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Diet and trophic relationships of top predators in the oceanic region of the archipelago of Madeira
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Os estudos de dieta são um elemento importante na compreensão da ecologia de uma espécie, mas são muitas vezes negligenciados e a sua importância subestimada. A dieta de um organismo permite conhecer o habitat em que o animal vive e as suas adaptações ao meio ambiente, mas também permite saber sobre interações com outras espécies. É a combinação de estudos de dieta de várias espécies que permite construir cadeias alimentares e compreender a estrutura e o funcionamento dos ecossistemas. A aplicação deste tipo de estudos no meio marinho tem importância acrescida, uma vez que estudos baseados em observação directa não são suficientes. Esta importância é ainda mais acentuada nas regiões oceânicas, onde as densidades de organismos são baixas e o seu estudo é um desafio.
Esta tese tem como objetivo estudar a dieta e as relações tróficas entre os predadores de topo e as suas presas na região oceânica do arquipélago da Madeira. A dieta e as áreas de alimentação de espécies de importância ecológica e económica, como atuns, aves marinhas e pequenos peixes pelágicos, foram estudadas na tentativa de compreender a sua posição e o seu papel na cadeia alimentar deste ambiente oceânico.
A dieta de duas espécies de peixes pelágicos, a cavala Scomber colias e o carapau-negrão Trachurus picturatus, foi analisada com base na identificação do conteúdo estomacal de indivíduos capturados ao longo de um ano, nas proximidades da ilha da Madeira. Ambas as espécies são planctívoras e piscívoras, alimentando-se sobretudo de copépodes calanóides e ciclopóides, e de pequenos agulhões Scomberesox saurus, clupeídeos, apara-lápis Macroramphosus scolopax e mictofídeos.
A dieta do patudo Thunnus obesus e do gaiado Katsuwonus pelamis, foi estudada identificando o conteúdo estomacal de indivíduos apanhados na Madeira e confirmada através da análise de mercúrio nos tecidos dos atuns e de outros dois peixes epipelágicos, a bicuda Sphyraena viridensis e o charuteiro Seriola rivoliana. A dieta de ambas as espécies de atum é composta principalmente por espécies epipelágicas, como a cavala e o peixe-rei Atherina sp., o que contrasta com o comportamento e dieta mesopelágicos do patudo em outras regiões do mundo.
Também investigámos a utilização de recursos marinhos pela gaivota-de-patas-amarelas Larus michahellis atlantis no arquipélago da Madeira. Esta ave costeira mostrou uma grande dependência antropogénica, com elevado uso de ambientes terrestres e associados ao homem. As poucas vezes que utilizou o meio marinho foi, principalmente, à noite e em associação com embarcações de cerco. A sua dieta refletiu este comportamento, sendo maioritariamente composta por resíduos e alguns peixes.
O estudo da variação na composição de espécies epipelágicas na vasta região da corrente das Canárias, foi realizado utilizando as aves marinhas como indicadores. Com recurso a viagens de alimentação e regurgitos de cagarras Calonectris borealis a nidificar nas Ilhas Selvagens, recolhidos por um período de sete anos, este estudo conseguiu detetar uma alteração na comunidade de peixes pelágicos, com um aumento acentuado da população de apara-lápis nos arredores das ilhas Selvagens, em 2017/2018.
Por fim, desenvolvemos um modelo baseado no equilíbrio de biomassas de espécies encontradas na Zona Económica Exclusiva do arquipélago da Madeira, utilizando o software Ecopath with Ecosim. Este capítulo reuniu as informações recolhidas nos outros capítulos e juntou-as aos dados disponíveis na literatura. O nosso modelo estimou uma elevada biomassa de produtores primários, zooplâncton, outros crustáceos e invertebrados, mas também de pequenos peixes pelágicos e mesopelágicos, que foram considerados o principal alimento de predadores de topo. O ecossistema caracterizou-se por um baixo número de ligações entre os níveis tróficos, que está relacionado com uma dieta mais especializada por parte de organismos como golfinhos, aves marinhas e grandes peixes pelágicos. Apesar de o ecossistema ter apresentado um nível trófico médio baixo, o nível trófico das pescas foi bastante alto por, na Madeira, predadores de topo como atuns e peixe-espada, serem os mais pescados. Os predadores de topo também foram considerados importantes modeladores do ecossistema, sendo designados por espécies-chave.
Diet studies are important for the understanding of the ecology of species, but commonly overlooked and their importance underestimated. The diet of an organism reveals its prey composition, but also its habitat and foraging behaviour. It also allows to have an insight on the interactions with other species and its surroundings. The combined information of diet studies of several species allows to build food webs and understand the structure and functioning of ecosystems. The application of these studies in the marine environment is particularly important, as pure observational studies are not enough. Such importance is even greater in oceanic regions where densities of organisms are low, and their study is challenging. This thesis aims to study the diet and trophic relationships of top predators and their prey in the oceanic region of the archipelago of Madeira. The diet and foraging areas of ecologically and economically important species, like tunas, seabirds and small pelagic fish, were studied in the attempt to understand their position and role in the food web of this oceanic environment. The diet of two species of mackerels, the Atlantic chub mackerel Scomber colias and the Blue jack mackerel Trachurus picturatus, was assessed using stomach contents of individuals caught throughout a year in the vicinities of Madeira island. These are planktivorous and piscivorous species, feeding mostly on calanoid and cyclopoid copepods, and on very small Atlantic saury Scomberesox saurus, clupeids, Longspine snipefish Macroramphosus scolopax, and myctophids. The diet of the Bigeye tuna Thunnus obesus and Skipjack tuna Katsuwonus pelamis, was assessed using stomach contents and confirmed using mercury analysis in the tissues of tunas and two other epipelagic fish, the Yellowmouth barracuda Sphyraena viridensis and the Longfin yellowtail Seriola rivoliana. The diet of both species was mainly composed by epipelagic prey, like Atlantic chub mackerel and Sand smelts Atherina sp., which contrasts with the mesopelagic behaviour and diet of Bigeye tunas elsewhere. We also investigated the use of marine resources by the Yellow-legged gull Larus michahellis atlantis in the archipelago of Madeira. This coastal bird showed a great anthropogenic dependence, with high use of terrestrial and human-associated environments. The few times it used the marine environment, it was mostly at night and in association with purse-seine vessels. Its diet reflected this behaviour, being mainly composed by human residues and few fish. The variation in the composition of epipelagic species through a period of seven years in the broad Canary current region was studied using foraging trips and regurgitations of Cory’s shearwaters Calonectris borealis nesting on Selvagens Islands. This study was able to depict a shift in the community, with a steep increase of the population of Longspine snipefish in the surroundings of Selvagens islands in 2017/2018. Finally, we developed a mass-balanced model of the Exclusive Economic Zone of the archipelago of Madeira, using the software Ecopath with Ecosim. This chapter brought together the information collected in the other chapters with data available in the literature. The Madeira system was characterized by high biomass of primary producers, zooplankton and other crustaceans and invertebrates, together with small epipelagic and mesopelagic fish, which were also the main prey of top predators. The food web was characterized by a more linear-like food chain, in opposition to a more web-like food chain, with a large proportion of more specialized organisms, like dolphins, shearwaters and large pelagic fish. Despite the low mean trophic level of the system, the mean trophic level of fisheries was very high, targeting mainly top predators, like tunas and Black scabbardfish, which were also the components with most impact in the ecosystem.
Diet studies are important for the understanding of the ecology of species, but commonly overlooked and their importance underestimated. The diet of an organism reveals its prey composition, but also its habitat and foraging behaviour. It also allows to have an insight on the interactions with other species and its surroundings. The combined information of diet studies of several species allows to build food webs and understand the structure and functioning of ecosystems. The application of these studies in the marine environment is particularly important, as pure observational studies are not enough. Such importance is even greater in oceanic regions where densities of organisms are low, and their study is challenging. This thesis aims to study the diet and trophic relationships of top predators and their prey in the oceanic region of the archipelago of Madeira. The diet and foraging areas of ecologically and economically important species, like tunas, seabirds and small pelagic fish, were studied in the attempt to understand their position and role in the food web of this oceanic environment. The diet of two species of mackerels, the Atlantic chub mackerel Scomber colias and the Blue jack mackerel Trachurus picturatus, was assessed using stomach contents of individuals caught throughout a year in the vicinities of Madeira island. These are planktivorous and piscivorous species, feeding mostly on calanoid and cyclopoid copepods, and on very small Atlantic saury Scomberesox saurus, clupeids, Longspine snipefish Macroramphosus scolopax, and myctophids. The diet of the Bigeye tuna Thunnus obesus and Skipjack tuna Katsuwonus pelamis, was assessed using stomach contents and confirmed using mercury analysis in the tissues of tunas and two other epipelagic fish, the Yellowmouth barracuda Sphyraena viridensis and the Longfin yellowtail Seriola rivoliana. The diet of both species was mainly composed by epipelagic prey, like Atlantic chub mackerel and Sand smelts Atherina sp., which contrasts with the mesopelagic behaviour and diet of Bigeye tunas elsewhere. We also investigated the use of marine resources by the Yellow-legged gull Larus michahellis atlantis in the archipelago of Madeira. This coastal bird showed a great anthropogenic dependence, with high use of terrestrial and human-associated environments. The few times it used the marine environment, it was mostly at night and in association with purse-seine vessels. Its diet reflected this behaviour, being mainly composed by human residues and few fish. The variation in the composition of epipelagic species through a period of seven years in the broad Canary current region was studied using foraging trips and regurgitations of Cory’s shearwaters Calonectris borealis nesting on Selvagens Islands. This study was able to depict a shift in the community, with a steep increase of the population of Longspine snipefish in the surroundings of Selvagens islands in 2017/2018. Finally, we developed a mass-balanced model of the Exclusive Economic Zone of the archipelago of Madeira, using the software Ecopath with Ecosim. This chapter brought together the information collected in the other chapters with data available in the literature. The Madeira system was characterized by high biomass of primary producers, zooplankton and other crustaceans and invertebrates, together with small epipelagic and mesopelagic fish, which were also the main prey of top predators. The food web was characterized by a more linear-like food chain, in opposition to a more web-like food chain, with a large proportion of more specialized organisms, like dolphins, shearwaters and large pelagic fish. Despite the low mean trophic level of the system, the mean trophic level of fisheries was very high, targeting mainly top predators, like tunas and Black scabbardfish, which were also the components with most impact in the ecosystem.
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Palavras-chave
Dieta Áreas de alimentação Relações tróficas Predadores de topo Cadeia alimentar Diet Foraging areas Trophic relationships Top predators Food web