Enhancing the science to understand species-specific growth patterns to improve our stock assessment models
Knowing the age of harvested tunas and billfish as well as how fast they grow is crucial to the accuracy of the assessment models used to define stock status for WCPO fisheries. FAME, in partnership with CSIRO in Australia and research agencies across the region, continues to pursue the development of reliable methods for ageing tropical tunas and billfish, estimate their growth rates, and understand how the environment influences growth.
Much of the ageing work to date has taken advantage of the growth increments (rings) that form in otoliths - small calcium carbonate ear “stones” - located on either side of the fish’s head. As the fish grow, growth rings build up on the otoliths, similar to the rings on a tree trunk. By counting these growth increments, we can estimate the age of the fish, depending on the species. We use these age estimates alongside measurements of the fish to derive relationships between fish length and age, and we can then fit mathematical models to these data to describe the fish growth. As with humans, all fish don’t grow at the same rate, and so we also need to consider this variation when estimating growth rates. This understanding of growth patterns is important for our stock assessments as gives us an indication of population processes, such as natural mortality and reproductive output.
Whilst otolith-based methods to estimate age and growth have proven very useful for some tunas (albacore, Thunnus alalunga) they have been less effective for others (skipjack, Katsuwonus pelamis). There are several approaches available to improve our confidence in these estimates. For example, we can analyse chemical markers that are found in these otoliths to validate the deposition rate of the increments, we can make improvements in decimal age algorithms, and integrate otolith and tag-recapture data.
Yet, there is a clear need for further direct age validation studies, and exploration of alternative approaches continues, such as using bomb radiocarbon (14C) dating to test the validity of age estimates from purported annual growth increments in otoliths from yellowfin (T. albacares) and bigeye (T. obesus) tuna sampled across the WCPO. A further opportunity lies in exploring the potential of applying DNA-based ageing methods on temperate and tropical tunas. If successful, this approach could provide a non-lethal, low-cost alternative to age estimation in WCPO tunas, complementing and further validating existing methods.