Airplane-based abundance and distribution of seabirds off western iberia - monitoring towards conservation
- Autores: Hélder Araújo Fernandes
- Directores de la Tesis: Catarina Eira (dir. tes.), Pedro Rodrigues Correia (codir. tes.)
- Lectura: En la Universidade de Vigo ( España ) en 2021
- Idioma: inglés
- Tribunal Calificador de la Tesis: Alberto Velando Rodríguez (presid.), Vitor Hugp rodrigues paiva (secret.), Isabel Afán Asensio (voc.)
- Programa de doctorado: Programa de Doctorado en Ciencias Marinas, Tecnología y Gestión por la Universidad de A Coruña; la Universidad de Santiago de Compostela; la Universidad de Vigo; Universidade de Aveiro(Portugal); Universidade de Porto(Portugal); Universidade de Trás-os-Montes e Alto Douro(Portugal) y Universidade do Minho (Portugal)
- Materias:
- Enlaces
- Tesis en acceso abierto en: Investigo
- Resumen
Seabirds are the most threatened group of birds, since nearly half of the species are experiencing population declines (Croxall et al. 2012). Of the 359 extant seabird species, 47% are currently declining and 37% are threatened (Croxal et al. 2012). According to Dias et al. (2019), 84% of all seabird species are affected by at least one threat, including invasive alien species, bycatch, hunting/trapping, climate change/severe weather, disturbance, pollution, overfishing and problematic native species.
Seabirds are presently protected under national and international legislation. However, despite their global importance within marine ecosystems, the analysis of their population trends has been typically limited to relatively small spatial and temporal scales (Paleczny et al. 2015). Hence, the evaluation of the conservation status of seabird populations along their geographical range is of crucial importance and is required to meet international regulations and agreements, including the Birds Directive (2009/147/EC) and the Marine Strategy Framework Directive (MSFD, 2008/56/EC). Also, accurate and comparable population abundance estimates are needed to assess the removals of animals from populations (Garthe & Hüppop 2004). Furthermore, understanding species distribution and abundance is essential when developing management plans and conservation programs, that will lead to the implementation of effective actions on target areas.
This thesis aims to improve the knowledge about the abundance, habitat use and spatial distribution of seabirds and to estimate seabird fisheries bycatch in Western Iberian Waters (WIW), with a particular focus on the Balearic Shearwater (Puffinus mauretanicus) and the Northern Gannet (Morus bassanus).
The WIW ecosystem it is particularly important to migratory birds, since it is a wintering and stop-over area, as well as a migratory flyway for almost 1/5 of the world’s seabird species (de Jauna & Garcia, 2015). For some of these species, WIW foster significant numbers of their global or European populations. Such is the case of the Balearic Shearwater (Puffinus mauretanicus), the Northern Gannet (Morus bassanus), the Great Skua (Catharacta skua), the Mediterranean gull (Ichthyaetus melanocephalus) and the European storm petrel (Hydrobates pelagicus) (Arcos et al. 2009, Meirinho et al. 2014). Although, WIW seem to be less important breeding grounds, when compared with other European Atlantic areas (OSPAR Commission 2000).
The Balearic Shearwater is considered one of most threatened seabird species in the world (BirdLife International 2021a). The species is classified by the International Union for Nature Conservation as Critically Endangered (BirdLife International 2018a, 2021a), with an estimated annual decline of approximately 14%, and an average extinction time of 61 years, if the current trend is maintained over time (Genovart et al. 2016, BirdLife International 2018a). The main threats to the Balearic Shearwater population are lack of breeding habitat, predation when breeding and bycatch (Cooper et al. 2003, Oro et al. 2004, Arcos 2011a, Abelló & Esteban 2012). Pollution, including light pollution, and fish stock depletion are among the most recently identified threats to Balearic Shearwaters (Boué et al. 2013, Rodríguez et al. 2015, Costa et al. 2016). Balearic Shearwaters use WIW as migratory flyways, stopover sites and wintering areas, especially on the post-breeding and wintering seasons (June-October), although non-breeders are present all year round (Guilford et al. 2012, Oppel et al. 2012, ICNF 2014).
The Northern Gannet European population is estimated at 683 000 breeding pairs, which equates to 1 500 000 - 1 800 000 mature individuals, and it appears to be increasing (BirdLife International 2021b). During migration, Northern Gannets move to southern Atlantic areas, using relatively wide migration corridors along the coastline. Throughout winter, the species occurs over large areas, from the European Atlantic to the Occidental African coastal waters and the Mediterranean (Veron & Lawlor 2009, Fort et al. 2012). Northern Gannet is one of the most abundant pelagic predators in the WIW (Arcos et al. 2009, Meirinho et al. 2014). The Northern Gannet is present in WIW predominantly between late July and early December (de Juana & Garcia 2015), which is in accordance with the usual increase of gannet numbers in marine animal rehabilitation facilities during that period of the year, in the eastern Portuguese coast (Costa et al. 2020). In WIW, Northern Gannets are exposed to several anthropic threats, such as fisheries bycatch (Valeiras et al. 2009, Oliveira et al. 2015) and several types of pollution (Mendes et al. 2008), including marine litter leading to entanglement (Rodríguez et al. 2013, Costa et al. 2020).
There are several techniques to estimate seabird abundance or distribution patterns. However, dedicated ship-based and aerial transect-based surveys are the most commonly used approaches to generate abundance estimates across a wide range of species and geographical areas (Rogan et al. 2018). Aerial survey campaigns have proved to be an effective method to provide unbiased seabird abundance estimates and probability distribution maps (e.g. Pettex et al. 2017, Rogan et al. 2018). This type of surveys is essential to assess abundance and demographic trends outside breeding grounds and their relationship with the breeding population. Also, data obtained through aerial surveys can be used to build seabird Species Distribution Models (SDMs) (Pettex et al. 2017, Rogan et al. 2018). SDMs are useful to identify areas of high and low probability of occurrence, benefiting the priorization of conservation actions and planning in protected areas (Hyrenbach et al. 2000, Cleasby et al. 2020).
In the first chapter seabird species composition in WIW were described and their distribution and abundance in the late summer/early autumn were accessed thorough aerial surveys, using a line transect methodology. Out of the 17 identified seabird groups (species or higher-level taxonomic groups), it was possible to estimate annual overall abundances for nine seabird groups (Balearic Shearwater, Great Shearwater, Cory’s Shearwater, Shearwaters, Northern Gannet, Sabine’s gull, Great Skua, Storm-petrels and Red Phalarope) in Western Iberian Waters. The Northern Gannet was found as the most abundant species (89 630 individuals, CV=6.28%), followed by the Cory’s Shearwater (25 044 individuals, CV= 7.56%) and the Balearic Shearwater (13 632 individuals, CV=20.81%). The remaining taxonomic groups showed variable annual abundances. The results confirmed the importance of WIW for several seabird species in their migratory corridors or as a wintering area. The WIW show particular importance during the post-breeding period of the Balearic Shearwater, the Cory’s Shearwater, the Northern Gannet and the Great Skua. The results provided baseline estimates to inform conservation policies and instruments, such as the Birds Directive or the Marine Strategy Framework Directive.
In the second chapter, abundance and distribution of Balearic Shearwaters in Portuguese Continental Shelf Waters were evaluated. Between 2010 and 2014, five aerial surveys were conducted during the post-breeding period when migrating birds are outside the Mediterranean Sea. Following a line transect method, observers recorded a total of 181 Balearic Shearwaters sightings. Using Distance sampling software, an overall species abundance (2010-2014) of 10 182 (CV = 26.58) was estimated (ranging between 2 338 in 2010 and 23 221 individuals in 2012). During the 2012 post-breeding period, the Portuguese Continental Shelf Waters were used by up to 96.8% of the latest migratory population assessment (24 000-26 500 individuals, Arroyo et al. 2014). Considering Balearic Shearwater estimates per sampling block, was found a preference for the North and Center sectors of the Portuguese coast (respectively 7 058 and 1 366 individuals) where several SPAs were already designated (Ria de Aveiro, Aveiro-Nazaré, Berlenga, Cabo Raso, Cabo Espichel). Annual and overall habitat predictive models for Balearic Shearwaters were computed using a maximum entropy algorithm on MaxEnt software. In all models, the Balearic Shearwater distribution was best predicted by mean chlorophyll concentration. Balearic Shearwaters was mostly present in shallow shelf and coastal waters particularly in the widest portions of the continental shelf. These areas are strongly influenced by upwelling, which concurs with the chlorophyll concentration. The results are in line with the Balearic Shearwater abundances recently estimated by Arroyo et al. (2014) and demonstrate that Portuguese Continental shelf waters are used by a large proportion of the population. Overall, the study area seems to be one of the most important post-breeding grounds for the species.
The chapter III primarily aimed at evaluating the importance of WIW within the Northern Gannet migration range while also evaluating the species distribution and habitat preferences in the study area. Aerial surveys were carried out in all 5 blocks of the study area (Galicia, North, Center, Alentejo and Algarve) in 2011 and 2012. In 2010, 2013, 2014 and 2015 aerial surveys were carried out in the 4 blocks that correspond to the Portuguese portion of the study area. Using a line transect methodology, 3 672 Northern Gannet sightings were recorded along 10 496.3 nm. Considering the total number of Northern Gannet sightings, immature individuals and adults corresponded to approximately 87% of all sightings. Using Distance sampling, an overall abundance was estimated in 89 930 birds (CV = 6.26%), varying between 58 010 individuals in 2014 (CV = 10.61%) and 128 140 individuals in 2015 (CV= 10.92%). The highest annual abundance value (recorded in 2015) represents 7.12 to 8.54% of the Northern Gannet global population (1 500 000 – 1 800 000 individuals, BirdLife International 2018b). The annual abundance values show some fluctuation, particularly between 2013 and 2015. Most Northern Gannet sightings were recorded in the North block of the surveyed area (1 149 sightings) representing 32.49% of all sightings. The North block also showed the highest encounter rate value. However, the species abundance was higher in the Center block (an average of 25 447 individuals; CV=10.36%) whereas the lowest abundance was registered in the Galicia block. In all surveyed years and blocks, the variance in Northern Gannet density was primarily affected by the encounter rate, followed by the detection probability and group size. The Northern Gannet was mostly present in shallow shelf waters particularly in the widest portions of the continental shelf between 3 and 20 nm away from the shore. Habitat suitability models for Northern Gannets in the study area (computed using MaxEnt software) were tested using several abiotic and biotic variables (available at ecologically relevant spatial and temporal scales), which are known important predictors of seabird distribution. Therefore, the following eco-geographical variables were selected: Sea Surface Temperature (SST) (assumed as a proxy for physical processes or features driving prey distribution); Chlorophyll Concentration (Chl), an index of marine productivity (Afán et al. 2014); Slope, assumed as a proxy for upwelling and known to influence seabird distributions (Nishizawa et al. 2017); Distance to ports, as a proxy for central-place foraging (Pettex et al. 2019); Wind, an important factor influencing seabird distribution at global (Davies et al. 2010) and regional scales (Weimerskirch et al. 2012). In all models, Chlorophyll concentrations were found to contribute the most to explaining annual Northern Gannet occurrence probabilities. Modelling results indicate that the Northern Gannet is mostly present in shallow shelf waters particularly in the widest portions of the continental shelf located between 3 and 20 nm away from the shore. Using the habitat suitability maps and the uncertainty maps generated by MaxEnt, a global interannual spatial analysis was carried out using the software Zonation. This global interannual spatial analysis demonstrated the core areas for conservation of the Northern Gannet in WIW. Several areas were consistently selected as preferential habitats in all models including the north/central region of Portugal (i.e. between Porto and Nazaré), south of Berlenga island, Cape Raso, south of Cape Espichel, Cape S. Vicente and, to some extent, the Finisterra Cape and the Cies Islands in Galicia, Spain. Although this study spans a 6-year period, it was not attempted to evaluate abundance trends, due to sample size limitations. Moreover, the results demonstrate that Western Iberian Waters are used by almost 10% of the Northern Gannet global population, corroborating the existence of seabird hotspots during the non-breeding period, along migration corridors and at their wintering grounds.
The Chapter IV evaluates the potential fisheries bycatch effect on two of the most emblematic seabird species wintering off continental Portugal, the critically endangered Balearic Shearwater and the Northern Gannet. Bycatch information was collected by on-board observers and voluntary logbooks kept by fishing boat captains (on Fixed Gears, Purse Seine, Bottom Trawl, Bottom Longline and artisanal Beach Seine fleets). Each species’ Potential Biological Removal (for the study area) was calculated based on the respective population abundance estimated through aerial surveys. Spatiotemporal overlap between Balearic Shearwater and Northern Gannet distributions maps (obtained from aerial survey data) and fishing effort (obtained from AIS-based fishing effort dataset), were preformed to evaluate the potential for interactions between fisheries and the studied seabirds. For each pair-wise comparison, were calculated the degree of overlap, defined between 0 (no overlap) and 1 (strong overlap). The Bycatch Risk Index corresponded to the overlap of high species occurrence probability and high-density fishing. According to the overlap degree, the Bycatch Risk Index was low (0-0.25), moderate (0.25-0.75) or high (0.75-1). The highest Balearic Shearwater bycatch rate was produced by the fleet using Fixed Gear. The purse seine fleet presented a lower Balearic Shearwater bycatch rate in relation to the Fixed gears fleet. However, the highest number of bycaught individuals was recorded in Purse Seines. The artisanal Beach Seine fishery presented the lowest mortality rate and does not seem to have a high impact on species mortality. No Balearic Shearwater bycatch was recorded in Trawlers and Longliners. The PBR estimates point to an overall sustainable removal of 41 Balearic Shearwaters per year (ranging between 9 individuals in 2010 and 85 individuals in 2012). However, the estimates revealed that on average, 2 070 individuals were removed annually due to bycatch, which is 50.5 times higher than the obtained PBR values. The number of Balearic Shearwaters in the study area in late summer/early autumn, the so-called post-breeding period, was used as the reference population abundance in the PBR estimate. Obviously, the global population is larger (24 000-26 500 individuals, Arroyo et al. 2014). However, by using the regional total numbers as reference points, the calculated thresholds yield the range of extra mortality likely sustained by the regional and total populations (Genovart et al. 2016). All the examined fisheries potentially overlapped with the Balearic Shearwater predicted distribution areas, except for Longliners since they operate beyond the 25 nm distance from the shore (Larger AIS-equipped boats mostly operating in bottom longline areas). The largest overlap was found in Fixed Gears and Purse Seines, 83.6 and 81.4% respectively. Along with Trawlers (with an overlap of 44.1%), Fixed Gears and Purse Seines have the largest overlap with the Balearic Shearwater distribution inside or around the SPAs Ria de Aveiro, Aveiro-Nazaré, Cabo Raso and Berlengas. Fixed Gears reveal the highest Balearic Shearwater bycatch risk (High and Moderate risk combined), followed by Purse Seines and Trawls. In the future, when planning for management measures dedicated to Balearic Shearwater bycatch mitigation, priority should be given to high and moderate risk areas. As for Northern Gannets, the highest observed bycatch and mortality rates were recorded for Fixed Gears (including gill and trammel nets) and Longlines. Data reported in the Fixed Gear fleet show that this fishery on average removes a significant number of individuals (58 observed individuals extrapolated to 1 383 individuals). As for Longliners, even though this fleet had the lowest percentage of monitored fishing events, this fleet presented a high bycatch rate. When extrapolating for the entire fleet, Longliners potentially captured on average 2 286 Northern Gannets per year. This value fulfilled almost the total threshold PBR for the species in the study area (2 345 individuals per year considering the overall period). The overlap between the analysed commercial fisheries and Northern Gannet distribution revealed fishery-specific High-Risk areas. The lowest overlap with Northern Gannet distribution was obtained for Purse Seiners, corroborating the lowest bycatch numbers extrapolated to the entire fleet. Nevertheless, part of this overlap was detected inside the Berlengas SPA where purse seine interactions were found to involve adults in the migration period and immature individuals that remain in the area throughout the year (Calado et al. 2021). The overlap with Trawlers occurred mostly in the south coast between the 200m and 1000m bathymetry, where the trawling intense effort coincides with Northern Gannet migratory movements to the Mediterranean and to the western African coast (Fort et al. 2012). Fixed Gear fisheries overlapped with the Northern Gannet predicted distribution throughout the study area except for the south coast. The Longline and Northern Gannet overlap was more pronounced around the 1000m bathymetry, an area of high fishing activity. Overall, Longlines and Fixed Gear fisheries presented the highest Northern Gannet bycatch risk (areas of High and Moderate risk combined).
In this thesis, multi-annual seabird distribution and abundance were investigated for the first time in WIW using aerial surveys. Each seabird distribution and abundance estimates were presented with the corresponding coefficient of variation (CV) and 95% confidence interval (CI) calculations. MaxEnt software was used to compute seasonal and inter-annual Habitat Suitability models for the Balearic Shearwater and the Northern Gannet. The predicted species distribution was based on occurrence probability in relation to habitat suitability. The use of models with different sets of environmental variables are particularly important in the marine environment because variability in ocean conditions may lead to identifiable species distribution changes (Becker et al. 2017). This modelling approach proved to be effective in determining areas of high and low use by the target species. These data are crucial to policy makers and stakeholders involved in conservation planning, particularly when defining marine protected areas (including SPAs) and threat mitigation measures.
In comparison to vessel platforms, airplane surveys are more cost-effective, use lower effort time and produce a lower responsive effect on seabirds. Data collected in airplane surveys allow estimating absolute and relative abundances and associated errors. Data also allow establishing temporal trends and they can be used to model species distribution. If a double platform is used, airplane surveys reduce the number of animals that are not detected on the line or have evasive responses. However, the availability of suitable platforms for aerial surveys is low, observers must be trained and experienced and the identification of similar species (e.g., Storm-petrels, some Larids and Shearwaters) is difficult. Also, there may be problems in estimating angles to sightings in high-density areas and in collecting additional oceanographic or climatic data. For less common species, there is a risk of high variability in the error associated with sampling, which can hamper the detection of small fluctuations. Overall, line transect aerial surveys proved to be a time and cost-effective method allowing for robust seabird abundance and density estimates across a large study area.
The information collected in this thesis contributed to understand the distribution and abundance of the target species, being necessary for EU member states, such as Portugal and Spain, so that they can meet the European Marine Framework Strategy Directive and the Birds Directive. Furthermore, this information can help decision-makers designate or review proposed marine protection areas. In this study it was also possible to estimate Balearic Shearwater and Northern Gannet PBR in relation to bycatch. Areas of higher bycatch risk were mapped and results emphasised that Balearic Shearwaters bycatch mortality was above the PBR threshold (mostly due to set nets in the Polyvalent fleet). Northern Gannet bycatch mortality was at the PBR limit (mostly due to the Longliner and Polyvalent fleets). These data highlighted the necessity for implementation of mitigation measures to reduce seabird bycatch in fisheries, defined and enforced by European or national governmental agencies, to particularly with focus on the critically endangered Balearic Shearwater.
