Impact of diet-index selection and the digestion of prey hard remains on determining the diet of the Steller sea lion (Eumetopias jubatus)

Abstract

Nine prey species (n = 7431) were fed to four captive female Steller sea lions ( Eumetopias jubatus (Schreber, 1776)) in 11 feeding trials over 75 days to investigate the effectiveness of different methods used to determine diet from prey hard remains. Trials aimed to replicate short (1–2 days) and long feeding bouts, and consisted of single species and mixed daily diets. Overall, 25.2% ± 22.2% (mean ± SD, range 0%–83%) otoliths were recovered, but recovery rates varied by species (ANOVA, P = 0.01) and were linearly related to otolith robustness (R2 = 0.88). Squid beaks were recovered at higher frequencies (mean 96%) than the otoliths of all species. Enumerating both non-otolith skeletal structures and otoliths (together termed bones) increased species recovery rates by twofold, on average (P < 0.001), with increases up to 2.5 times for Pacific herring ( Clupea pallasii Valenciennes in Cuvier and Valenciennes, 1847) and 3–4 times for salmonids. Using bones reduced interspecific differences (P = 0.08), but recovery varied among sea lions. Bones were distributed over more scats per meal (mean 2.9 scats, range 0–5) than otoliths (mean 1.9 scats, range 0–4). In three different 15-day mixed diet trials, biomass reconstruction (BR) indices performed better than frequency of occurrence indices in predicting diet fed. Applying our experimentally derived numerical correction factors (to account for species differences in complete prey digestion) further improved BR estimates, resulting in all 12 unweighted comparisons within 5% (for otoliths) and 12% (for bones) of the actual diet fed.

Spatial and temporal variation in the diet of Steller sea lions in the Kodiak Archipelago, 1999 to 2005

“…Identification of all diagnostic skeletal structures in addition to otoliths, as used in our study, increases the detection rate of many species (Olesiuk et al 1990, Cottrell & Trites 2002, Tollit et al 2007). However, more robust bone structures from species such as pollock may have greater recovery rates compared to fragile structures of smaller species such as capelin and sand lance (Tollit et al 2003(Tollit et al , 2007. Therefore, the importance of small forage species may be underestimated in the present study.…”

“…Therefore, the importance of small forage species may be underestimated in the present study. In addition, prey that leave no or few hard remains, or are preferentially regurgitated, such as cephalopods, will be underestimated (Tollit et al 2007). In our study, only small cephalopod beaks and eye lenses were recovered from scats, but fragments of much larger beaks were sometimes found.…”

“…Prey recovered from scats are also thought to be biased towards prey consumed by seals just prior to hauling out and may not be representative of all prey consumed during a foraging trip. Crude passage rates for prey remains in captive Steller sea lions range from 2 to 3 d, although some prey may be retained for longer and deposited over a greater period of time (Tollit et al , 2007. Thus, the consumption of some prey species may be quantified more accurately than that of others.…”

“…Provided sample sizes are relatively large, FO indices can provide useful dietary information in the absence of biomass estimates (Olesiuk et al 1990, Sinclair & Zeppelin 2002, Tollit et al 2007). However, the limitations and biases associated with scat analysis and the indices used should be taken into consideration when interpreting our results.…”

Methods of estimating marine mammal diets: A review of validation experiments and sources of bias and uncertainty

“…Other measures of diet, such as frequency of occurrence of prey, generally fail to provide reliable estimates of the proportion by mass of prey consumed (Tollit et al . ). Although the experimental evidence comes mainly from pinnipeds, we expect the results will apply to odontocetes, other marine mammal taxa, and other fish eating predators.…”