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The Agreement for the Establishment of the Indian Ocean Tuna Commission was approved by the Council of the Food and Agriculture Organization of the Un

Scientists develop model to predict movement of tuna

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Indian Ocean Tuna Commission - the organization responsible for the management of tunas and tuna-like species in the Indian Ocean region is conducting new research to better understand the movement of yellowfin tuna, a highly valuable yet heavily exploited species. By mapping out more detailed dynamics of fish movement, the research aims to improve the way the stock is assessed, which is pivotal to the sustainable exploitation and management of the fishery resources.

Yellowfin tuna. Fabio Fiorellato, IOTC Secretariat

Researchers primarily utilize data from historical mark-recapture programs, where fish were tagged and released with a unique identification. Fish tagging is known to be very useful in providing valuable information on life history traits and population estimates, and assisting management decisions for fish stocks. “Tagging is a powerful tool for studying highly migratory species, such as tropical tuna, which can migrate thousands of kilometers to seek suitable habitats for foraging and spawning”, says Paul de Bruyn, the Science manager at IOTC.

During the last 50 years, tuna tagging using dart tags have been implemented in the major oceans. In the Indian Ocean, a large-scale tropical tuna tagging program (IOTTP), funded by the European Union, tagged and released over 200,000 tropical tunas between 2007 and 2009, 30% of which were yellowfin tuna. Despite issues related to reporting of tag recoveries and tag losses, a recapture rate of over 15% has been observed, with times at liberty up to 7 years and distances between release and recovery of over 2000 nautical miles. In addition to that, a series of small-scale tagging programs were developed across the Indian Ocean Coastal states throughout the 2000s. These tags have offered remarkable insights into the population dynamics of tropical tuna species.   

 Yellowfin tuna movement from tagging. Dan Fu, IOTC Secretariat

“The large amount of data recorded during the IOTTP has enabled and led to a suite of analyses including studies on fish growth, natural mortality, and exploitation rates. The results are fed into stock assessments undertaken by IOTC scientists, underpinning some important sustainable management measures of these resources”, says Fabio Fiorellato, IOTC Data Coordinator.

So far, a considerable progress has been made in understanding the behavior of yellowfin tuna from analyzing these data. Yet there remains a question as to whether the tagged individuals were randomly distributed throughout the relevant portion of the population – a rather critical assumption when the tags are used inform the stock size.  “The tag mixing rate is difficult to estimate due to the non-random nature of fishery distribution and variable reporting. As the assessments get more complex, we have to be increasingly careful to make sure that the data we use are well understood and adequately explored”, says Dan Fu, the fishery officer at IOTC who is coordinating the research project.

To evaluate if the released tags mix homogeneously within the broader population, the researchers build a high-resolution computer model to simulate the dispersion of yellowfin tuna over time and space. They calibrate the model with the actual data to identify potential environmental drivers such as seasonal oceanic conditions. A range hypothesis on the movement and dispersion pattern of yellowfin tuna at finer spatial scale are evaluated.  “Simulation experiments using spatial movement models can provide a useful tool to assist in the evaluation of bias caused by non-random distribution of tags”, says Dan Fu.

Yellowfin tuna dispersion. Dan Fu, IOTC Secretariat

The tag project receives funding from the European Union under a Science Grant to support the scientific research at IOTC.  “These activities have improved scientific information on the fisheries for tuna and tuna-like species operating in the Indian Ocean and contribute to a better scientific knowledge of the ecosystems managed by the IOTC”, says Paul de Bruyn.

Yellowfin tuna support one of the largest tuna fisheries in the Indian Ocean, where they are harvested with a variety of gear types, with current total catches over 400,000t. The stock is estimated to be overfished and IOTC introduced a rebuilding plan in 2016 to reduce the overall fishing pressure on the stock. Engagement with fishing industry, communities, government, and scientific institutions through sound and robust scientific research to address the health of yellowfin tuna stock in the Indian Ocean is an important component to end overfishing and support rebuilding efforts.



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