ASSESSING THE USE OF HARD PARTS IN FECES TO IDENTIFY HARBOR SEAL PREY- RESULTS OF CAPTIVE-FEEDING TRIALS

Faeces were collected from four captive harbour seals (Phoca vitulina) that consumed known amounts of herring (Clupea harengus), walleye pol lock (Theragra chalcogramma), Pacific hake (Merluccius productus), sur f smelt (Hypomesus pretiosus), and juvenile chinook salmon (Oncorhynch us tshawytscha). The goal was to determine which structures (hard part s) passed through the digestive tract (e.g., eye lenses, scales, verte brae, otoliths), and which of these could be used to determine the typ e and number of fish consumed. Nearly 5000 fish were consumed, from wh ich over 50000 hard parts were recovered from seal faeces. Scales were the most numerous of the 23 structures recovered (> 20000), followed by vertebrae, eye lenses, and otoliths. Morphological distinctiveness and digestive erosion of the structures varied among fish taxa. Two to five structures accounted for over 90% of the taxon-specific elements recovered, depending upon the species of fish consumed. Otoliths, whi ch are used routinely to characterize pinniped diets, accounted for on ly 17% of the identified taxon-specific hard parts. The variation in t ypes of structures and rates of recovery across taxa underscores the i mportance of using several types of hard parts to identify prey. Ident ifying several different prey structures increases the likelihood of i dentifying a prey type.