Size of Steller Sea Lion Prey

Researchers are uncertain about the size of prey that Steller sea lions consume, and to what extent fisheries might be targeting the same size. Size of fish can be derived from the length of fish bones because the two are proportional to each other. Measurement of fish otoliths (ear bones) recovered from stomach or scat (fecal) samples have often been used to calculate the size of fish originally eaten. However, the scats of Steller sea lions contain relatively few otoliths, and most of them are severely eroded by the process of digestion. Alternative skeletal structures (such as jaw bones and vertebrae) can be used, but like otoliths, these structures are also found in a variety of states of digestion, making it difficult to accurately estimate prey size.

Captive feeding experiments undertaken by Dr. Dom Tollit and Susan Heaslip in collaboration with researchers at the National Marine Fisheries Service have led to keys that define the grade or degree of digestion of seven key skeletal structures from both walleye pollock and Atka mackerel. They have derived size correction factors (with confidence intervals) for each grade of digestion based on loss of various morphological features (see bone figure).

Examples of three digestion grade categories for two structures of pollock, interhyal (left) and quadrate (right). Grades are presented left to right in increasing level of digestion.

They applied the grading and size correction technique to pollock bones recovered from 531 scats collected in Southeast Alaska between 1994-1999. Measurements of all 247 otoliths recovered (and taking no account of the level of digestion) suggested that sea lions ate juvenile and adolescent fish (mean length 20.2 cm). However, the majority of otoliths (>86%) were severely digested.

Using six alternative structures and applying size correction factors changes this conclusion. Resulting estimates suggest that the sea lion diet was in fact dominated by adult pollock (i.e., the fish were about twice the size of the original estimate: mean length 42.7 cm, n = 875). Applying correction factors to original bone sizes led to an average increase in estimated fish length of ~23% (see histograms). These preliminary results highlight the value of using bones other than otoliths, and the importance of developing appropriate correction factors to accurately estimate the size of prey eaten.

Frequency length (cm) distribution of pollock eaten by Steller sea lions in Southeast Alaska, estimated with and without size correction factors (all bones graded "severely digested" were excluded; filled bars denote otoliths, n = 34, and clear bars denote six other selected structures, n = 875).

6 January 2003