Growth, age estimation and feeding of captive Sevengill Sharks, Notorynchus cepedianus, at the Monterey Bay Aquarium

Abstract

Five methods, all based on the von Bertalanffy growth function (VBGF) model, were used with captive sevengills to characterize growth and to estimate both age and age at sexual maturity. Fabens' method (MI) gave a high k = 0.25 ± 0.04 year^-1 and a low L_∞= 202 ± 13 cm and W_∞ = 41 ± 11 kg. Determination of the third VBGF parameter, L₀ or W₀, requires age to be known.

Reported estimates of size at birth were used to estimate ages of individual sharks. For the 'individual VBGF fit' method (M2), the age axis was fixed by using size at birth. For the 'derivative' method (M3), a growth rate was calculated from observed size when the sharks were received and size at birth. The results of either method confirmed that two sharks were first-year class and three sharks were second-year class when they were received. Both M2 and M3 required the use of a combination of total length (TL) and weight data via the 'TL-scaled cube root weight' method (M4) to obtain adequate statistics for individual sharks.

The 'exponential regression' method (M5) was used to predict the sizes and ages of larger and older sharks. Age at sexual maturity for males was estimated by using L_∞ = 225-250 cm and k = 0.174 to 0.132 year^-1, which gave 4.3 to 5 years, in good agreement with the age estimated from clasper-length measurements from a single male. Age at sexual maturity for females was estimated by using L_∞ = 275-300 cm and k = 0.107 to 0.090 year^-1, which gave 11-21 years.

Feeding habits have been monitored for captive sevengill sharks since 1985. Pups consumed up to 2% of their body weight (BW) per day, juveniles about 0.6% BW day^-1, and adults about 0.2% BW day^-1. Food consumption and, more importantly, total efficiency changed with age. The observed large food consumption and the observed high total efficiency of pups could be substantiated on the basis of a theoretical calculation of food intake based on von Bertalanffy growth. Adult sevengills showed a 'feast or famine' feeding pattern that followed a cycle of 5-7 days and that may have been influenced by water temperature and previous meal size.

We conclude that growth rates in captivity and in the field are similar, based on the available growth data and feeding behaviour of sevengills in captivity and in the field.