After a volcanic eruption and the cooling process, the crater fills with water. And at some undetermined time, someone discovers this new lake. And finds it has fish in it. How did they get there?
I looked for recorded examples and found out that after the eruption of Mt. St. Helen's volcano, the lake below was filled by the avalanche of debris. There is no way any fish could have survived the eruption or the period afterwards when the lake was depleted of oxygen by bacteria. But fish were later found here. It's likely that fish could have survived in the ice covered waters near to the lake and were washed down into the lake to re-colonize it with fish. Perhaps this is how fish end up in volcanic lakes - they are washed in from another area of water. If the lake is fed by a stream then this could be an explanation, but where rain appears to be the only source of water, it is harder to explain. It is also a possibility that the lakes are illegally stocked with fish by keen fishers, although this is unlikely. Who wants to climb a volcano just for a spot of fishing?
A more interesting explanation could be that twisters or whirlwinds pick up fish from one lake and carry them to another. This may seem a little far fetched but there are plenty of examples of where it has literally rained fish. Obviously it would have to be a pretty strong whirlwind to carry a fully grown salmon-sized fish (it would really hurt if one of these dropped out of the sky onto you!) but smaller fish are more easily picked up by the wind. During thunderstorms small whirlwinds and mini-tornados are formed easily and when they move across water they pick up small debris such as fish, fish roe and frogs. The little creatures can be carried for miles before the clouds open and rain brings them back to Earth.
Another method of transportation could be via birds. After a picking up a tasty snack in a lake, Mr Bird proudly carries it home to show his kids but unfortunately is caught in a freak gust of wind and the slippery fishy manages to wriggle out of his beak and falls to Earth to populate a previously uninhabited lake. Or maybe the birds spread the fish roe between lakes. Lets see what Charles Darwin has to say on the matter in his 'Origin of the Species':
"When ducks suddenly emerge from a pond covered with duck-weed, I have twice seen these little plants adhering to their backs; and it has happened to me, in removing a little duck-weed from one aquarium to another, that I have unintentionally stocked the one with fresh-water shells from the other. But another agency is perhaps more effectual: I suspended the feet of a duck in an aquarium, where many ova of fresh-water shells were hatching; and I found that numbers of the extremely minute and just-hatched shells crawled on the feet, and clung to them so firmly that when taken out of the water they could not be jarred off, though at a somewhat more advanced age they would voluntarily drop off. These just-hatched mollusks, though aquatic in their nature, survived on the duck's feet, in damp air, from twelve to twenty hours; and in this length of time a duck or heron might fly at least six or seven hundred miles, and if blown across the sea to an oceanic island, or to any other distant point, would be sure to alight on a pool or rivulet."
So fish's eggs could potentially get stuck to birds who have recently visited one lake and be deposited in the next. Well traveled little fish!
