Invasive species appearing out of nowhere. What should I do?

Hello everyone. A few months ago I decided to create a pond for wildlife observation which had some great results. There’s a lot of animals that go there daily to have a drink and bathe, or even to live there. In the thread I created I talked about introducing Argentine Pearlfish (Austrolebias bellottii) since they are present in the area but people argued against it, saying that they might eat everything in sight and minimize biodiversity.
So the pond went on to be naturally filled with tadpoles, water beetles and plenty of other animals, except for fish. It is currently the middle of summer here and we had been experiencing heavy drought until a few days ago when there was intense rain, which naturally filled up the pond (I had been adding non-chlorinated water to it to keep it from drying out cause of the hot weather).
Yesterday when I went to check it out I noticed that there were some strange looking “tadpoles”, very small and weird. I then spotted some similar looking animals but bigger, so I got my net and tried to get a few. Lo and behold, what I managed to caught where crearly some type of fish. I was very confused as to how they got there, since there isn’t any natural body of water connecting the pond to nearby ditches or puddles, and plus those were all dried out from the drought. So how did this fish manage to get here? I didn’t put them there and neither did my family. So, thinking they were probably the same species of fish that I had seen in nearby ditches but wanting to be sure I took some pictures and uploaded them to iNat. They have since been identified as West Mosquitofish (Gambusia affinis) which are an invasive species originally introduced to Argentina around 1945 to control mosquito larvae population.
Doing some research I found a paper which studied Mosquitofish’s diet in the argentine province of Salta (im in Buenos Aires province) and it seems to indicate that they feed mostly on plants and occasionally on insect larvae. Even if they seem to be a naturalised species here, it seems that in other places like Spain they out-compete native fish species and can cause a lot of ecological damage. They are also on the 100 most invasive species list by the IUCN. Overall there seems to be a lack of information on their actual ecological impact in my area.

So here I have a big dilemma. Should I drain the pond in order to kill off all the mosquitofish and likely kill a lot of native species in the process (I will try to save and reintroduce as many tadpoles and water beetles as I can, but there’s gonna be casualties regardless) or allow the mosquitofish to exist here and have them potentially spread on further? If I am to take action it should be now that it is summer season, so I would let the pond dry out on its own and save the natives once the water level is low enough.

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Unless you can figure out how they got there, I would probably do nothing. Birds are known to eat fish eggs and some number of those can survive (see: https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecy.2774), so it might be a losing battle.

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If you noticed ducks in the pond, they are a likely explanation. A small amount of fish eggs that ducks eat survive the digestive system and are pooped out somewhere else, where they hatch and thrive. I would do nothing about them. Try to scare away ducks for some time.

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Is it possible that your rain was heavy enough to trickle in water from a nearby source of the fish? Wouldn’t take much flow to move a few fish eggs.

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Gambusia affinis is a life bearing species. But other then that, the explanations of transfer by ducks (in the feathers and on the legs for example) are pretty likely.

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I haven’t seen any type of waterfowl or aquatic birds near the pond. I have seen ducks fly by on occasion, but they are a rare sight and never come down here, probably because of all the people.

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I do not think so, no. Mostly because other water sources are at a lower height than the ground my pond is on. And the pond is around 80-100 meters away from the nearest possible water source (a ditch that was completely dry before the rains). The only nearby water filled fitches before the storm were about 400-500 meters away at least and way lower than the ground around the pond.

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Any chance someone passing by might have put them in? If it was publicly accessible and looked like empty land that’s a possibility. It’s been a persistent problem here in Hawaii where Gambusia is also invasive.

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It seems highly unlikely. The property is fenced in and the pond is away from the nearby road. Also it’s a less populated area since its almost in the middle of the countryside. The only other people that may have access to it are the gardeners (which have been told not to cut the grass around the pond so they shouldn’t even go close to it and have no reason to carry fish around) or a neighbor since the pond is some 5 meters from the fence that connects our two properties, but I do not see them yeeting a small fish above the fence to try and get them into the pond. I may ask if I see them tho!

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Two more theories.

1: If you were near another pond lately that has fish, you could have had an egg stick on you. Highly unlikely, but plausible.

2: Maybe another songbird bathed in a puddle, got eggs stuck in its feathers and transported them here while bathing. This is very likely, as it is migration season, and many birds are moving around.

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What @Ajott said is true, as far as I know. Gambusia affinis is a live-bearing, not egg-laying, species. I don’t know how they may have made it to your pond. I am not an expert on fish so there may be other ways hatchling Gambusia may have made it to your pond, but I’m not sure. I am very curious though!

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If the fish embryo arrival was as massive as I understand, I would naively say that a human intervention seems likely. I mean that someone may think that your pond is a source of mosquitoes. Unless birds eat huge numbers of eggs… I do not know.

Whatever the origin of this invasive fish population, my (again) naive suggestion would be to introduce a native predator, notably a native fish that would predate on their eggs or larvae. I have no expertise, but a fish would definitely stay in the pond (contrary to birds or snakes) and there are papers showing that solutions like this can be efficient, for example: “A small native predator reduces reproductive success of a large invasive fish as revealed by whole-lake experiments” in 2019.

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Being stuck in plumage of waterfowl is listed as a potential dispersal vector for these: http://issg.org/database/species/ecology.asp?si=617&fr=1&sts=sss

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@parped Now this here makes a lot more sense! Barn swallow (Hirundo rustica) flock here at the start of spring time, whilst during winter they migrate north, into Canada and the US. I have seen them many times diving into the pool, getting their chest feathers wet so they may land somewhere else and drink the water. Whilst I don’t remember actually seeing them around the pond it seems likely that they use it in a similar manner as well. I don’t know if just diving would be enough to attach fish spawn to their feathers tho.

@lucareptile From what I’ve read they are indeed ovoviviparous, but the female may either spawn live fish or lay the eggs and they would hatch shortly after, so the egg explanation is still a possibility.

@odole it is possible that the smaller fish appeared some time prior to when I first noticed them, as they seem to be 2mm in size and almost transparent. But I spend a lot of time staring into the pond, so I think I would have noticed them either way. And sadly from what I was able to find no mosquitofish predator species are native to this area. I don’t want to introduce another exotic species to try and solve this (least I end up like that one scene in The Simpsons) and whilst I have considered adding argentine pearlfish they are none present right now (since they lay their eggs deep into the mud so they may hatch in the rainy season, but the adults die during summer) and plus they are bottom feeders and almost the same size as mosquitofish, so I highly doubt they would control their population. Best case scenario some small snake finds the pond and has a feast, but I have yet to see any reptile approach it.

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Is it a small enough pond that you could use a minnow trap or other means to trap them out, or at least significantly reduce their numbers, so you don’t have to drain the pond?

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The pond is medium small, however it has a lot of varying depths and places where fish could hide like logs and rocks (since I designed it with the idea of making it a full on livable ecosystem for wildlife). Also, the fish are incredibly small, with the adults being around 2-3 cm and the babies around 2 mm, so I don’t think that fishing them out would have an actual impact on the population since most would pass thru the net. It would likely turn into a vicious cicle of me having to constantky kill the adults and then have the population bounce back up. Even if it did partially work overall I find it to be potentially more cruel than just killing them all at once by drying out the pond.

Would you not have dragonfly larvae in your pond? They are voracious feeders.

It is a possibility, yeah. I have seen many dragonflies around the pond so the chances they laid eggs there are pretty big. However if there are indeed dragonfly larvae they are not eating enough. Here is a short video my mother sent me today, she fished out as many mosquitofish as she could, but this are mostly all adults, so there’s bound to be many more left.

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I just realized by reading the first post of your previous thread that your pond is quite small (as you mentioned : “3m long and 1,5m width”). Thus I fear that in order to survive it will often need your direct interventions, besides adding water in summer, a bit like a big aquarium. The advantage is that you should feel free to transiently collect all relevant animals into a couple of Noah’s Arks (to separate preys and predators), then flood the pond or dry it up with the purpose to remove these fishes, and then put back normal water levels and the animals you selected. I hope this makes sense.

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@odole has the best answer. That is how we have dealt with the threat the Gambusia affinis posed to Fundulus julisia in some locations here in the USA. The Fundulus were all removed to secure indoor ponds at a conservation facility while the wild location was allowed to dry up, effectively killing all of the Gambusia. Once the water levels returned the Fundulus were reintroduced and it is now one of the strongest wild populations although the site is still at risk of drought from time to time.

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