Slow warm up this year....They haven't had a reason to leave the lake as the temps aren't much warmer in the tribs. Cats have no problem spawning in the lake if trib temps don't draw them in. Concentrate on the first deep hole or two close to the lake in the afternoon/evening this weekend and you may find a good pod of them
Speaking for the smaller tribs, Otter, Catfish and other tribs that size, there aren't many true "resident" cats of any size. There are a percentage of fish that will head back to the lake after spawning and some will stick around, particularly in warmer years. If temps drop too much many cats will head back to the warmer lake waters after the spawn, possibly returning if they are again drawn in by increased flows and significantly warmer creek water. On the other hand, larger rivers like the Thames and Grand do have significant resident populations...
Josh