Fuel consumption and air emissions in one of the world’s largest commercial fisheries

Highlights

• We modelled the annual fuel consumption in tuna purse seiners of the Indian Ocean.

• Days at sea, numbers of operations, vessel length and age affect fuel consumption.

• Fuel Use Intensity and consumption showed strong variability over 1981 to 2019.

• Greenhouse gas emissions were about 600,000 t y⁻¹ of CO₂-eq over 2015 to 2019.

• Sulphur limits reduced annual SO₂ emissions from >1500 t to almost none since 2018.

Abstract

The little information available on fuel consumption and emissions by high seas tuna fisheries indicates that the global tuna fleet may have consumed about 2.5 Mt of fuel in 2009, resulting in the production of about 9 Mt of CO₂-equivalent greenhouse gases (GHGs), i.e., about 4.5–5% of the global fishing fleet emissions. We developed a model of annual fuel consumption for the large-scale purse seiners operating in the western Indian Ocean as a function of fishing effort, strategy, and vessel characteristics based on an original and unique data set of more than 4300 bunkering operations that spanned the period 2013–2019. We used the model to estimate the total fuel consumption and associated GHG and SO₂ emissions of the Indian Ocean purse seine fishery between 1981 and 2019. Our results showed that the energetic performance of this fishery was characterized by strong interannual variability over the last four decades. This resulted from a combination of variations in tuna abundance but also changes in catchability and fishing strategy. In recent years, the increased targeting of schools associated with fish aggregating devices in response to market incentives combined with the IOTC management measure implemented to rebuild the stock of yellowfin tuna has strongly modified the productivity and spatio-temporal patterns of purse seine fishing. This had effects on fuel consumption and air pollutant emissions. Over the period 2015 to 2019, the purse seine fishery, including its support vessel component, annually consumed about 160,000 t of fuel and emitted 590,000 t of CO2-eq GHG. Furthermore, our results showed that air pollutant emissions can be significantly reduced when limits in fuel composition are imposed. In 2015, SO₂ air pollution exceeded 1500 t, but successive implementation of sulphur limits in the Indian Ocean purse seine fishery in 2016 and 2018 have almost eliminated this pollution. Our findings highlight the need for a routine monitoring of fuel consumption with standardized methods to better assess the determinants of fuel consumption in fisheries and the air pollutants they emit in the atmosphere.

Introduction

Oceangoing ships constitute a significant source of air pollution through the emission of greenhouse gases (GHGs) such as carbon dioxide (CO₂) and nitrous oxide (N₂O), and other air pollutants such as sulphur dioxide (SO₂) (Corbett and Fischbeck, 1997; Corbett et al., 1999). Global shipping emissions have major effects on the environment, including ocean acidification and contribution to climate change, and on human health (Corbett et al., 2007; Jägerbrand et al., 2019; Tian et al., 2013). With about 2.5 million motorized vessels out of 4.6 million vessels in operation (FAO, 2018; Rousseau et al., 2019), the global fishing fleet annually consumes about 30–40 million tonnes (Mt) of fuel and accounts for more than 1% of the global marine fuel demand (Parker et al., 2018; Tyedmers et al., 2005). Global emissions from fuel combustion by fishing vessels have been estimated at about 180–200 Mt of CO₂-equivalent GHGs every year (Parker et al., 2018). Furthermore, total emissions related to fishing go beyond the direct emissions of fuel combustion because of indirect effects of upstream and downstream activities, e.g., emissions generated during fuel processing and refining, fish product packaging and transport (Winebrake et al., 2007). Fishing and water transport are therefore considered among the most air-polluting industries, per unit of wealth created, in particular for CO₂ and SO₂ (Bagoulla and Guillotreau, 2020). To improve air quality and global health, global sulphur limits of 0.5% (mass/mass) in fuel oil have been recently imposed by the International Maritime Organization (IMO) under the MARPOL convention (Annex VI) to reduce the emissions of both sulphate aerosols and sulphur-containing particles (Chu Van et al., 2019).

Industrial tuna fisheries are one of the most highly capitalized fisheries in the world (Miyake et al., 2010). High seas fishing vessels, typically longer than 25 m, travel long distances to search and catch highly migratory tuna and billfish widely distributed across the world’s oceans (Fonteneau, 2010). Energy costs make up to 20% or more of total running costs in the high seas fishing industry (Miyake et al., 2010). However, little information is available on fuel consumption and emissions by high seas tuna fisheries. This said, a survey-based study indicated that the global tuna fleet may have consumed about 2.5 Mt of fuel in 2009, resulting in the production of about 9 Mt of CO₂-equivalent GHGs, i.e., about 4.5–5% of the global fishing fleet emissions (Tyedmers and Parker, 2012). Although large-scale purse seiners represent a very small component of the global tuna fleet (∼700 vessels in 2020; Justel-Rubio and Recio (2020)), they accounted for more than two thirds of the global catch of tuna since the late 2000s. In 2009, the global tuna purse seine fishery was responsible for the release of more than 3 Mt of CO₂-equivalent GHGs into the atmosphere (Parker et al., 2015b).

The global tuna purse seine fishery has significantly changed over the last decade. The purse seine catches of tropical tuna increased from about 2.8 Mt in the late 2000s to more than 3.2 Mt in the late 2010s, with about two thirds of the catch coming from fish aggregating devices (FAD) and the rest from free-swimming schools (FSC) and schools associated with dolphins (Taconet et al., 2018). In the Indian Ocean, the catch of the tuna purse seine fishery, composed of about 50 vessels larger than 65 m, increased from 280,000 t in the late 2000s to almost 500,000 t in 2018 (Fiorellato et al., 2019). In particular, the advent and increasing use of echo-sounder buoys attached to the FADs deployed at sea has greatly increased the efficiency and catchability of purse seiners over the last decade (Lopez et al., 2014; Wain et al., 2020). Furthermore, 20 support vessels assist the purse-seine fishing fleet by maintaining a network of FADs. These support vessels have proved to be instrumental in increasing fishing success, although they consume additional fuel energy and produce more GHG emissions (Assan et al., 2015; Ramos et al., 2010). Over the last decades, an increasing proportion of FAD-caught tuna has been observed in the Indian Ocean purse seine fishery. Since 2017, the use of FADs has been further accentuated by a shift in the fishing strategy to target more skipjack tuna (Katsuwonus pelamis) (Assan et al., 2019; Baez et al., 2018; Floch et al., 2019). This change occurred following the implementation of a total allowable catch on yellowfin tuna (Thunnus albacares) by the Indian Ocean Tuna Commission (IOTC) with the aim of rebuilding the yellowfin tuna stock. Yellowfin tuna compose the large majority of FSCs while tuna schools associated with FADs are dominated by skipjack tuna (Fonteneau et al., 2013). Such a change in fishing strategy may have affected the fuel consumption and air pollutant emissions as purse seiners targeting schools associated with FADs have been shown to consume more fuel per ton landed than purse seiners targeting FSCs at global scale (Parker et al., 2015b).

In this context, the overarching objective of the present study was to estimate with more accuracy the GHG and SO₂ emissions of the tuna purse seine fishery of the Indian Ocean over the period 1981 to 2019 and assess how they vary with fleet structure, fishing strategy and productivity. First, we developed a model of fuel consumption of tropical tuna purse seiners based on a unique large data set of bunkering operations that took place in the Seychelles between 2013 and 2019. Secondly, we used the model to estimate the direct total fuel consumption and associated GHGs and SO₂ emissions for the western Indian Ocean purse seine fishery over the last four decades (1981–2019), including the fuel consumed by the fleet of support vessels. Finally, we assessed the extent of the reduction in SO₂ emissions following the mandated reduction in sulphur content of the marine diesel oil delivered in the Seychelles.