ReviewGlobal patterns of sex- and age-specific variation in seabird bycatch
Graphical abstract
Introduction
Fisheries are one of the primary threats to marine biodiversity, impacting ecosystems from the open ocean to the coast, and from the poles to the tropics (Halpern et al., 2008, Jackson et al., 2001). Commercial fishing has resulted in severe and widespread ecosystem disruption primarily as a result of over-harvesting, habitat degradation and the mortality of non-target species, also called bycatch (Hall et al., 2000, Halpern et al., 2008, Jackson et al., 2001). Populations of large marine vertebrates, such as sea turtles, sharks, marine mammals, and seabirds, are particularly susceptible to bycatch because of a combination of their attraction to fishery bait and discards, and their naturally slow reproductive rates rendering them sensitive to even small increases in mortality (Hall et al., 2000, Lewison et al., 2004). The impacts are so extensive that the recent declines of many large marine vertebrates resulting from bycatch have been compared to the historical extirpations and extinctions of terrestrial megafauna by human hunting (Lewison et al., 2004, Lewison et al., 2014).
Seabirds are particularly at risk from fisheries, as they are bycaught in a wide range of gear types (Croxall et al., 2012, Montevecchi, 2002, Phillips et al., 2016). For example, drift nets set by Japanese, Korean and Taiwanese vessels are estimated to have killed up to 40 million sooty (Ardenna grisea) and short-tailed (A. tenuirostris) shearwaters in the North Pacific between 1952 and 2001 (Uhlmann et al., 2005). Coastal gillnet fisheries are also a major source of mortality, with > 400,000 seabirds killed annually, worldwide (Žydelis et al., 2013). Global longline fisheries are estimated to have killed at least 160,000, and potentially 320,000 seabirds annually, mainly albatrosses, petrels and shearwaters (Anderson et al., 2011). Trawl fisheries are also a threat, with about 9300 birds, mostly albatrosses, estimated to be killed annually just in the waters off South Africa by wet fish trawls (Maree et al., 2014, Sullivan et al., 2006, Waugh et al., 2008). These levels of mortality have led to severe declines in many populations and are clearly unsustainable (Croxall et al., 1998, Cuthbert et al., 2005, Delord et al., 2008, Phillips et al., 2016, Piatt and Gould, 1994, Rolland et al., 2010, Žydelis et al., 2009, Žydelis et al., 2013).
The impact of bycatch depends not only on the number of individuals killed, but also on the components of the population that are impacted (Bugoni et al., 2011, Lewison et al., 2012). For example, because seabird life histories are characterized by delayed maturation, high survival and low rates of reproduction, mortality of adults will have greater population-level impacts than mortality of immatures (Lewison et al., 2014). Moreover, because seabirds are monogamous, with obligate biparental care, sex-biased mortality in fisheries can reduce the effective population size (Mills and Ryan, 2005, Weimerskirch et al., 2005). Sex- and age-biases in seabird bycatch are reported in a number of fisheries (Awkerman et al., 2006, Gales et al., 1998, Ryan and Boix-Hinzen, 1999, Stempniewicz, 1994), and there has been a review of adult sex-ratios (ASR) in bycatch of albatrosses and petrels (Bugoni et al., 2011). However, there has been no comprehensive review of sex- and age-biases in bycatch of seabirds in general, even though a better understanding of their nature and extent is required to determine the full impact of bycatch on populations and communities. Indeed, this has been identified as one of the highest priority research questions in the field of seabird ecology and conservation (Lewison et al., 2012, Phillips et al., 2016).
Accordingly, the aim of the current study is to provide the first global review of age- and sex-specific bycatch in seabirds. This will contribute towards a better understanding of the frequency and magnitude of these effects across taxa, regions and fishery gear-type, as well as the implications for management and conservation. We predict that larger and more dominant individuals, usually adult males, will have higher bycatch rates than adult females, or younger birds of either sex, because they are better able to compete for discards and baits while attending fishing boats (Awkerman et al., 2007b, Bregnballe and Frederiksen, 2006, Croxall and Prince, 1990, Montevecchi, 2002). However, bycatch rates will also be influenced by region. Many studies have shown that females and immatures tend to travel further from their breeding sites, or to lower latitudes, compared with males and adults (Hedd et al., 2014, Phillips et al., 2004, Phillips et al., 2005). Therefore, because the majority of seabirds breed at high latitudes (Schreiber and Burger, 2002) we broadly predict that bycatch in subpolar (sub-Arctic and sub-Antarctic) areas will tend to be skewed towards males and adults, whereas in subtropical regions, bycatch will be biased towards females and immatures.
Section snippets
Literature review
We reviewed the literature for studies reporting sex and age composition of seabird bycatch in fisheries from around the world. We searched Thomson Reuters Web of Science and Google Scholar using the following search terms: topic = (seabird* OR albatross* OR petrel* OR penguin* OR shearwater*) and (sex OR age OR female OR male OR adult OR juvenile) and (fishery* OR bycatch OR mortality) and (bias); timespan = all years. To ensure the best possible coverage of the bycatch literature, we supplemented
Results
We found 44 studies, published between 1990 and 2016, that reported sex and age composition of seabird bycatch in fisheries, of which 35 (79%) were in the southern hemisphere and 9 (21%) in the northern hemisphere (Fig. 1, Table 1). Data were available from four main types of fishery: 14 studies for pelagic longline (32%), nine for demersal longline (20%), nine for gillnet (20%), two for trawl (5%). Seven (16%) studies reported data for more than one fishery, separated according to gear type.
Discussion
Sex- and age-biases in seabird bycatch have been reported in a number of fisheries (Delord et al., 2005, Gales et al., 1998, Nel et al., 2002a, Phillips et al., 2010, Ryan and Boix-Hinzen, 1999), and there is growing interest in both the underlying mechanisms and the potential demographic consequences (Bugoni et al., 2011, Lewison et al., 2012). Here we provide the first global synthesis of both sex and age-specific variation in seabird bycatch rates by different fisheries and in diverse
Conclusion
Sex and age biases in seabird bycatch are common features across global fisheries, mainly related to differential at-sea distributions. Overall, bycatch of adults and males was higher in subpolar regions and closer to colonies, whereas immatures and females were caught more frequently in subtropical waters. We found no compelling evidence that differences in sex and age ratios are related to the competitive advantage of males and adults foraging at fishing vessels, or to naivety of young birds
Acknowledgments
We thank Stuart Bearhop for providing constructive comments on the manuscript, and Hanna Nevins and Peter Ryan for providing valuable literature. We also thank Fabio Olmos for providing pictures of some of the birds composing our figures, and the three anonymous referees for their helpfull suggestions for improvements to the paper. This work is part funded via a scholarship to DG from the Sciences Without Borders Program (CNPq/Brazil, Proc. 246619/2012-0). The study represents a contribution to
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2021, Biological ConservationCitation Excerpt :Some studies suggest juveniles may scavenge disproportionally behind vessels because of lower foraging efficiency, or that they are less able to avoid fishing gear (Österblom et al., 2002; Bregnballe and Frederiksen, 2006). However, age-specific variation in distribution appears to be the main driver of observed age-specific susceptibility to bycatch in seabirds at a global level (Gianuca et al., 2017). Soon after fledging (May–June), the tracked juveniles in our study reached the southern limit of high-intensity fishing effort in the southeast Atlantic, including the reported bycatch hotspot for this species in ICCAT subareas 6, 7 and 8 (Inoue et al., 2012; Katsumata et al., 2017).
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2021, Ocean and Coastal ManagementCitation Excerpt :In the northeastern Atlantic, gillnets were associated with the collapse of the Common guillemot Uria aalge (Pontoppidan, 1763) population in Iberia (Munilla et al., 2007) and decreased adult survival of the European shag Gulosus aristotelis (Linnaeus, 1761) in Galicia, Spain (Velando and Freire, 2002). In the Atlantic Iberian coast, seabird-fishery interactions are poorly known (see reviews in Gianuca et al., 2017; Pott and Wiedenfeld, 2017; Žydelis et al., 2013; but see Oliveira et al., 2015; Valeiras, 2003) despite the importance of this area as a valuable foraging ground for several breeding species and as a key migratory corridor for non-breeding species (Pereira et al., 2018). This coast is characterized by high productivity and intense seasonal upwelling, targeted by both seabirds and fisheries (Veiga-Malta et al., 2019).