The study took place within the Exclusive Economic Zone of the Republic of Cyprus, in the marine areas off Larnaca Bay and Paphos – Limassol (southeastern and western coasts of Cyprus, respectively to a maximum distance of 40 nautical miles from the nearest shore. The total fishing effort of the entire fleet targeting albacore tuna in 2018 was 600 days. The albacore tuna fishing period lasts approximately three months, from late May to August. The pelagic longline fleet consisted of 30 vessels of lengths between 12 and 18 meters capable of using various gears (polyvalent vessels) permitted to adjust fishing activities according to the season and the presence and movement of various species of fish. The main fishing gear used is the drifting longline and secondary gears are trammel nets, gillnets, bottom longlines, and traps. The longline fishery is active within small distances from the shore, targeting demersal and mesopelagic species using bottom longline and nets, and away from the shore and outside the territorial waters, using drifting longlines, mainly targeting swordfish and albacore tuna, and other pelagic species (Department of Fisheries and Marine Research, 2019). The swordfish fishing period is between September to December whereas demersal species are targeted between December to May. Sardine is always used as a bait for fishing albacore tuna, and mackerel and squid is always used for swordfish fishing.

The analysis of the data was conducted in four phases. First, the rates and estimates for depredation and economic loss were calculated. Second, correlations and comparisons between parameters and variables were conducted. The third step aimed to qualitatively and quantitatively understand depredation, bycatch and mitigation practices based on fishers’ knowledge using descriptive statistics. Fourth, a non-parametric spatial analysis method was used to identify possible fisheries-dolphin conflict areas.

All the statistical analyses were performed using the R software (R Core Team, 2020). Statistical significance set at a<0.01 was used for all statistical tests to account for the increased probability of type I error multiple testing. The standard deviation (SD) for all averages is given unless specified otherwise.

The findings of this study have shown that the species that are most frequently involve in depredation and cause the highest impact on fishing operations are the common bottlenose dolphin and the striped dolphin. The common bottlenose dolphin is the most frequently reported species to interact with small-scale fishing activities in the Mediterranean Sea (Lauriano, 2004; Díaz López, 2006; Brotons et al., 2008; Rocklin et al., 2009; Blasi et al., 2015; Pennino et al., 2015; Snape et al., 2018; Pardalou and Tsikliras, 2020), probably due to its opportunistic feeding habits and wide spatial distribution (Barros and Odell, 1990). This species has a flexible and cosmopolitan diet (Shane et al., 1986; Barros and Odell, 1990; Connor et al., 2000) and its distribution is often related to prey distribution, which in many cases overlaps with the distribution of species targeted by a fishery (Barros et al., 2000), and thus, inevitably leading to conflicts between dolphins and fisheries. There are several reports worldwide of the common bottlenose dolphin interacting with fishing gear involving hook and line (Werner et al., 2015). Interactions between the striped dolphin Stenella coeruleoalba and fisheries have been also documented in the Mediterranean with small-scale and purse seine fisheries (Magnaghi and Podestà, 1987; Di Natale and Notarbartolo di Sciara, 1994; Bearzi, 2002; Northridge et al., 2013; Crosti et al., 2017; Bruno et al., 2021).

The results revealed that there is an estimated mean economic loss per day of about 313 EUR and an estimated annual economic loss for the entire fleet of about 260,000 EUR from depredation. In other regions, the economic loss by cetacean depredation on set demersal longlines targeting halibut (Reinhardtius hippoglossoides), arrowtooth flounder (Atheresthes stomias) and Patagonian toothfish (Dissostichus eleginoides) has been estimated to range between 1,034 USD to 8,449 USD per day (Roche and Guinet, 2007; Tixier et al., 2010; Hamer et al., 2012 and references therein), which shows substantially higher economic loss per day from the findings of the current study. Regarding the percentage of depredation rate (D%), we found that it ranges from 0 to 100 (mean D% = 17%), which is similar to the depredation rate (17.7%) found by Tixier et al. (2010) regarding the longline depredation on Patagonian toothfish, while other studies which included depredation on species of tuna, provided a broad range of depredation rates (0.5-100%) (Secchi and Vaske, 1998; Purves et al., 2004; Williams et al., 2009; Hamer et al., 2012) It has to be addressed that, the actual cost and rate of depredation from the current study, may be underestimated, as fish completely removed from the hooks were not possible to quantify. The rate of depredation may be affected either by the number of fish available or by the size of the group or from both factors. Both, D% and LPUE decrease from June to August, whilst the fishing effort increases. This can be explained by the albacore tuna migration at the end of the fishing season in August which corresponds to less catch for fishers and LPUE, and subsequently less available depredation resources for dolphins and D%.

Currently, pelagic longline fishers in Cyprus are being compensated annually by the government 300 EUR per fisher, for the damage caused on their fishing gear due to dolphin depredation (Ministry of Agriculture, Rural Development and the Environment, Department of Fisheries and Marine Research, 2020). This is considered by the fishers to be very low compared to the actual damage they are experiencing. Our findings support their arguments since we estimated the mean economic loss, during the fishing seasons, to be 313EUR daily per fisher. Personal author’s communication with fishers, unveiled that many fishers expressed aggression towards dolphins, often considering harming them, especially during fishing trips on days with high depredation. However, this was not always the case as a minority of fishers consider themselves as “outsiders”, meaning that they did not naturally belong in the dolphin’s environment and that they should respect and protect them.

The photographic evidence collected from onboard observations confirmed the depredation by the common bottlenose dolphin and striped dolphin, the shortfin mako shark and the neon flying squid on albacore tuna. Depredation by the cetacean species was distinguishable from a shark- and squid-damaged fish. Odontocetes removed the entire torso and left only the head of the fish up to the gills and in some cases up to the jaw. In other cases, dolphins tore the body of the fish, leaving bites with ragged borders and with the head of the fish left on the hook. Also, in all cases, marks from their pencil-like teeth were visible on the wounded flesh ( Figure 1A ). Apart from that, depredation from dolphins is in almost all cases verified by the presence of dolphins close to the vessel.

The depredation caused by squid was distinguished from dolphins and sharks, by the small beak bites throughout the torso of the caught fish. In most cases, squids attack the belly and middle body of the caught fish, possibly aiming for the highly nutritious eggs ( Figure 1B ). Another distinguishable feature was the visible marks from the squid suckers on the body and head of the caught fish ( Figure 1C ). The squid was also reported by fishers to depredate on the bait and often get caught on the hook. Apart from the photographic evidence showing an individual neon flying squid that was bycaught during depredation ( Figure 2 ), the fishers who participated in the interviews were asked to confirm the species by showing them pictures of the Mediterranean squids and they all identified it as O. bartrami. Fishers also reported catches of the neon flying squid of individuals weighing between 15 - 20 kg. Taking this information into consideration as well as the specimen photographed in Figure 2 , no other known squid species in the Mediterranean can reach this body weight.

Shark depredation was distinguishable by the clean cuts, and bite-shaped portionsof flesh from the torso of the caught fish, leaving the surrounding body of the fish undamaged ( Figure 1D ). The species of the shark was identified by the observer since the depredation event happened exactly at the time of towing the longline. The shark arrived at the surface to depredate on the caught fish while towing the longline and dove back in the water. Similar results on the identification of odontocetes and shark depredation on the depredated catch with almost identical marks have been reported in the literature from other parts of the world (Dalla Rosa and Secchi, 2007; Gilman et al., 2007; Hamer et al., 2012).

Pelagic shark and squid species have been also reported in the literature to depredate on the catch on different fisheries and fishing gears. Shark depredation on the longline fishery targeting mainly different tuna species, swordfish and toothfish, has been previously documented (Gilman et al., 2008; Mandelman et al., 2008; Hamer et al., 2012; Rabearisoa et al., 2018; Tixier et al., 2021) however, such information was none existed in the Mediterranean Sea until very recently. Malara et al. (2021) reported for the first time shark depredation in the Mediterranean Sea on the swordfish harpoon fishery. The shark species reported in the study that potentially depredated on swordfish were the blue shark (Prionace glauca), the shortfin mako (I. oxyrinchus) and great white shark (Carcharodon carcharias) (Malara et al., 2021). The blue shark, the shortfin mako and the bigeye thresher (Alopias superciliosus) are the most commonly bycaught shark species in the Cypriot pelagic longline fishery (Department of Fisheries and Marine Research 2016 - 2021). The very limited published information on squid depredation mentioned the depredation by the colossal squid (Mesonychoteuthis hamiltoni) on longlines targeting the Antarctic toothfish in the Southern Ocean (Remeslo et al., 2015) and by the purpleback flying squid (Sthenoteuthis oualaniensis) on gillnets targeting tuna in the Northern Arabian Sea (Moazzam, 2019). However, squid depredation has never been reported before in the Mediterranean fisheries.

The results from the interviews showed that the species causing depredation on the albacore tuna were the T. truncatus, followed by S. coeruleoalba, O. bartrami, I. oxyrinchus and G. griseus. The results from the interviews confirmed the findings from the logbooks and onboard observations. The interviews also revealed another species that is involved in depredation for which little is known about as it appears scarcer in the Levantine basin, the Risso’s dolphin (Boisseau et al., 2017). Even the though the depredation by this species is considered less frequent and rare as described by some fishers, there are a few known areas that this species is known to inhabit of which the fishers of the area know and try avoiding fishing there. The depredation by the neon flying squid is becoming a great concern for longline fishers as this species depredates on bait and on the albacore tuna as well as on swordfish (Xiphias gladius). However, the species is edible, not prohibited to catch and sell, and according to fishers, taste-wise it is highly valued. This could be an opportunity for fishers to target the species in the future and support their income, in case of available market niche.

The results also showed the high probability of dolphin bycatch during depredation on both bait and catch. Fishers reported that in most cases the bycaught dolphins were juveniles that often drowned. The entanglement and hooking of cetaceans on longlines have been previously reported as an occasional event often leading to serious injuries and mortality (e.g. Forney and Kobayashi, 2007; Garrison, 2007). Even if the fishers cut the branchline to release the bycaught dolphin, it is very likely that the hook and the line will be ingested, injure vital organs and hence, the animal will die as a result (Wells et al., 2008). Considering that fishers have reported a decrease in tuna and swordfish stocks and significant damage on gear, catch and bait from depredation, it is very likely that they resort to extreme measures, using harmful methods to prevent depredation and sustain their operational costs. Practices to prevent depredation, such as direct shooting or using explosives have been reported previously in the literature (e.g. Northridge and Hofman, 1999; Gilman et al., 2006; Zollett and Read, 2006). This is a major concern for cetacean conservation because mortality and injury resulting from those actions may have significant consequences on a population level, especially on small, isolated populations associated, with islands such as Cyprus. However, there are no official records of cetacean harassment in Cyprus.

Although both dolphin species are responsible for depredation, it is unclear from the data which species has the greater impact. Fishers participated in the study reported that depredation by the common bottlenose dolphin is more frequent than the striped dolphin, however, they mentioned that in many cases have observed both species in the same group. Therefore, it is particularly important to understand the precise depredation rate by each species as well as their population dynamics. This will require intense sampling of direct observations by trained and experienced observers.

The study also aimed to understand fishing practices and strategies that are employed by fishers which seek to minimize interactions, depredation and bycatch of the odontocetes with the pelagic longline fishery. It was possible to identify several applicable mitigation measures that if applied alone or in combination could potentially decrease longline-dolphin interactions. However, cetaceans are well known for their adaptive capabilities and a single, universal solution is unlikely to exist.

The ‘move-on’ rule is another practical method to avoid longline-dolphin interactions that is applied among fishers and require collaborative behaviour on their part. When a triggered event occurs between dolphins and fisheries, all fishers of the area are informed and are expected to avoid this specific fishing ground for a specific period (Gilman et al., 2006; Dunn et al., 2014). This method requires good communication and coordination by the fishing industry and fishery associations (Gilman et al., 2006). The method is already practised by fishers in Cyprus as they retain good collaboration among them but with low effectiveness on avoiding dolphin interactions. This is mainly because the tuna fishing grounds are restricted to specific areas and the options for the tuna longline fishery are limited. Additionally, when a pod of dolphins identifies a vessel or a fishing activity it can follow the vessel for miles away.

Moreover, the cause of acoustic discomfort to dolphins with the use of acoustic deterrent devices is a promising mitigation technique, although scientific evidence of their effectiveness is contradicting (Northridge et al., 2003; Brotons et al., 2008; Gazo et al., 2008; Buscaino et al., 2009) and habituation to the acoustic harassment device may occur (Tixier et al., 2015). The current study identified deterrent devices that are currently used by longline fishers in Cyprus and their effectiveness in mitigating depredation ( Table 2 ). Although no scientific experiment was conducted in the current study to prove their effectiveness, the feedback from fishers is that only one type of high-intensity acoustic deterrent devices (DDDs/DiD from STM industrial Electronics) seem to be effective in mitigating interaction with the common bottlenose dolphin and striped dolphin, even after a long period of usage (more than two years). Other studies have tested the DDDs devices and showed positive results on cetacean bycatch and depredation (Buscaino et al., 2009; Northridge et al., 2011). Based on fishers’ feedback, dolphin acoustic deterrent devices are considered for the moment as the only practical solution to mitigate interactions and depredation by dolphins. It is important to mention that in 2020 the government of the Republic of Cyprus announced a call for grant proposals for the purchase of repellent equipment (specifically dolphin anti-depredation pingers) for the protection of marine protected species. The percentage of co-funding to the beneficiaries (fishers) ranged from 30% to 80%, depending on the case and the remaining percentage was private participation (Thalassa 2014-2020, 2020). However, the potential ecological adverse effects of using high-intensity acoustic deterrent devices have not been assessed in the current study and the current knowledge on their short- and long-term effects is limited. It is likely that using such devices could alter the cetacean foraging strategies and distribution and in some cases damage hearing (Gilman et al., 2006).

Another alternative option, less invasive than deterrent devices, is the predictive forecasting; a method that identifies areas with fishing-dolphin conflicts to avoid these areas in the future through habitat modelling (Peterson and Carothers, 2013; Passadore et al., 2015). However, this method requires a high effort of data collection and analysis over a long period and the quality of such models is variable. In this study, a map ( Figure 11 ) was developed to show areas of highly probable longline-dolphin interactions and the probability of dolphins’ occurrence in these areas based on fishers’ experience. This approach for data collection and analysis was low-cost and could be easily applied to other regions and fishing gears; these results could be used to inform fishers about conflict-prone regions and could also be applied for management measures, such as temporally spatial closures. It is also important to mention that areas identified as longline-dolphin conflict areas are also important fishing grounds with high catchability rates making any management recommendations of spatial closures difficult. It is strongly suggested that any decisions taken by policy-makers should ideally result in following consultations with stakeholders, in this case, the fishers.

The current study showed that feeding on the albacore tuna caught on pelagic longlines represents an alternative foraging strategy for dolphins. The albacore tuna is not included in the diet neither of the common bottlenose dolphin (Blanco et al., 2001; Nowacek, 2002; Bearzi et al., 2005; Díaz López, 2006; Bearzi et al., 2009; Pardalou and Tsikliras, 2020) nor of the striped dolphin (Würtz and Marrale, 1993; Blanco et al., 1995; Spitz et al., 2006; Perrin et al., 2008; Matsuda et al., 2020), however, both species are known to exhibit high dietary plasticity. Other studies from the Mediterranean Sea and around the world have shown similar foraging strategies between cetaceans and static nets (Lauriano, 2004; Díaz López, 2006; Brotons et al., 2008; Bearzi et al., 2011; Maccarrone et al., 2014; Snape et al., 2018), trawling (Zollett and Read, 2006) and longlines (Dalla Rosa and Secchi, 2007; Ramos-Cartelle and Mejuto, 2008; Rabearisoa et al., 2015). Such findings support the optimal foraging theory where an animal makes foraging decisions according to prey type and availability and based on its individual fitness benefits with the aim to increase the net energy intake per unit of time spent foraging (MacArthur and Pianka, 1996). Considering that the literature on the diet of the common bottlenose dolphin and the striped dolphin the albacore tuna is not included; depredation as a foraging strategy intends to increase foraging efficiency while decreasing foraging energy costs.

All fishers interviewed in the study strongly believed that dolphins were able to identify their fishing ground by hearing the engine sound and even follow their vessels from hours to days. Fishers who had previously used low-intensity acoustic deterrent devices (from Fishtek Marine) reported that they were effective only at the beginning while after some time of usage they became ineffective which even acted as a signal for food for dolphins that were previously exposed and had learned the sound. Similar findings were reported by Pardalou and Tsikliras (2018). The behaviour of dolphins to relate acoustic signals from the vessel engines and acoustic deterrent devices to the presence of prey is clear evidence of the ‘dinner-bell’ effect (e.g., Visser, 2000; Cox et al., 2004; Carretta and Barlow, 2011; Wargo Rub and Sandford, 2020).

The gradual increase of depredation incidents could be the result of foraging decisions and due to cetaceans advanced learning abilities and knowledge transfer within populations (Whitehead et al., 2004; Pennino et al., 2013). However, depredation incidents may have also become more frequent due to the increased fishing effort as a result of the increasing demand for seafood worldwide (FAO, 2020). The increased incidents of cetacean depredation have also motivated researchers worldwide to study cetacean depredation and since the 2000s, the number of studies published on the topic has significantly increased (Hamer et al., 2012).

The main limitation of the study is the data collection methodology. Certainly, logbooks cannot replace the quality and accuracy of the data that can be collected from onboard observations by experienced researchers. To achieve optimal coverage of onboard observations and collect sufficient data that are representative of the entire fleet, it is often very expensive, especially when fishers require compensation for their services. On the other hand, self-reporting (logbooks) and interviews are relatively inexpensive, but the data gathered can be inaccurate and biased. However, this approach is widely used and has proven to be an important tool of research if its adequately implemented and with clear protocols for the surveys (e.g. Azzurro et al., 2011; Azzurro et al., 2019; Lopes et al., 2019). To limit the factors that could lead to inaccurate and biased data, the participants in our study were carefully selected and under certain criteria (see Methods section). In addition, the fishers who participated in the self-reporting were previously actively participating and collaborating through the actions of the Cyprus Bycatch Project (2018 – 2022) and other projects (in previous years) and this was also an important criterion during the selection process. The existing trust and respect that was built between the authors of this study and the selected fishers also played a significant role in the quality of the data recorded in logbooks. We rule out any bias in the data arising from the professional relationship with the interviewees (i.e., reporting what is expected to conform to the norm); the onboard surveys confirmed the veracity of the data. This was also another reason why only four out of the 30 licensed vessels/fishers were selected for the self-reporting and anonymity was assured. Definitely, the more the participants the better the extrapolations and representation of the fleet. Nevertheless, the selected participants are among the most active pelagic longline fishers in Cyprus.

Even though the identification of the different depredators from the fish carcasses is in most cases obvious as shown in Figure 1 , there might be cases which are not. For this reason, a possible misidentification might occur when fishers reported dolphin depredation in their logbooks. To limit this bias, fishers were asked to send photographic evidence of the depredated tunas when possible.

Another limiting factor is that the studies undertaken to assess the economic costs from cetacean depredation on pelagic longlines are very limited and for the Mediterranean do not exist yet. In addition to that, the lack of a standardized methodology that assesses depredation rate and costs is preventing any direct comparison with other studies.