Scientists Seek to Reduce Disturbance on Rookeries

To better understand the diet of wild Steller sea lions populations, Consortium scientists visit haulouts and rookeries to collect fecal (scat) samples, which they then analyze for prey remains. However, scat collection often requires displacing the animals that are onshore, which can be disruptive in breeding areas (rookeries) at certain times of year.

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To reduce disturbance on rookeries, scientists proposed using scats from adjoining bachelor male (non-breeding) haulouts as a proxy. If the scats of non-breeding males are similar enough in composition it may eliminate the need to disturb breeding females. This hypothesis was recently tested and published in the journal Aquatic Mammals by Dr. Andrew Trites (University of British Columbia) and Donald Calkins (formerly at the Alaska Department of Fish and Game, and now at the Alaska SeaLife Center).

Sampling Protocol
“We sought to develop a scat-sampling protocol that would minimize disturbance to Steller sea lions during the breeding season yet would yield accurate data about what sea lions eat,” the authors write. They compared scats from sexually mature Steller sea lions at one male haulout with those from three female-dominated rookeries at Forrester Island in southeast Alaska.

If the diet of bachelor bulls was similar enough to that of nearby breeding females, it could be useful as a proxy in future studies. That is, it could help to predict the diet of females without sampling the breeding population and with minimal intrusion on the nearby bachelor bulls. Further, if the scats indicated a similar diet among the females, samples collected from one rookery could help to predict the diets of all breeding females in the vicinity, further reducing disturbance.

tritesShared Diet
“We found that it is not necessary to sample scats from all of the breeding sites that make up the Forrester Island complex because females using these rookeries all have the same diet,” the authors reported. Female diets contained gadids, salmon and small oily forage fish, with fewer rockfish, flatfish, cephalopods, and other fishes.

trites_2Compared to females, males consumed significantly fewer salmon and more pollock, flatfish, and rockfish. These dietary differences suggest that males and females may forage in separate areas, or that their relative physical size affects their ability to capture certain prey.

This study shows that female diets can be determined from samples collected at a single site within a rookery complex. Thus, a rotating schedule of scat collection from one breeding group could determine the diet of all local breeding females while reducing long-term disturbance in the population. Unfortunately, the male diet at Forrester Island differed sufficiently from that of breeding females to prevent its use as a proxy for female diet.

To ensure the long-term conservation of the species, scientists must balance the need to accurately determine what Steller sea lions eat with the cost of obtaining this information. Future comparisons of dietary information from other rookeries and haulouts in Alaska will help to determine how many rookeries and haulouts need to be sampled to accurately determine what Steller sea lions are eating.

June 2, 2008

SEE PUBLICATION:

Diets of mature male and female Steller sea lions differ and cannot be used as proxies for each other.
Trites, A.W., and D.G. Calkins. 2008.
Aquatic Mammals 34:25-34.
abstract
Disturbance of otariid breeding sites (rookeries) to determine diet from fecal remains (scats) could be eliminated if the diets of males using adjoining bachelor haulouts could be used as a proxy for diets of breeding females. We collected scats from sexually mature Steller sea lions (Eumetopias jubatus) at one male resting site (haulout) and three female dominated breeding sites (rookeries) at Forrester Island, Southeast Alaska (June-July, 1994–1999) to test whether the diets of bachelor bulls differed from that of breeding females. Female diets were fairly evenly distributed between gadids, salmon and small oily fishes (forage fish), and contained lesser amounts of rockfish, flatfish, cephalopods and other fishes. Female diet did not differ significantly between the 3 rookeries, but did differ significantly from that of males. Males consumed significantly fewer salmon, and more pollock, flatfish and rockfish compared to females. The males also consumed larger pollock compared to females. These dietary differences may reflect a sex-specific difference in foraging areas or differences in hunting abilities related to the disparity in physical sizes of males and females. The similarity of the female diets between rookeries suggests that female diets can be determined from samples collected at a single site within a rookery complex. Unfortunately, summer diets of breeding females cannot be ascertained from hard parts contained in the scats of mature male Steller sea lions.
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