Fuel consumption and air emissions in one of the world’s largest commercial fisheries
Graphical abstract
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.
Section snippets
Fuel data
All bunkering operations in Port Victoria are recorded by the Seychelles Petroleum Company (SEYPEC) and include the vessel name, type of gasoil (i.e., sulphur content), volume (l) and weight (t) delivered, and the date and location of delivery. All purse seiners and support vessels considered in the study use the same marine diesel oil, a marine fuel composed of various blends of distillates and heavy fuel oil. Except for sulphur, the general composition of the marine fuel delivered in Port
Fuel delivery in Port Victoria
From 2013 to 2019, mean total weights of 139,000 t (SD = 20,000 t) and 8000 t (SD = 3000 t) of fuel were delivered annually to the purse seiners and support vessels calling on Port Victoria, Seychelles, respectively (Table 1). Over that period, the support vessels represented 5.4% of the total fuel purchased in Port Victoria. For purse seiners, the mean fuel quantity delivered during an operation was 264 t and the maximum was 858 t (Table 1). For support vessels, the mean was 79 t and the
Discussion
Our results provide a four decade perspective on the air pollutant emissions of one of the world’s largest commercial fisheries, the Indian Ocean purse seine fishery, responsible for about half a Mt of tropical tuna catch in 2018. Based on an original and unique data set of more than 4300 bunkering operations spanning the period 2013–2019, we developed a model of annual fuel consumption for a subset of large-scale purse seiners based in Port Victoria, Seychelles, as a function of fishing
Conclusion
Our model of purse seiner fuel consumption allowed us to reconstruct the history of air pollutant emissions of the Indian Ocean purse seine fishery over four decades. The FUI predicted by our model is in line with that found in earlier studies, but it also shows a great inter-annual variability according to environmental and fishing conditions that should be taken into greater consideration. The shifting structure of the fleet towards larger vessels assisted by support vessels and more
Author contributions
Conceptualization, EC, SA; Funding Acquisition, EC, SA, JL; Project Administration, EC, SA;
Sampling and Data Acquisition, EC, SA, JL, CA, MM; Data Curation and Analysis, EC, SA, JL,
CA, NB; Writing – Original Draft Preparation, EC; Writing – Review & Editing, All co-authors
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We are grateful to all the people involved in the collection and management of the fisheries data used in the present study, including the purse seine fishing associations ANABAC, OPAGAC and ORTHONGEL. Special thanks to Patrice Dewals for his dedication to the curation of the fisheries data. We are deeply grateful to Joan Didon for her work on the bunkering data as well as Sarah Romain, Xerxes Pardiwalla and Alexandre Barbier from SEYPEC for their support and assistance with the fuel data set.
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