Detached Olive, Halford

Shell Fly, Berners
masking hatch   English
  A fly fisher's term mentioned in the excellent-book "Fly Fishing Strategy" by Doug Swisher and Carl Richards in 1975.
  Difficult and frustrating situations can happen once in a while in fishing. That is, an angler is unable to catch fish even if he throws an exact imitation and the fly floats down naturally where the aquatic insects are hatching and trout are preying on them repeatedly. In such a case it is likely that other aquatic insects are hatching simultaneously, and trout may be feeding on them selectively. In many cases, the former insect is large and is highly visible, and the latter insect is small and inconspicuous. Swisher and Richards called the hatch of larger insects a masking hatch (hatch to cover), and the hatch of smaller insects as an invisible hatch. In effect they said that we should not be confused by the hatch of a large and showy aquatic insect. In addition the invisible hatch appears to be very similar to Vincent Marinaro's hidden hatch.
  In the aforementioned book, Swisher and Richards called it a compound hatch when two kinds of aquatic insects hatch simultaneously and making fishing difficult, and they further divided it into masking hatch and mimicking hatch. Readers are referred to mimicking hatch and compound hatch for details. By the way, they described another hatch pattern that is difficult to fish. It is a complex hatch in which three or more kinds of insects hatch at a same time.
  In a situation like that, a fly fisherman is obliged to devote himself to fly fishing and struggles to avoid getting lost in the operation of nature.
ReferenceFly fishing strategy, 1975. The tactics of fly fishing, 1980. A modern dry fly code, 1950.
invisible hatch, mimicking hatch, hidden hatch, compound hatch, complex hatch
Selected Headings

A native char of Japan, which inhabits in a limited area in Japan. 
It has white spots in the head as well as the body.