“Freshwater Killer Whales”: Beaching Behavior of an Alien Fish to Hunt Land Birds

et al. (2012) ''Freshwater Killer Whales'': Beaching Behavior of an Alien Fish to Hunt Land Birds. PLoS ONE 7(12): e50840. doi:10.1371/journal.pone.0050840 ''Freshwater Killer Whales'': Beaching Behavior of an Alien Fish to Hunt Land Birds Julien Cucherousset 0 Ste phanie Boule treau 0 Fre de ric Aze mar 0 Arthur Compin 0 Mathieu Guillaume 0 Fre de ric Santoul 0 Dirk Steinke, Biodiversity Insitute of Ontario - University of Guelph, Canada 0 1 CNRS, Universite Paul Sabatier, ENFA, UMR 5174 EDB (Laboratoire E volution & Diversite Biologique) , Toulouse, France, 2 Universite de Toulouse, UMR 5174 EDB, Toulouse, France, 3 Universite de Toulouse, INP, UPS , EcoLab (Laboratoire d'Ecologie Fonctionnelle et Environnement) , Toulouse, France, 4 CNRS, EcoLab, Toulouse , France The behavioral strategies developed by predators to capture and kill their prey are fascinating, notably for predators that forage for prey at, or beyond, the boundaries of their ecosystem. We report here the occurrence of a beaching behavior used by an alien and large-bodied freshwater predatory fish (Silurus glanis) to capture birds on land (i.e. pigeons, Columbia livia). Among a total of 45 beaching behaviors observed and filmed, 28% were successful in bird capture. Stable isotope analyses (d13C and d15N) of predators and their putative prey revealed a highly variable dietary contribution of land birds among individuals. Since this extreme behavior has not been reported in the native range of the species, our results suggest that some individuals in introduced predator populations may adapt their behavior to forage on novel prey in new environments, leading to behavioral and trophic specialization to actively cross the water-land interface. - To capture, handle, kill and/or swallow their prey, predators have developed numerous strategies, including pack hunting, venin, cooperation, trapping webs and the use of tools. For instance, savannah chimpanzees have been reported to construct wooden spear-like tools to hunt their preys [1] while New Caledonian crows can use stick tools to capture highly energetic wood-boring beetle larvae [2]. However, perhaps the most astonishing strategies are when the preys targeted by the predator are located outside of the predators ecosystem boundaries. These strategies can be grouped into three broad categories. First, the predators can passively lie in ambush until the preys cross the water-land interface (voluntarily or accidently) and capture them. This is notably the case of crocodiles that capture migrating wildebeest crossing rivers and of terrestrial predators such as birds, spiders and bats that capture emerging aquatic insects [3,4]. Second, some predators may develop strategies that force the prey to enter their ecosystem. For instance, archerfish (Toxotes jaculatrix Pallas) have developed complex optical and morphological adaptations to shoot down insects located on trees by expelling droplets on the insects that will subsequently fall in the water [5]. Third, and most spectacularly, predators can actively cross the water-land interface to capture the prey. Some marine predators such as killer whales (Orcinus orca L.) and bottlenose dolphins (Tursiops sp.) display intentional beaching behavior to catch prey on beaches [6,7]. In many predators, these extreme hunting behaviors represent a form of ecological specialization [8,9] that is displayed only by a subset of individuals in the populations [7]. Here, we report the occurrence of a hunting behavior, analogous to the intentional beaching of marine mammals, in an alien freshwater fish species (European catfish Silurus glanis L., the worlds third largest and Europes largest freshwater fish) [10,11] to capture land birds. Additionally, we demonstrate the existence of trophic niche variability within the population with only some individuals foraging on land birds. Materials and Methods (a) Behavioral Monitoring European catfish originates from Europe (east of Rhine River) and has been introduced in many ecosystems of Western Europe, including Spain, Italy and Southwestern France [11]. We conducted the present study in the Tarn River (Southwestern France) within the historical city center of Albi, a UNESCO World Heritage Centre. European catfish were introduced in the Tarn River in 1983 and have since established self-sustained populations [12]. Behavioral monitoring was performed from a bridge above a gravel island where pigeons (Columbia livia Gmelin) regroup for drinking and cleaning (43u 559 51.770 N, 2u 089 41.830 E). At the studied stretch, the Tarn River is approximately 100 m wide with a mean depth of 3 m (maximum depth 5.4 m) and belongs to a protected area where angling is prohibited. In total, 24 surveys (approx. 3-hour long on average, total observation and filming time of approx. 72 h) were performed from June 30th to October 19th 2011 in the morning or in the afternoon. The number and success rate of beaching behavior were determined by filming the predatory fish nearby the Figure 1. European catfish displaying beaching behavior to capture land birds. Several individuals were observed swimming nearby the gravel beach in shallow waters where pigeons regroup for drinking and cleaning (large picture). One individual is seen approaching land birds and beaching to successfully capture one (small pictures). doi:10.1371/journal.pone.0050840.g001 gravel island. Throughout the survey, river discharge was low and the water was clear, allowing full observation of all displayed behavior (Figure 1 and Movie S1). (b) Stable Isotope Analyses Tissue samples of European catfish and their potential prey were collected in the observation site from 18th September to 14th October 2011 to quantify the contribution of land birds to the catfish diet using stable isotope analyses. Sampling was allowed by the permit Arrete Prefectoral no. 2011018. Stable isotope analyses (principally d13C and d15N) are now commonly used by ecologists to provide reliable estimates of long-term dietary patterns of predators [13,14], including trophic specialization [15,16] and the ecological impacts of non-native fish species [14,17]. A total of 14 European catfish located within the study area (estimated body size ranging from 900 to 2000 mm) were sampled. Fin clips were collected by angling from 5 individuals while muscle samples were collected on the flank of 9 additional individuals by scuba divers using a spear gun equipped with a 40mm length and 27-mm diameter stainless steel biopsy tip. Fin and muscle samples were pooled in the analyses since their stable isotope values do not differ significantly in this species [12]. No lipid correction was performed since samples have low and homogeneous C:N ratios (3.260.2 SD) [18]. Muscle samples of the putative aquatic and terrestrial prey present in the study area were also collected. These included crayfish (Orconectes limosus Rafinesque n = 3) and Cyprinids prey fish (Blicca bjoer (...truncated)