“The organization and development of fisheries was a matter of politics.  It meant competitive exploitation of water resources and manpower, and here the princes and landowners stepped in; already in the twelfth century the piscatura was a form of lordship along the coast, on rivers and lakes, jealously guarded against encroachments by poachers, and protected from overfishing by bans on certain kinds of net.  The fish itself was an acceptable token of power, and could be paid in tribute or tithe; the islanders of Oland, off south-east Sweden, made their sole acknowledgment of loyalty to the king of Uppsala by sending him an annual present of herring, and in the 1170s Bishop Absalon of Roskilde let the Slavs of Rugen present him with a single fish in recognition of his sea-patrols, which enabled them to bring in their catch unmolested.  When the warriors of the king of Poland reached the Pomeranian coast in 1107 they sang of their conquest in this terms:

Salted fish and stinking, once theought us from afar,
Now the boys have caught ’em fresh, and all alive they are!”

Eric Christiansen, The Northern Crusades, pg. 13-14

For most of human history the relationship between other animals and humans has been a highly charged one; much of mythology, folklore, etc. is based on it, and it isn’t surprising that everything from fish to unicorns to lions to dragons to etc. figure in heraldry, etc.  For example the pelican (which in the Middle Ages was said to pluck the flesh from its own breast)  was analogized to Jesus on the cross throughout much of Christian history; and Shakespeare uses the pelican as a means to describe the relationship between King Lear and his errant daughters:

Death, traitor! nothing could have subdued nature
To such a lowness but his unkind daughters.
Is it the fashion, that discarded fathers
Should have thus little mercy on their flesh?
Judicious punishment! ’twas this flesh begot
Those pelican daughters.