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Is EPO Tuna Migrating In Westward Direction To WPO?

Recently, some tuna fishermen are claiming that lower catches in the Eastern Pacific Ocean (EPO) are the result of a new upcoming “El Nino”, whereas others blame climate change. Assumptions have been made that the tuna is migrating in westward direction away from the EPO into the WPO, and leaving South American based tuna vessels with empty nets.  

In addition, some research performed by Dr. Daniel Pauly from University of British Columbia predicted that climate change through 2055 may lead to a 30% to-70% increase in catches at higher latitudes and a drop of up to 40% in the tropics.

However, when it comes to tuna schools, researchers from the Inter-American Tropical Tuna Commission (IATTC) disagree on tuna mobility from the EPO to the Western Pacific Ocean (WPO).

The stock structure report from 2008 has demonstrated regional fidelity for bigeye, yellowfin and skipjack tunas “with low levels of mixing expected with stocks in the Central and Western Pacific Ocean”.

The available scientific information about the stock structure of these three species in the EPO has been reviewed, evaluated, and compiled in this report. The evidence indicates there are probably northern and southern sub-stocks of bigeye (with separation at about 10°N), yellowfin (with separation at about 15°N), and skipjack (with separation at about 15°N), based on tagging data.

“The spatial extent of those stocks and the levels of mixing are not yet well defined. Stock boundaries most likely oscillate (swing) within a few degrees of latitude relative to seasonal and annual variability in oceanographic conditions”, stated the report.

The results of bigeye, yellowfin and skipjack tagging studies in the eastern and western Pacific Ocean demonstrate restricted movements, with very limited mixing of fish between areas separated by distances greater than about 1.000 miles (1.609 Km).

The bigeye tagging studies recently undertaken in the equatorial EPO demonstrate that movements are restricted primarily to the equatorial region, and no movement from the southern to the northern region of the longline catch distribution was observed.

Yellowfin studies in the EPO, on the other hand, demonstrated restricted movements, but with fidelity to northern and southern regions of the EPO with limited mixing between them. Same goes for skipjack: restricted movements, with fidelity to northern and southern regions of the EPO.

It is undeniable that climate changes are currently ongoing and will affect tuna fisheries in the future. Oceans acidification is one of the effects. More frequent and unseasonal El Ninos are another. Nevertheless, the strongest external factor affecting tuna stocks according to scientific evidence is still overfishing. Not allowing tuna stocks to reproduce in a healthy speed will bring each time less and smaller fish to the fishermen.