The diet and foraging ecology of harbour seals (Phoca vitulina) in the Moray Firth, Scotland

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Sizes of walleye pollock (<em>Theragra chalcogramma</em>) consumed by the eastern stock of Steller sea lions (<em>Eumetopias jubatus</em>) in Southeast Alaska from 1994-1999

Lengths of walleye pollock (Theragra chalcogramma) consumed by Steller sea lions (Eumetopias jubatus) were estimated by using allometric regressions applied to seven diagnostic cranial structures recovered from 531 seats collected in Southeast Alaska between 1994 and 1999. Only elements in good and fair condition were selected. Selected structural measurements were corrected for loss of size due to erosion by using experimentally derived condition-specific digestion correction factors. Correcting for digestion increased the estimated length of fish consumed by 23%, and the average mass of fish consumed by 88%. Mean corrected fork length (FL) of pollock consumed was 42.4 +/- 11.6 cm (range = 10.0 - 78.1 cm, n = 909). Adult pollock (FL &gt; 45.0 cm) occurred more frequently in seats collected from rookeries along the open ocean coastline of Southeast Alaska during June and July (74% adults, mean FL = 48.4 cm) than they did in seats from haulouts located in inside waters between October and May (51% adults, mean FL = 38.4 cm). Overall, the contribution of juvenile pollock (5:20 cm) to the sea lion diet was insignificant; whereas adults contributed 44% to the diet by number and 74% by mass. On average, larger pollock were eaten in summer at rookeries throughout Southeast Alaska than at rookeries in the Gulf of Alaska and the Bering Sea. Overall it appears that Steller sea lions are capable of consuming a wide size range of pollock, and the bulk of fish fall between 20 and 60 cm. The use of cranial hard parts other than otoliths and the application of digestion correction factors are fundamental to correctly estimating the sizes of prey consumed by sea lions and determining the extent that these sizes overlap with the sizes of pollock caught by commercial fisheries.</p

Sizes of walleye pollock (<em>Theragra chalcogramma</em>) consumed by the eastern stock of Steller sea lions (<em>Eumetopias jubatus</em>) in Southeast Alaska from 1994-1999

Lengths of walleye pollock (Theragra chalcogramma) consumed by Steller sea lions (Eumetopias jubatus) were estimated by using allometric regressions applied to seven diagnostic cranial structures recovered from 531 seats collected in Southeast Alaska between 1994 and 1999. Only elements in good and fair condition were selected. Selected structural measurements were corrected for loss of size due to erosion by using experimentally derived condition-specific digestion correction factors. Correcting for digestion increased the estimated length of fish consumed by 23%, and the average mass of fish consumed by 88%. Mean corrected fork length (FL) of pollock consumed was 42.4 +/- 11.6 cm (range = 10.0 - 78.1 cm, n = 909). Adult pollock (FL &gt; 45.0 cm) occurred more frequently in seats collected from rookeries along the open ocean coastline of Southeast Alaska during June and July (74% adults, mean FL = 48.4 cm) than they did in seats from haulouts located in inside waters between October and May (51% adults, mean FL = 38.4 cm). Overall, the contribution of juvenile pollock (5:20 cm) to the sea lion diet was insignificant; whereas adults contributed 44% to the diet by number and 74% by mass. On average, larger pollock were eaten in summer at rookeries throughout Southeast Alaska than at rookeries in the Gulf of Alaska and the Bering Sea. Overall it appears that Steller sea lions are capable of consuming a wide size range of pollock, and the bulk of fish fall between 20 and 60 cm. The use of cranial hard parts other than otoliths and the application of digestion correction factors are fundamental to correctly estimating the sizes of prey consumed by sea lions and determining the extent that these sizes overlap with the sizes of pollock caught by commercial fisheries.</p

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

Nine prey species (n = 7431) were fed to four captive female Steller sea lions (Eumetopias jubatus (Schreber, 1776)) in I I 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 (R-2 = 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 &lt; 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.</p

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

Nine prey species (n = 7431) were fed to four captive female Steller sea lions (Eumetopias jubatus (Schreber, 1776)) in I I 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 (R-2 = 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 &lt; 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.</p

A method to improve size estimates of walleye pollock (<em>Theragra chalcogramma</em>) and Atka mackerel (<em>Pleurogrammus monopterygius</em>) consumed by pinnipeds: digestion correction factors applied to bones and otoliths recovered in scats.

The lengths of otoliths and other skeletal structures recovered from the seats of pinnipeds, such as Steller sea lions (Eumetopias jubatus), correlate with body size and can be used to estimate the length of prey consumed. Unfortunately, otoliths are often found in too few scats or are too digested to usefully estimate prey size. Alternative diagnostic bones are frequently recovered, but few bone-size to prey-size correlations exist and bones are also reduced in size by various degrees owing to digestion. To prevent underestimates in prey sizes consumed techniques are required to account for the degree of digestion of alternative bones prior to estimating prey size. We developed a method (using defined criteria and photo-reference material) to assign the degree of digestion for key cranial structures of two prey species: walleye pollock (Theragra chalcogramma) and Atka mackerel (Pleurogrammus monopterygius). The method grades each structure into one of three condition categories; good, fair or poor. We also conducted feeding trials with captive Steller sea lions, feeding both fish species to determine the extent of erosion of each structure and to derive condition-specific digestion correction factors to reconstruct the original sizes of the structures consumed. In general, larger structures were relatively more digested than smaller ones. Mean size reduction varied between different types of structures (3.3-26.3%), but was not influenced by the size of the prey consumed. Results from the observations and experiments were combined to be able to reconstruct the size of prey consumed by sea lions and other pinnipeds. The proposed method has four steps: 1) measure the recovered structures and grade the extent of digestion by using defined criteria and photo-reference collection; 2) exclude structures graded in poor condition; 3) multiply measurements of structures in good and fair condition by their appropriate digestion correction factors to derive their original size; and 4) calculate the size of prey from allometric regressions relating corrected structure measurements to body lengths. This technique can be readily applied to piscivore dietary studies that use hard remains of fish.</p

A method to improve size estimates of walleye pollock (<em>Theragra chalcogramma</em>) and Atka mackerel (<em>Pleurogrammus monopterygius</em>) consumed by pinnipeds: digestion correction factors applied to bones and otoliths recovered in scats.

The lengths of otoliths and other skeletal structures recovered from the seats of pinnipeds, such as Steller sea lions (Eumetopias jubatus), correlate with body size and can be used to estimate the length of prey consumed. Unfortunately, otoliths are often found in too few scats or are too digested to usefully estimate prey size. Alternative diagnostic bones are frequently recovered, but few bone-size to prey-size correlations exist and bones are also reduced in size by various degrees owing to digestion. To prevent underestimates in prey sizes consumed techniques are required to account for the degree of digestion of alternative bones prior to estimating prey size. We developed a method (using defined criteria and photo-reference material) to assign the degree of digestion for key cranial structures of two prey species: walleye pollock (Theragra chalcogramma) and Atka mackerel (Pleurogrammus monopterygius). The method grades each structure into one of three condition categories; good, fair or poor. We also conducted feeding trials with captive Steller sea lions, feeding both fish species to determine the extent of erosion of each structure and to derive condition-specific digestion correction factors to reconstruct the original sizes of the structures consumed. In general, larger structures were relatively more digested than smaller ones. Mean size reduction varied between different types of structures (3.3-26.3%), but was not influenced by the size of the prey consumed. Results from the observations and experiments were combined to be able to reconstruct the size of prey consumed by sea lions and other pinnipeds. The proposed method has four steps: 1) measure the recovered structures and grade the extent of digestion by using defined criteria and photo-reference collection; 2) exclude structures graded in poor condition; 3) multiply measurements of structures in good and fair condition by their appropriate digestion correction factors to derive their original size; and 4) calculate the size of prey from allometric regressions relating corrected structure measurements to body lengths. This technique can be readily applied to piscivore dietary studies that use hard remains of fish.</p

Estimating diet composition in sea lions: which technique to choose?

Reliable estimates of diets are vital to monitor impacts of sea lion populations on their ecosystems and their interactions with fisheries, and to understand the role of food to animal nutrition and health. Approaches include using (1) prey remnants in stomach contents, spews and scats; (2) prey DNA in scats; (3) fatty acid signatures in blubber; and (4) stable isotope ratios in predator's tissue. Each methodology has particular advantages and limitations, many of which can be assessed and improved through controlled captive feeding trials. Analysis of prey remnants from captive sea lion scats have shown significant variability in digestion between and within prey species, which, coupled with preferential regurgitation and enumeration biases, can confound accurate diet quantification, but does not prevent spatial or temporal comparisons. Correction for partial digestion and use of additional structures besides otoliths can provide reliable prey size estimates. Prey DNA can be consistently isolated from soft remains in scats from captive sea lions, and with further development this approach may allow quantification of diet. Genetic methods can be expensive and representative of only one to two days foraging (like prey remnant analysis), but may be less affected by differential digestion and can identify prey in scats that could not be identified through structural remnants. Validation of fatty acid signature analysis to quantify diet at longer temporal scales in sea lions is ongoing. This new technique promises to be particularly useful to assess biases in traditional methods, identify the onset of weaning, and highlight the prey that most contribute to lipid reserves. Stable isotope analysis of predator tissues gives only trophic level data, but can provide data on diet changes on many temporal scales. Remote video monitoring of foraging events and lavage/enema techniques can provide valuable diet information, but, like many newer techniques, animal capture is required. Ideally a suite of techniques should be used to study diet. While methods and correction factors developed for Steller sea lions can likely be applied to the other five sea lion species, they should be verified experimentally.</p

Estimating diet composition in sea lions: which technique to choose?

Reliable estimates of diets are vital to monitor impacts of sea lion popu-lations on their ecosystems and their interactions with fisheries, and to understand the role of food to animal nutrition and health. Approaches include using (1) prey remnants in stomach contents, spews and scats; (2) prey DNA in scats; (3) fatty acid signatures in blubber; and (4) stable isotope ratios in predator’s tissue. Each methodology has particular ad-vantages and limitations, many of which can be assessed and improved through controlled captive feeding trials. Analysis of prey remnants from captive sea lion scats have shown significant variability in diges-tion between and within prey species, which, coupled with preferen-tial regurgitation and enumeration biases, can confound accurate diet quantification, but does not prevent spatial or temporal comparisons. Correction for partial digestion and use of additional structures besides otoliths can provide reliable prey size estimates. Prey DNA can be con-sistently isolated from soft remains in scats from captive sea lions, and with further development this approach may allow quantification of diet. Genetic methods can be expensive and representative of only one to tw