I have also heard the air bladder thing. That being said, I'm guessing you might have been seeing HAKE !!! (Pacific Whitefish marketing ploy)
There is more of them,,, and I don't think the doggies bunch up till breeding time --spring and early summer......... <img border="0" alt="[Goldfish]" title="" src="graemlins/goldfish.gif" />
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Tip Up ---- 'Peri'