various mesh sizes used to capture the sharks.

The von Bertalanffy growth curves of male and female gummy shark were significantly different, but the growth curves of male and female school shark were not. The growth curves suggest that growth rates of male and female gummy shark in Bass Strait were lower during 1986-87 than during 1973-76 and that the growth rates of male and female gummy shark and school shark in Bass Strait during 1986-87 were lower than those in South Australia at the same time. These apparent temporal and spatial differences in growth patterns of gummy shark are explained by the 'Phenomenon of Apparent Change in Growth Rate'. It is concluded that the growth curves determined for 1986-87 are distorted by the effects of a long history of high and length-selective fishing mortality and that actual growth patterns of gummy shark are better represented by the von Bertalanffy growth equation determined for shark caught in Bass Strait during 1973-76, when fishing mortality was much lower.

Verification of age estimates was attempted by comparing von Bertalanffy growth curves derived from age-length data with those derived from tag release-recapture length-increment data, but these comparisons highlight the limitations of using tag data for this purpose. Although reasonable agreement was found between such growth curves for gummy shark, it appears that school shark older than 11 years cannot be aged accurately from stained whole or sectioned vertebrae. Sectioned vertebrae from a school shark recaptured 35.7 years after being tagged and released and calculated as having an age exceeding 45 years gave estimates of only 18-20 years of age.