The printed fish technique
Gyotaku 魚 拓
The origins of the practice of gyotaku are still nebulous, its past is not very clear, and the present is divided between various schools that weakly communicate with each other.
Making a fish print (魚 gyo = fish + 拓 taku = print)
it is originally a tradition linked to fishermen in Japan to preserve the memory of an important catch, like a snapshot of a trophy. The fish was painted with a black soot based ink, resin and glue (sumi) on which the sheet of paper (washi) was then placed on (Takugi-ga); the image was then accompanied by information related to the fish such as species, size and weight, the name of the fisherman, and (sometimes) the date. The two oldest examples are dated 1839 (Tempo Era). From the 19th century onwards many gyotaku were made by the inhabitants of the Yamagata prefecture (Shonai).
The artistic gyotaku was born from this practice and it's carried out by a few masters:
The direct method
the evolution of the traditional method is the addition of color instead of black ink. Through a sequence of dark (kyo), medium (Tyu) and light (Jiaku) shades blended with the aid of only the brush, the livery of the species in question is painted directly on the animal; the resulting imprint is obtained by spreading a sheet over the colored fish and, through rubbing and pressure with the fingers, the anatomical details are obtained. At the end of this procedure the eye is painted freehand. Master Masatzu Matzunaga is the proponent of this technique. He founded the Takuseikai art gyotaku association in 1977 in Osaka, and is the master from whom I learned the direct colored technique. Printing defects, such as white parts, or unclear details, are considered peculiar: "Either it goes, or it breaks it". Touching up the defects would be equivalent to not capturing the snapshot and imprint appearance of the fish. Timing is essential: based on the relative humidity you have a maximum time of 30/40 minutes to print, and the result is always a surprise for the operator too, as you work on the back of the sheet.
The indirect method
Developed in the 1940s by Master Ryuzabuto Takao in Tokyo but bonded to an ancient chinese tradition Taku-hon (拓本), it consists of preparing the cleaned fish and applying the support (paper or cloth) directly on it; subsequently, the imprint of the fish is obtained by dabbing the surface with slightly diluted color using dabbers; also in this case, the steps of the various colors are coded by a sequence. Unlike the direct method, you have total control of the result, since you work directly on the result side, and you have all the time to intervene on the desired effect. The name of this method is Tatsunoko-kai and includes artists such as Ryutaro Ohno, Haryu Ide and Mineo Sakamoto. 
In both methods there is a ban on retouching the impression, with the exception of the eyes. If the image is retouched it is not considered gyotaku.