This year we harvested most all adult fish from our 20 year-old pond, and have embarked on a big rebalancing. The main feature is that we're trying to get YP established, so on May 23rd we stocked 150 6-8" pellet trained YP (plus 50 3-4" RES). See full thread on my pond's revamp if interested in more background info:
http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=506290

All perch survived the trip and stocking and we see maybe 50-60 per day during feeding time (hard to really estimate given how quickly they zip around and wander from one spot to the next). The issue is that we still have some perch dying, and have no idea why. I want to figure out why these fish are dying, if it's my fault, and if there's anything I can do about it, because we paid good money for these fish, so it would be nice if a couple actually survived.

We had a steady trickle of YP dying over the first two weeks after stocking. I saw/confirmed 12 dead (out of the 150 stocked). Then, we had no new dead perch seen during week 3. Now tonight, after feeding, we saw 1 perch come up floating, belly-up, and he had the energy to try to swim and flip himself rightside-up, but for some reason he couldn't flip over. I assume he will also be dead shortly, so now we're at 13 "confirmed" dead. This particular perch came up in an area where we had just pellet fed heavily, he had quite good body condition and the blue tint that pellet eaters have, and the school in this particular spot was (abnormally) eating lots of pellets off the top (ours usually only eat sinking pellets). I saw this same belly-up behavior with 1 other perch that died a couple weeks ago.

Questions: What are some potential reasons why these YP died, especially considering the strange behavior of floating belly-up while still apparently being strong enough to swim and try to flip over right-side-up? Could it be that he gorged on pellets and they're just buoyant enough to make his belly/underside be too buoyant? Could it be that he was eating too many off the top and gulped too much air into his stomach? Could I have overfed him, and he died from overeating? Could it be my pellet hydrating technique (hydrating in ziploc bags using well water)? I also squeeze my pellets into globs about the size of a ping pong ball before throwing in, so the pellets from each throw are concentrated and sinking in a tight formation, which means that the school has to be a bit competitive/aggressive, so could this tight formation and "fighting" over pellets stress fish too much to the point of killing one on occasion?

Also, see this thread for water chemistry test results if you think that could be the issue:
http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=507377#Post507377

Make sure the food is not moldy; either as dry food or aged hydrated pellets. Early onset mold on pellets is hard to notice. Do not store hydrated pellets longer than 48hrs even 36hrs to be conservative. The deaths in 1st 2 weeks were very likely handling strssors. Deaths in first two weeks are almost always handling problems. Your few subsequent deaths 'could' still be due to handling, likely from the source.

The only time I have seen the strange behavior you describe is when some YP were over laden with eggs. Gulping air is not an issue. Eating too many floating pellets could be a possibility, but this would be extremely rare. If the problem was water quality lots more fish would be dying, not just a casual death.

Hope you figure it out Drew. Following with interest.

Bill,
We hydrate our pellets immediately before use each time, so any mold issue would have to be present in the dry food. How would one identify mold in dry pellets? It's not readily visible and I don't think the pellets smell weird, and I don't think the pellets have gotten wet; at the hatchery, they were stored in plastic on a pallet and our bags came from the top, and at our place they've been stored in a plastic storage container (a strictly dry bowl is used to scoop pellets).

Thanks for the comments on likely causes of the mortalities. I imagined that the deaths in the first week or so were from stocking/handling stress, but it just worried me to see another dead one after 3 weeks. I'll report back on YP mortalities later on when I see if this is an ongoing trend, or just a freak incident.

Hope ya figure out what's going on Drew, the knot in the gut sucks.

I have seen pellets become moldy when removed from the bag and stored in buckets with tight fitting lids (plastic storage containers). This happens especially in warm, humid weather. This in one of the main reasons to be watchful of older food from production date especially when it is stored in containers with tight fitting lids. Feed in the original bag should have a production date somewhere on the bag/tag. Old food IMO would be more than 10 months old. I have seen feed get moldy when 3 to 6 months old when not stored properly esp in humid conditions.
Having just one or an occasional YP die after 3 weeks is not alarming. A latent death a couple weeks after stocking is not uncommon due to what perch have to go through when net harvested, held, transported, and restocked. The warmer the water for YP the harder it is on their metabolism esp 70F+.

Since I am from the south, I have no experience growing yp so I can only pass along what I have seen at my place when feeding fish. A couple yrs back I bought a bag of Cargills that's was a different size from my normal Cargills feed. Added it to my feeder and shortly after that I started seeing some small 3" cnbg dead and floating and like you I could not figure it out. And then I was watching as the feeder went off and saw some fish float to the top and act as you described. It took me awhile but I discovered the fish were choking on that sized fish food. And you said bait was rolled into a ball. It might be they are getting too big of a bite and choke on it. Hay, it may sound crazy but think about the larger sized fish that choke on too big of a sized forage fish and they choke and die. I have seen that at my pond also. At my pond the HSB, Northern LMB and my CNBG are all very aggressive when I feed them. I can see where a yp might be the same. And a soft hydrated ball of feed is there and they attack it and get too big of a bite and choke. I would stop making the ball and feed it as it comes in the bag and see if it changes things.

If I'm interpretting the code at the bottom correctly, this batch of Optimal was produced on 4/10/2019 and it's been in that plastic container in our basement, which stays fairly cool, so I'm thinking we're probably okay on this particular bag of feed.

It's early to report back, so take this with a grain of salt, but we haven't seen any more dead YP since that last one, so it doesn't appear to be any big mass extinction going on. Thanks Bill for being the force of calm here and sharing your expertise; it's too soon to say for sure, but I believe it's probably just as you say -- a stray latent death still due to all the stocking stress.

Tracy, your experience in the South with a different panfish species is still relevant and great food for thought. Most of the YP will knock into the pellet ball to break it up before they eat, but it only takes one overambitious YP on occasion trying to eat a giant bite to kill a fish. I appreciate your observation, so I'm still squeezing pellets to get the air out so they'll sink, but now I'm trying to loosen the ball up before I toss it in so that it breaks up right away upon hitting the water so I don't create so much of a fish choking hazard. Thanks!

If there are any additional developments please let us know. Stocking morts (sure appears to be the cause) should be over now.

Just to circle back: No new mortalities since 6/12. So, we had just shy of 10% die, that we were able to see.