Toothed whale and shark depredation indicators: A case study from the Reunion Island and Seychelles pelagic longline fisheries

Abstract

Depredation in marine ecosystems is defined as the damage or removal of fish or bait from fishing gear by predators. Depredation raises concerns about the conservation of species involved, fisheries yield and profitability, and reference points based on stock assessment of depredated species. Therefore, the development of accurate indicators to assess the impact of depredation is needed. Both the Reunion Island and the Seychelles archipelago pelagic longline fisheries targeting swordfish (Xiphias gladius) and tuna (Thunnus spp.) are affected by depredation from toothed whales and pelagic sharks. In this study, we used fishery data collected between 2004 and 2015 to propose depredation indicators and to assess depredation levels in both fisheries. For both fisheries, the interaction rate (depredation occurrence) was significantly higher for shark compared to toothed whale depredation. However, when depredation occurred, toothed whale depredation impact was significantly higher than shark depredation impact, with higher depredation per unit effort (number of fish depredated per 1000 hooks) and damage rate (proportion of fish depredated per depredated set). The gross depredation rate in the Seychelles was 18.3%. A slight increase of the gross depredation rate was observed for the Reunion Island longline fleet from 2011 (4.1% in 2007–2010 and 4.4% in 2011–2015). Economic losses due to depredation were estimated by using these indicators and published official statistics. A loss of 0.09 EUR/hook due to depredation was estimated for the Reunion Island longline fleet, and 0.86 EUR/hook for the Seychelles. These results suggest a southward decreasing toothed whale and shark depredation gradient in the southwest Indian Ocean. Seychelles depredation levels are among the highest observed in the world revealing this area as a “hotspot” of interaction between pelagic longline fisheries and toothed whales. This study also highlights the need for a set of depredation indicators to allow for a global comparison of depredation rates among various fishing grounds worldwide.

When large marine predators feed on fisheries catches: Global patterns of the depredation conflict and directions for coexistence

“…Rocklin et al, 2009;Söffker et al, 2015;Tixier, Burch, et al, 2019;Tixier, Giménez, et al, 2019) and for sharks (e.g. MacNeil, Carlson, & Beerkircher, 2009;Rabearisoa et al, 2018;Ryan, Taylor, McAuley, Jackson, & Molony, 2019). In some regions, the use of satellite tagging increased knowledge on preferred areas of predators, and therefore, on areas where fishers were the most likely to be subject to depredation (e.g.…”

“…While reported potentially successful in 14 of these 17 fisheries, their long‐term effectiveness at reducing depredation sometimes lacked data support (Table 3). For example, low‐cost fish protection systems showed promising results in pelagic longline fisheries but conclusions were limited by experimental trials being too short to produce sufficient evidence (Hamer, Childerhouse, McKinlay, Double, & Gales, 2015; Rabearisoa, Bach, Tixier, & Guinet, 2012). Despite these limitations, some systems, like the so‐called “cachalotera” or “umbrella” (Brown, Brickle, Hearne, & French, 2010; Goetz et al, 2011; Moreno, Castro, Mújica, & Reyes, 2008), have been fully implemented in several fisheries, suggesting some level effectiveness.…”

“…This greater availability of assessments is likely due to marine predators partially consuming catches when depredating in tuna/swordfish pelagic longline fisheries, leaving quantifiable evidence. In these fisheries, catch losses were, for instance, reported as being equivalent to USD 500,000 per year off the Seychelles (Rabearisoa, Sabarros, Romanov, Lucas, & Bach, 2018) and between USD 390,000-690,000 per year per vessel off Hawaii (Gilman, 2007).…”

Patterns of depredation in the Hawai‘i deep‐set longline fishery informed by fishery and false killer whale behavior

“…As in other pelagic longline fisheries experiencing odontocete depredation (e.g., Secchi and Vaske 1998, Rabearisoa et al. 2018), depredation by toothed whales is rarely observed directly in the deep‐set fishery (Bayless et al. 2017), but rather inferred by characteristic damage to individual caught fish retrieved during the haul (Forney et al.…”

Molecular tools for identification of shark species involved in depredation incidents in Western Australian fisheries

“…Depredation is an ecological and economic issue worldwide, occurring in commercial and recreational fisheries [9, 10]. In north-western Australia, anecdotal observations from recreational, charter and commercial fishers suggest that depredation by sharks may have increased in recent years leading to the assumption by some of a general increase in shark abundance.…”