Winterin distribution of Balearic shearwater (Puffinus yelkouan mauretanicus, Lowe 1921) off the north-east coast of Spain

Abstract

The distribution and numbers of the Balearic Shearwater during the Novembgr-February period are discussed. Until the 1980’s this subspecies seemed to be concentrated along the coast south of Valencia. Since 1991-1992 this pattern has changed, and up to 10465 birds have been recorded northwards, along the Catalan coast. The subspecies is distributed along the Valencian and Catalan coasts in 9 main areas and numbers vary, reaching a total of ca. 8000-11000 birds. The accuracy of these figures and subspecies identification is discussed. An increase in the wintering population of Balearic shearwaters in Catalan waters has been noted. No parallel increase in breeding populations has been reported. Therefore, a northwards displacement of its main wintering area is a possible explanation. A trophic hypothesis is suggested to explain this winter distribution, given that some important changes in clupeid stocks of the extreme western Mediterranean have been noted. Observed numbers match the known breeding population and post-breeding counts, and thus almost the whole population of the subspecies may occur during the winter in the study area.

Se discute la distribucion y poblaciOn de la Pardela Mediterränea (subespecie balear) en el periodo noviembre-febrero. Hasta los años ochenta, la subespecie se concentraba en las costas valencianas hacia el sur. Desde 1991-1992, Ia situaciOn cambia y hasta 10.465 ayes son censadas en las costas de Cataluna. Actuahnente se distribuye a lo largo de las costas catalano-valencia­ nas en 9 areas principales cuya importancia numérica varla, totalizando unas 8.000-11.000 ayes. Se discute Ia precision de las cifras obtenidas y de la identificaciOn de la subespecie. La poblacidn invernal de pardelas mediterraneas en las costas catalanas ha aumentado. Dado que no se ha citado un aumento de su poblacidn nidificante, se sugiere un desplazamiento hacia el norte del area de invernada principal. Se postula una hipOtesis trOfica para explicar esta distribuciOn invernal, ocasionada por los cambios en los stocks de clupeidos en el extremo occidental del Mediterraneo. La poblaciOn observada concuerda con Ia nidificante conocida y con conteos post-nupciales publicados, por lo que casi toda la poblacion de la subespecie podrIa estar concentrada en la zona durante el invierno