On Fish Dispersal and the Perpetual Evil of the Duck

Image Credit: Norbert Nagel, CC BY-SA 3.0, Image Credit

Woe betide my fishy ancestors, for I am come here today to vent my grievances at a paper so dastardly it has cast a tepid patina of anxiety on a LOT of the structured squabbling my colleagues and I call ‘research’.

Actually, I shouldn’t vent too harshly on the sarcopterygiites, those ancient lobe-finned ancestors of ours and their close cousins the regular fish. Birds, as always, are the main culprit here. An abhorrent series of mutations that messed up a perfectly good reptile.

Theatrics aside, here’s what happened. This week a stupid (actually it’s not stupid it’s brilliant and elegant in its simplicity) paper basically showed us that ducks (mallard ducks, which are evil incarnate) are capable of ingesting carp eggs, then carping them out, upon which many of the eggs could still hatch and produce a viable population. Congratulations Mr. Ádám Lovas-Kiss and friends, you’ve messed everything up for people, not least myself.

Read: Experimental evidence of dispersal of invasive cyprinid eggs inside migratory waterfowl

Why is this important? Why am I so pissed off? It’s because this adds another layer of complication into our understanding of fish dispersal, which forms the core of my thesis.

If you don't believe that mallard ducks are evil and that birds in general are stupid, you are fooling yourself. Here is a male sexual deviant and his captor. May you never meet a person whose favourite animal is the duck (Image Credit: Fcb981, CC BY-SA 3.0)

If you don’t believe that mallard ducks are evil and that birds in general are stupid, you are fooling yourself. Here is a male sexual deviant and his captor. May you never meet a person whose favourite animal is the duck (Image Credit: Fcb981, CC BY-SA 3.0)

In brief: as waters warm and hobby fishers keep shifting fish around the globe, fish species are more and more likely to spread, often outcompeting and even wiping out (at least on a local level) other fish species. In the area I live in, Cyprinid fish species (like the carp used in this experiment) are of particular concern, as they often do well in warmer waters and can upend ecosystems in freshwater lakes. If we’re going to stop this happening, we need a bunch of data and some very competent models, in order to figure out where it will happen. The good news is, that over the course of my PhD I’ve started to realise that (given the right data) we can figure out where it will happen. We can figure out which lakes fish species are most likely to pop up in, whether they’re moving naturally as the climate warms or being introduced by humans who should know better.

But right now, we can’t account for some idiot duck who should really know better (and who probably has jovial Skype meetings with Rodrigo Duterte) just shitting out a bunch of eggs into a lake.


As you can see here, the male duck is at fault (Image Credit: Lovas-Kiss et al., 2020)

We actually had a direct encounter with this previously. The paper that I’ve recently had accepted in Diversity and Distributions was initially rejected by another journal. The paper (hopefully openly available next week) studied recolonisation of lakes which were treated by rotenone. Rotenone is a fish-killing chemical used to remove invasive species from lakes, often deployed in Sweden and Norway to remove pike or perch. We compared the slope downstream from the lake to whether or not that lake was recolonised by the pike or perch. It essentially lets us know how steep a slope has to be before it’s insurmountable by a fish species. If the only access to a lake is a very steep stream, then we can deprioritise it in terms of prevention of invasion by fish species. Reviewer Two took issue with our assumption, as they felt that surely successful colonisation could have happened by some other means, say “by mud on a duck’s feet”.

We laughed it off at the time. There was very little empirical evidence for this, as the paper which is the current subject of my ire concedes. It is, however, not so funny now. Though it’s not the feet we have to worry about. Make what you will of the mud part.

To be clear, cyprinids like carp are very different from species like pike and perch. And while this paper (which, again, is actually very good and we’ll be summarising it next week) shows that dispersal of fish eggs through duck faeces is possible, I don’t think it happens to the extent that it will invalidate the vast expanses of work that has been done on fish dispersal. Even if a single duck does manage to bring fish eggs into an isolated lake, they have to be able to survive and establish a population. But it is something we need to start considering.

More importantly, the paper answers the question of how fish species often turn up where they shouldn’t. I look forward to hearing more about the traits of fishes for which this is a viable dispersal mechanism.

But I still maintain that wiping out all ducks is just as warranted a path for further research.

Sam Perrin is a freshwater ecologist currently completing his PhD at the Norwegian University of Science and Technology who is vaguely vegetarian but will choke out a duck at the slightest provocation. You can read more about Sam’s research on his Ecology for the Masses profile here, and follow him on Twitter @samperrinNTNU.

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