A shallow-diving seabird predator as an indicator of prey availability in southern California waters: A longitudinal study

• Prey deliveries tracked changes in three species based on independent survey data.
• Prey deliveries signaled an increase in pipefish and a decrease in anchovy.
• The Elegant Tern diet did not change in diversity, only composition, over time.
• The Elegant Tern is an effective sampler of prey in ocean surface waters.
• The Elegant Tern serves as a sentinel of changes in prey composition.

We tested the hypothesis that the Elegant Tern (Thalasseus elegans), a plunge-diving predator, is an indicator of changes in the prey community in southern California coastal waters. Shannon diversity (H′) of the tern's diet determined from dropped fish collected variously at the three nesting sites for 18 years over a 21-year interval (1993–2013) showed no significant change in diet diversity. Based on a species–accumulation curve, total diet species represented about 70% of an extrapolated asymptotic richness. Abundance patterns of five prey species making up > 75% of prey numbers for all years were compared with abundance patterns of the same species in independent surveys obtained from zooplankton tows, bottom trawls and power-plant entrapments. Three of the five species – northern anchovy, kelp pipefish and California lizardfish – showed significant, positive correlations between diet and survey abundances. Even though the tern's diet has been dominated by anchovy and pipefish, its diet is still broad, with prey taxa representing > 75% of the 42 species groups making up the California shelf fish fauna. Altogether, our results support the hypothesis that the Elegant Tern, with its flexible diet, is a qualitative indicator, a sentinel, of changes in the prey communities in southern California coastal waters.