"Now, creature of the surface shall thy praise be sung, O mighty silure, whom, with back glistening as though with olive oil of Attica, I look on as a dolphin of the river--so mightily thou glidest through the waters and canst scarce extend thy trailing body to its full lenght, hampered by shallows or by river weeds."*
This account by fourth-century poet and teacher at the Imperial Court, Ausonius of Bordeaux, is from The Moselle, a long hymn to the Mosel River. One of the many remarkable things about this poem is a list of all of the fish of the Mosel, with their attributes and characters poetically described. He continues: "But when thou urgest thy peaceful course in the stream, at thee the green banks marvel, at thee the limped waters [marvel]. Intrigued by this "gentle whale of our Moselle" that "brings glory to the mighty stream," I figured surely this was a poetical exaggeration, part of Ausonius' attempt to portray the Mosel as bigger an better than not only other rivers, but also the sea.But as I started to explore some fish data, it became increasingly clear that he silure was most likely the silurus glandis, otherwise known as the sheat-fish or the wels catfish. 
These fish that like both fresh and brackish water, can grow to over three meters and live up to 100 years. They are smooth, big, ugly, and indeed a bit "whale-like" in their bulk and appearance. (Do a google image search--I dare you!) But recently, Ausonius' gentle giant has become a monster! It is "built to kill" according to Animal Planet, is a potential "man-eater" and even (gasp!) attacks defenseless dachshunds. Headlines scream "Hunt for a man-eater"; "Der Monster-Wels aus den Schlachtensee" and that "Giant catfish in India turn to preying on humans." So how does a perception of a fish change so much over time?
In part, this shift is due to some high profile incidents from the last decade. In 2008, German swimmers claimed to have been attacked by something big while swimming. One young girl was reportedly drug underwater, and several adults had their legs bitten badly. An article from the Spiegel magazine includes this information: "We don't go in the water any more," Berlin student Stephanie Kahl told Bild. "We just stay near the shore, where the catfish can't go." Jeremy Wade, in his book River Monsters (he's also host of an Animal Planet show with the same name) wrote, “Although there is no physical evidence to support any of these stories, the attacks in Germany seemed to lend them credence and revive speculation that there might be a freshwater man-eater in the heart of Europe.” It would in fact be easy to simply blame our modern fascination with "killer" nature--see for example this new weather channel series. But though this is clearly a contributing factor, the modern focus on the wels as "man-eater" is, surprisingly, nothing new!
An 1853 essay from Fraser's Magazine on the wels and some other fishes pointed out that there were rumors of small boys being found inside their stomachs, “and from the paunch of another, who had fattened for sixteen years in a hole under a gentleman’s kitchen, and was eight cubits long,a man’s hand, with three gold rings on the fingers, was pulled out." However, the anonymous author went on to note, the neighbors were not overly quick to blame the fish, since "the circumstance of finding any part of a man stowed away in such a pantry was sufficient to create a strong suspicion of violence and unfair play.” So, a littoral example of feeding ones enemies to the fishes?
Clearly, by the 1700s (when it was scientifically named) and 1800s, there was already an association between the wels and not only monstrous size but also monstrous (and, I should note, barrier crossing) behaviors--in this case eating (or, as is more likely, scavenging) of humans, but many modern you tube clips and articles emphasize the ability of the wels to "beach" itself to catch small prey on land--breaking the barrier of fish "safe" in the water. Granted, the medieval sample of references to the wells is much smaller, but if Ausonius is at all representative, these ideas were not foremost in the experience of the wels before the 1500s. So--what changed?
Though I must here admit to being only at the start of trying to unpack this, I suspect that we have here a change in human perceptions of an animal in part driven by changes in climate and river use that first caused the wels population to shrink dramatically in Western European rivers and then, more recently, to return. Though the wels is not treated as a full "invasive" species, it is considered an "introduced" species now in many parts of Europe where it was native before the Little Ice Age made river temperatures too low for successful wels populations. As an interesting part of this temperature dyamic, part of the reason the wels might now be returning is the increasing temperatures of rivers like the Rhine due in large part to industrial activity and waste (see Marc Cioc's excellent book on the Rhine for the many effects of these developments). Introduction has been largely connected to sport fishing (for example to the Ebro river in Spain and the Scottish Lake District (Nessie, anyone?) Thus, the wels it no longer a familiar part of the river, but a foreigner whose behaviors (protecting the eggs in the grasses during mating season, or scavenging dead bodies) are unknown and decontextualized, becoming "attacks."
This is just a small moment from my research, but is at least leaving me hopeful that I will not only find interesting things about rivers in medieval sources, but will also find meaningful connections between the tangible ways that people related (or not) with their ecosystems and the ways that they inscribed nature into their cultural imaginations.
(*note translation from the Loeb edition of Sidonius Apollinaris' poems--I haven't been working on my own yet)