Rainbows and spawning

[Thomas]

Hello everybody!'

I have a question about rainbows and their ability to spawn - here in Sweden rainbows don´t spawn. Why? I know that a lot of the fish that are being stocked in our put and take fisheries are triploids (?) that are unable to spawn everywhere. But not all of them are. So what is it about out lakes, rivers or rainbows that prohibit spawning? Is it anybody here that knows something about this?

/Thomas

[Paul Arden]

Hi Thomas,

I think they only manage to reproduce in a couple of places in the UK. A long time ago now I had an email from a NZ scientist asking me what I knew about this subject because they had noticed that while brown trout spawn well in NZ, rainbows were having a real problem.

I think it’s an unknown. I suspect water temperature. But I don’t really know. I know in NZ they spawn later than browns and by the time they’ve finished spawning the browns are back on tip-top condition. But why they are not invasive to the UK I don’t know. Of course most are female and many are triploid but this doesn’t explain it. I think in the UK it’s probably a good thing by the way!!

Cheers, Paul

[James9118]

How's this for a hypothesis (and it is only that) - due to mankind's interference with the spawning of stocked rainbows they are no longer a viable species for survival in the wild?

The fish that are stocked into put and take fisheries are effectively 'domesticated' - selectively bred over hundreds of generations to pile on weight fast but with no input from the environment regarding their survival.

It's a bit like comparing a French bulldog to a fox or a dingo, all dogs but the French bulldog would have no chance in the wild. (Genuine wild rainbows being the equivalent of the fox).

James

[Paul Arden]

The problem with that hypothesis is that it doesn’t explain why 140 years ago they didn’t demolish brown trout populations in the way that grey squirrels did with red ones. Perhaps brown trout are just hardier?

[James9118]

Hi Paul - I don't see the contradiction?

[Tangled]

This is a nice article about it,

https://www.wildtrout.org/content/rainbow-trout

Jame's could be onto something; the original batch of rainbows put into the River Wye here in the UK that has become self-sustaining was very genetically diverse.

"According to American fish biologist Robert Behnke ​‘the 1885 shipment to the U.K. would have been highly diverse in ancestry with high heterozygosity for natural and artificial selection to effect rapid adaptive change’. The rainbows in the River Wye certainly did adapt well and now make up around 25% of the fish caught on the Derbyshire Wye, co-existing alongside the indigenous wild brown trout population."

The brown trout in Argentina were introduced, are self-sustaining and also bloody enormous. They co-exist with introduced rainbows there too. They introduced Atlantic salmon but they didn't work out. The introduced browns are sea going so maybe they outcompeted them? I doubt anyone actually knows why some work and some don't.

https://www.patagoniariverguides.com/hi ... patagonia/

[Paul Arden]

Sorry, I don’t disagree with you James and I’m quite that’s the case particularly in Europe. I think however that there must be additional reasons that they don’t transport well. I was surprised to hear about that being the case in parts of NZ, but you can see similar results in Tasmania where browns also have fared much better. Why this is the case I don’t know but I do know they spawn at different times during the winter. Browns straight after Autumn and Rainbows just prior to the Spring.

I recall reading that rainbows push browns out by being more aggressive but I’m not sure how valid that is as a general rule.

Cheers, Paul

[Bendix]

Hi

I also think James is on to something here.

Here in Denmark, the Rainbow Trout that sometimes escape the fish farms, have apparently become so domesticated, that they can’t survive or reproduce with very good success in the wild. Or at least, that is the predominant theory.
Also, they compete with the wild Trout and Salmon, which is a very big problem, whenever they escape in large numbers. They destroy the spawning grounds, and eat the eggs of the wild Trout and Salmon. Which is why, a lot of fishermen are making an effort to remove the old fish farms in the rivers, and limit the number of new ocean fish farms being build.

/Bendix

[Tangled]

The most recent introductions in the UK are Pink Salmon, humpies. Every couple of years - in odd numbered years - we get immigrants from the White Sea. They don't seem to have settled yet, though they breed and create reds. Much discussion about whether this is good or bad. They're not even native to Russia, introduced for food in the 60's and 70's.

[Thomas]

Hello everybody and thanks you for your answers! It seems like I´m not the only one who doesn't understand why rainbows have these difficulties with spawning. My fishingclub have - with the help from some biologists and geneticists - been comparing the dna of the trout in "our" stream with the dna of trout in other nearby waters. I´ll ask the geneticist about this hypothesis of genetic diversity. Perhaps he might be able to throw some light on this issue.

/Thomas