Ethics statement

The mitigation trials were conducted in two commercial vessels, so all fishing activities were performed by the fishermen. We had the approval of the captains to carry out the experiments in their vessels and complied with the relevant requirements to go onboard commercial fishing vessels as an occasional crewmember. This study did not require any special permission since fishing operations were conducted in strict conformity with the local rules and regulations.

Mitigation measure trials

The experiments were mainly carried out on a medium-scale artisanal demersal longliner (Cona C.B., 11.10 m length) from north-western Mediterranean between May to July 2013 and 2014 (41°08’N, 1°53’E, Vilanova i la Geltrú, Catalonia, Spain). In addition, one of the trials (tori line) was also tested on another medium-scale demersal longliner in May 2014 (Mar endins, 14.5 m length) that operates in the Gulf of Lion area (42°20’N, 31°30’E, Llançà, Catalonia, Spain). The areas and period were chosen to be among those with the highest risk of bycatch within the study area [18]. The fishing grounds are located within important foraging areas of the most susceptible species (the three endemic shearwaters) [20–22], while the study period overlapped with the prelaying and incubation stage of Scopoli’s shearwaters and the chick-rearing and post-nuptial migration stage of the Puffinus shearwaters. In addition, trials performed with one of the longliners (Cona C.B.) coincided with a trawling moratorium period, which should exacerbate bycatch probability [15,23]. Both vessels used a fishing gear configuration called the “Piedra—Bola” system (Floated), which is composed of a series of floats and weights to keep hooks at different heights from seabed (Fig 1), following a zigzag structure (see [18]). They also used the same size of hook: 3.17 and 1.40 cm of total length and of gape, respectively.

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Fig 1. Longline types used by the artisanal demersal fleet.

Floated: “Piedra–Bola” configuration composed by a sequence of floats and weights. Non-floated: fishing gear typically used by small-scale vessel where only weights are attached to the mainline. The position of the time-depth recorders (TDRs) is also indicated by a red square in the floated (Float, Mid and Weight positions) and non-floated (Mid) longlines. Drawing by Toni Mulet.

https://doi.org/10.1371/journal.pone.0196731.g001

Hooks were baited with European pilchard (Sardina pilchardus) or European anchovy (Engraulis encrasicolus) to target European hake, although other species with less commercial value were also caught, such as blackmouth catshark (Galeus melastomus), European conger (Conger conger), blackbelly rosefish (Helicolenus dactylopterus) and small-spotted catshark (Scyliorhinus canicula). There were some differences in the fishing gear configuration linked to the particular practices of the fishermen on the different boats (Table 1). The main differences were the distance between consecutive weights, as well as the distance from the weight to the float.

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Table 1. Characteristics of the fishing gears used in demersal longliners in which the sink rates were recorded.

https://doi.org/10.1371/journal.pone.0196731.t001

Trials were performed under typical fishing conditions to test different mitigation strategies. A description of the methods used is detailed in the Supporting information (S1 Table). For each mitigation measure, paired longline settings were performed consecutively: one line was normally set without any mitigation measure (control setting) and the other one using the tested mitigation measure (experimental setting). In case of night setting trials, the night set was scheduled to be completed 1 hour before the sunrise (when nautical dawn begins: sun angle is 12° below horizon), while the day set was performed between 1 and 3 hours after the night set, coinciding with the sunrise or the first hours of the day. The duration of each setting (control or experimental) was about 30 minutes. In case of the Cona C.B., longlines were left in the water until the night of the following day (around 15 hours), starting with the first line set the previous day. On the Mar endins, the soak time of the longlines was 2–3 hours and the order of the hauling operations was the same as for the settings.

The artificial baits used in the trials were manufactured by the Spanish company Arom Bait^®, and consisted of brown, triangular pieces composed of a mix of products derived from fish.

During setting operations, the number and species of seabirds attending the vessel and the attacks on bait performed by each species every 10-min period were recorded. For each time interval, the number and distance astern of overall attacks on bait were also registered and were ranked in six intervals: < 5, 5–10, 11–20, 21–50, 51–100 and > 100 m. During the night, counts of birds and attacks were limited by the distance from the stern at which these could be detected given the illumination provided by the boat (< 50 m). Additionally, a 12-volt halogen spotlight was used to confirm seabird species and increase the detection of bait attacks during the night settings. Position, depth and setting speed were also recorded at every count interval. A detailed description of the fishing gear used and the meteorological data were registered for each fishing trip. Seabird catches were recorded during the hauling, together with the number of the fish commercial catches, their size (from the tip of the snout to the tip of the longer lobe of the caudal fin), condition and the discards (fish catches returned to the sea). Seabird catches observed during the setting but not retrieved during the hauling were also considered. Discards were classified into four different causes: (1) bad condition of the specimens, (2) low commercial value, (3) small size and (4) non-commercial or protected species (fish bycatch). Moreover, the body mass of a sample of 470 European hakes were measured to obtain the relationship between the weight and the size, and then estimate the body mass from those individuals that were not weighed. Potential function showed a best fit of these data (R² = 0.958; p < 0.005):

The effectiveness of each mitigation method in reducing seabird attraction and bait attacks was assessed comparing results between the control and experimental lines. The practical applicability was also evaluated for all mitigation measures tested, while the effects on fish catches was only assessed in those measures that involved a change in the longline configuration and fishing habits (night setting, weighted lines and artificial baits). Given that seabird incidental catches only occur occasionally (12% of the fishing days, [18]), it was not possible to obtain enough data to compare bycatch rates between control and experimental settings. Instead, the bait attacks were used to infer seabird bycatch risk [18].

Determining longline sink rate

Sink rates were measured to 10 m depth with Time-Depth Recorders (TDRs, long-life G5 data storage tags, CEFAS technology limited, UK). This reference depth was based on the dive ability of the Balearic shearwater [22]. Each TDR measured 35.5 mm length and weighed 2.5 g in seawater. TDRs were programmed to record depth within 0.5 m every second. Devices were attached onto the monofilament of the branchline at 5–10 cm from the hook.

Sink rates were estimated for the three different longline configurations used in demersal longline vessels of the study area (Table 1 and Fig 1). Most data were collected from the control longlines (Floated configurations) of the mitigation trials conducted on the Cona C.B. (Floated 1) and Mar endins (Floated 2). TDRs were also deployed on a small-scale vessel called Vigilante (6.81 m length) from Mallorca (Balearic Islands) that used a different longline configuration (Non-floated). In this case, the longline only had weights attached at regular intervals, which keeps the hooks level over the seafloor (Fig 1). Moreover, in contrast to other configurations studied, the weights used by this vessel were less heavy and more widely spaced (Table 1). This longline type is typical in the polyvalent small-scale vessels from the western Mediterranean to target a wide variety of demersal fishes, such as common pandora (Pagellus erythrinus), toothed bream (Dentex dentex) or gilt-head bream (Sparus aurata) (see [18]).

TDRs were placed in different sections of the longline: initial (first 50 hooks), middle and final (last 50 hooks). Within each section, TDRs were placed at different hooks inside the float-weight sequence (see Fig 1): near to the float (Float), between the float and the weight (Mid) and near to the weight (Weight). In the case of the non-floated longlines, the TDRs were located between consecutive weights (Mid). During the trials of the additional lead weights, TDRs were deployed in the two-paired longlines of each sample (control and experimental). To check if the lead weights increased the sink rate of the hooks, the devices were placed at the same section of the longline and position inside the float-weight sequence.

From the sink rate data and the setting speed of the vessel, it was possible to determine the distance behind the vessel at which shearwaters are most vulnerable to becoming hooked, namely the seabird access window (vessel speed x seconds to 10 m depth). This allowed the greatest bycatch risk area behind the stern during the setting for each longline type to be defined.

Tori line

The tori line was designed following the guidelines and recommendations for demersal longline vessels [24,25], and it was then adapted to the traits and size of local vessels with the help of the fishermen. Before the experiments, the height of the tori line, length and distance between streamers were modified until adequate protection of the baited hooks was achieved. The aerial extent of the streamers determines the efficiency in deterring birds from the sinking baited hooks [24], so the tori line was set as high as possible to reach sufficient aerial extent to cover as much as possible of the seabird access window. The final configuration of the tori line (Fig 2) was composed of 11 paired branch streamers distributed along a 6 mm-wide twisted rope of 70 m length. Rope was fixed on a pole located at the stern 5 m above the water surface. Brightly coloured long streamers were used to scare birds from flying under the aerial extent of the tori line [24,26]. Each streamer was 12.5 mm thermoplastic red tubing, decreasing in length with increasing distance from the vessel (3 m to 1 m long) and attached to the rope at intervals of 4 m. All streamers reached the sea-surface in calm conditions. A float was added at the end to create drag and tension of the rope. This kept the line off the water for some distance and, consequently, increased the aerial extent of the streamers. The main purpose was to cover a minimum of 20 m behind the vessel, as most seabird attacks (89%) in demersal longliners operating in the area occur within 5–20 m behind the vessel [18]. In each sample, consecutive settings with or without the tori lines deployed were conducted in the same order (first set without the tori line) to compare its effect on seabird interaction (details in Supporting information). Moreover, during the trials, additional tori line settings were also performed to better assess its adaptation to the vessels.

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Fig 2. Design of the tori line used in the trials.

Drawing by Toni Mulet.

https://doi.org/10.1371/journal.pone.0196731.g002

Statistical analysis

Generalized linear mixed models (GLMMs) were used to test the effect of the mitigation measure on the number of bait attacks, the number and total kilograms caught of hake, the number of the most common commercial non-target species (catsharks and congers) and the proportion of fish discarded. The seabird species not vulnerable to bycatch, such as the Mediterranean storm-petrel (Hydrobates pelagicus melitensis), individuals discarded of target species and the samples of the fish catches data in which the paired lines differed more than 20 m depth were not considered in the analyses. Each model included the use of the mitigation measure as a binary fixed effect. “Trip” was included as a random term to correlate the observations of paired longline settings. When the number of hooks differed between paired longline settings, this variable was considered as an additional factor (seabird observations in tori line trials) or offset (fish catches in weighted lines). The number of birds following the boat was used as an offset [27,28], except in the case of the night-setting trials due to the difficulty in detecting birds in low-light conditions. Moreover, in this latter case, the effect of the measure on seabird interactions was assessed considering only the attacks occurred in the first 50 m from the stern, that is the distance in which the attacks could be detected during the night by the illumination of the boat. Therefore, the number of birds that attacked baits at this distance is unknown as these counts were not divided by distance intervals as it was done for the bait attacks. For GLMMs, significance tests for fixed effects were performed using Wald Z-tests [29]. Models were fitted with the laplace approximation using the package “glmmADMB”. A negative binomial distribution was assumed for the response variable of the seabird observations, given that count data typically show a skewed distribution and overdispersion [30,31]. For the fish catches, the distribution of the response variable of each dataset was previously checked for normality using Shapiro-Wilk test. In case of no deviation from normality, linear mixed models (LMMs) were applied by using the “lmer” function from the “lme4” package. Then, a likelihood-ratio test (LT) was performed to obtain the p-values. Otherwise, negative binomial GLMMs were used in accordance with the type of mean–variance relationship found in the response variable [31]. On the other hand, a gamma distribution was used for the kilograms of hake and a binomial distribution for the proportion of fish discarded [31]. In case of the congers, due to their relative lower abundance, the probability of catching the species was modelled using binary data with a Bernoulli distribution.

The effect of the measure on the type of fish discarded was also assessed by dividing the discards into different reasons (fish damaged, low value, small size, bycatch) and comparing their proportion between control and experimental settings. Moreover, the effect of the tori line on the distance astern where the bait attacks occurred was evaluated comparing the proportion of attacks inside (< 50 m) and outside (> 50 m) the area covered by the streamers. These two last analyses were assessed using a chi-square test.

Sink rate differences among (1) longline configurations, (2) the positions along the float-weight sequence and (3) the lines with and without additional lead weights, as well as the differences in the extent of the seabird access window between longline vessels, were compared using an analysis of variance (ANOVA) and a post–hoc analysis (Tukey’s test). P-values were corrected by Bonferroni method. To compare sink rates between sections a Welch's t-test was used. Differences between longline configurations were evaluated by considering the values obtained in the middle sections and between consecutive weights (Mid positions). However, only the sink rates obtained in the Floated 1 configuration were used to assess the differences between the sections and the positions along the mainline. Values obtained in the Mid position were used to compare sections, while in the case of the comparison between hook positions in the float-weight sequence the data from the initial section was used. The data from the final section and Weight sink speed values from the middle section were not considered because of their small sample size. On the other hand, to assess the effects of lead weights on hook sinking speed, the sink rates recorded on hooks located in the middle section and near to the floats (Float) were considered. All statistical analyses were conducted using R version 3.2.4.

Bait attacks

Setting at night was the only measure that significantly reduced the number of bait attacks (see Table 2 and Fig 3, Negative binomial GLMM: coefficient = -4.87, SE = 1.51, Z = -3.23, p <0.001). Unfortunately, only a small numbers of bait attacks were registered in the trials of the additional lead weights and artificial baits, so it was not possible to assess their effects on seabird interactions. This is because bait attacks occur in relatively low frequency in the demersal longlines of the study area (38% of the sets, [18]) and, in addition, both trials were performed at the end of the peak season of bycatch.

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Fig 3.

Number of bait attacks in the night setting and tori line trials (a and b). Attacks are shown per 1000 hooks set. In each trial, paired longlines were set; “Control” denotes the settings without mitigation measure. The middle line of boxes shows the median and the filled circles represent outliers.

https://doi.org/10.1371/journal.pone.0196731.g003

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Table 2. Bait attacks registered by species (those most susceptible to bycatch) and mitigation measure.

https://doi.org/10.1371/journal.pone.0196731.t002

During night settings, seabirds following the vessels were those that are known to have nocturnal foraging habits (Table 2), such as Scopoli’s shearwaters [32] and Audouin’s gulls (Larus audouinii) [33]. Nevertheless, only Scopoli’s shearwater was seen to attack bait in only one of the samples performed during the new moon.

The number of bait attacks did not differ between control and tori line settings (Fig 3, Negative binomial GLMM: coefficient = 0.19, SE = 0.51, Z = 0.37, p = 0.71). Nonetheless, the tori line increased the distance at which bait attacks occurred (Fig 4), scaring seabirds away from the area covered by the streamers lines (> 50 m; χ² = 14.56, df = 1, p < 0.005). This was particularly true for shearwaters (Fig 4), which continued to attack baits when the tori line was deployed but just further away from the stern. However, the attacks on bait also occurred inside the tori line area (< 50 m, 48% of the attacks observed).

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Fig 4. Total number of attacks registered at different distance intervals from the vessel stern.

Shearwaters (triangle; Scopoli’s, Balearic and Mediterranean shearwater) and gulls (square; Audouin’s and yellow-legged gulls) were considered separately. Dark refers to the control settings without tori line (No deterrent, n = 12 settings), while grey refers to experimental settings with the tori line installed (Tori line, n = 12). Dashed line denotes the limit of the area covered by the streamers.

https://doi.org/10.1371/journal.pone.0196731.g004

During the trials, three Mediterranean shearwaters were caught in one of the samples; one of them in a control setting and the other two in a tori line setting, and one Scopoli’s shearwater was caught in another sample during the tori line setting. On the same day, three birds were caught (one Scopoli’s shearwater, one Mediterranean shearwaters and one Balearic shearwater) during an additional tori line setting performed with out the paired comparison settings. All seabird catches occurred on a calm day. Bait attacks occurred with the same frequency inside of the area covered by the streamers in calm (< 50m, 51%, N = 102 attacks) and windy days (45%, N = 76). However, these were proportionally greater in the first 20 m from the stern on calm days (< 20 m; χ² = 5.43, df = 1, p < 0.05). In addition, attacks near the stern were also favoured when fishermen stopped the boat for changing the box where longlines are stored (each box contains at about 420 hooks). During this time lapse, the aerial extension of the tori line was reduced considerably, allowing birds to attack baits closer to the boat.

We observed that the aerial coverage of the streamer lines varied with weather conditions and setting speed, but it usually covered 30–45 m astern. Taking into account the mean setting speed and sink rate, the streamers covered the sinking hooks until they reached 2 m depth below the surface.

Commercial catches

Target catches did not differ between control and night settings or lead weight settings (Table 3). However, we found a significant increase of the Blackmouth catshark in longlines with additional weights (Negative binomial GLMM: coefficient = 0.75, SE = 0.34, Z = 2.19, p < 0.05) and higher likelihood of catching European congers when longlines were set at night (Bernoulli GLMM: coefficient = 18.75, SE = 5.52, Z = 3.40, p < 0.001). On the contrary, we found a significant reduction in the number (Negative Binomial GLMM: coefficient = -1.49, SE = 0.19, Z = -7.83, p < 0.001) and total weight caught (Gamma GLMM: coefficient = -1.54, SE = 0.20, Z = -7.69, p < 0.001) of the European hake (target species) during the artificial bait setting. Therefore, the catches obtained in artificial baits represented only a quarter of the total hake caught when fishermen used sardine or anchovy bait.

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Table 3. Fish catches registered for those mitigation measures that involved a change in the longline configuration and fishing practices.

https://doi.org/10.1371/journal.pone.0196731.t003

The proportion of catch discarded was similar between the control and experimental settings of the mitigation measures tested. However, we found a significantly lower proportion of discards due to fish bycatch when longlines were set at night (χ² = 7.97, df = 1, p < 0.005) and in lines with additional weights (χ² = 8.13, df = 1, p < 0.005). In night setting trials, these catches were mostly registered in the control lines (N = 13 individuals, 39% common stingray Dasyatis pastinaca, 31% the ocean sunfish Mola mola, 15% swordfish Xiphias gladius, 8% Blue shark Prionace glauca and 8% Atlantic bluefin tuna Thunnus thynnus), while only one ocean sunfish was bycaught during the settings at night. Conversely, in the weighted line trials, all fish bycatch occurred in the control lines (N = 12, 50% swordfish, 25% common stingray and 25% the ocean sunfish).

Practical applicability

We detected some operational problems during the tori line and weighted line tests. In strong wind (> 17 knots) and cross wind conditions the tori line was displaced towards the fishing gear. Under these conditions, the captain refused to deploy the tori line alleging it could entangle with the longline and compromise the fishermen’s safety. Moreover, fishermen perceived an increase of entanglements between the branchlines and hooks during the setting operations when they use longlines with the additional weights.

Longline sink rate

The sink rate of longlines to 10 m depth varied according to the line configuration considered (Table 4, F_2,51 = 64.01, p < 0.005). Non-floated longlines sank at a significantly slower rate than the Floated (post-hoc Tukey’s test: p < 0.005, Floated 2; p <0.005). In addition, the sinking of hooks from the Floated 2 was significantly slower than the Floated 1 (post-hoc Tukey’s test: p < 0.005).

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Table 4. Time and sink rate to 10-m depth and 10-m access window by longline configuration.

https://doi.org/10.1371/journal.pone.0196731.t004

The distance astern at which longline hooks were within 10 m of the surface (AW, Table 4) differed between the longline types (F_2,48 = 9.25, p < 0.005). Floated 2 lines showed the greatest extent of the access window (post-hoc Tukey’s test: Floated 1; p < 0.05, Non-floated; p <0.005). Conversely, we found that the seabird access to baited hooks was shortest in the Non-floated configuration. Nevertheless, the shorter distance in Non-floated was not significant when it was compared to the Floated 1.

Hooks showed different sink rates depending on the longline section, and the position with respect to the weights and the floats (Fig 5). Hooks sank faster in the initial section (Mid_{initial-middle}: t_17.728 = 2.998, p < 0.05), and those near to the weight showed greater sink rates than those located near to the float or between the float and the weight (F_2,35 = 8.98, p < 0.005; post-hoc Tukey’s test: Float_initial−Weight_initial, p < 0.005; Mid_initial−Weight_initial, p < 0.005). However, the proximity to the float did not reduce the sinking speed (Float_initial—Mid_initial, ns; Float_middle−Mid_middle; ns). This could be due to the influence of the weight placed close to the float (see Table 1).

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Fig 5. Sink rates of the baited hooks in the first 10 meters depth.

The values (mean + CI 95%) came from the hooks located at (1) different positions inside of the float-weight sequence (Float = near to the float, Mid = between the float and the weight, Weight = near to the weight) and (2) longline sections (initial, middle and final). The hook sink rates were collected in the same vessel and from gear with a “Piedra–Bola” configuration (Floated 1). Numbers inside bars denote the sample sizes.

https://doi.org/10.1371/journal.pone.0196731.g005

The sinking speed of the hooks increased significantly when a lead weight of 10 g or 20 g was added to the branchlines (F_2,34 = 15.83, p < 0.002, Fig 6), reaching the same sink rate for the two types of weights (10 g– 20 g = 0.20 m/s). Thus, in both cases, the addition of the weights led to a significant reduction of the seabird access window of at least 30 m (F_2,34 = 22.02, p < 0.002; Mean ± SD; Control = 163.54 ± 7.27 m, 10 g = 131.53 ± 17.96 m, 20 g = 131.50 ± 14.31).

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Fig 6. Sink-rate comparison of the longlines set with or without additional weights.

The values (mean ± CI 95%) were registered in the control (n = 13) and experimental longlines with an added weight above hooks of 10 g (n = 14) and 20 g (n = 10). Data were collected from hooks located in the middle section of the longline and near to the floats.

https://doi.org/10.1371/journal.pone.0196731.g006

Efficacy of mitigation tests on seabird interaction and fish catches

Night setting was the only method tested in the present study that reduced the frequency of bait attacks with no effect on target catches, and also had positive effect on fish bycatch species. In general, setting at night has proved to be very efficient in demersal longliners, especially in the Southern Ocean [34,35]. A recent study carried out in U.S. West Coast has also shown that night setting significantly reduces albatross catches without compromising target catches [36]. Nevertheless, its efficacy is particularly high among seabird species foraging only during daylight hours, but less effective on nocturnal seabirds. Some seabirds do forage at night, particularly during bright moonlight nights, which may compromise the efficacy of night setting [34,37,38]. In the Mediterranean, although all species are mainly diurnal, there are several species showing some nocturnal activity, such as Scopoli’s shearwater [32] and Audouin’s gull [33]. On-board observations conducted on demersal longliners in the same area showed the occurrence of nocturnal bycatch of these species and the Balearic shearwater in different moonlight conditions, but most of them occurred on full moon nights (71%, [18]). Even so, bird catches at night were far less frequent than catches at dawn or daytime. These findings were also found by Sánchez & Belda [39], which showed an important reduction of bait losses caused by Scopoli’s shearwater in longlines set at night. Consequently, both studies considered night setting as the most promising measure to reduce bycatch in the Mediterranean longliners. In the present study, we did not record any bycatch events during night settings, but we detected a large number of bait attacks by Scopoli’s shearwater in one of the night settings performed during a non-moon day, suggesting this measure may not completely avoid seabird catches and bycatch might not be restricted to moonlit nights. Furthermore, setting the longline at night may affect catches of commercial species, leading to economic losses for fishermen [35]. In this regard, this is the first study in the Mediterranean showing night setting having no effects on target catches. However, it increased the probability of catching the European conger, probably due to its nocturnal feeding behaviour [40]. This fact could lead to negative effects on fishermen activity if the night setting substantially increases the conger catches, since this species has low commercial value. Moreover, night setting reduced the catches of fish bycatch species, such as swordfish, ocean sunfish and blue shark. These species perform diel vertical migration so at night these fish stay close to the surface [41–43], making an encounter with demersal longline gear less likely. Nevertheless, these findings should be confirmed for longliners targeting fish other than European hake, since the effect of setting time may differ for other fish species with different activity rhythms [44]. Apart from reducing seabird catches, the potential benefits of night setting for fishermen is well illustrated by the fact that some fishermen operating near seabird breeding colonies voluntarily changed to setting their longlines at night to avoid bait losses caused by seabirds, suggesting benefits obtained by reducing bait losses compensate for any decrease in commercial catches or practical problems.

Tori lines have also been demonstrated to be a suitable mitigation method in demersal longliners [7]. However, its effectiveness may vary depending on the seabird assemblage and fishing gear configuration [7,24,36]. In this study, the deployment of the tori lines did not influence the overall number of bait attacks. Nonetheless, seabird attacks were displaced further astern, occurring beyond the 45 m covered by the streamers, and they were mainly performed by shearwaters. This can be explained by the greater diving ability of the different shearwater species compared to gull species which only feed on surface. Shearwaters can dive several metres and reach baited hooks at a considerable distance astern (mean ± SD; Scopoli’s shearwater: 0.78 ± 0.79 m [45], Balearic shearwater: 5.6 ± 4.1 m [46]). Indeed, Puffinus shearwaters can dive > 10 m depth (up to 28 m) [22], which means that according to the sinking rates recorded in the present study, shearwaters could still reach baited hooks after the 45 m covered by the streamers lines (when baited hooks are at 2 m depth) and up to 190 m behind the vessel (10 m depth). Therefore, the use of tori lines would appear to be ineffective in avoiding shearwater bycatch because baited hooks would still be accessible to these species. Previous studies also found that streamers were not able to reduce the catches of diving birds, only those of surface feeding birds [8,24,47]. Moreover, Gladics et al. [36] found that the tori line alone does not protect adequately the baits from bird attacks in the floated demersal longlines. The floats slow the sinking speed of the baited hooks, making them available to seabirds well beyond the area protected by the streamers lines, while also increasing the risk of fouling with the tori line. In addition, the effectiveness of the tori line may be compromised in low wind conditions, when birds attacked baits at shorter distances astern. This fact may also occur when fishermen reduce the vessel speed to change the box with the baited hooks. Shearwaters are the only group of seabirds bycaught during the trials, but all catches registered in tori line settings occurred under calm conditions. On windless days, the streamers did not move and did not deter birds away [48]. In addition, a lower turbulence of the water during calm days could favour the visibility of the baits to seabirds, and hence lead to an increase in bycatch risk [35]. Similar results were previously found by a relevant study [48], suggesting the efficacy of tori lines is limited to windy days. However, strong crosswinds may also compromise the effectiveness of the tori line by displacing it from the setting area, leading to entanglements with the gear, or by bringing it out of its ideal position to protect the baited hooks [7,24,49]. In fact, during these trials fishermen were concerned about the risk of tangles and refused to deploy the streamer line in strong winds (> 17 knots). To avoid the adverse influence of calm days and crosswinds, some studies recommended the use of paired scaring lines [7,24,50]. However, the installation of two tori lines on small artisanal demersal longliners would be impractical, compromising the fishing operations and increasing the chance of tangles [36].

Previous studies illustrated that weighted lines is the most suitable method to reduce catches of diving seabirds in demersal longliners, since it increases longline sink rate and reduces the seabird access window [7,51,52]. In addition, the increase in the longline sinking speed could also enhance the fishing efficiency, particularly for those vessels targeting demersal species of very deep waters, since the gears reaches the seafloor earlier, which maximises the bait attractiveness [53]. In these experiments, it was not possible to make a direct assessment of the effect of adding weights on bait attacks due to the low number of birds interacting with the vessel. Alternatively, to evaluate its effect on the bycatch risk we used the hook sink rates and its potential to prevent seabird access to baited hooks. We found that the lines with additional lead weights sank 25% faster than control lines, reducing by 20% (30 m) the seabird access. Even so, the sinking speed obtained (0.20 m/s) was not enough to reduce the catches of the shearwaters as they still had a large access window. Previous studies recommended achieving a longline sink rate of > 0.3 m/s to minimize seabird catches [54], but even though it does not entirely eliminated bycatch risk [7]. Consequently, some studies recommended the weighted line in combination with a tori line to achieve an effective reduction in the seabird catches, especially of diving species [7,24,50]. Regarding fish catches, weighted lines probably affected the way the line was settling on the seabed, since the lead weights could keep the branchlines in a more vertical position bringing the baited hooks closer to the seabed, ultimately affecting fish catches. Indeed, weighted lines increased blackmouth catshark catches, possibly because catsharks forages in the near bottom layer and on the seabed [55]. In addition, we found an interesting reduction of non-commercial or protected fish species (common stingray, swordfish and ocean sunfish) in experimental lines, which might also result from the different way the line arrangement on the seabed.

Artificial baits have been proposed as a potential method to reduce seabird bycatch in longline fisheries [3], however, there is little information about its effectiveness in reducing seabird bycatch and its effect on commercial catches. In this study, similar to the conditions of weighted lines, it was not possible to assess its effect on seabird interactions due to the low number of birds recorded during the control and experimental settings. However, artificial bait setting showed a 77% reduction of the target catches relative to control lines. This result could be explained by differences in the release rate of the chemical feeding stimulants between baits, since this attribute greatly determines catch efficiency [56]. Therefore, this method may constitute an extra cost for fishermen to buy the bait, overall rendering it inappropriate for the profitable exploitation of European hake in the study area. Nevertheless, the potential usefulness of other types of artificial bait cannot be completely dismissed.

Effects of longline configuration on seabird access area

The seabird access area to baited hooks may vary between vessels according to the longline configuration used, the setting speed and the mainline location of the hooks. Regarding the longline configuration used, the distance between consecutive weights was an important factor in sink rate variation since the increase in distance between weights reduced the sink rate when we compared line configurations with similar weights and set at the same speed. In this regard, a previous study conducted in the Balearic Sea found that longline configurations with a greater distance between the weights have a higher seabird bycatch risk [18]. Therefore, our study provides evidence that this difference in the bycatch risk with regard to the distribution of the weights could be mainly driven by the variation in the sink rate and its related seabird access window to the baited hooks. Moreover, a previous study found higher attack rates in longlines with floats in comparison with non-floated longlines, since the use of floats slow the sink rate of the hooks and, hence, increasing the seabird access to baits [36]. Conversely, in this study, the longline type used in the small-scale vessel (Non-floated) sank significantly slower than the “Piedra–Bola” system (Floated), mainly due to the combination of lighter weights and the greater distance between them. Besides, in the case of the floated longlines considered in this study (zigzag structure), the placement of a heavy weight (2.3 kg) at a short distance of the float (7–11 m) could have also led to an increased sink rate in this type of configurations. However, these results cannot be extrapolate to other longlines configurations with floats used by the fleet (e.g. the pyramidal structures [18]), since the space between the weight and the float could be larger.

Setting speed also influenced sink rates but to a lesser extent [57]. The seabird access window in the Non-floated longline was significantly shorter than in the Floated 2 (139 m vs. 203 m) and similar to Floated 1, probably caused by its lower setting speed, thus indicating that lowering the setting speed of the vessels can decrease bycatch risk by reducing the seabird access window. However, setting speed also determines the final arrangement of the fishing gear on the seabed; faster speeds allow a tension of the mainline which keeps the longline straight and expanded along the sea bottom [57]. Vessel speed is sometimes adjusted to obtain an appropriate arrangement of the gear due to the speed and direction of the currents or other operational reasons, and so regulating setting speed to manage bycatch risk would thus be impractical. The mainline location of the hooks was also an influential factor on bait exposure, since we found differences in sinking rates between different sections of the mainline. The first section of the longline together with the hooks near to the weight sank much faster. As a result, the middle section and the hooks located near to the float and between the float and the weight were more exposed to seabird bait attacks. In contrast, a previous study [58] showed the opposite results, as they found a higher number of seabird catches in the first part of the longline, probably because the propeller turbulence slows the sinking speed of the hooks. In our study, however, the hooks located in the first section sank faster due to the influence of the anchor, since at the beginning of the setting the anchor has not yet reached the seabed so it pulls the longline towards the bottom at higher speed.