If you have an Argy, fly over the otter and pick it up with the argy's beak ('C' on pc). Land the argy and let the otter hang out in its beak while you bring fish over. I found I needed to drop the fish and pick it back up to target the otter. After having done this by following around a scampering otter with a dead fish pinwheeling wildly in my hands for 10 minutes the first time, I will never ever attempt it again without my argy.

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