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Stunted / runt fish... why???

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Magnum Man

Fish Herder
Jun 21, 2023
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Southern MN
there is a current thread going about stunted or runt angel fish... & I'm raising Tilapia... the supplier says if the fry / fingerlings aren't kept at the optimum temperature for their metabolism, & fed correctly they will stunt... I'm sure there are other fish, in the hobby that are the same... & science has progressed a lot since I was into fish 20 years ago, but in my circles at the time, is was said that some fish excrete a hormone that will cause the fish to slow or stop growing, if the tank is too small, or water was not changed often enough...

back to the future... so with my tilapia. I used the recommended foods, & kept the fish at the recommended temperatures, yet out of 100, I got at least 2 stunted / runts... mine ended up getting culled out in the drain valves, which I didn't start using until the bulk of the fish were big enough to not have problems with the valves... but I noted one, one day, back when the bulk of the fish were 4-5 inches long, one that was hanging around one of the pipes, that had only grown to about an inch & a half... so way less than half the size of the others, even though they were in the same tank, & I took steps to ensure everyone got food, by feeding as much as they would eat ( my skimmers regularly had just a few pellets in them & ( at the times the pellet size changed ) I fed the larger size pellets 1st, when I was switching over, giving what could be eaten in the recommended time, then added a scoop of the smaller pellets that were being switched out, thinking if the big ones had the bigger pellets hanging out of their mouths, ( they will eat until they can't pack any more in their mouths ) they couldn't gulp up the smaller pellets, making sure the smaller fish got a chance to eat... BTW... my Tilapia supplier said that I could raise 75 fish to harvest size in each tank I have ( I have 2 ) so I equally split the (100) fingerlings when they came, so at 50 per tank, I'm not putting more fish in the tank than recommended...

so I'm wondering if in general aquarium fish runts, are that way genetically, from being bred so much in captivity, as it doesn't seem to be solely environment or food based??? Thoughts???
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There is variation in every batch. That is why Lakeway Tilapia grades them and the runts are sold as pond grade because they stay small and keep eating algae. Food grade grow at the fastest and most consistent rate but they are separated out of each batch for fastest growth and maturity traits. They are not just genetically superior fish that would keep giving same type offspring.
You can make runts with bad practices, but even in the best of set ups, you always have a few fry that don't grow like the ones we want to keep, especially with food fish. It's just variation.

The growth suppression hormones are a great reason for water changes with young fish, and we have issues of overall water quality, feeding, etc to deal with. When I was a teenager, I thought I had a line of dwarf X. variatus platys breeding true. Then I read some articles on this new idea (way back then) of doing water changes in tanks, and when I decided to experiment with it, I no longer had dwarf variatus! After several generations as 1/3 sized fish, they suddenly reverted to normal size.

But there, it was 100% small to normal growth. In a batch of 100 fish, a few will always grow very slowly.
mine were Lakeway graded as aquaponics grade, but there were still about 2% stunted fish
I didn’t mean this to be solely a tilapia discussion which is why I started with the angel fish

I’m not proud that I still own some glo fish, but even on these man made fish there can be a huge variation in size ( I bought about 25 to start with in one purchase for my football themed tank ) they varied in size from a US dime, to a US silver dollar the smallest ones never grew… of course they could also be diseased, or carrying parasites that could stunt their growth

Also maybe we weren’t so far off on the science 20 years ago, about the fish secreting hormones in some cases… for example my friend and I each had Red Breasted Pacu’s bought at the same time… mine graduated to bigger tanks, as he grew, my friends started & stayed in a 55 gallon tank… mine eventually end up in a custom 220 gallon tank, when the fish died ( roughly the same time ) and both cared for as well as we knew how back them, his was 10 inches long, and mine had grown to nearly 2 feet long

Going back to the tilapia, mine were supposed to be graded, but I’ll admit I was disappointed at how small they were when they came… Lakeway called them fingerlings, I would have called them fry yet… they were little 3/4 inch pieces of wire… they hadn’t gotten their typical “fish” shape yet… so they must be pretty good at sorting them, but apparently a 2% stuntting rate is acceptable…

I’m hoping to breed my own fry for next year, as they were not cheap… I’d be curious if a stunted fish were use for breeding, if a higher percentage of fry would be stunted ( thinking parrot fish here ) since I’m after fish that will grow best in my conditions, I’ll be starting with the biggest, healthiest looking fish to pick a breeding colony… as far as fish production, it seems wrong to take your best fish out of harvest, and it’s hard to care for bigger fish in an aquarium rather that choose a mid sized pair… but if I were breeding pigs or cattle, I’d be breeding for traits I wanted ( I do & have bred most farm animals )

Most Angel fish found in the hobby are tank raised ( I will acknowledge there are still wild caught ) but those are fewer and generally harder to keep

This goes back to a thread I had a week or so ago, that the gene pool is likely very small on a lot of bred fish, and my tilapia will be no different in this case, as they were all purchased at the same time, so while I’m going to be choosing my breeding fish, they are likely to be from the same batch of eggs ( brother/ sister ) or at least likely the same father... maybe next year, I can order a few fingerlings from a completely different supplier, & replace this years male, in hopes of widening my gene pool...

It just seems we are likely to encounter increased stunting or defective fish when the gene pool is so small ( back to the parrot fish... balloon Mollies, etc. that were likely a single defect to start with ) but this should also include any unique colored or patterned aquarium fish, that has been line bred to achieve...
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