O arrojamento de animais marinhos mortos são uma importante fonte de informação, especialmente acerca da sua biologia (Vingada et al., 2011). A carcaça de um animal numa praia contém informações sobre a anatomia, história de vida, genética, doenças, contaminantes, parasitas, predadores e ecologia alimentar (Perrin & Geraci, 2009). É desta forma que se obtêm os estômagos para avaliar a dieta de aves, visto que a análise dos conteúdos estomacais é uma das técnicas mais utilizadas para esse fim, através da identificação de partes duras de presas (ossos e otólitos) resistentes à digestão, sendo possível fazer uma análise quantitativa e qualitativa da dieta (Pierce et al., 2004).
O estudo da dieta de aves marinhas é essencial, pois para além de fornecer informação sobre a ecologia trófica, constitui um meio de obter dados interessantes sobre as suas presas. Este aspecto é muito importante quando se trata de espécies com interesse comercial, mas é igualmente relevante para outras presas que, pela sua abundância ou distribuição, constituem elementos-chave das cadeias tróficas marinhas (Montevecchi & Myers, 1995).
Com este estudo foi possível obter uma análise qualitativa e quantitativa da dieta da torda- mergulheira, através de estômagos de aves arrojadas mortas ao longo da costa centro/norte portuguesa, no período de 2012 a 2014. A sua dieta é constituída exclusivamente por peixe, principalmente Clupeidae, provavelmente representados na sua maioria pela sardinha (Sardina pilchardus). Resultados diferentes foram obtidos por outros autores noutros locais, mostrando que existe uma variação geográfica na dieta da torda-mergulheira.
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A causa de morte da maioria das aves amostradas foi a captura acidental. Isto exemplifica mais uma vez a interacção com artes de pesca, sobretudo pesca ilegal que apresenta taxas mais preocupantes de captura acidental, sendo necessárias medidas de mitigação que diminuam as capturas acidentais e promovam a conservação dos predadores marinhos.
Tendo em conta que o presente estudo é apenas o segundo a ser realizado na costa portuguesa sobre a dieta da torda-mergulheira, seria interessante a continuação de esta monitorização no futuro. Para além de acompanhar a variação da dieta desta ave ao longo do tempo, seria também uma forma de detectar possíveis alterações na abundância das presas ingeridas. Talvez o mais adequado seria combinar diferentes métodos de análise de dieta, de modo a que os resultados obtidos não fossem enviesados pelas falhas da utilização de um só método. Poderia ser também interessante a construção de uma colecção de referência com espécies-presa juvenis da nossa costa bem como a elaboração de rectas de regressão para estimar o tamanho e o peso das presas.
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