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Fish: Rainbow Trout

Trout eggs biology question (female with OLD eggs)

Post By: eholm      Posted: 11/14/2023 5:16:18 PM     Points: 17793    
I recently kept and ate a female rainbow trout, caught in a reservoir. I was surprised to find two sets of eggs in her, an old one and a new one. Bear in mind, this is mid-November, and as far as I know this reservoir only contains spring-spawning rainbows (among other species).

The old set of eggs were turning yellow, and all of them were split open. They looked a bit like soggy corn kernels. Essentially they were spoiled and decomposing, but still there. The newer eggs were normal size and color for eggs that are well developed and close to spawn time (Not early-development tiny ones).

It was actually pretty gross, but the flesh seemed fine so I ate it anyway.

I did some googling, and found minimal information on this topic. The information I found seemed to imply that this is a common condition among farmed trout, or those that don't have a place to spawn.

I also found it stated that brown trout can reabsorb such unlaid eggs, but rainbow trout cannot. Therefore female rainbows in such condition will die with 2 years of unlaid eggs.

I didn't find any scientific studies to back up these claims. To my knowledge, rainbow trout do typically lay their eggs in reservoirs even when more "natural" spawning conditions do not exist. This was a large reservoir that stocks rainbows annually. I'm assuming this is an unusual case, as I've never seen it before.

Questions - Anyone else seen this? Any more information about it, or known causes?
 Reply by: eholm      Posted: 11/15/2023 10:13:32 AM     Points: 17793
A few bits of info I found...

Quoted from a Utah report about Colorado River Cutthroat breeding at hatcheries:

"A number of females expressed old, ripe, and green eggs all at once. Many of these fish were from the 2017/2018 cohort that experienced a delayed photoperiod in 2021. Some of the old eggs observed in 2022 were likely eggs that matured after the fish were moved out to the exterior raceways in 2021 and weren’t expelled. These dead eggs were counted as green eggs, but were never viable. Holding eggs from 2021 adjacent to eggs produced in 2022 in the body cavity may have impacted the survival of new eggs." Source: [log in for link]

Quoted from Fishbio website, which is a fisheries consulting company:

"Rainbow trout that inhabit lakes and reservoirs may either migrate into rivers and streams to spawn (adfluvial) or may spawn in the gravel at inlets and outlets (fluvial). They generally spawn in the winter/spring (January to June), depending on the climate. The timing is dictated by increasing temperatures and spawning usually occurs when daily water temperatures exceed 42-44°F (6-7°C)." Source: [log in for link]

Quoted from a 2002 fisheries management paper from Western Australia:

"Brown trout have the ability to reabsorb eggs if suitable spawning gravel is not available and a new batch of eggs are produced in the following year. Rainbow trout, however, do not have this ability."

"Female egg production normally detracts from growth and eggbound female rainbow trout usually die as 2+ year olds in farm dams."

Source: [log in for link]

Quoted from an aquaponics forum, which also referenced the above paper:

"Over the years I have dissected many female Rainbow Trout from farm dams and most of the older females have had two distinct sets of eggs in their cavity. The new eggs are easily distinguished from the old dead eggs thru the difference in size and colour." Source: [log in for link]
 Reply by: panfishin      Posted: 11/16/2023 3:35:31 PM     Points: 9369
I don't keep many but I have had a couple later in the year that still had a fell set of eggs. They all looked normal and not the yellow color that you described. I think both that I have harvested like that came out of Antero in either late Auguste or September.
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