En halieutique, la déprédation se définit comme le prélèvement total ou partiel des poissons ou des appâts sur les engins de pêche par les requins, les odontocètes, les calmars ou les oiseaux. Ces interactions portent préjudice à la fois aux espèces impliquées (captures accidentelles, modification du régime alimentaire et de la stratégie de chasse), aux pêcheurs (pertes économiques liées aux dommages aux captures et à la recherche de nouvelles zones de pêche) et à la gestion des stocks des ressources exploitées (sous-estimation des déclarations des captures).
La déprédation qui impacte la pêche palangrière pélagique réunionnaise engendre des pertes financières conséquentes pour cette filière qui a vivement sollicité les scientifiques pour étudier le phénomène et trouver des solutions. En réponse à cette demande nous proposons dans ce projet, de nous intéresser à la déprédation par les odontocètes et les requins autour de trois axes de recherche :
(1) analyse de l’étendue du phénomène dans l’espace et dans le temps afin d’identifier des fenêtres spatio-temporelles de fortes intensités
(2) analyse de l’impact économique du phénomène, en prenant en compte les coûts directs et indirects de la déprédation
(3) analyse de la perception de la déprédation par les pêcheurs
(4) mise en place d’un protocole scientifique de tests de dispositifs innovants de lutte contre la déprédation.
Rapport scientifique (pdf)
Depredation (damage or removal of fish from fishing gear by predators), raises concerns about conservation, fisheries profitability and stock assessment. There is a lack of knowledge about depredation impacting pelagic longline fisheries, especially in the southwest Indian Ocean (SWIO). Therefore, the development of accurate depredation indicators is needed. In La Reunion and Seychelles, local pelagic longline fisheries targeting swordfish (Xiphias gladius) and tuna (Thunnus spp.) are affected by toothed whale and shark depredation. Fishery data collected between 2004 and 2015 were considered to estimate depredation indicators. For both fisheries, the Interaction Rate (depredation occurrence) was higher for shark depredation, but when toothed whale depredation occurred, the Depredation Per Unit Effort (number of fish depredated per 1000 hooks) and the Damage Rate (proportion of fish depredated per depredated set) were greater for toothed whale depredated sets. The Seychelles Gross Depredation Rate was 18.3% (9.3% from toothed whales, 8.1% from sharks and 0.9% from both predators). A slight increase of the GDR was observed for La Reunion since 2011, when the local longline fleet concentrated its fishing effort around La Reunion EEZ and the east coast of Madagascar (GDR was 4.1% in 2007-‐2010 and 4.4% in 2011-2015). Therefore a southward decreasing toothed whale and shark depredation gradient is highlighted in the SWIO. Seychelles depredation levels would be among the highest observed in the world revealing this area as a « hotspot » of interactions between pelagic longline fisheries and toothed whales. Economic losses were estimated from those indicators and from official statistics. 0.09 €/hook and 0.63 €/hook were lost to depredation for La Reunion and Seychelles fleets, respectively. This accounted for 20% and 54% of the landed price per hook. This study enlightens the need of indicators setup to allow a global standardization of depredation rates for comparison purposes between various areas and fisheries.
Depredation, defined as the damage or removal of fish from fishing gear by predators, is an issue leading to negative impacts on animals and fisheries. Toothed whale depredation in pelagic longline fisheries targeting swordfish (Xiphias gladius) and tuna (Tuna spp) involves short-finned pilot whales (Globicephala macrorhynchus) and false killer whales (Pseudorca crassidens). In the Seychelles archipelago, a database was built to assess the extent of this phenomena and to analyse how fishing practices could be involved in. Data analyzed came from fishermen logbooks and scientific cruises and covered the 2002-2006 period, representing a total of 705 fishing operations. The proportion of sets impacted by toothed whale depredation was 16% while the proportion of damaged fish reached 58% when toothed whales interacted with longlines. Toothed whale global depredation rate reached 10.7%. Logistic regression analysis and generalized additive models showed that both depredation occurrence and rate were positively related to the abundance of target fish, suggesting the co-occurrence of toothed whales in areas of high concentration of pelagic fish. Obviously, alternative fishing practices cannot be considered as an efficient way to mitigate depredation. Consequently we investigated fishing gear improvement by deploying a technology designed to physically protect the hooked fish by hiding them to predators. The efficiency of “spiders” was tested during a fishing trial of 26 longline fishing operations when 12480 hooks and 1970 devices were set. 117 fish were caught on branchlines equipped with spiders and among those devices, 87.3% were correctly triggered and 80% of capture were correctly protected. While more trials should be carried out to deeply verify the efficiency of “spider” devices, we remain convinced that the consideration of the fishing gear technology might be more actively investigated to propose innovative measures to mitigate toothed whale depredation in pelagic longlining.
Depredation is defined as the damage or removal of fish from fishing gear by predators, and is an issue leading to negative impacts on animals involved in depredation and fisheries. Depredation on pelagic longliners targeting swordfish (Xiphias gladius) and tuna (Tuna spp) involves short-finned pilot whales (Globicephala macrorhynchus), false killer whales (Pseudorca crassidens) and some pelagic sharks. In the Seychelles archipelago, a database was built to assess its extent on the pelagic longline fishery and to identify fishing practices likely to influence depredation. Data were collected from pre-existing databases, fishermen logbooks and satellite-based Vessel Monitoring System information, and covered the 2002-2006 period, representing a total of 705 fishing operations. The number of sets impacted by shark depredation was significantly greater than the number of sets involving toothed whale depredation. Nevertheless, when depredation occurred, the proportion of fish damaged by toothed whales was significantly greater. Depredation occurrence was analyzed by using logistic regression. Toothed whale depredation occurrence was positively related to the abundance of target fish and the fishing area (being more frequent in the south-west of the archipelago). Shark depredation was more frequent in the north-west of the archipelago. Depredation rate was analyzed by implanting Generalized Additive Models. Toothed whale depredation rate depended on the fishing area. Shark depredation rate was negatively related to the longline length and the latitude. Both depredation types occurred in areas of high CPUE (Catch Per Unit Effort), suggesting the co-occurrence of sharks and toothed whales in areas of high concentration of pelagic fish. Otherwise, toothed whales damaged significantly more swordfish than tuna, whereas sharks equally damaged both species. The global depredation rate reached 19.3% (10.7% were due to toothed whales and 8.6% were due to sharks). No obvious answers can be given by modifying fishing practices and other solutions have to be investigated to reduce depredation impacts.
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 ªhotspotº 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.
Depredation is defined as the damage or removal of fish from fishing gear by predators, and is a crucial issue leading to negative impacts on both animals involved in depredation and fisheries. Depredation in longline pelagic fisheries targeting swordfish (Xiphias gladius) and tuna (Thunnus spp.) involves short-finned pilot whales (Globicephala macrorhynchus), false killer whales (Pseudorca crassidens) and some pelagic sharks. Since no long-term solution could be found to date, we investigated fishing gear improvement by deploying a technology designed to physically protect the hooked fish by hiding it to predators: the DMD (depredation mitigation device). Two types of DMDs were designed: “spiders” and “socks”. The efficiency of “spiders” was tested in November 2007 during a fishing trial of 26 longline fishing operations when 12,480 hooks and 1970 devices were set. The efficiency of “socks” was tested in October 2008 during a fishing trial of 32 longline fishing operations when 13,220 hooks and 339 devices were set. 117 and 24 fish were hooked on branchlines equipped with spiders and socks, respectively and among those devices, 87.3% versus 69.2% were correctly triggered and 80% versus 15% of the fish were correctly protected. A low entanglement rate of the spiders with the fishing gear was found (3.6%), but a higher one was associated to the socks (17.8%). Operational constraints to routinely deploy “spiders” were examined. The number of sets impacted by shark depredation was significantly greater than the number of sets involving toothed whale depredation. However, when depredation occurred, the proportion of fish damaged by toothed whales was significantly greater. While more trials should be carried out to deeply verify the efficiency of DMDs, we remain convinced that considerations of fishing gear technologies might be more actively investigated to propose innovative measures to mitigate toothed whale depredation in pelagic longlining. For this type of gear, innovative technology is an important issue of the ecosystem approach to fisheries (EAF) framework.
Depredation is defined as the damage or removal of fish from fishing gear by predators and raises concerns about the conservation of marine protected species involved, fisheries yield and profitability, and stock assessment of target species. There is an obvious lack of knowledge about depredation impacting pelagic longline fisheries, especially in the southwest Indian Ocean. Thus, there is a real need for the development of accurate indicators to assess its impact in a given fishery. In Reunion Island, local pelagic longliners targeting swordfish (Xiphias gladius) and tuna (Thunnus spp.) are affected by short-finned pilot whale (Globicephala macrorhynchus), false killer whale (Pseudorca crassidens) and pelagic shark depredation. Catch and depreda- tion data collected during self-reporting, commercial and experimental cruises between 2007 and 2015 were used to compute depredation indicators such as the depredation occurrence (Interaction Rate), the proportion of fish depredated among the overall catch (Gross Depredation Rate), the average proportion of fish depredated per depredated set (Damage Rate) and the number of fish depredated per 1000 hooks (Depredation Per Unit Effort). Here we show that shark depredation impacted more fishing sets (IRs=31%) than toothed whale depredation (IRc=14%), but when depredation occurred, toothed whale depredation impact was higher: the number of fish damaged per 1000 hooks and the average proportion of fish damaged per set were greater for toothed whale depredated sets (DPUEc*=2.7 fish and DRc*=16.8%) than for shark depredated ones (DPUEs*=1.2 fish and DRs*=6.4%). Since 2011, when the pelagic fleet concentrated its fishing effort around the Reunion EEZ and the east coast of Madagascar, the gross depredation rate increased and ranged from 4 to 6.3%. In areas of high toothed whale depredation rates, the amount of fish lost per fishing operation was high (DPUEc*=3.2 fish/1000 hooks, DRc*=18%). In areas of low toothed whale depredation rates, the amount of fish lost per fishing operation was low (DPUEc*=1.8 fish/1000 hooks, DRc*=4.6%). Shark depredation has low impact on commercial CPUE. However, one should keep in mind that these are minimum depredation estimates, since several uncertainties could not be taken into account: total depredation leaving no trace on the hook, bait depredation by small delphinids indirectly leading to catch loss, toothed whale presence scaring fish away or additional running costs when leaving a fishing area to avoid predators. Thus, combined with marginal profits, increased running costs and low fish prices, toothed whale depredation has disastrous effects on Reunion pelagic longline fishery.
Cette thèse étudie la déprédation par les globicéphales tropicaux (Globicephala macrorhynchus), les faux-orques (Pseudorca crassidens) et certaines espèces de requins pélagiques à laquelle font face les flottilles palangrières pélagiques ciblant le thon (Tuna spp.) et l’espadon (Xiphias gladius) opérant dans le sud-ouest de l’Océan Indien. Le travail qui y est présenté étudie des données issues de campagnes de pêche commerciales et scientifiques collectées entre 2002 et 2010 dans le Sud-Ouest de l’Océan Indien.
Dans un premier temps, une analyse de la déprédation exercée par les requins et les odontocètes pour identifier des pratiques de pêche et des facteurs environnementaux influençant son occurrence a été réalisée par le biais de régressions logistiques. Les évènements de déprédation par les requins sont plus fréquents, mais les odontocètes endommagent plus de poissons sur les palangres. 19,5 % des captures sont ainsi perdus aux Seychelles, ce qui en fait un « hot-spot » de la déprédation. Ces interactions mettent en évidence une synchronie spatiotemporelle entre l’activité de pêche et l’abondance des prédateurs, notamment les odontocètes.
Au vu des multiples conséquences liées à ce phénomène, différentes mesures antidéprédation ont été mises en place dans d’autres régions pour essayer d’en limiter les impacts, mais à ce jour, aucune n’a montré de réelle efficacité à long terme. Des dispositifs innovants reposant sur une modification des engins de pêche et visant à protéger les captures ont été conçus et testés. Trois campagnes ont été menées aux Seychelles et à la Réunion pour évaluer l’efficacité des « araignées », des « manches » et des « effaroucheurs » face à la déprédation par les odontocètes. Les premiers essais ont souligné les contraintes fonctionnelles liées à l’utilisation de ces dispositifs en situation de pêche. Néanmoins, des résultats encourageants nous poussent à poursuivre le développement de ce type de dispositifs, une démarche qui correspond à une approche durable d’une pêche responsable qui répond à des problématiques de conservation et d’économie du secteur de la pêche palangrière pélagique.
Nos données de déprédation ont aussi été analysées pour améliorer nos connaissances sur l’écologie des faux-orques et des globicéphales tropicaux. Par analogie avec les relations prédateurs-proies-charognards étudiées dans les écosystèmes terrestres, nous avons développé un modèle similaire pour évaluer de manière indirecte l’abondance des groupes d’odontocètes impliqués dans les évènements de déprédation. Les groupes mis en cause sont probablement des groupes de chasse stables, unités de base composant les populations de faux-orques et de globicéphales tropicaux. Nous avons observé des tailles de groupe plus importantes aux Seychelles et aux abords des limites du plateau continental, soulignant l’attrait de ces espèces pour les zones biologiquement les plus productives.
Ce travail a permis de mettre en lumière un phénomène peu étudié, et de renforcer les bases nécessaires à l’élaboration de mesures visant à limiter les impacts de la déprédation. Il s’agit d’un objectif majeur qui rentre dans le cadre de l’approche écosystémique des pêches, conciliant des objectifs économiques à court terme avec des objectifs de durabilité des espèces exploitées et de conservation des espèces non cibles.
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