Roaches (Särki in Finnish) are one of the most common fish, along with perches. They weigh around .2 lbs, and even if 'delicious' when cooked, will only fill a sliver of your hunger bar. While large quantities of roaches (in the vein of 10 - 20 roaches) can fill your hunger bar, it is far more efficient to just keep fishing until you catch a larger fish, such as a pike. Roaches are so inexpensive that enormous amounts of dried or smoked roaches can be traded for only a few staves or clubs, which can hold off starvation in desperate times.
In terms of nutrition, roaches are actually better than some larger fish on a pound-to-pound basis. Roaches are also more filling pound-to-pound, meaning that when at medium to low nutrition levels, eating roaches will fill up the hunger bar without satisfying your nutrition requirements.
Roaches are rarely used for anything other than bait due to their small nutrition value and low weight. They are also useful as sacrifices to appease the spirits.