2015 El Niño threatens tuna fishery

Guest student blogger: Juan Carlos Villaseñor-Derbez


tuna1Imagine a helicopter circling a fleet of small skiff boats surfing the waves down below. The radio is on, blaring loud instructions: “go left, faster, faster, faster!”, “Now, catch’em! Catch’em!” In the distance, a large vessel swings in the waves, its captain coordinating every detail of this perfectly planned operation, frantically communicating over the radio. This is not a James Bond movie; this is the big leagues of the fishing industry: tuna.

In Mexico, this fishery is vital not only to the economy, but supports a way of life.  Canned tuna is a cheap protein source, essential for a person to meet their basic food needs. But changes in ocean conditions due to El Niño are threatening fishermen with empty nets.


Fishing for Tuna

Purse-seineTuna live in the uppermost layer of the ocean, rarely swimming deeper than 100 meters, at which point the water rapidly becomes colder with each descending meter. This cold water acts as a physical barrier for tuna, and fishers take advantage of that.

Helicopters are used to locate the tuna, small boats herd the fish with direction from above, and two large vessels cast a net around the fish, in a process known as purse seining. When fishermen cast their nets, tunas find themselves “trapped” between the ocean’s surface, a floor of cold water, and a 150 meter deep net that encircles them. Fishermen only need to close the bottom of the net and haul the fish up to the boat.  When the operation is well coordinated, a single catch may yield up to 100 tons of tuna.

While fishing effort remains relatively constant, an increased number of “null sets,” or zero catch, during El Niño seasons has been observed.  Typical years have about 10% of null sets, while El Niño seasons present up to 25%.


Null Sets Demystified

Fishery scientists from Universidad Autónoma de Baja California might have found an explanation to this phenomenon. Compiling more than 10 years of data (1992-2013) on null sets from the Mexican tuna purse seine fleet, scientists were able to identify patterns of this occurrence in Mexican fisheries.

Null sets occur in areas of downwelling, where the cold deep water that act as a barrier for tuna is pushed down by warmer water at the surface. The data revealed an increased number of null sets in areas associated with downwelling.

One of the big influencers changing these ocean conditions is El Niño.  During El Niño events, a deeper portion of the water column in the Eastern Tropical Pacific is warmer than usual. Therefore, the cold water floor that traps the tuna is deeper than fishermen’s nets, and fish are able to escape. The costs of these unsuccessful attempts, along with the invested time and money for fishing, cause the price per kilogram of tuna to rise, without increasing the total catch.



When the floor of cold water is deeper, fish can escape under the net. During El Niño years this cold water is often deeper than usual.

This Year’s Challenges

Reports from NOAA2 indicate that this year’s El Niño is one of the three strongest events on record. In Mexico, where tuna represents the third most important fishery by volume and value3, the 2015 El Niño could pose a serious threat by increasing the likelihood of null sets.  With null sets occurring up to 20% of the times for mild El Niño events1, the economic implications of a strong El Niño could be huge.

El Niño events directly impact many fishing industries, but these effects can ripple and reach society. Drops in tuna production and increases in price would make an affordable protein inaccessible to a large portion of the Mexican population.  The main problem is not the lack of fish for an industry and a society, but the uncertainty of how this might impact on the economy and management. While we don’t know exactly what this year’s climate phenomenon has in store, the likely aggregate impact of an El Niño event will be reduced catches.


Juan Carlos  Villaseñor-Derbez is a Latin American Fisheries Fellow (LAFF) at the Bren School. He is an first year MESM student (2017) specializing in Coastal Marine Resource Management.


1 Dreyfus-León, M.J., Mejía-Trejo, A., Villaseñor-Derbez, J.C. 2015. Analysis of null sets (zero catch) made by the Mexican tuna purse seine fleet (2000-2013). Ciencias Marinas, 41(2): 85-92, doi: 10.7773/cm.v41i2.2471
2 [NOAA] National Oceanic and Atmospheric Administration. 2015. El Niño/Southern Oscillation Diagnostic Discussion. [Cited 29 November 2015]. Available from http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/index.shtml
3 [SEMARNAT] Secretaría de Medio Ambiente y Recursos Naturales. 2000. Pesquería del atún. [Cited 29 November 2015]. Available from: http://app1.semarnat.gob.mx/dgeia/estadisticas_2000/informe_2000/07_Aprovechamiento/7.3_Recursos_pesca/data_recursos_pesca/recuadro4.htm

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