Fishing behaviours and fisher effect in decision-making processes when facing depredation by marine predators

International audienceFishers aim to optimise cost– benefit ratios of their behaviour when exploiting resources. Avoidance of interactions with marine predators (i.e. their feeding on catches in fishing gear, known as depredation) has recently become an important component of their decisions. How fishers minimise these interactions whilst maximising fishing success is poorly understood. This issue is addressed in a sub- Antarctic, long- line fishery confronted with extensive depredation by sperm whales Physeter macrocephalus and killer whales Orcinus orca by examining a 15- year data set. Whereas a broad range of behaviours was identified from spatio- temporal and operational descriptors, none combined high fishing success with low frequency of interactions. With experience, fishers favoured exploitation of productive patches with high frequencies of interactions over avoidance behaviours. Such decisions, although potentially optimal in the short term, are likely to intensify pressures on fish stocks and impact depredating whales. Therefore, the present study provides additional evidence to inform management decisions pertaining to the coexistence between fisheries and marine predators

Evidence of deep-sea interactions between toothed whales and longlines

International audienceToothed whales (odontocetes) feeding on fish caught on hooks in longline fisheries is a growing issue worldwide. The substantial impacts that this behaviour, called depredation, can have on the fishing economy, fish stocks and odontocetes populations, raise a critical need for mitigation solutions to be developed. However, information on when, where and how odontocete depredation occurs underwater is still limited, especially in demersal longline fisheries (fishing gear set on the seafloor). In the present study, we investigated depredation by killer whales (Orcinus orca) and sperm whales (Physeter macrocephalus) on demersal longlines in the French Patagonian toothfish fishery (Southern Ocean). Using a combination of animal-borne behavioural and longline-attached data loggers, we demonstrated that both species are able to depredate longlines on the seafloor. This study, therefore, suggests that odontocetes whales-longline interaction events at depth may be unrecorded when assessing depredation rates from surface observations during hauling phases only. This result has implications for the management of fisheries facing similar depredation issues as underestimated depredation rates may result in unaccounted fish mortality in fish-stock assessments. Therefore, while further research should be conducted to assess the extent of deep-sea whale-longline interaction events during soaking, the evidence that depredation can occur at any time during the whole fishing process as brought out by this study should be considered in future developments of mitigation solutions to the issue

Role of sociality in the response of killer whales to an additive mortality event

In highly social top predators, group living is an ecological strategy that enhances individual fitness, primarily through increased foraging success. Additive mortality events across multiple social groups in populations may affect the social structure, and therefore the fitness, of surviving individuals. This hypothesis was examined in a killer whale (Orcinus orca) population that experienced a 7-y period of severe additive mortality due to lethal interactions with illegal fishing vessels. Using both social and demographic analyses conducted on a unique long-term dataset encompassing periods before, during, and after this event, results indicated a decrease in both the number and the mean strength of associations of surviving individuals during the additive mortality period. A positive significant correlation between association strength and apparent survival suggested that the fitness of surviving individuals was impacted by the additive mortality event. After this event, individuals responded to the loss of relatives in their social groups by associating with a greater number of other social groups, likely to maintain a functional group size that maximized their foraging success. However, these associations were loose; individuals did not reassociate in highly stable social groups, and their survival remained low years after the mortality event. These findings demonstrate how the disruption of social structure in killer whales may lead to prolonged negative effects of demographic stress beyond an additive mortality event. More importantly, this study shows that sociality has a key role in the resilience of populations to human-induced mortality; this has major implications for the conservation of highly social and long-lived species.</p

Fisheries interaction data suggest variations in the distribution of sperm whales on the Kerguelen Plateau

The emergence of longline fishing for Patagonian toothfish (Dissostichus eleginoides) on the Kerguelen Plateau over the past two decades is concomitant with the development of depredation-type interactions by sperm whales (Physeter macrocephalus). Through a unique collaboration between the French and the Australian fisheries operating respectively around Kerguelen, and Heard Island and McDonald Islands (HIMI), this study preliminarily investigated the spatio–temporal variations of the rate of occurrence of sperm whale depredation on the Kerguelen Plateau. Between 2011 and 2016, sperm whales depredated toothfish on 29.1% of all longline sets and over 49.4% of the fished area. The probability of vessels to experience depredation decreased with the latitude and decreased in winter. Vessels operating in Kerguelen experienced significantly higher rates of occurrence of sperm whale depredation (33.2 ± 4.5% of sets; 48.2 ± 7.2% of the area) than vessels operating in HIMI (3.1 ± 1.2% of sets; 5.4 ± 2.0% of the area) over the 2011–2016 period, but also during any season of the year. The results suggested that heterogeneity in the distribution of sperm whales is likely a key driver of depredation. The Kerguelen Plateau fisheries represent a unique opportunity to investigate the spatial factors influencing this distribution, and therefore to predict the occurrence of depredation.  L’émergence de la pêche palangrière à la légine australe (Dissostichus eleginoides) sur le plateau de Kerguelen ces 20 dernières années coïncide avec l’intensification des interactions de type déprédation avec les cachalots (Physeter macrocephalus). Grâce à une collaboration unique entre les pêcheries françaises et australiennes opérant respectivement autour des îles Kerguelen, et des îles Heard et McDonald (HIMI), cette étude préliminaire a examiné les variations spatio–temporelles du taux d’occurrence de la déprédation par les cachalots sur le plateau de Kerguelen. Entre 2011 et 2016, les cachalots ont interagi avec 29,1% des palangres posées et sur 49,4% de la zone de pêche. La probabilité d’exposition des navires à la déprédation a baissé avec la latitude et diminué pendant l’hiver. Pendant la période 2011–2016, mais aussi pendant toutes les saisons de l’année, les navires opérant à Kerguelen ont été exposés à des taux d’interaction avec les cachalots nettement plus élevés (33,2 ± 4,5% des palangres ; 48,2 ± 7,2% de la zone) que les navires opérant dans les HIMI (3,1 ± 1,2% des palangres ; 5,4 ± 2,0% de la zone). Les résultats suggèrent que l’hétérogénéité de la distribution naturelle des cachalots constitue probablement un facteur clé de l’occurrence de la déprédation. Les pêcheries du plateau de Kerguelen représentent une situation unique pour étudier les variables spatiales qui influencent cette distribution, permettant ainsi de prédire l’occurrence de la déprédatio