New Zealand has a world class tuna fishery within range of recreational fishers off the eastern Bay of Plenty and Cape Runaway. Over the last 4 years a southern bluefin tuna (SBT) survey has collected catch information and ask for crews returning to Waihau Bay to donate fish heads for use in an international tuna ageing study.
Over the last 4 years 248 otolith (balance bone) samples were collected and sent to Australia for aging. This is part of a wider study funded by Fisheries New Zealand needed to convert the size of fish in commercial and recreational catch into age for use in the global stock assessment.
Thin slices through the centre of the otolith bone are used to count annual growth rings. These are quite wide as the fish grows fast in the early years and then closer together as fish approach maximum size and growth slows.
The youngest SBT from Waihau Bay were 3 years old and about 30 to 45 kg. The oldest was 23 years old and 121 kg, while the heaviest was 155.1 kg and 21 years old. Growth rates are variable, with a range of lengths at each age plotted in the graph below.
Thanks to all those people fishing from Waihau Bay who donated southern bluefin tuna (SBT) heads. If you know the date and weight of the tuna you caught you maybe able to find the age in the table below.