The researchers say fisheries managers have wrongly treated upper limits for catches as target levels for fishing, contributing to global declines and the threatened status of some species.
Internationally, sea fishing is regulated and managed by regional fisheries management organisations (RFMOs), most of which have the power to set limits for fish catches.
RFMOs are important in regulating fishing of widespread, migratory species such as tuna.
The EU plays an active role in six RFMOs focused on tuna fishing1. For over a decade, the UNs Food and Agriculture Organization has recommended that RFMOs treat one widely recognised reference point for fish catches the maximum sustainable yield (MSY) as an absolute upper limit.
However, many tuna RFMOs have not implemented specific limits, and have viewed MSY as a target.
The value of the MSY is supposed to provide a theoretical limit for the amount of fish that can be caught before the population is unable to replenish itself through breeding.
It is difficult to estimate accurately2 and therefore regarding MSY as a target level can lead to fisheries catching more than can be replenished.
It is well known that tuna populations are declining as a result of overfishing.
However, some species are more seriously affected than others.
The study charts population decline (or growth) of 26 related species - 17 species of tuna and 9 species of mackerel and Spanish mackerel between 1954 and 2006, based on stock assessments. Some funding for the study was received through the EU METAOCEANS project3.
Overall, global stocks of these 26 populations decreased by more than half (52%) during this time, despite the relative resilience of one particular tropical species, the West Pacific skipjack tuna, which has been declining, but more slowly.
Discounting the skipjack tuna population, global stocks fell by over 60%.
Tropical species have been more resilient in general, declining less steeply than temperate species, such as the Southern bluefin tuna and Albacore tuna.
An evaluation by the International Union for Conservation of Nature (IUCN) recently led to Southern and Atlantic bluefin tunas being added to the Threatened category of the IUCN Red List.
When the researchers categorised the 26 species they studied according to exploitation status, West and East Atlantic bluefin tuna and Southern bluefin tuna were some of the most overexploited, meaning they are being fished above the level of MSY.
It is thought that the longer life spans, shorter breeding seasons and more restricted breeding areas of the temperate species make them more vulnerable.
Continued losses of tuna and mackerel may have unknown effects on marine ecosystems.
As well as revising management strategies to work appropriately within MSY limits, the researchers suggest that the Convention on International Trade in Endangered Species (CITES)4 and other conservation tools should work alongside existing fisheries management frameworks to ensure sustainable fishing.
Notes
1 http://ec.europa.eu/fisheries/cfp/international/rfmo/index_en.htm
2 www.fao.org/docrep/005/y3427e/y3427e07.htm
3 METAOCEANS (Elucidating the structure and functioning of marine ecosystems through synthesis and comparative analysis) was supported by the European Commission through the Sixth Framework Programme.
www.azti.es/proyectos/metaoceans-elucidating-the-structure-and-functioning-of-marine-ecosystems-through-synthesis-and-comparative-analysis.html
4 See: www.cites.org
Source: Juan-Jord, M., Mosqueira, I., Cooper, A.B. et al. (2011). Global population trajectories of tunas and their relatives. Proceedings of the National Academy of Sciences. 108(51): 20650-20655.
