I have never liked the notion that fish eggs get dispersed via birds. The idea is that water birds somehow acquire fish eggs when they wade around in fish spawning areas and then transport the eggs to further water bodies. Sticky eggs allegedly adhere to bird feet and/or become trapped under feathers. It’s not hard to come up with other mechanisms, either, such as eggs being released from a dead fish when it is regurgitated by an egret feeding its young.
But none of those ideas wash with me. Sure, the eggs of some fish species are sticky, but lots of other species’ eggs are not and yet isolated water bodies can still have fish, even live-bearers. Indeed, the mosquitofish (Gambusia affinis), which is found in more aquatic habitats and more isolated surface water bodies than any other fish in the Coastal Plain, is a live bearer.
And the feathers of birds in the water are so tightly appressed that it is hard to see how eggs could get under them and hitchhike around. It is certainly reasonable to presume that some fish eggs get transported by birds, but it must be rare compared to the innumerable isolated water bodies out there harboring multiple fish species.
Then there’s the inconvenient fact that many small mountain streams and their little manmade reservoirs do not contain fish. This is because fish cannot pass tall waterfalls (and is the reason that salamanders are abundant and species-rich in the Appalachians). Furthermore, many reservoirs constructed here have existed for decades and are frequently visited by water birds, yet they do not have fish.
Thus, I welcomed the publication of research focusing on a review of the scientific literature that concluded there is no evidence for fish eggs migrating via birds (tinyurl.com/ya3m4whv). Fortunately, a lit review like this often sets the stage for experiments to prove or disprove a theory. I can hardly wait!
The notion that fish themselves hitchhike rides on birds is even less likely than their eggs doing so. Water birds are largely fish-eaters and have excellent eyesight.
And then there’s the notion that fish can disperse via underground conduits. Sure, that can work with manmade culverts and to a lesser extent where streams running through vadose caves connect surface water bodies, but fish are much less likely to get around through phreatic cave passages. Nonetheless, the vast majority of isolated water bodies are underlain by dirt, which fish are totally unable to migrate through. Overall, the underground dispersal of fish must be trivial.
I believe that fish disperse almost exclusively by swimming. Many surface water bodies that today are isolated were connected in the past, which would explain how fish reached them. Less obviously, a connected wetland can fool the casual observer into wrongly thinking that it is isolated today; however, any competent civil engineer, geologist, or field biologist can spot tiny channels called “drains” that flow only during and immediately after storm events. Having been out in the woods in the rain numerous times, I can attest that many wetlands and ponds that look isolated are actually connected by tiny temporary surface streams during strong rain events during wet seasons.
I have personally seen several kinds of fish swimming upstream in such natural drains, including the mosquitofish and its taxonomic live-bearing cousins the least killifish (Heterandria formosa) and sailfin molly (Poecilia latipinna), plus bowfin (Amia calva) and various bream species (Lepomis spp.). I have also seen small fish swimming overland in the inundated ruts of a trail road that imitated forest “drains” during a downpour. Fish were coming from a river and going uphill to a series of small ponds.
Thus, I can think of numerous ways that fish can disperse by swimming, instances when apparently isolated water bodies are not actually isolated, and instances where fish do not exist in truly isolated places that are frequented by water birds. Case closed? Naw, now gotta prove it!