Climate change, reproductive performance and diet composition of marine birds in the southern California Current system, 1969-1997

We studied the effects of low-frequency climate change on the reproductive performance of 11 species of marine bird in the southern California Current system, 1969-1997. Reproductive performance of Brown Pelican (Pelecanus oc cidentalis) and Double-crested Cormorant (Phalacrocrax auritus) in southern California demonstrated an increase in the 1970s and early 1980s, attribut able to recovery from organochlorine contamination (primarily DDE). Brandt' s Cormorant (Phalacrocorax penicillatus) in central California was the only species to demonstrate a secular increase in performance through time, a p attern that remains unexplained. Ashy Storm-petrel (Oceanodroma homochroa) and Pelagic Cormorant (Phalacrocorax pelagicus) demonstrated curvilinear pa tterns of change, with decreasing reproductive performance in the past deca de. All other species including Western Gull (Larus occidentalis), Pigeon G uillemot (Cepphus columba), Xantus's Murrelet (Synthiloboramphus hypoleucus ), Common Murre (Uria aalge), Cassin's Auklet (Ptychoramphus aleuticus) and Rhinoceros Auklet (Cerorhinca monocerata) showed diminishing reproductive performance through time. Patterns of change for the murre and auklets were not significant, presumably because of a lack of reproductive variation fo r these species, which display a conservative breeding effort (i.e. single- egg clutches). Changes in the birds' abilities to provision young and maint ain chick survival during May-July each year appeared most closely related to overall changes in reproductive performance. Dietary change indicated a decline in use of juvenile rockfish (Sebastes spp.) by marine birds in cent ral California. There was also significant interannual variability in consu mption of juvenile rockfish and the euphausiid Thysanoessa spinifera. Patte rns of change in marine bird reproductive performance were generally concor dant between southern and central California after considering the period o f recovery for Brown Pelican and Double-crested Cormorant. The decline in r eproductive performance and changes in diet composition do not appear direc tly related to the polarity reversal of the Pacific Decadal Oscillation in 1976/1977. Instead, reproductive performance and dietary characteristics in dicate substantial change in the late 1980s, suggesting another regime-shif t at that time. (C) 2001 Elsevier Science Ltd. All rights reserved.