El Niño-Southern Oscillation can affect tuna fish in the Indian Ocean, says FAO report

El Niño, a weather phenomenon that occurs in the central and eastern equatorial Pacific Ocean, has been known for impacting agricultural production, leading to harvest loss. However, until now, not much has been known about El Niño’s impacts on marine fisheries and aquaculture.

The Food and Agriculture Organization (FAO) of the United Nations has been focussing on early warning systems as a disaster preparedness measure to help farmers. Along the same lines, the international food agency has published a report titled El Niño Southern Oscillation (ENSO) Effects on Fisheries and Aquaculture, which captures the impacts of ENSO events on global fisheries.

India has a coastline of 7,516 km, an exclusive economic zone (EEZ) of 2.02 million square km and more millions of people live in the coastal areas. According to the last estimates for 2017, the fisheries sector provided about 13 million jobs, with women representing about 32 percent of the people employed in the sector. In terms of total fisheries and aquaculture production, in 2008-2010 and in 2012, India was the second major producer in the world, and in 2017 ranked third. Fish production reached an estimated level of 11.6 million tonnes in 2017.

While the impact of ENSO on island fisheries and aquaculture in India has not been reported so far, it does not mean that ENSO does not have any effect on the country. Certain El Niño events usually reduce monsoonal precipitation and alter the probability of extreme events, which may affect inland fisheries and aquaculture. Dedicated studies with quality data are required to assess the exact influence of ENSO in each region, said Florence Poulain, fisheries officer, FAO Fisheries and Aquaculture Department in Rome.

Tracing El Niño’s history

El Niño, described as an unusual and warm ocean current by South American fishers in 1600, off the coast of Peru, triggered transformations in the pre-Colombian civilisation.

ENSO is characterised by a warming phase in the central and eastern equatorial Pacific as well as the opposite cooling phase known as La Niña. During El Niño, “Tropical species arrive from the north while native fish species either migrate southwards or collapse, the most famous case being that of the Peruvian anchovy or anchoveta, populations of which typically collapse during these events,” the report notes. Thus, seabirds and mammals along the Peruvian coast that depend on anchoveta for food can also decline.

In the Pacific Ocean, during the warm (El Niño) phase, the thickness of the top layer of the eastern Pacific Ocean classically increases, thus preventing cold and nutrient-rich deep waters to reach the surface, dampening production on which fish depend for food, said Arnaud Bertrand, senior scientist at the Institute of Research for Development (IRD), and one of the authors of the FAO report.

In the eastern Pacific, small pelagic fish such as anchovy can be dramatically impacted while more tropical pelagic fish, like tuna or dolphin fish, are favoured and become more accessible to fishers close to the coast. In the same way, the tropicalisation during strong El Niño events has differential impacts on benthic and demersal species, which results in those that survive (for example scallops, octopus, shrimp and lobsters) as well as those that don’t (for instance crabs, clam and some hake species). The problem with ENSO is that its incidences are rarely similar in nature, the report says. Besides this, sea surface temperature anomalies that have accompanied these events appear to have intensified in the past two decades, with some climate models suggesting that their frequency is on course to double.

The authors suggest that every ENSO event is different in intensity, duration and consequences. They also agree that ENSO events generally worsen with the effects of climate change on fish and fisheries, but the evidence is not yet conclusive enough. In this context, the identification of five broad types of ENSO: Extreme El Niño, Moderate Eastern Pacific (EP) El Niño, Moderate Central Pacific (CP) El Niño, Coastal El Niño and Strong La Niña, is of vital importance.

“The impact of an ENSO event can be positive, null or negative, according to the type of event,” said Poulain. For example, while extreme El Niño and a strong La Niña have negative and positive effects on Peruvian anchovies respectively, El Niño in the central Pacific has no detectable impact.

Impact on fisheries

For marine fisheries, the difference in fish catch is typically more than four million tonnes depending on the type of the ENSO event, the report notes. While the bulk of the net change is on Pacific fisheries, there are impacts on some fish populations in the Atlantic Ocean as well as on tuna fisheries in the Indian Ocean.

“In average an Extreme El Niño event lead to a negative anomaly of 3.2 millions of tonnes in world marine fish catches (it can reach 6 millions of tonnes during a given extreme event, i.e., El Niño 1997-98) while strong La Niña leads to an average positive anomaly of 1.3 millions of tonnes (up to about 2 millions of tonnes for a given event). Therefore on average, there is a difference higher than 4 million tonnes between an extreme El Niño year and a strong La Niña year,” said Bertrand in an email to Mongabay-India.

However, the report urges policymakers to realise “interactions between different sectors that provide food for the global populations”. The report explains that modern agriculture and aquaculture have both been reliant on marine capture fisheries for the provision of fish by-products for feed. Apart from marine fisheries, ENSO events can also significantly impact aquaculture output.

The report champions institutional adaptation, livelihood adaptation and risk reduction and management for resilience for offsetting ENSO impacts. Currently, reasonable forecasts can be made up to nine months in advance, but with very little ability to predict which ENSO type will occur.

The response to an El Niño warning must be tailored to the type of event, or otherwise, the impact of adaptation measures may have the opposite effect, the authors said.

Capacity building is critical to assess, warn, disseminate and communicate potential threats to end-users through early warning systems. Supporting participatory or collaborative monitoring of changes (for instance in sea surface temperature, in weather and climate patterns) and learning are key to inform fisheries policy, management and regulation over time. Response capabilities also need to be strengthened. This can range from supporting adaptive management approaches and regulatory capacity (for example temporarily halting fishing at any time) to enhancing people’s adaptive capacity by promoting the diversification of livelihood activities and by strengthening education, building economic safety nets, raising awareness and developing disaster risk reduction measures specific to each type of ENSO ahead of the event.

Impact on tuna in the Indian Ocean

The report says that ENSO events will affect tuna fish in the Indian Ocean. According to Matthieu Lengaigne, senior scientist at IRD in France, the economic importance of tuna in the Indian Ocean allowed researchers to gather quality data on this specific species. However, globally tuna catches are not impacted by ENSO.

“Tuna have the capability to cope with ENSO events by migrating to other areas. Consequently, ENSO can cause strong increase or decrease in tuna catch in a given region with important consequences for local communities,” said Lengaigne.

According to Paul de Bruyn, science manager at the Indian Ocean Tuna Commission in Seychelles, India does have a significant catch of tuna and tuna-like species. According to India’s national report, the total landings of tuna and tuna-like species along the Indian mainland coast for 2018 was estimated at 2,08,928 tonnes.

The oceanic tunas comprising three species (yellowfin, skipjack and big eye tunas) contributed to 35.65 percent of the total tuna landings during 2018.  The neritic tunas (coastal tunas) comprising four species contributed to 27.76 percent during the same period. The Spanish mackerels also contributed significantly (21.57%) to the total tuna and tuna-like species landings of India during the year 2018. The billfishes, including Indo-Pacific sailfish, marlins and swordfish collectively formed 8.79 percent, whereas the other species (5.78%) and pelagic sharks (0.46%) constituted the rest of the landings.

Information gleaned from the ESSO-INCOIS also says landings have been continually low. ESSO is Earth System Science Organization under the Ministry of Earth Sciences; Indian National Centre for Ocean Information Services (INCOIS) is an autonomous organisation under the government of India.

Lengaigne said it is generally the lack of data that prevents the characterisation of fish or fisheries’ response to ENSO. “Regarding the decline observed over the recent years in the Indian Ocean, it is not necessarily related to ENSO, but could also be attributed to the growing pressure of overfishing on the fish population or climate change, which has been reported to decrease the productivity over the Arabian Sea, and may hence have reduced the fish population there,” he added.

The report says that the Pacific and the Indian oceans are connected via the Indonesian Archipelago. But Lengaigne says that the main impact of El Niño on the Indian Ocean is mediated by the atmosphere. An El Niño usually induces warming over the Indian Ocean, as it reduces the cloud cover over this ocean by modifying the atmospheric circulation.

El Niño is also responsible for the generation of the Indian Ocean Dipole in the Indian Ocean (though it is not the only generating mechanism), which is known to shift the tuna population as reported in the document.

The impact of ENSO on tuna fisheries occurs indirectly mainly via the Indian Ocean Dipole, which results in a shift of the tuna population in the Indian Ocean rather than an overall decrease or increase over the entire basin. That is why ENSO does not affect the whole fisheries landing when averaged over the entire Indian Ocean, Lengaigne explained.

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“Tunas have a preferred temperature range. Tropical tunas (yellowfin, skipjack and bigeye) prefer higher temperatures than temperate tunas like albacore. Neritic tunas (found in shallow seas) have a variety of temperature ranges depending on the species. Warming water tends to change the distribution of the species. They move to areas where they can find their preferred water temperature. This may affect migrations, feeding and spawning (reproduction), and so may have a profound effect on the long-term viability of the populations,” said Bruyn.

Some species of tuna may migrate large distances. Species such as yellowfin, bigeye, skipjack and albacore are considered pelagic and wide-ranging species. The neritic tunas tend to migrate smaller distances and are more often associated with the coastal shelf, added Bruyn.

The worrying factor is that the Indian Ocean Dipole can also cause changes in fish production irrespective of ENSO. While half of the Indian Ocean Dipole events tend to be triggered by ENSO, half of them develop without any ENSO signal in the Pacific. These ENSO-independent Indian Ocean Dipole events are expected to have the same impacts as the ENSO-dependent ones, authors conclude.

“In the Indian Ocean, the main mode of variability is the Indian Ocean Dipole. It is only partly connected to ENSO variability, which blurs the effects. In addition, there is a lack of specific studies based on fair data set to fully evaluate ENSO impact on fisheries in India. However, in addition to tuna, a clear effect is related to marine heatwaves that are fostered by ENSO and destroy coral reefs and associated fish communities and fisheries,” said Lengaigne.

One of the main conclusions of the report is that there is indeed a lack of quality data and dedicated studies to fully assess the influence of ENSO on fisheries and aquaculture in general.

But experts feel that if a response plan is ready in place, a few months’ advance notice allows taking appropriate measures ahead of the peak, as for example transferring fishing effort from species that are expected to be threatened to others that should be favoured as well as evacuating vulnerable sites. All these measures must be part of a long-term fisheries and aquaculture development plan to be effective and readily available at short notice.

Banner image: Fisherfolk and their traditional vessel in the Bay of Bengal. Photo by E. Amalore/FAO.