Tuna and transshipment: a global analysis to explore the links between tuna diversity and transshipment vessel location
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Transshipment at sea is a practice where refrigerated cargo vessels, also known as reefers, meet with fishing boats to exchange catch, fresh water, food and crew. Transshipment makes economic sense as it greatly extends the time a vessel can spend at sea fishing. However, it compromises the transparency pertaining to environmental and social sustainability within the seafood industry as catch, both legal and illegal, from several fishing vessels are mixed onboard the reefer. Hence, transshipment often participates and services illegal, unregulated and unreported (IUU) fishing, which is detrimental to the sustainability of globally traded fish stocks, such as tuna. Currently six out of the seven main tuna species stocks are fully fished or overfished, making tuna more vulnerable to IUU fishing activities. This study aims to connect transshipment vessels and distribution patterns of seven tuna species by correlating tuna distribution and spawning grounds with reefer activity. Satellite-based Automatic Identification System (S-AIS) data was used to track transshipment vessels, and published data on the distribution of tuna was used to map tuna presence, spawning areas, and diversity. Tuna and reefer patterns were tested for correlations on a global scale. No correlation between tuna diversity and reefer presence was found, however reefer presence was higher in regions with high levels of documented IUU fishing. Lack of transparency due to transshipment effects the monitoring process of stocks, which are already under high fishing pressure and could result in a collapse of fish populations without proper management. Improved traceability of fish processed by reefer vessels would be an important step toward increased sustainability.