Foraging Ecology of Seabirds at Sea: Significance of Commercial Fisheries in the NW Mediterranean.

[eng] Seabirds feeding ecology was studied at sea to assess the significance of two commercial fisheries, trawlers (demersal fishery with diurnal activity) and purse seiners (pelagic fishery with nocturnal activity), in the NW Mediterranean. Trawlers offered high amounts of discards (mostly fish of small size) on a predictable basis, and consequently attracted large numbers of seabirds. At the Ebro Delta, discards provided in average 65% of the energy requirements of the seabird breeding community, being most important for large gulls and of little significance for terns. Purse seiners produced few discards, on an irregular basis, and attracted lower numbers of seabirds during discarding (after dawn). During the fishing operation, at night, only Audouin's Gulls Larus audouinii attended these vessels, capturing live fish concentrated near the sea surface, attracted by powerful lamps. During the breeding season, Audouin's Gulls preferentially attended trawlers in spite of potential competition with the more aggressive and opportunistic Yellow-legged Gulls Larus cachinnans, since the former showed to be more efficient at capturing discards and reduced competition by showing a wider range of feeding techniques. Only during trawler closures, Audouin's Gulls occurred in important numbers at purse seiners (thus acting as a secondary food resource for most individuals). During the non-breeding season Audouin's Gulls shifted to preferentially attend purse seiners, presumably because competition at trawlers strengthened at this period. The rare and threatened Balearic Shearwater Puffinus mauretanicus also made significant use of discards, which represented more than 40% of the energy requirements of the whole population during the breeding season. Seabirds feeding upon discards showed higher levels of mercury, as discards make accessible to them demersal fish that is more polluted than epipelagic fish (natural prey for most seabirds). Thus, the risks of toxicity by this metal are higher for those species capturing discards. Given the significance of commercial fisheries for seabirds in the NW Mediterranean, and considering the increasing ecosystem-based approach to fisheries management, fisheries managers should regard these birds when designing fishing policies

Relative abundance and distribution of fisheries influence risk of seabird bycatch

Fisheries provide an abundant and predictable food source for many pelagic seabirds through discards, but also pose a major threat to them through bycatch, threatening their populations worldwide. The reform of the European Common Fisheries Policy (CFP), which intends to ban discards through the landing obligation of all catches, may force seabirds to seek alternative food sources, such as baited hooks from longlines, increasing bycatch rates. To test this hypothesis we performed a combined analysis of seabird-fishery interactions using as a model Scopoli's shearwaters Calonectris diomedea in the Mediterranean. Tracking data showed that the probability of shearwaters attending longliners increased exponentially with a decreasing density of trawlers. On-board observations and mortality events corroborated this result: the probability of birds attending longliners increased 4% per each trawler leaving the longliner proximity and bird mortality increased tenfold when trawlers were not operating. Therefore, the implementation of the landing obligation in EU waters will likely cause a substantial increase in bycatch rates in longliners, at least in the short-term, due to birds switching from trawlers to longliners. Thus the implementation of the landing obligation must be carefully monitored and counterbalanced with an urgent implementation of bycatch mitigation measures in the longline fleet

Sex-specific foraging behaviour in response to fishing activities in a threatened seabird

Some seabird species have learnt to efficiently exploit fishing discards from trawling activities. However, a discard ban has been proposed as necessary in Europe to ensure the sustainability of the seas. It is of crucial importance for the management and conservation purposes to study the potential consequences of a discard ban on the foraging ecology of threatened seabirds. We assessed the influence of fishing activities on the feeding habits of 22 male and 15 female Audouin's gulls ( Larus audouinii ) from the Ebro Delta (Mediterranean Sea) during the breeding period using GPS loggers together with Stable Isotope Analysis (SIA), which provided new insights into their foraging behavior and trophic ecology, respectively. GPS data revealed different sex-specific foraging patterns between workdays and weekends. Females were highly consistent in that they foraged at sea throughout the week even though discarding stops at weekends. In contrast, males switched from foraging at sea during the week (when discards are produced) to an increased use of rice field habitats at week- ends (when fishermen do not work). This sex-specific foraging behavior could be related to specific nutritional requirements associated with previous egg pro- duction, an energetically demanding period for females. However, on a broader time scale integrated by the SIA, both sexes showed a high degree of individual specialization in their trophic ecology. The need to obtain detailed information on the dependence and response of seabirds to fishing activities is crucial in conservation sciences. In this regard, sex-specific foraging behavior in relation to fisheries has been overlooked, despite the ecological and conservation impli- cations. For instance, this situation may lead to sex differentiation in bycatch mortality in longlines when trawlers do not operate. Moreover, any new fisher- ies policy will need to be implemented gradually to facilitate the adaptation of a specialized species to a discard ban scenario

Meta-population feeding grounds of Cory's shearwater in the subtropical Atlantic Ocean: implications for the definition of Marine Protected Areas based on tracking studies

Aim Apical pelagic species forage in predictable habitats, and their movements should signal biologically and ecologically significant areas of the marine ecosystem. Several countries are now engaged in identifying these areas based on animal tracking, but this is often limited to a few individuals from one breeding population, which may result in biased portrayals of the key marine habitats. To help identify such foraging areas, we compiled tracking data of a marine top predator from the main breeding colonies in the Central Macaronesia. Location North‐east Atlantic Ocean. Methods Over seven years, we tracked the foraging movements of Cory's shearwaters (Calonectris borealis) from several populations during the chick‐rearing period using global positioning system and platform terminal transmitter devices. Results We obtained foraging trips from 174 shearwaters breeding on six important colonies representative of the range occupied in the Macaronesian Archipelagos of Madeira, Salvages and Canaries. Our results show that birds orient and move rapidly towards the closest neritic waters over the African continental shelf. Birds from different colonies show substantial spatial segregation in their foraging grounds but consistently overlap in some specific foraging areas along the Canary Current characterized by high productivity. By weighting the use of foraging grounds according to the size of each study population, we inferred the overall exploitation of such areas. Main conclusions Our meta‐population approach provides a more comprehensive picture of space use from both perspectives: the studied species and the Canary Current system. Foraging grounds consistently used by several populations may not be adequately identified by tracking a single population, and therefore, multiple population tracking studies are needed to properly delineate key conservation areas and inform conservation planning in the marine ecosystem. Finally, we highlight the long‐term stability and sustainability of identified foraging areas and propose that countries with geographical jurisdictions over the Canary Current area should work towards multilateral agreements to set management plans for this key marine ecosystem

Modelling terrestrial and marine foraging habitats in breeding Audouin's gulls Larus audouinii: timing matters

Although the breeding ecology of Audouin's gull has been widely studied, its spatial distribution patterns have received little attention. We assessed the foraging movements of 36 GPS-tracked adult Audouin's gulls breeding at the Ebro Delta (NW Mediterranean), coinciding with the incubation period (May 2011). This also coincided with a trawling moratorium northwards from the colony. We modelled the distribution of the gulls by combining these tracking data with environmental variables (including fishing activities from Vessel Monitoring System, VMS), using Maxent. The modelling range included both marine and terrestrial areas. Models were produced separately for every 2h time interval across the day, and for 2 fishing activity scenarios (workdays vs. weekends), allowing to assess the spatio-temporal distribution patterns of the gulls and the degree of association with fisheries. During workdays, gull distribution at sea fully matched with fishing activities, both trawling (daylight) and purse-seining (nightime). Gulls tended to avoid the area under trawling moratorium, confirming the high influence of fisheries on the distribution patterns of this species. On weekends, gulls made lesser use of the sea and tended to increase the use of rice fields. Overall, Audouin's gull activity was more intense during dailight hours, although birds also showed nocturnal activity, on both workdays and weekends. Nocturnal patterns at sea were more disperse during the latter, probably because these gulls are able to capture small pelagic fish at night in natural conditions, but tend to congregate around purse-seiners (which would enhance their foraging efficiency) in workdays. These results provide important insight for the management of this species. This is of particular relevance under the current scenario of European fisheries policies, since new regulations are aimed at eliminating discards, and this would likely influence Audouin's gull populations

Seabirds and demersal longliners in the northwestern Mediterranean: factors driving their interactions and bycatch rates

Bycatch mortality in longline fisheries is considered the main threat at sea for numerous seabird species. These incidental catches occur worldwide, but mortality levels are mainly determined by the specific traits of the fishery operating in each area and the feeding behaviour and local abundance of seabirds. In the Mediterranean, demersal artisanal longliners are known to catch several seabirds, but bycatch rates and the main factors influencing both the probability and the level of seabird bycatch are poorly known. From 2011 to 2015 we conducted 220 trips onboard demersal longline vessels of the Balearic Sea, aiming to study their interaction with seabirds, as well as to understand the detailed procedures of the fishery and the factors that might influence seabird bycatch. Additionally, we recorded bird catches reported by fishermen. We found an average overall bycatch rate of 0.58 birds per 1000 hooks (0.13-1.37, 95% CI), which would imply a conservative estimate ranging from 274 to 2198 seabirds caught annually on demersal longliners in the study area. The most affected species were the 3 endemic and threatened Scopoli's, Balearic and Mediterranean shearwaters of the Mediterranean (Calonectris diomedea, Puffinus mauretanicus and P. yelkouan, respectively), likely due to their highly aggregative behaviour and diving capabilities. Overall, the main factors influencing bycatch risk were season and time of day. Other influential factors were bait type, wind conditions, gear configuration (specifically, distance between weights), proximity to the breeding colony and the number of hooks. This study shows that mortality caused by demersal longliners is high and may be jeopardizing the viability of the shearwater populations. Therefore, the identification and implementation of mitigation measures is urgently required

Seabirds and demersal longliners in the northwestern Mediterranean: factors driving their interactions and bycatch rates

Bycatch mortality in longline fisheries is considered the main threat at sea for numerous seabird species. These incidental catches occur worldwide, but mortality levels are mainly determined by the specific traits of the fishery operating in each area and the feeding behaviour and local abundance of seabirds. In the Mediterranean, demersal artisanal longliners are known to catch several seabirds, but bycatch rates and the main factors influencing both the probability and the level of seabird bycatch are poorly known. From 2011 to 2015 we conducted 220 trips onboard demersal longline vessels of the Balearic Sea, aiming to study their interaction with seabirds, as well as to understand the detailed procedures of the fishery and the factors that might influence seabird bycatch. Additionally, we recorded bird catches reported by fishermen. We found an average overall bycatch rate of 0.58 birds per 1000 hooks (0.13-1.37, 95% CI), which would imply a conservative estimate ranging from 274 to 2198 seabirds caught annually on demersal longliners in the study area. The most affected species were the 3 endemic and threatened Scopoli's, Balearic and Mediterranean shearwaters of the Mediterranean (Calonectris diomedea, Puffinus mauretanicus and P. yelkouan, respectively), likely due to their highly aggregative behaviour and diving capabilities. Overall, the main factors influencing bycatch risk were season and time of day. Other influential factors were bait type, wind conditions, gear configuration (specifically, distance between weights), proximity to the breeding colony and the number of hooks. This study shows that mortality caused by demersal longliners is high and may be jeopardizing the viability of the shearwater populations. Therefore, the identification and implementation of mitigation measures is urgently required

Sex-specific foraging behaviour in response to fishing activities in a threatened seabird

Some seabird species have learnt to efficiently exploit fishing discards from trawling activities. However, a discard ban has been proposed as necessary in Europe to ensure the sustainability of the seas. It is of crucial importance for the management and conservation purposes to study the potential consequences of a discard ban on the foraging ecology of threatened seabirds. We assessed the influence of fishing activities on the feeding habits of 22 male and 15 female Audouin's gulls ( Larus audouinii ) from the Ebro Delta (Mediterranean Sea) during the breeding period using GPS loggers together with Stable Isotope Analysis (SIA), which provided new insights into their foraging behavior and trophic ecology, respectively. GPS data revealed different sex-specific foraging patterns between workdays and weekends. Females were highly consistent in that they foraged at sea throughout the week even though discarding stops at weekends. In contrast, males switched from foraging at sea during the week (when discards are produced) to an increased use of rice field habitats at week- ends (when fishermen do not work). This sex-specific foraging behavior could be related to specific nutritional requirements associated with previous egg pro- duction, an energetically demanding period for females. However, on a broader time scale integrated by the SIA, both sexes showed a high degree of individual specialization in their trophic ecology. The need to obtain detailed information on the dependence and response of seabirds to fishing activities is crucial in conservation sciences. In this regard, sex-specific foraging behavior in relation to fisheries has been overlooked, despite the ecological and conservation impli- cations. For instance, this situation may lead to sex differentiation in bycatch mortality in longlines when trawlers do not operate. Moreover, any new fisher- ies policy will need to be implemented gradually to facilitate the adaptation of a specialized species to a discard ban scenario

Relative abundance and distribution of fisheries influence risk of seabird bycatch

Fisheries provide an abundant and predictable food source for many pelagic seabirds through discards, but also pose a major threat to them through bycatch, threatening their populations worldwide. The reform of the European Common Fisheries Policy (CFP), which intends to ban discards through the landing obligation of all catches, may force seabirds to seek alternative food sources, such as baited hooks from longlines, increasing bycatch rates. To test this hypothesis we performed a combined analysis of seabird-fishery interactions using as a model Scopoli's shearwaters Calonectris diomedea in the Mediterranean. Tracking data showed that the probability of shearwaters attending longliners increased exponentially with a decreasing density of trawlers. On-board observations and mortality events corroborated this result: the probability of birds attending longliners increased 4% per each trawler leaving the longliner proximity and bird mortality increased tenfold when trawlers were not operating. Therefore, the implementation of the landing obligation in EU waters will likely cause a substantial increase in bycatch rates in longliners, at least in the short-term, due to birds switching from trawlers to longliners. Thus the implementation of the landing obligation must be carefully monitored and counterbalanced with an urgent implementation of bycatch mitigation measures in the longline fleet

Modelling terrestrial and marine foraging habitats in breeding Audouin's gulls Larus audouinii: timing matters

Although the breeding ecology of Audouin's gull has been widely studied, its spatial distribution patterns have received little attention. We assessed the foraging movements of 36 GPS-tracked adult Audouin's gulls breeding at the Ebro Delta (NW Mediterranean), coinciding with the incubation period (May 2011). This also coincided with a trawling moratorium northwards from the colony. We modelled the distribution of the gulls by combining these tracking data with environmental variables (including fishing activities from Vessel Monitoring System, VMS), using Maxent. The modelling range included both marine and terrestrial areas. Models were produced separately for every 2h time interval across the day, and for 2 fishing activity scenarios (workdays vs. weekends), allowing to assess the spatio-temporal distribution patterns of the gulls and the degree of association with fisheries. During workdays, gull distribution at sea fully matched with fishing activities, both trawling (daylight) and purse-seining (nightime). Gulls tended to avoid the area under trawling moratorium, confirming the high influence of fisheries on the distribution patterns of this species. On weekends, gulls made lesser use of the sea and tended to increase the use of rice fields. Overall, Audouin's gull activity was more intense during dailight hours, although birds also showed nocturnal activity, on both workdays and weekends. Nocturnal patterns at sea were more disperse during the latter, probably because these gulls are able to capture small pelagic fish at night in natural conditions, but tend to congregate around purse-seiners (which would enhance their foraging efficiency) in workdays. These results provide important insight for the management of this species. This is of particular relevance under the current scenario of European fisheries policies, since new regulations are aimed at eliminating discards, and this would likely influence Audouin's gull populations