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I'm in the process of adding the fun parts to my world, and I'm a bit perplex on this one;

Premise

Let's assume a fantasy ooze

An ooze is a mindless carnivorous blob of jello - think cube-shaped or formless jellyfish. It has no natural predators, and feeds passively - if organic matter touches it, it gets sucked in and digested until there is no trace left. Oozes are quite slow, and the bigger they grow, the slower they get. Main way to tell different species apart is by colour, but the danger resides in the fact that they tend to blend in the environment.

Let's say an ooze grew so big it became mostly stationary and sipped in a lake, filling the bed of it from the bottom. It is translucent-blue, so it blends very well in its environment. Water has to flow, so there is still a ~2m layer of it above the ooze and thus, the lake in itself is not fatal. The locals learned not to skinny dip in it, sometime a child goes missing, but overall, people and the ooze can coexist without much problem.

But what about aquatic life? Obviously, there is no algae in that lake below a 2m depth. I'm curious regarding migrating fish species making their way upstream for reproduction. Would they evolve to avoid that very specific lake? Would we see any behavioral changes over a couple of centuries?

Question restriction

We are talking regular biology and animal behaviour here. The ooze in itself is a magical creature and heavily handwaved. We are not discussing the potential ecological problem in a few millennia when that ooze grows so big it begins to destroy the ecosystem if not dealt with. We are not discussing how the ooze is biologically unsustainable. I am just asking what the fish will do when confronted with an unusual, localized area that is highly hostile to them in their migration path.

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    $\begingroup$ Human cities are an example of this scenario. $\endgroup$ Oct 26 at 21:20
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We actually have real world examples of migratory fish ceasing to breed in particular lakes and streams. These are mostly caused by dams not predatory slime but the result, fish aren't able to make it up the waterway to breed, and can't make it to the ocean to mature. We don't say that the fish have evolved to avoid the dams. We say that the dams have made the fish locally extinct.

Migratory fish return to the waters they hatched to breed. If suddenly something prevents them from doing this, via an obstruction or eating them, there will quickly be no fish to return to those waters, no genetic or behavioral change required.

While it's technically possible to physically navigate the waterway without getting preyed upon, normal fish behavior puts them in the danger zone. Fish fry stick close to the bottom to reduce the effect of the current and avoid predation. You see a similar behavior when salmon swim upstream, as close to the bottom where the current is slower and so that eagles and bears have a harder time eating them.

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  • $\begingroup$ I mean, fishes can still physically cross that lake. They can hardly breed in it, but it's not a natural obstacle the way a dam is. Otherwise, there wouldn't be a real point to this question, would it? $\endgroup$
    – Nyakouai
    Oct 25 at 16:41
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    $\begingroup$ When discussing managing fish populations the passthrough rate is an important metric. This is the number of fish who attempt to return to span vs the number of fish that make it to the spawning site. Like your critter, most dams have a non-zero passthrough rate, through systems like fish ladders, but they impact the passthrough rate enough to make migratory fish locally extinct. It will be the same for your lake. If you want a more detailed analysis you'd need to provide us with an estimated passthrough rate. $\endgroup$
    – sphennings
    Oct 25 at 16:47
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    $\begingroup$ "Migratory fish return to the waters they hatched to breed." Okay, that's a point I did not know about! Don't have any clue regarding the passthrough rate. Stupid question tho; let's say a new upstream way appears, will it ever be colonized by the fish (over a long period of time)? I assume accidents happen at least and they do not always return to the exact same stream due to circumstances? $\endgroup$
    – Nyakouai
    Oct 25 at 17:08
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    $\begingroup$ You're talking very long odds. Two fish would need to make it upstream by accident at the same time, and survive their trip across the lake, just to be able to spawn. Under normal circumstances only 0.079% of hatched salmon fry live to adulthood. Now factor in that these fry need to make it past the lake, while avoiding the slime, which puts them at greater risk of being preyed upon by other predators, grow to adulthood, and then swim back through the lake. The odds of getting to a second generation are ridiculously tiny. $\endgroup$
    – sphennings
    Oct 25 at 17:16
  • $\begingroup$ Ok, thanks for the detailled explanation, that clarified things! $\endgroup$
    – Nyakouai
    Oct 25 at 17:17
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No migratory fish, maybe no fish at all

In a zone where there is a winter cold enough to freeze the lake, there would be no fish. During winter fish seek the lower water for warmth and would then intersect with the blob and be eaten.

Migratory fish such as salmon would not return to the lake. These types of fish lay their eggs on the bottom of the lake, ie: where the blob is. As fish migrate back to the area they were spawned, this entire subset of fish would vanish, leaving only the occasional "lost" individual from other lakes to wander in, go try to lay eggs, and be eaten by the blob. So in that sense yes the salmon/whatever would "evolve" to avoid that lake.

Most other fist would likewise vanish, as most fish lay their eggs either on the lakebed (eaten) or attached to marine plants which are on the lakebed (which have also been eaten). You could conceivably have some small minnows or other fish which lay "drifting" eggs, but even those would lose out as eggs drifted downward. Given the timeframe involved I don't think there's a chance evolutionarily or behaviorally for most species to adapt. Too many eggs would be lost in the first few generations, and that combined with natural predation would probably clear the lake of everything but possibly shallows-dwelling minnow species.

My guess is it would quickly be full of frogs (which lay their eggs in the shallows/on the surface and avoid the blob) and algae blooms which may or may not be enough to keep the lake oxygenated. (do massed tadpoles eat enough to counter-balance the lack of fish? Does the algae by itself make up for whatever other lower-level oxygenators have been eaten? no idea!)

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  • $\begingroup$ That's a nice and detailled first answer, taking short and long term into accounts! $\endgroup$
    – Nyakouai
    Oct 25 at 17:20
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There could be a slime slim chance that fish change their behavior, if we further restrict the environment:

  1. Ice does not become too thick in wintertime, leaving enough room for the fish to swim freely during the coldest weather.

  2. No predators exist in the area that can access a tasty fish meal in 2m of water.

  3. The fish species can spawn in 2m of water or less.

  4. The blob has settled into the deepest part of the lake, but the lake is shallower near the edges, which would provide a blob-free habitat for animals and plants.

Perhaps fish could spawn in deeper waters as well as shallower waters, but one year the blob shows up and eats all of the fish and eggs spawned in deep water. That leaves the fish that spawned in shallower waters to survive and reproduce the next year. Year after year, fish that can spawn successfully in shallower waters survive. Those who choose deeper waters do not.

Bonus points for people and animals using the lake frequently and not getting hurt, but legends exist about a monster in the deepest part of the lake.

Don't go over your head!

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  • $\begingroup$ Oh, locals very well know about the slime. Best part is that since it eats only organic matter, everything mineral can subsist... including coins, jewelry and other shiny that accumulated over time in the slime. Still debating whether or not they use it for executions. I like that slime. $\endgroup$
    – Nyakouai
    Oct 25 at 17:26
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I suspect that the fish population will very rapidly evolve (more accurately those that don't get culled) into a shallow-water, surface-dwelling variant.

After all, the lake down to 2m is a perfectly normal water lake, right? Only if you dive below 2m, and touch the ooze, do you get eaten.

The exact equivalent already exists in real-life lakes, where the deeper waters of a lake are still and anoxic, yet the surface waters thrive. The boundary is usually a bit deeper than 2m (more like 15-20m), but that's a difference in degree, not type.

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If there are alternate routes that avoid the problem completely, the fish will eventually evolve to avoid that route instinctively. It's unclear how long that will take, but since fish (like salmon) tend to be one generation per year, probably on the order of thousands to hundreds of thousands. Migratory birds will also avoid it on about the same timescale.

Probably a dearth of all aquatic life is possible with this, even insects will seem to avoid it (though that would be the appearance, it's more likely that they just get wiped out immediately and comprehensively when they attempt to re-colonize).

Larger animals will, even if aware, take their chances. Many antelopes and zebras and so forth will sip at the edge of seasonal lakes even though they're intelligent enough to be aware of the dangers of crocodiles. If in a temperate area, this would be more along the lines of deer and bears, but again if there are no other decent sources of fresh water, they might just dare. With mixed results.

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  • $\begingroup$ Last I checked most water systems were acyclic. With the exception of manmade canals it's incredibly rare to find a obstruction across a whole river and be able to find an alternate water route to upstream of the obstruction. $\endgroup$
    – sphennings
    Oct 25 at 16:53
  • $\begingroup$ No, but there are other rivers to go upstream. To the fish, it's the same as an alternative route: there is another path to a breeding ground. Not the same sure, but a breeding ground is a breeding ground to a fish. $\endgroup$
    – Nyakouai
    Oct 25 at 16:55
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    $\begingroup$ @Nyakouai That's not how migratory fish work. If you block off a stream, all the fish that hatched in that stream will try to get to their spawning ground until they die. Not to mention that other spawning sites have their own established populations. $\endgroup$
    – sphennings
    Oct 25 at 17:06
  • $\begingroup$ @sphennings That's not quite true either. If it were, then there would be no fish anywhere... they can't colonize anything without an established population. Occasionally a few get lost/confused, and establish new populations. Their offspring may or may not return to the original spawning waters. And over time, new populations are naturally established. $\endgroup$
    – John O
    Oct 25 at 17:14
  • $\begingroup$ @JohnO You're right that fish had to colonize streams, and they do this by getting lost/confused, but recolonizing streams is a rare occurrence without direct human intervention. $\endgroup$
    – sphennings
    Oct 25 at 17:18
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Fish will notice if the lake floor is suspiciously dead below 2m. Not much evolving is necessary for them to avoid diving down there.

And since the lake is close to dead, there's no need to stay there, so the fish will rather quickly move further up or downstream during migration, both of which is perfectly possible.

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