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Estimates for the parametric terms depth (m), SST, and fishing time (h) obtained by GAM for the probability of dolphin depredation.
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Depredation by cetaceans is a growing problem that may have serious economic implications for fisheries and for dolphin conservation. We investigated depredation by Risso's dolphin (Grampus griseus) in the hand-jig squid fishery around the Azores to determine the factors that may influence depredation behaviour and impacts on the fishery, and condu...
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... ( n 42) of the fishing trips and depredated 33% ( n 32) of the trips. In six trips with depredating Risso’s dolphins, blue sharks ( Prionace glauca ) were also observed depredating squids and on another two trips, bottlenose depredation was also documented. Depredation by dolphins was easily distinguished from depredation by sharks or other veined squid because dolphins pull the squid from the jig pins removing it whole or leaving the head and tentacles, whereas sharks make clean cuts and squids cause only superficial damages to the squid flesh. Risso’s dolphins were observed depredating squid 1.5 + 1.6 h after the fishing activity started and remained around the fishing boats for 1.5 + 1.9 h. When a squid was hooked, dolphins approached the boat rapidly, dived presumably to remove the squid, and then surfaced away from the boat. There were on average three dolphin attacks per fishing trip with a time interval between attacks of 1 + 0.85 h. On average, dolphins depredated 4.4 squids per fishing trip, corresponding to a loss of 4.8 kg of squid. Risso’s dolphin depredation was not statistically associated with lower squid catches (trips with depredation: 44.6 + 40.9 kg; trips without depredation: 55.4 + 51.7 kg; t 1⁄4 2 1.026, d.f. 1⁄4 94, p 1⁄4 0.308). Trips with dolphin depredation showed slightly lower cpue (1.9 + 1.3 kg fisher 2 1 h 2 1 ) than trips without depredation (2.1 + 2.0 kg fisher 2 1 h 2 1 ), but the difference was not significant ( t 1⁄4 2 0.492, d.f. 1⁄4 94, p 1⁄4 0.624). Depredation occurred more frequently south and northeast of S. Miguel, where fishing effort was more concentrated. The AIC-based results of the stepwise selection procedure from GAM suggested that only three variables—depth, SST, and fishing time—significantly explained the probability of dolphin depredation in the hand-jig squid fishery. The best fitting GAM explained 22% of the deviance and included parametric terms for depth and SST, and a smoother for fishing time (Supplementary Table S2). Depredation probability increased linearly with depth and decreased with SST, being more likely to occur in areas deeper than 165 m and when the water temperature was , 18.5 8 C (Figure 5). The duration of the fishing trip affected the likelihood of depredation, but the relationship was non-linear. The probability of dolphins depredating squids decreased rapidly with fishing time, reaching the minimum for fishing trips of 5 –7 h but increased slightly in longer trips. We then developed a quasipoisson GLM to investigate which factors influenced the quantity of depredated squid by dolphins. The model included only fishing trips with depredation to eliminate the excess of zeros that are difficult to model when sample size is small. Instead of building a model with all available explanatory variables, we included only a subset of covariates describing two aspects of the fishery—effort and catch—that we expected to be in- fluential on the number of squids depredated. The number of hand- jigs, total catch, and number of fishing boats in the area had no effect on the number of squids depredated and fishing time was the only variable retained in the final model (GLM: t 1⁄4 2.470, p , 0.05). The model predicted greater catch losses as duration of fishing trips increased, with . 5 squids depredated for trips longer than 6 h (Figure 6). Depredation rate for the period from 2009 to 2011 was calculated at 0.32 (standard error 1⁄4 7.9 × 10 2 5 ), meaning there was a loss of 3.2% of the total squid catch (in kg) per trip. The highest depredation estimate was in 2010 with over 12 t of squid depredated. Combining data for the 3 years, depredation by dolphins repre- sented a catch loss of 30 t of squid and an economic loss of about E 152 000 for the squid fishing community of S. Miguel (Table 4). A total of 154 trials were performed during 45 fishing trips to test the effect of pinger condition and brand on the catch and on dolphin depredation (Supplementary Table S3). On several occasions, it was not possible to carry out the five trials during a fishing trip due to degradation of sea state or shifts in fishing area. An average of 2 + 1.4 squids fisher 2 1 h 2 1 was captured during the trials. Cpue for the control condition was 2.17 + 1.37, compared with 2.19 + 1.49 for pinger-inactive and 1.71 + 1.19 for pinger-active, with the linear mixed-effects model, suggesting that pinger condition had no effect on cpue (Supplementary Table S4). While the Fumunda- active showed some reduction in cpue compared with the control and other treatments, changes in cpue across pinger treatments were not significant ( p . 0.05; Figure 7; Supplementary Table S4). Depredation was recorded in 33% ( n 1⁄4 15) of the fishing trips and Risso’s dolphins were responsible for all depredation events. Risso’s dolphins depredated on average 3 squids per trip. About 20% of the control trials and 19% of the trials with pingers present (independently of being active or not) were depredated. Pinger condition and brand had no effect on the probability of dolphins depredating the catch ( p . 0.05; Figure 8; Supplementary Table S5). The linear mixed-effects model predicting the proportion of depredated squid also did not suggest a significant effect of pinger condition and pinger brand (Supplementary Table S5). There was no documented change in dolphin behaviour (0, no reaction) relative to the presence of active or inactive ...
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... Such approaches result in sampling bias, as defined in Table 2. Purposive sampling was used in 29.6% of studies (n=27) ( Table 1). For example, Cruz et al. (2014) sampled islands where squid (Loligo sp.) catch was higher and Lunneryd and Westerberg (1997) interviewed fishers with a high likelihood of bycatch. Such sampling approaches would produce a biased sample, unless the sampling frame was corrected to only reflect the type of fishers interviewed. ...
... Here, e.g. villages or islands were used as clusters (Cruz et al., 2014;Leeney et al., 2015). If cluster sampling is used, and clusters are of different sizes, then fishers from different strata will have different probabilities of being sampled. ...
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... Furthermore, dolphins are evidenced to predate fish from nets or hooks, resulting in potential damage of fishing gear and in loss of capture and resulting revenue (e.g., Pardalis et al., 2021;Snape et al., 2018). However, Cruz et al. (2014) studied interactions between Risso's dolphins in the Azorean artisanal fishery for veined squid, and showed that there was no difference in the mean weight of squid landed in trips with and without dolphin depredation and that damage to fishing gear was infrequent. ...
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AVAILABLE AT: https://doi.org/10.1093/icesjms/fsac173
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... The effect of the Banana Pingers on the depredation of squid hand-jig lines was variable. This is consistent with studies of depredation occurrence (Lapiccirella et al., 2018) and of the effectiveness of pingers emitting sound continuously on pelagic cetaceans (Cruz et al., 2014); in particular, this gear is poached by several dolphin species, including T. truncatus, S. coeruleoalba, Grampus griseus and Globicephala melas, regardless of pinger presence. In the Southern Tyrrhenian Sea, striped dolphin attack squid hand-jig lines throughout the year, disturbing fishing operations and plundering the catch (Di Natale & Navarra, 2019). ...
... In the second test period -when trials were performed between Vulcano Island and Sicily, which is highly frequented by striped dolphin as well as fishers (Fortuna et al., 2007) -the pingers not only failed to keep the dolphin away, but they actually appeared to attract them. Cruz et al. (2014) have reported that two pinger models emitting sound continuously had no effect on Risso's dolphin interactions with the squid fishery, neither scaring them away nor attracting them. Despite the limited literature on squid hand-jig lines, it cannot be excluded that some physical features of pelagic habitats may affect pinger emissions (Cruz et al., 2014). ...
... Cruz et al. (2014) have reported that two pinger models emitting sound continuously had no effect on Risso's dolphin interactions with the squid fishery, neither scaring them away nor attracting them. Despite the limited literature on squid hand-jig lines, it cannot be excluded that some physical features of pelagic habitats may affect pinger emissions (Cruz et al., 2014). Although pingers have clear effects on coastal marine mammals (Battaglia et al., 2010;Akande, 2018;Björklund Aksoy, 2020), several studies have described failures due to the interference of background noise or to starving-driven depredation due to habitat degradation (Hardy & Tregenza, 2010;Carretta & Barlow, 2011;Snape et al., 2018). ...
In a test-control study conducted in the Aeolian Archipelago (Southern Italy), acoustic deterrent devices (pingers) were applied to four gear types typical of local artisanal fisheries to assess their effectiveness in mitigating dolphin-fishery interactions. In this area ecosystem degradation and overfishing have been increasing bottlenose dolphin (Tursiops truncatus) and striped dolphin (Stenella coeruleoalba) conflict with fishers. Banana Pingers were applied to Spicara maena gillnets, trammel nets, “lampara” nets and hand-operated squid jig lines (“totanara”) in trials conducted from April to September 2017. Dolphin depredation events were greatly reduced in the gillnet (100%) and the “lampara” net (86%), resulting in a strong increase in catch weight (kg) and revenue (€). In the squid hand-jig line trials, severe depredation events (60%) markedly reduced catch and revenue. In the trammel net, catch weight and revenues were not significantly different in the test and control nets. Despite the absence of dolphin damage, the fish species that are part of the dolphin diet were more abundant in the test net. Our findings suggest that pinger effectiveness may be influenced by a variety of factors including dolphin species, season, habitat and fish species distribution. Notably, the discards of trammel nets account for nearly 50% of the catch and include potentially valuable bycatch species, like Sparisoma cretense, which however commands a low price on the local market. We suggest that together pingers and the local sale of non-target species could mitigate the economic loss due to dolphin damage, although this requires appropriate planning.
... Risso's dolphin prey veined squid (Loligo forbesii), for example, was only detected over the island slope at relatively shallow depths (100 to 500 m), matching its known habitat, as well as depth of catches from local fisheries (33,40). As a confirmed prey species of the Azorean population of Risso's dolphins (41), its absence offshore may help explain the species' preference for mesopelagic foraging over slope versus bathyal waters. Strictly deep-sea species such as Planctoteuthis levimana and Chtenopteryx sp. were only recorded at large depth (1600 m) (33). ...
Fundamental insight on predator-prey dynamics in the deep sea is hampered by a lack of combined data on hunting behavior and prey spectra. Deep-sea niche segregation may evolve when predators target specific prey communities, but this hypothesis remains untested. We combined environmental DNA (eDNA) metabarcoding with biologging to assess cephalopod community composition in the deep-sea foraging habitat of two top predator cetaceans. Risso’s dolphin and Cuvier’s beaked whale selectively targeted distinct epi/meso- and bathypelagic foraging zones, holding eDNA of 39 cephalopod taxa, including 22 known prey. Contrary to expectation, extensive taxonomic overlap in prey spectra between foraging zones indicated that predator niche segregation was not driven by prey community composition alone. Instead, intraspecific prey spectrum differences may drive differentiation for hunting fewer, more calorific, mature cephalopods in deeper waters. The novel combination of methods presented here holds great promise to disclose elusive deep-sea predator-prey systems, aiding in their protection.
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Shark depredation, where a shark partially or completely consumes an animal caught by fishing gear before it can be retrieved to the fishing vessel, occurs in commercial and recreational fisheries worldwide, causing a range of negative biological and economic impacts. Despite this, it remains relatively understudied compared to other fisheries issues. This is the first review of the literature relating to shark depredation, which also includes an overview of the potential mechanisms underlying its occurrence and options for mitigation. Furthermore, this review highlights key research gaps that remain to be investigated, thereby providing impetus for future research. In total, 61 studies have been published between 1955 and 2018, which include information on shark depredation. These studies recorded quantitative rates of depredation between 0.9 and 26% in commercial and recreational fisheries and during research fishing, identified 27 shark species from seven families that were responsible for depredation and discussed potential factors influencing its occurrence. Information from research into bycatch mitigation and the testing of shark deterrent approaches and technologies is also presented, in the context of applying these approaches to the reduction of shark depredation. This review presents an holistic overview of shark depredation in fisheries globally and, in doing so, provides a central resource for fisheries researchers and managers focusing on this topic to stimulate further collaborative research on this important fisheries issue.
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Small-scale artisanal fisheries can have a significant negative impact in cetacean populations. Cetacean bycatch has been documented in the pole-and-line tuna fishery in the Azores with common dolphins being the species more frequently taken. Based on data collected by observers on ∼50% of vessels operating from 1998 to 2012, we investigate the influence of various environmental and fisheries-related factors in common dolphin bycatch and calculate fleet-wide estimates of total bycatch using design-based and model-based methods. Over the 15-year study dolphin bycatch occurred in less than 0.4% of the observed fishing events. Generalized additive modelling results suggest a significant relationship between common dolphin bycatch and duration of fishing events, sea surface temperature and location. Total bycatch calculated from the traditional stratified ratio estimation approach was 196 (95% CI: 186–205), while the negative binomial GAM estimated 262 (95% CI: 249–274) dolphins. Bycatch estimates of common dolphin were similar using statistical approaches suggesting that either of these methods may be used in future bycatch assessments for this fishery. Our work shows that rates of common dolphin bycatch in the pole-and-line tuna fishery in the Azores are low, despite considerable variations between years. Dolphins caught were released alive although the fate of these individuals is unknown. Continued monitoring will provide a better understanding of dolphin bycatch and more accurate estimates essential in the development of potential mitigation measures.
... Considering that the pole-and-line tuna fishery was worth approximately €7 million per year during this period, this represents approximately 4% of the value of tuna landed. Cruz et al. [53] estimated that Risso´s dolphin depredation in the artisanal squid fishery in São Miguel island, Azores (where most of the fishery takes place) affected 33% of the fishing events from 2009-2011, representing an estimated loss of €50 thousand per year, 1.9% of total landed value of this fishery over this time. Thus, compared to Risso ´s dolphin interaction in the squid fishery, common dolphin interaction in the tuna fishery is less frequent but has a greater economic impact. ...
... However, the effectiveness of acoustic deterrents at decreasing cetacean interaction is still unclear [57,59,60]. Recently, Cruz et al. [53] showed that depredation rates by Risso's dolphins on the squid fishery in the Azores did not decrease with the use of various pingers. Moreover, the effect of these devices on tuna behaviour would need to be investigated before this mitigation measure could be considered. ...
Common dolphins (Delphinus delphis) are responsible for the large majority of interactions with the pole-and-line tuna fishery in the Azores but the underlying drivers remain poorly understood. In this study we investigate the influence of various environmental and fisheries-related factors in promoting the interaction of common dolphins with this fishery and estimate the resultant catch losses. We analysed 15 years of fishery and cetacean interaction data (1998–2012) collected by observers placed aboard tuna fishing vessels. Dolphins interacted in less than 3% of the fishing events observed during the study period. The probability of dolphin interaction varied significantly between years with no evident trend over time. Generalized additive modeling results suggest that fishing duration, sea surface temperature and prey abundance in the region were the most important factors explaining common dolphin interaction. Dolphin interaction had no impact on the catches of albacore, skipjack and yellowfin tuna but resulted in significantly lower catches of bigeye tuna, with a predicted median annual loss of 13.5% in the number of fish captured. However, impact on bigeye catches varied considerably both by year and fishing area. Our work shows that rates of common dolphin interaction with the pole-and-line tuna fishery in the Azores are low and showed no signs of increase over the study period. Although overall economic impact was low, the interaction may lead to significant losses in some years. These findings emphasize the need for continued monitoring and for further research into the consequences and economic viability of potential mitigation measures.
... Cetaceans are legally protected from hunting in the Azores, and fisheries bycatch is negligible (Silva et al. 2011). However, depredation by Risso's dolphins has been documented in the hand-jig squid fishery, and this may be a potential source of conflict (Cruz et al. 2014). A possible way to address it would be to establish time limitations to squid fishing in the core areas delineated in the present study. ...
Knowledge of the residency patterns of marine mammals is an important element for management and conservation strategies. Here we investigate a population of Grampus griseus off Pico Island, Azores. Our data set covers the period 2004–2007, based on at-sea observations of 1,250 individually identified animals, 303 of known or assumed sex. Using photo identification and GPS locations we calculated mean monthly sighting rates and lagged identification rates to analyze temporal patterns, and estimated kernel density to study the home range. Our results show site fidelity and relatively restricted home ranges, which corroborate the existence of a resident population on the study site. We further document sex differences, including a higher number of males present in the area at any given time but females staying for longer consecutive periods, and male home ranges with significantly less overlap than those of females. These observations are consistent with a mating system based on multimale pods defending areas where females periodically return. We hypothesize that squid distribution is a major factor in structuring these patterns. These findings reinforce the need for a precautionary management approach that would include limiting pressure from commercial activities.
