International Seafood Sustainability Foundation

Posted by Iago Mosqueira[1]
12 December 2011

The foundation of a well-managed fishery is to assess its status and compare it to target and limit reference points, thresholds for taking management action in order to maintain a stock’s health. This baseline, if well-defined, provides support to necessary management or harvest control rules used by management bodies to determine fishing effort and mortality.

In a new research paper published this week in the Proceedings of the National Academy of Sciences, titled ‘Global population trajectories of tunas and their relatives‘, a group of researchers summarized the global status of tunas, mackerels and Spanish mackerels, the Scombrids, and looked at the factors that have led to increased catches and population declines, such as the efficiency of fishing methods and the ability of vessels to travel much greater distances for longer periods of time. The paper identified two groups with populations whose decline has taken them beyond their safety limits:

“… the largest, longest lived, highest value temperate tunas and the smaller, short-lived mackerels, both with most of their populations being overexploited.”[2]

That means overfishing and poor management has negatively impacted populations of large tunas, such as Atlantic bluefin and southern bluefin. At the same time:

“The remaining populations, mostly tropical tunas, have been fished down to approximately maximum sustainable yield levels, preventing further expansion of catches in these fisheries.”[3]

These other populations are tuna species like skipjack and yellowfin, which in most cases can continue to be fished at or close to the current levels, but likely can not support any meaningful increase in fishing pressure.

One of the chief concerns highlighted by the authors is a lack of target and limit reference points for most stocks of tuna andtuna-like species. It is noted that few regional fisheries management organizations (RFMOs) have adopted these measures and instead use maximum sustainable yield (MSY) – the greatest amount that a stock can be fished indefinitely given certain conditions – as an “upper limit” of catch.

“We recommend the development of well-defined management strategies involving harvest control rules and the associated decision rules that can keep the fishery within defined limits.”[4]

These rules need to be tested by simulation, and their performance carefully followed, but they need to be adhered to once adopted.

On capacity, the authors are unequivocal in their support for controlling and reducing the size of fishing fleets and the need for increased management measures to further reduce fishing’s impact on the greater marine environment.

“The long-term sustainability of tunas and their relatives can only come from stricter management measures to treat MSY-related levels as a limit rather than a target management objective, to reduce the overall fishing capacity, and to rebuild overexploited populations, as well as further implementing regulations to minimize the collateral impacts of these fisheries on marine ecosystems.”[5]

The worst-case scenario of species on the path toward commercial extinction, which has drawn considerable media attention, overshadows the real message – tuna species can be fished sustainably, but must be managed according to well defined reference points and tested harvest control rules, and through a reduction in fishing capacity.