Were namazu real fish?

Most Japanese sources I have read stress the metaphoric nature of namazu and point out that namazu in popular art prints and other depictions might correspond to a variety of large bottom-dwelling fish, or to no specific real-life fish at all. Cornelis Ouwehand was somewhat more certain about the identity of namazu, and, based on standard Japanese guides to fish, describes them as follows:

The namazu, as shown on the prints, is undoubtedly the Parasilurus asotus (Linné) [Silurus asotus, Silurus japonicus (Temminick & Schlegel)], family Siluridae, order Nematognathi. Kawabara and Okada give the following particularities for the Parasilurus asotus: mud fish, catfish. Dark greenish brown, sometimes with irregular dark mottlings, belly of lighter colour. Big, depressed head with large mouth. Sharp teeth form bands on both jaws and on the vomer. Lower jaw protruding. On both lower and upper jaw one pair of barbels. Smooth, scaleless, elongate and compressed body. Long ventral fin merging without interruption into tail fin. The poison glands on the pectoral spines produce intense pain when one is pricked. Inland waters Japan, Korea, Manchuria, China. 40-150 cm (and more, in China).

Although the Siluridae were originally fresh-water fish, several have developed into salt-water forms. Remarkably enough, however, it is preponderantly genera and species of other families which are popularly indicated in Japan as "sea-namazu" (umi namazu, oki namazu). These are, for example, Brotulidae and Gadidae, which are all ground-fish and show in their external appearance resemblances to the Siluridae , although they are unrelated to them. [. . . ]

I have deliberately dwelt on these names because it is clear than when the people speak of "namazu" there are, apart from the namazu depicted on the prints, various actual possibilities involved, and along the seacoast and in inland waters particular existing fish may be meant. (Namazu-e and Their Themes: An Interpretative Approach to Some Aspects of Japanese Folk Religion [Leiden, Netherlands: E. J. Brill, 1964], pp. 4-5.)

Notice that although Ouwehand seems certain of the identity of some namazu, he also acknowledges the wide variety of fish to which the word "namazu" might refer.