Preliminary age estimation of New Zealand white shark

Summary

White shark (Carcharodon carchariasI, Māori: mangō-taniwha, ururoa, and tuatini) is a wide-ranging coastal and pelagic shark, occurring throughout most of New Zealand’s waters, from the Kermadec Islands to Campbell Island/Motu Ihupuku and the northern Macquarie Ridge. White sharks are susceptible to population reduction because of their low productivity and naturally low population size. Understanding age and growth is vital for the conservation and management of white sharks and can be used to estimate population growth rates and other important demographic parameters, and to inform developing technologies such as close-kin mark recapture.

Here, vertebral banding patterns and microCT imaging were used to estimate age and growth for New Zealand white sharks for the first time. Vertebrae were obtained from white sharks reported dead from sources including commercial and recreational fishing vessels, and beach cast specimens over a 30-year period (1991 to 2021). Most white shark samples were collected around the North Island and were sampled throughout the year. The final sample (n = 38) included 20 females (1.52 to 5.36 m total length, TL), 12 males (1.87 to 4.85 m TL), and six unsexed sharks (2.26 to 3.0 m TL).

Vertebrae were difficult to read, particularly when counting the narrow increments near the margin of the vertebrae from old sharks. There was strong agreement between readers for age estimates of young New Zealand white sharks, but large disagreement for older sharks. Growth was modelled for both readers separately. Nearly half of the individuals were young (1–2 years old) and only six sharks were estimated to be older than 10 years of age. One shark (1.53 m TL) had no fully formed growth bands or distinct birth band, and was likely captured shortly after birth. Maximum age estimates from the band counts for Reader 1 and Reader 2, respectively, were 30 and 45 years for males (4.85 m TL) and 19 and 44 years for females (5.36 m TL).

The preliminary work here suggests New Zealand white sharks are relatively fast growing initially, and possibly long-lived. The relationship between length and growth was found to be nearly linear for young New Zealand white sharks. White sharks are born at approximately 1.5 m TL during the summer months (January, February) and deposition of opaque banding likely occurs in the winter months (May to August). White sharks were estimated to double their birth length to 3 m TL within five years, equating to an annual growth rate of approximately 30 cm per year. This rate of growth is similar to estimates from previous studies from Australia, South Africa, and California. Growth appeared to slow at approximately 3 m in length, which may be indicative of changes in diet, movement or habitat, or a reallocation of energy from somatic growth to reproductive development (i.e., maturity). Age-at-maturity could not be assessed here because of the small sample size, particularly for large individuals. However, based on known length-at-maturity estimates, age- tmaturity may occur at 7–10 years for males and 14+ to 22+ years for females. Additional samples of large sharks will be needed to comprehensively understand age and growth of white sharks that inhabit New Zealand waters.

Age estimates could not be validated and bomb radiocarbon dating is unlikely to provide any useful insight here because samples are unlikely to be old enough for this validation technique. New Zealand white sharks should continue to be sampled when accidentally captured, or when found dead (e.g., beach cast specimens) to increase the sample size, particularly for larger sharks. A combined New Zealand-Australia study should be carried out to characterise the life history parameters of the South Pacific white shark population.