Dietary response of the elegant tern (Thalasseus elegans ) to changing ocean conditions and prey populations in Southern California.

摘要

Elegant Tern diets were compared at three southern California nesting sites in 2004, a neutral El Nino Southern Oscillation (ENSO) year, to help determine whether ocean conditions have shifted from a warm to cool regime and whether this abundant tern can indicate oceanographic regime changes. Previous studies have shown northern anchovy is this tern's major prey, but, during warmer years, Pacific sardine can become a more important forage item. If the northeastern Pacific Ocean entered a cool regime in 1999, I expected a higher anchovy to sardine ratio in this tern's diet at all three sites, but more pronounced at the northerly locations (Los Angeles Harbor and Bolsa Chica). I also expected Elegant Terns to have a broader diet at the more southerly site (San Diego) because of subtropical influences. Diets were assessed at the colonies by identifying fish dropped by the birds and regurgitated by the chicks. Anchovy to sardine ratios from the Los Angeles and Bolsa Chica samples were higher, but not significantly, than those from the San Diego samples. No significant difference in diversity was shown among the three sites, but a significant difference was found in species composition, mainly driven by anchovies and sardines. Species composition of the Elegant Tern diet based on data collected from 1993 to 2005 was not significantly different during warm and cool Pacific Decadal Oscillation (PDO) conditions or ENSO conditions. This study shows the potential to use the Elegant Tern diet as an indicator of local prey abundance, especially for northern anchovy.