Balearic Shearwater

Balearic Shearwater, Puffinus mauretanicus

Balearic shearwaters are endemic to the western Mediterranean, and are among the region’s rarest seabirds (1).  Until recently, the Balearic Shearwater was considered a subspecies of the Manx shearwater (Puffinus puffinus) or the yelkouan shearwater (Puffinus yelkouan) (1,2).   They are now recognized as a distinct species but their ecology has not been well studied (2).  The population of Balearic shearwaters was estimated at about 3000 breeding pairs in 1991 (3 in 1) but is now closer to 2000 pairs (4 in 5).  The population is now declining at a rate of 7.4% annually with a mean extinction time of about 40 years (6).  Balearic shearwaters have been classified as critically endangered by the World Conservation Union since 2004 (7).

Like other Procellariiformes (petrels, albatrosses, shearwaters and fulmars), Balearic Shearwaters are long-distance foragers characterized by longevity and high parental investment in a single offspring (8).  Adult survival is unexpectedly low, suggesting that incidental mortality in fishing gear is an important factor in the species’ decline (6).   Interactions between seabirds and longlining gear have not been as well documented in the Mediterranean as in high-latitude regions (9,10).  Balearic shearwaters are capable of diving to pursue baited longline hooks, and may become hooked or entangled and eventually drown.  The extent of longlining mortality in the Mediterranean, and throughout the species’ migration route and wintering grounds along the Atlantic coast of western Europe, is unknown (6).  Bycatch of Balearic shearwaters has been documented in the past, and the species is particularly vulnerable due to its small population size and limited range (7,10).  Consequently, longlining mortality is cited by the World Conservation Union as one of the major justifications for listing the Balearic shearwater as critically endangered (7).

Balearic shearwaters have a complicated relationship with Mediterranean fisheries because they benefit from opportunistic foraging on fishery discards.  They rely heavily on fishery discards from purse seines and demersal trawlers to meet their energetic requirements during the breeding season, when surface productivity is naturally low in the Mediterranean (2).  Although scavenging is probably secondary to foraging as a feeding strategy (5), the availability of discards during the breeding season is one determinant of breeding success (8).  The relative importance of discards in the Balearic shearwater’s diet varies, as the abundance of small pelagic fish fluctuates seasonally and in response to oceanographic conditions (2,8,11).  The availability of fishery discards can be influenced by changes in fishery policies, and a reduction in fishing effort could decrease the amount of discards produced but increase the availability of forage fish.  The net effect is uncertain (2,8)

Improving adult survival rates of Balearic shearwaters should be made a primary conservation objective (8).  General techniques for preventing seabird bycatch in longline fisheries focus on deterring scavenging birds, and decreasing the visibility or availability of baited hooks (12).  These strategies are most effective when developed and implemented on a fishery-specific basis (13).  Marine zoning based on core-buffer marine protected areas (MPAs) is suggested as a strategy for focusing conservation efforts in areas where seabirds aggregate, while protecting them throughout their foraging rage (5).  A comprehensive conservation strategy for Balearic shearwaters will also need to address the species’ reliance on fishery discards and the implications of changes in fishery policies, as well as habitat loss, predation by rats and feral cats, and hybridization or competition with other species (particularly Puffinus yelkouan) (7,14).  Balearic shearwaters are protected by the European Union’s Directive on the Conservation of Wild Birds (1979) and the Habitats Directive (1992).  These two directives require member states to designate Special Protection Areas and Special Areas of Conservation, respectively, which are included in the European Union’s Natura 2000 network of protected areas. (5) 

References and Additional Resources:

1.  Aguilar, J.S., S. Benvenuti, A. Dall’Antonia, M. McMinn-Grivé, and J. Mayol-Serra. 2003. Preliminary results on the foraging ecology of Balearic shearwaters (Puffinus mauretanicus) from bird-borne data loggers. Scientia Marina 67 (Suppl. 2): 129-134

2.  Arcos, J.M. and D. Oro. 2002. Significance of fishery discards for a threatened Mediterranean seabird, the Balearic shearwater Puffinus mauretanicus.  Marine Ecology Progress Series 239: 209-220

3.  Aguilar, J.S. 1991. The Atlas of the Breeding Seabirds of the Balearic Islands. An. Ornit. Bal. 6: 17-28

4.  Ruiz, A. and R. Martí. 2004. La Pardela Balear. SEO/BirdLife-Consellaria de Medi Ambient del Govern de les Illes Balears. Madrid, Spain.

5.  Louzao, M., K.D. Hyrenbach, J.M. Arcos, P. Abelló, L.G. de Sola, and D. Oro. 2006. Oceanographic habitat of an endangered Mediterranean Procellariiform: Implications for marine protected areas. Ecological Applications 16 (5): 1683-1695

6.  Oro, D., J.S. Aguilar, J.M. Igual, and M. Louzao. 2004. Modelling demography and extinction risk in the endangered Balearic shearwater. Biological Conservation 116: 93-102

7.  BirdLife International 2005. Puffinus mauretanicus. In: IUCN 2007. 2007 IUCN Red List of Threatened Species. <www.iucnredlist.org>. Downloaded on 1 April 2008.

8.  Louzao, M., J.M. Igual, M. McMinn, J. Salvador, R. Triay, and D. Oro. 2006. Small pelagic fish, trawling discards and breeding performance of the critically endangered Balearic shearwater: Improving conservation diagnosis. Marine Ecology Progress Series 318: 247-254

9.  Cooper, J., N. Baccetti, E.J. Belda, J.J. Borg, D. Oro, C. Papaconstantinou, and A. Sánchez. 2003. Seabird mortality from longline fishing in the Mediterranean Sea and Macaronesian waters:  A review and a way forward. Scientia Marina 67 (Suppl. 2): 57-64

10. Belda, E.J. and A. Sánchez. 2001. Seabird mortality on longline fisheries in the western Mediterranean:  Factors affecting bycatch and proposed mitigating measures. Biological Conservation 98: 357-363

11. Mourino, J., F. Arcos, R. Salvadores, A. Sandoval, and C. Vidal. 2003. Status of the Balearic shearwater (Puffinus mauretanicus) on the Galician coast (NW Iberian Peninsula). Scientia Marina 67 (Suppl. 2): 135-142

12. Bull, L.S. 2007. Reducing seabird bycatch in longline, trawl and gillnet fisheries. Fish and Fisheries 8: 31-56

13. Gilman, E., Brothers, N., and D.R. Kobayashi. 2005. Principles and approaches to abate seabird by-catch in longline fisheries. Fish and Fisheries 6: 35-49

14. Genovart, M., Juste, J. and D. Oro. 2005. Two sibling species sympatrically breeding :  A new conservation concern for the critically endangered Balearic shearwater. Conservation Genetics 6: 601-606