Monitoring temporal and spatial variability in sandeel (Ammodytes hexapterus) abundance with pigeon guillemot (Cepphus columba) diets

We evaluated pigeon guillemots (Cepphus columba) as monitors of nearshore f ish abundance and community composition during 1995-1999 at Kachemak Bay, A laska. We studied the composition of chick diets at 10 colonies and simulta neously measured fish abundance around colonies with beach seines and botto m trawls. Sandeels (Ammodytes hexapterus) formed the majority of the diet a t one group of colonies. Temporal variability in sandeel abundance explaine d 74% of inter-annual variability in diet composition at these colonies and 93% of seasonal variability. Diets at other colonies were dominated by dem ersal fish. Among these colonies, 81% of the variability in the proportion of sandeels in diets was explained by spatial differences in sandeel abunda nce. Pigeon guillemots exhibited a non-linear functional response to sandee l abundance in the area where these fish were most abundant. Temporal and s patial variability in demersal fish abundance was not consistently reflecte d in diets. Spatial differences in the proportion of different demersal fis hes in the diet may have been driven by differences in guillemot prey prefe rence. Prey specialization by individual pigeon guillemots was common, and may operate at the colony level. Inter-annual variability in sandeel abunda nce may have been tracked more accurately because the magnitude of change ( Il-fold) was greater than that of demersal fish (three-fold). (C) 2000 Inte rnational Council for the Exploration of the Sea.