This project scaled up and improved electronic tag attachments to striped marlin. In all 24 more striped marlin were tagged and tracked, 20 of these carried two types of satellite tag to record detailed positions as well as detailed temperature, depth and light level. The data from the two kinds of tags were distinctly different, and surprisingly challenging to combine.
Striped marlin movement was categorised into four types of behaviour. Fast and slow straight segments, and fast and slow non-straight segments. Migration, chasing and fleeing are likely to be represented by straighter segments and were called ‘Transiting Behaviours’.
Localised foraging and resting are consistent with high turning frequencies and were simply be called ‘Area Restricted Behaviours’ (ARB). Diving patterns often changed as these fish switched between the categories, which provides a measure of confidence that the model is estimating different behaviours relatively well.
Six different striped marlin reversed directions or halted northerly progress in the tropics between 20-21°S during each tagging season (2005-2008), within all months from April-August. Analysis of the oceanography associated with this behaviour indicated changes in mixed layer depth, and local bathymetric gradients contributed to this behaviour. Also evident was that water temperatures are not the only factor at play, as they ranged from 24-29° C at the time these fish changed course.
The fascinating new information collected here has wide spread value, ranging from new basic biological understanding to critical insights to help inform species management in the southwest Pacific Ocean. At least seven different exclusive economic zones were crosses by striped marlin from this research program, as far afield as French Polynesia. The highly migratory nature of the species means that population management cannot occur in isolation, with individual nations setting national priorities without regard to wider regional implications. The recreational value of the species in New Zealand is indisputable, and the recreational community has made critical contributions towards ensuring its value is recognized by supporting this research.