Seasonality in marine ecosystems: Peruvian seabirds, anchovy and oceanographic conditions
Résumé
In fl uctuating environments, matching breeding timing to periods of high
resource availability is crucial for the fi tness of many vertebrate species, and may have
major consequences on population health. Yet, our understanding of the proximate environmental
cues driving seasonal breeding is limited. This is particularly the case in marine
ecosystems, where key environmental factors and prey abundance and availability are
seldom quantifi ed. The Northern Humboldt Current System ( NHCS ) is a highly productive,
low- latitude ecosystem of moderate seasonality. In this ecosystem, three tropical seabird
species (the Guanay Cormorant Phalacrocorax bougainvillii , the Peruvian Booby Sula variegata ,
and the Peruvian Pelican Pelecanus thagus ) live in sympatry and prey almost exclusively
on anchovy, Engraulis ringens . From January 2003 to December 2012, we monitored 31
breeding sites along the Peruvian coast to investigate the breeding cycle of these species.
We tested for relationships between breeding timing, oceanographic conditions, and prey
availability using occupancy models. We found that all three seabird species exhibited
seasonal breeding patterns, with marked interspecifi c differences. Whereas breeding mainly
started during the austral winter/early spring and ended in summer/early fall, this pattern
was stronger in boobies and pelicans than in cormorants. Breeding onset mainly occurred
when upwelling was intense but ecosystem productivity was below its annual maxima, and
when anchovy were less available and in poor physiological condition. Conversely, the
abundance and availability of anchovy improved during chick rearing and peaked around
the time of fl edging. These results suggest that breeding timing is adjusted so that fl edging
may occur under optimal environmental conditions, rather than being constrained by nutritional
requirements during egg laying. Adjusting breeding time so that fl edglings meet
optimal conditions at independence is unique compared with other upwelling ecosystems
and could be explained by the relatively high abundances of anchovy occurring throughout
the year in the NHCS .