Study suggests tuna gillnet fisheries deadly for dolphins, but evidence unconvincing

  • A study published in January this year, assessed how many dolphins were caught in tuna gillnets fishery in the Indian Ocean. Gillnets are one of the most common fishing gears used to catch tuna in the region.
  • The study concluded that up to 80,000 dolphins were caught every year in the Indian Ocean and around 10,000 dolphins were caught in India each year by tuna gillnet fisheries.
  • But the study acknowledges that it is based on limited data and several assumption about dolphins and tuna fisheries in the region.

A study published the journal Endangered Species Research, assessed the state of dolphin bycatch in tuna gillnet fisheries in the Indian Ocean. The results are grim.

Tuna fisheries is a multibillion dollar industry, with the Indian Ocean contributing 20% of the global catch.  A variety of fishing gears are used to catch tuna including simple fishing rods, longlines, purse seines and the most important gillnets. All fishing methods can result in some sort of accidental bycatch, but studies suggest that gillnets are the most deadly for marine mammals.

Gillnets are basically a wall of netting placed below the surface of the water; schools of tuna and other fish swim into the net and get trapped by their gills. Like the fish, cetaceans, i.e. dolphins and whales, find it difficult to locate these fine nets and often swim into them and get entangled. If they are unable to surface for oxygen within a few minutes, they can drown.

Around 33 percent of tuna catch in the Indian Ocean comes from gillnets, and according to this latest assessment the nets have a massive impact on the dolphins in the region. The assessment suggests that 80,000 dolphins may be accidentally caught every year by tuna gillnetting boats, with India alone contributing 10,000 dolphins annually. Approximately 4.1 million dolphins may have been caught since tuna gillnetting began in the Indian Ocean in 1950.

The researchers acknowledged that the data they worked with had limitations and they had to make many assumptions to make such an assessment. But not everybody is convinced by these results.

Dolphin bycatch in the Indian Ocean

Ideally to understand how many dolphins are caught by the tuna fishing industry, researchers would need data on the amount of tuna caught in the region, the number of dolphins caught in each fishing season and the number of dolphins present in the entire Indian Ocean. But such information is scarce in the region.

So, the researchers of this Indian Ocean assessment, relied on two different sets of data: tuna catch data from 24 countries in the region, recorded by the Indian Ocean Tuna Commission (IOTC) – an intergovernmental agency whose job it is to ensure tuna fisheries in the Indian Ocean remains sustainable and annual dolphin bycatch data from India, Pakistan, Australia and Sri Lanka.

The Indian Ocean Assessment estimates that up to 4.1 million dolphins have been accidentally caught in tuna fisheries. Photo in public domain courtesy National Aeronautics and Space Administration (NASA), United States.

Using a combination of these annual bycatch numbers and some historical information, the Indian Ocean assessment researchers calculated the rate of dolphin bycatch per 1000 tonnes of tuna.

The researchers then used this dolphin bycatch rate and tuna catch data to calculate the number of dolphins caught by tuna gillnets each year from 1950 to 2020 which increased from 10,000 dolphins in 1950 to around 80,000 in 1995 and has been fluctuating between 80,000 and 100,000 since then. These annual bycatch numbers were then added up to conclude approximately 4.1 million dolphins were caught by tuna gillnets in the Indian Ocean since 1950.

Data uncertainty

But to arrive at these figures for the entire Indian Ocean, the assessment depended on a handful of studies and in at least one case unclear calculations. Take for instance annual dolphin bycatch numbers estimated for India by 2008 study by K.S.S.M. Yousuf and other researchers from Central Marine Fisheries Research Institute (CMFRI) from India.

This study recorded 44 dolphins from three harbours in Chennai and Kakinada and Mangalore over the course of 80 days. Yousuf and team noted that approximately 400 gillnetting boats operated out of these harbours but in total India had approximately 14,183 gillnetting boats catching tuna for 240 days of the year. Using this information, they estimated that tuna gillnetters would be catching 9,000 to 10,000 dolphins each year. The Indian Ocean assessment used this figure for their other calculations for the Indian Ocean.

But reporter repeated these calculations and found that the bycatch number would be closer to 4,680 dolphins per year. Moreover, only 30 of the 44 dolphins that Yousuf et al recorded came from gillnetting boats; the rest were caught by another fishing method called purse seines. So, this number would further drop if those 14 dolphins were removed from the calculation.

Charles Anderson, the main author of the Indian Ocean assessment and a researcher based in Maldives, confirmed this reporter’s calculation. Anderson pointed out that the original study by Yousuf et. al. had only visited harbours for three hours every day and suggested that Yousuf et. al. had increased the sampling hours in their calculation to arrive at the 9000 figure but didn’t mention this in their text.

He had not clarified with the Yousuf et. al if this was indeed the case and didn’t address why included dolphins caught in an entirely different fishing method was included in calculations for gillnetting.

Mongabay-India reached out to K.S.S.M. Yousuf regarding this data but received no response.

Small datasets

The assessment also assumes that studies from select fishing locations of just four countries was representative of the entire Indian Ocean. But Rahul Muralidharan, a researcher from the Ashoka Trust for Research in Ecology and Environment (ATREE), not associated with this study, questioned whether bycatch data from the Yousuf et. al. study was even representative of tuna gillnetting in all of India.

The study spanned only a small part of the fishing season. The Chennai harbour was sampled for 20 days in September, the Mangalore harbour for 15 days between November and December and the Kakinada harbour for 15 days each in March, May and September.

“But if you’re going to extrapolate you should have had sampling across three or four seasons. Then that will sort of give me an insight into how this bycatch varies across seasons,” said Muralidharan who studies the impact of dolphin conservation policies on artisanal fishermen in the Gulf of Mannar. Gillnets also don’t operate in the same location throughout the year, Muralidharan pointed out. “So, the fishing effort varies over time and space” he said.

The second Indian study used in the assessment attempts to address some of these questions. In this study, the main author Mohammed Koya (also a CMFRI researcher and co-author of the Indian Ocean assessment) asked the skippers or captains of three tuna gillnetting boats in Gujarat to record all the dolphins they caught while fishing, between 2011 and 2016.

In the six year period, the skippers reported that they had caught 30 dolphins. Dolphins were caught in 22 of the 567 fishing voyages that the fishermen undertook. From this data, the Indian Ocean assessment calculated that 14,400 dolphins were caught in all of India per year.

Gillnets are the preferred tuna fishing gear in the Indian Ocean but are considered particularly prone to dolphin bycatch. Graphic by Goodfish Badfish.

Although the Koya et. al. study encompassed several fishing seasons, this extrapolation is also based on a small sample of only three boats.

“Sample size of our study is relatively scarce mainly because of erratic compliance of the skippers/boat owners as well as the limitations in fund (nominal compensation provided to skipper for his time),” said Koya. “However, the selected fishing boats (comprised of different capacities) having been fished actively throughout the years and across the Gujarat coast is representative of the gillnet fishery of the state,” he said. The views expressed by Mohammed Koya here are personal and not that of his employer.

Considering these numbers are drawn from studies with low sample sizes, could these numbers be an overestimate, at least for India? Anderson felt that the bycatch numbers could be overestimated or grossly underestimated.

“This is why we have included such large uncertainty (error bars) with our estimates,” he pointed out. “Incidentally, if we have indeed over-estimated Indian bycatch, this would imply that recent cetacean population levels are even lower than we think, i.e. that they have been more heavily overfished than we estimate.”


This inference is based on the assumption that dolphin and tuna populations overlap in the Indian Ocean. Both dolphins and tuna are known to prefer productive waters with high prey abundance. So, the authors assumed that an increase in tuna catch indicated an increase in dolphin catch. Since IOTC data suggests that tuna itself are being overfished, it was likely the same with dolphins. But as the assessment itself points out, there is no evidence for this assumption.

Another assumption made by the Indian Ocean assessment is that dolphin populations are evenly distributed across tuna fishing locations. Wherever fishermen catch tuna, there must be dolphin bycatch regardless of seasonality and fishing locations.

But the Koya et. al. study used in this assessment appears to contradict this. The study was able to identify that in Gujarat dolphins were most frequently caught in summer and in the open ocean at depths between 100-200 metres. The study also showed that locations and seasons when dolphin bycatch was highest was also the least productive in terms of tuna catch.

Koya also pointed out that Gujarat could be seen as the largest tuna gillnetting state in India. Much of this was restricted to coastal waters, the area with least dolphin bycatch according to his 2018 study. Fishermen from southern states like Kerala and Tamil Nadu do fish for tuna in the open ocean more than fishermen from Gujarat and Maharashtra.

“However, such tuna fishing fleet mainly comprise of long liners (though few vessels carry both the gears). Long lines are ideal gear for harvesting larger oceanic tuna resources and have relatively less interaction with sensitive species like cetacean and turtles (except some scheduled sharks are likely to be caught as sharks are predominant catch in longlines),” said Koya.

In contrast, the Yousuf et. al. noted that all the dolphin bycatch came from maximum depth of 70 metres. This could mean that dolphin bycatch was high in both coastal waters and the open ocean or it could mean that bycatch could vary between southern and northern parts of the Indian coastline. None of this variation is included in the assessment.

“It is true that the assessment didn’t use finer information such as the depth of operations and the season of abundance of cetaceans and I feel that the estimate would have been slightly different (likely to be lesser than 14,400 individuals) had these discreteness [been] made in the assessment. We hope to attempt such an assessment in the near future with latest data,” said Koya.

Anderson accepted that it was entirely possible that cetacean bycatch rates vary between areas and fisheries. “It is not clear what effect violation of this assumption will have on the final results – this is something for future analyses,” Anderson said.

Lack of data

There are two aspects that seem to be leading to these assumptions. One, there is no baseline information on dolphin populations in the Indian Ocean.

“It would be very valuable to have a good baseline estimate of cetacean abundance. Unfortunately, there were very few early quantitative cetacean surveys conducted in the Indian Ocean, and none in Indian waters,” said Anderson who planned to do such surveys in the future.

Without a clear understanding of where different dolphin species are distributed in the Indian ocean and which areas have high numbers it is difficult to know where tuna gillnetting can be detrimental and where it might be safe.

Second, there is no data on bycatch in tuna fisheries from different parts of the country or indeed the Indian Ocean region. Anderson noted that although IOTC member countries (including India) had an obligation to report cetacean no national fisheries department was carrying out this task effectively. In fact, independent reviews have pointed IOTC had one of the worst track records of fisheries monitoring in the world.

Tuna fisheries is a multibillion dollar industry primarily serving the canned tuna industry. Photo by Daniel Case/Wikimedia Commons.

In western nations, tuna fishing ships often have an independent observer on board to monitor bycatch. If bycatch from fisheries is too high, the government can impose restrictions on fisheries. But in India and most Indian Ocean countries fishing boats are too small to have anyone except fishermen.

Thus, the onus of reporting dolphin bycatch is on fishermen. But there are no clear guidelines for fishermen to report bycatch, according to Vincent Jain, from the South Indian Federation of Fishermen Societies. “Where will they report? Where is the reporting office?” Jain asked

The dolphin is also a protected species under the Wildlife (Protection) Act. Even accidental killing can result in punishment. So, there is little incentive to report bycatch. “All it does is causes problems for them [fishermen],” Jain added.

A political problem

Muralidharan suggested that the issue must be viewed as a political problem.

“Basically, governments or states see fishing in terms of trade; how much fish they can trade with another government or another state so they can make money. So, they’re not going to impose controls on how much gear that you’re carrying or the size dimensions.”

Anderson agreed. “I cannot speak for India, but in general most countries are reluctant to address this issue and will deny everything.”

On the other hand, the Forest Department, under the Ministry of Environment, Forests and Climate Change, in charge of conservation is ill-equipped to deal with the impact of fisheries on endangered marine animals according to Muralidharan. “They don’t understand the scale of fishing or what’s really happening in the sea,” he said.

According to Muralidharan the two agencies simply cannot reconcile their own objective. Trapped in between are fishermen, who are encouraged to increase fish catch as the only way of earning a livelihood but also told that there must be no harm to endangered marine species in the process.

In such a context Muralidharan worried that assessments such as this one with all its assumptions, would be alarmist. But there was cause for alarm according to Anderson.

“For me, it does not matter if the cumulative mortality [for the entire Indian Ocean] from tuna gillnet fisheries was 4.1 million, or 4.2 million, or even 3.1 million or 5.1 million. Whatever the exact number, the indisputable conclusion is that the scale of bycatch is immense and unsustainable,” said Anderson cautioning that more data would not change the need for action.

Lack of trust

But the problem is not everybody agrees with this evidence. Vincent Jain expressed little faith in the numbers that the assessment reports, especially data collected in landing centres. “Turtles and some other kind of endangered species like sharks and all we have seen many places, but we have never seen a dolphin in a landing place,” he said.

He also ruled out that the bycatch numbers could be underestimated because fishermen were extremely careful to avoid dolphins. “They feel dolphins are like children. So, why will they bring them to landing centre?” Jain asked.

This contradiction between Jain’s claims and the assessment was the consequence of fishing communities never being equal partners in monitoring fisheries, suggested Muralidharan

For instance, until this reporter called him, Jain had never heard of the study. No organisation or agency had attempted to report these results to the fishermen or sought their views on the issue. He viewed this study and conservation actions in general as an attempt to blame fishermen.

Anderson asserted that the purpose of the study was not to impact livelihoods of fishing communities. “However, the overexploitation of dolphins is just one facet of the wider overexploitation of our oceans: turtles, sharks, tunas and other fishes are all facing the same fate. If this wider issue is not addressed, the future of these fishermen is bleak indeed.”

But if this concern is genuine, then fishermen must be involved in assessing the state of the fisheries according to Muralidharan. “Are they active participants in this? No. if you are designing a study with their active participation, then the methodology would be completely different,” he said. 

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


Anderson, R. C., Herrera, M., Ilangakoon, A. D., Koya, K. M., Moazzam, M., Mustika, P. L., & Sutaria, D. N. (2020). Cetacean bycatch in Indian Ocean tuna gillnet fisheries. Endangered Species Research, 41, 39-53.

Koya KM, Rohit P, Vase VK, Azeez PA (2018). Non-target species interactions in tuna fisheries and its implications in fisheries management: case of large-mesh gillnet fisheries along the north-west coast of India. Journal of Marine Biological Association, India 60: 18−26.

Yousuf KSSM, Anoop AK, Anoop B, Afsal VV and others (2009) Observations on incidental catch of cetaceans in three landing centres along the Indian coast. Marine Biodiversity Records 2: e64.


Banner image: Lucrative tuna fishing in the Indian Ocean might be deadly for dolphins according to new study. Photo by Jean Beaufort via Public Domain Pictures.

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