Thin-sectioned otoliths reveal extended longevity of southern boarfish (Pentaceros richardsoni) and are used to investigate inter-oceanic differences in length and age structure and growth

Abstract

The age and growth of southern boarfish (Pentaceros richardsoni) was determined from samples collected by deep-sea trawlers over the Lord Howe Rise region in the Tasman Sea and Walters Shoal region in the south-west Indian Ocean. The number of growth (opaque) zones counted in whole otoliths and in thick sections (450 μm) of otoliths were lower than the counts of opaque zones in thin-sections (300 μm) of otoliths. Visual analyses and modelling of the trends in marginal increments on thin-sections of otoliths show that opaque zones are formed annually. The maximum ages determined for female and male P. richardsoni of 41 and 60 years respectively, greatly exceed the estimate of 14 years previously determined for this species based on counts of opaque zones in whole otoliths and the 2–8 years for its congener Pentaceros wheeleri from the North Pacific Ocean (NPO). The high maximum age for P. richardsoni and corresponding low natural mortality rates parallel those of Pentacerotids in coastal waters of southern Australia. Trends in the growth of P. richardsoni from the Tasman Sea demonstrate that this species exhibits rapid growth in the first 4–5 years of life with little or no subsequent growth throughout the remainder of their protracted life, which is analogous to other Pentacerotids. The truncation of the lengths and ages of P. richardsoni from the Walters Shoal suggests that the population of this species in these waters has experienced greater fishing mortality than the populations over the Lord Howe Rise region in the Tasman Sea, which is also confirmed by the far higher total mortality estimates in the former region. These findings for P. richardsoni, and the knowledge that P. wheeleri populations in the NPO have still not recovered from sustained, high fishing mortalities in the late 1960s to mid-1970s should inform the management strategies for P. richardsoni in the Southern Hemisphere.