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Marine Ecology Progress Series 639:215 Vol. 639: 215–232, 2020 MARINE ECOLOGY PROGRESS SERIES Published April 2 https://doi.org/10.3354/meps13284 Mar Ecol Prog Ser Trophic overlap between marine mammals and fisheries in subtropical waters in the western South Atlantic Rodrigo Machado1,2,3,*, Larissa Rosa de Oliveira1,4, Paulo Henrique Ott1,5, Manuel Haimovici6, Luis Gustavo Cardoso6, Lucas Milmann1,7, Maria Alejandra Romero8,9, Roberta Aguiar dos Santos10, Márcio Borges-Martins1,2 1Grupo de Estudos de Mamíferos Aquáticos do Rio Grande do Sul, Torres, RS, 95560-000, Brazil 2Universidade Federal do Rio Grande do Sul, Programa de Pós-Graduação em Biologia Animal (PPGBAN), Porto Alegre, RS, 91501-970, Brazil 3Universidade do Extremo Sul Catarinense (UNESC), Programa de Pós-Graduação em Desenvolvimento Socioeconômico (PPGDS), Criciúma, SC, 88.806-000, Brazil 4Laboratório de Ecologia de Mamíferos, Universidade do Vale do Rio dos Sinos, São Leopoldo, RS, 93022-000, Brazil 5Laboratório Biodiversidade e Conservação, Universidade Estadual do Rio Grande do Sul, Unidade do Litoral Norte, Osório, RS, 95520-000, Brazil 6Universidade Federal do Rio Grande, Instituto de Oceanografia, Laboratório de Recursos Pesqueiros Demersais e Cefalópodes, Rio Grande, RS, 96201-900, Brazil 7Universidade Estadual de Santa Cruz, Programa de Pós-Graduação em Ecologia e Conservação da Biodiversidade, Ilhéus, BA, 45662-900, Brazil 8Centro de Investigación Aplicada y Transferencia Tecnológica en Recursos Marinos ‘Almirante Storni’, Güemes 1030 (8520) San Antonio Oeste (RN), Argentina 9Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Buenos Aires, C1425FQB, Argentina 10Instituto Chico Mendes de Conservação da Biodiversidade (ICMBio), Centro Nacional de Pesquisa e Conservação da Biodiversidade Marinha do Sudeste e Sul (CEPSUL), Itajaí, SC, 88301-445, Brazil ABSTRACT: Marine mammals and humans are apex predators and both may compete for fish in ecosystems under continuous fishing pressure. We assessed the degree of trophic overlap between prey species found in the diet of 5 marine mammals (39 specimens of sea lion Otaria flavescens, 61 fur seals Arctocephalus australis, 76 franciscana dolphins Pontoporia blainvillei, 25 bottlenose dolphins Tursiops truncatus and 28 Lahille’s bottlenose dolphins T. gephyreus) and the catches of the 6 main commercial fishing gears used in southern Brazil (coastal gillnets, oceanic gillnets, purse seine, demersal pair trawling, bottom [single] trawl and double-rig trawling) between 1993 and 2016. An adjusted general overlap index indicated an overall moderate to high overlap. Specific overlap analysis showed that O. flavescens and T. truncatus presented high trophic relationships with fisheries, followed by T. gephyreus. Smaller interactions were observed for A. australis and P. blainvillei, even though they also exploit commercial fishing resources. Coastal gillnet and pair bottom trawling are the fisheries that most target the fish species favoured by O. flavescens, T. gephyreus and T. truncatus. The information presented in this study on trophic interactions may assist decision making for both fishery management and conservation measures for these apex predators. Commercial fishing activities are a major threat to marine mammals both regionally and globally. Current levels of fishing or its intensification may lead to dramatic changes in the coastal marine food web, including additional threats to coastal marine mammal populations in southern Brazil. KEY WORDS: Trophic interactions · Feeding ecology · Marine mammals · Fisheries · Marine conservation Resale or republication not permitted without written consent of the publisher *Corresponding author: [email protected] © Inter-Research 2020 · www.int-res.com 216 Mar Ecol Prog Ser 639: 215–232, 2020 1. INTRODUCTION on natural ecosystems and a lack of proper fishing management (e.g. Haimovici 1998, Froese et al. 2012, copy Marine mammals are an informal group which Pauly & Zeller 2016). Another factor that also poten- contains representatives of 3 orders, Cetartiodactyla, tiates this scenario is habitat loss in continental and Sirenia and Carnivora (Gatesy et al. 2013, Würsig et estuarine aquatic systems related to water pollution, al. 2018). They exhibit complex and heterogeneous deforestation, disordered urban development and Author distributions throughout the ocean, between polar improper management practices (e.g. Barletta et al. and equatorial regions, and coastal and estuary 2010). zones, as well in freshwater environments (Forcada All of these factors can exacerbate the conflict 2018). While some marine mammals have mostly between marine mammals and fisheries, and the monotypic diets (e.g. sea otters specializing on ben- interactions between them can be either operational thic invertebrates and baleen whales specializing on or biological. Biological (or ecological) interactions large zooplankton), others (e.g. pinnipeds and toothed refer to the indirect effects of competition for fish or whales) consume a variety of prey species including food resources, while operational interactions include mainly benthic invertebrates, fish and cephalopods direct contact between marine mammals and fish- (e.g. Pauly et al. 1998, Trites & Spitz 2018). However, eries. As a result of interactions with fisheries, marine in general, pinnipeds and toothed whales are mostly mammals can impact commercial fish stocks and opportunistic and generalist, whereas fish contribute damage commercial fishing gears. They may also get roughly to 50% of their diets (e.g. Pauly et al. 1998, entangled in discarded material, harassed by fishing Trites & Spitz 2018). The differences and similarities vessels or even attacked by fishermen (Wickens in the diets of the species reflect their evolutionary 1995, Lavigne 2003, Northridge 2018). adaptations to marine life, both morphologically and Trophic relationships (biological interactions) be- behaviourally (Trites & Spitz 2018), which are associ- tween marine mammals and fisheries are complex ated with different feeding strategies and habitats and little studied despite their importance for under- used by marine mammals while foraging (Heithaus standing how top predators coexist in an ecosystem et al. 2018). This foraging behaviour can sometimes under continuous and growing human pressures lead to interactions with humans. (Trites et al. 1997, Yodzis 1998). Although not a meas- Interactions between marine mammals and human ure of competition, trophic overlap indices may be activities occur when feeding, breeding, and resting used to provide an approximation of the levels of bio- areas overlap with areas used or impacted by humans. logical interaction between marine mammals and In this context, the main threats to marine mammals fisheries. This approach has been successfully used arising from interactions with human activities are in the western South Atlantic Ocean (e.g. Dans et linked to habitat loss from coastal development, water al. 2003, Szteren et al. 2004, Romero et al. 2011, pollution, noise pollution from military and seismic Machado et al. 2018). sonar, boat strikes, direct harvesting, marine debris, In the subtropical region of the western South effects of climate change, en tanglement in fishing Atlantic, the industrial fishery targeting coastal dem- gear and loss of prey or other food sources due to ersal fish species began with bottom trawling in the poor fisheries management (Polidoro et al. 2008, 1950s followed by the introduction of bottom gillnets Schipper et al. 2008). in the 1980s (Yesaki & Bager 1975, Vasconcellos et al. Interactions with fisheries are currently the main 2014). Coastal pelagic fish species, mainly bluefish conservation problem of most marine mammal spe- Pomatomus saltatrix (Linnaeus, 1766), began to be cies (Schipper et al. 2008, Northridge 2018), and this explored as a potential commercial species by purse conflict generally constitutes a global problem result- seiners in the 1960s (Yesaki & Bager 1975) and by ing in negative consequences for both parties. While pelagic gillnetters in the 1980s (Haimovici & Krug this competition could lead to a decrease in the rela- 1996). In southern Brazil, fisheries removal is esti- tive abundance of the marine mammals or changes in mated to account for about one-quarter of the total their diet composition, the production yield for fisher- primary production of continental shelf ecosystems men could also decrease (Wickens 1995, Lavigne (Vasconcellosa & Gasalla 2001). This value is typical 2003). Globally, the fishing industry has been experi- of intensely fished coastal mid-latitude regions encing an unprecedented crisis since the late 1990s. (Pauly & Christensen 1995). Since the 1970s, when A number of natural populations of fishery resources Brazilian trawlers were banned from fishing in have collapsed as a result of increased fishing effort, Argentinian and Uruguayan waters, coastal demer- overfishing of important stocks, impacts of fisheries sal fishing in southern Brazil intensified (Haimovici Machado et al.: Marine mammals and fisheries trophic overlap 217 et al. 2014). Intense fishing in the region led to the The present study evaluates the trophic relation- decrease in the abundance of the main species in the ships of 5 species of marine mammals along with 6 copy coastal demersal fisheries in the following decades, important commercial fisheries in the southern including Micropogonias furnieri (Vasconcellos & region of Brazil. In addition, we assessed the poten- Haimovici 2006), Cynoscion guatucupa
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