A STEADY stream of fish surfaced from the waters of Lake Erie last week as biologists aboard the research vessel Esox hauled in a gill net laden with hundreds of perch, bass, carp and freshwater drum. The bountiful sample, taken off Kelleys Island near the US-Canada border that bisects the lake, comes as a relief to Jeff Tyson, an Ohio fisheries biologist from nearby Sandusky (see Map). Yet Tyson knows a killer is lurking.