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 (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 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.
Rabearisoa, N., Bach, P., Lucas, V., Giroux, F., Vely, M., Romanov, E., Tixier, P., Guinet, C., 2009. Assessment of the efficiency of the physical protection of fish as mitigation measure to depredation by marine mammals in pelagic longlining, in: 3rd WIOMSA Scientific Symposium. Saint-Denis, La Réunion. (pdf)
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.
Depredation belongs to negative interactions between large marine megafauna and fisheries and is defined as the damage or removal of fish or bait from the fishing gear by predators, such as toothed whales, sharks, birds or squids. Those interactions lead to several negative impacts affecting the species involved (bycatch, shift in their feeding habits and hunting strategies), fishermen (financial loss induced by fish damage and search for new fishing areas) and stock assessment aspects (underestimation of catch report). Nevertheless, this issue remains poorly studied in the frame of pelagic longline fisheries, and especially in the Indian Ocean.
This work aimed at studying short-finned pilot whale (Globicephala macrorhynchus), false killer whale (Pseudorca crassidens) and pelagic shark depredation impacting pelagic longline fleets operating in the southwest Indian Ocean. Analyzed data were collected during scientific and commercial fishing trips from 2002 and 2010 in the southwestern Indian Ocean. First, an analysis of shark and toothed whale depredation was implemented and logistic regression models were used to identify potential correlates between depredation occurrence and covariates characterizing fishing practices and environmental factors. Shark depredation events were more frequent but toothed whales damaged more fish on the fishing gear. 19.5% of the catch was lost to depredation in the Seychelles archipelago, and this area is considered as a depredation “hot-spot”. Those interactions highlight a spatiotemporal synchrony between fishing activities and predators’ abundance, in particular toothed whales.
Because of the aforementioned consequences, several depredation mitigation measures have been tested so far but their long-term efficiency are still to be proved. Innovative devices based on fishing gear modifications and aiming at protecting capture were designed and tested. Three fishing trips were conducted in Seychelles and Reunion Island to assess the efficiency of “spiders”, “socks” and “DESAP” (DEpredation mitigation device by Scaring predators And Protecting capture) towards toothed whale depredation. Preliminary results underlined operational constraints to routinely deploy those devices during fishing operations. Nevertheless, the promising results obtained during the field tests gave us valuable insights to go further for the development of a new prototype, our approach being fully consistent with the ecosystem approach to fisheries.
Our depredation data were also used to give new insights in false killer whale and shortfinned pilot whale ecology. Predator-prey-scavenger models developed in terrestrial environment were used to implement a similar model aiming at indirectly assessing the relative abundance of toothed whales involved in depredation events. Odontocetes groups involved in depredation are presumably some stable hunting groups, a small sub-unit of false killer whale and short-finned pilot whale social structure. Greater group sizes were found in Seychelles and near the shelfbreak, underlining the attractiveness of those biologically richest areas.
This work highlighted a poorly studied issue and is paramount for the development of depredation mitigation measures. This crucial aim supports a sustainable approach to environmentally responsible fishing, where short-term economic goals meet conservation concerns.
