Composting fish carcasses!

We've had good luck composting mammal butchering scraps in 55 gal drums using wood chips to absorb the stink.  Gave it a try with some carp carcasses.  Holy stink!  Anyone else compost fish?  How do you do it?

Going to add charcoal next go round and see if that helps...

I've been composting for about 20 years and always avoided meat.  That said, if I were to stop avoiding meat, fish would probably remain on the "Danger, Will Robinson" list.  There are few things I've encountered in my life that stink worse than rotting fish.  There are things that do, but it's pretty high on the wretched stench list.  I have no doubt that activated carbon would help mitigate that.  But it seems like extra effort for little gain.  Like, put in 1 unit of compostable goodies, but also have to produce .9 units of work to make it worthwhile.  Also, if we can smell it even a little, it's probably like a lighthouse beacon for pests.

Fish compost is made with fish plant waste & woodchips commercially around here. A friend did it for a while as well.

Nothing but woodchips and said waste fish... but the ratio neds to be right, and you gotta mix it, like with an excavator!

A barrel is way too small to hot compost, I am guessing that is an important difference..

"Fish fertilizer" is a thing, so there must be a way to process it, but then again, fish fertilizer may be pretty stinky.

I've only ever composted small quantities of fish and relied on "lots of carbon and other stuff" to contain the smell. Wood chips take a long time to decompose. They require special lignin digesting microbes which aren't in fish. I do have to bury lots of fowl guts and bodies and I've found that wrapping stinky stuff in old feed bags (heavy brown paper) so there are multiple layers surrounding it has worked better for me. Having things like "chicken shit inoculated wood-chips" below, around and above, also seems to help.

I've had good results with bokashi fermentation of the carcass and guys prior to adding it to the compost pile

My mother's "fish barrel" (salmon offal and water left to ferment in the sun) was a legendary stink.

However, most of the heads and guts when the salmon were running got dug into trenches in the rows between raised beds, and got covered by lots and lots of decomposed sawdust.  The smell was pretty much restricted to that section of the garden, and not too dramatic.  However, digging root vegetables from those beds in September after burying the material in July was ... an adventure.  One wanted to be cautious about where to step or shovel.  

Back when the fishing was good on the East coast of Canada, it was common practice to bury a "junk" fish under each potato plant. The results supposedly spoke for themselves. Perhaps the stink spoke loudly as well -- I have not found written mention of it.

We have a neighbor that composts fish from the local fisherman, and uses wood chips keeps turning for about a month and has beautiful compost. He calls it fish and chips 😂

I took part in a Rainbow Trout mort composting project using a 3 bin system built from pallet wood at the College of Southern Idaho Fish Hatchery in the early 90s. Each chamber was approx. 1 cubic meter. We kept daily records of the weight of the morts added. Each day's layer of morts was covered by about 10 cm of moldy straw.
It was interesting to see how bin #1 would fill to halfway in fairly short order and then take much longer to fill all the way up from there. The pile would really start heating up once it was half way and it would just kind of collapse on itself and start to decrease in volume even as we added morts and straw. Temps of 60c to 68c (140f to 154f) in the center of the bin were quite common once the system got going.
The smell could be intense if the morts weren't covered sufficiently with straw.
By the time the material made it through bin #2 there was no evidence that it was composed of about 80% dead fish by weight.
There was always a waiting list of people who wanted the black earthy smelling compost from bin #3 once it cooled down.
The CSI Fish Hatchery still composts the morts, 25 years after the research project ended.

Douglas Alpenstock wrote:Back when the fishing was good on the East coast of Canada, it was common practice to bury a "junk" fish under each potato plant.

Even before you wrote this, I was already thinking about whether memories of plentiful fish make me an old man in this world.  The time we were gillnetting subsistence king salmon in enough quantity that disposal of the offal was a problem was forty-five years ago.  These days, it's my understanding that they do individual fish counts by sonar on the upper reaches of the Yukon in Eastern Alaska, so that enough escapement (into Canada) can happen to satisfy our treaty obligations.  I'm told that most years now, there are either zero fish available for local subsistence fishing or the number is so few, people are happy to get one or two fish for fresh eating.  The chum (dog) salmon typically fished in the fall for sled dog food and by people with food insecurity (they are smaller fish that get beat to hell by the first 800 miles of their spawning run, so they aren't very appealing) have until recently stayed reasonably plentiful, but the news out of Alaska in the last few weeks is that this year's run failed spectacularly.  People are organizing crowd funding for emergency dogfood air freight shipments to several rural communities, where many sled dogs are at risk of starvation.  

Sorry, didn't mean to divert the thread.  Just musing.

I have never composted fish but I buried a trout's remains near some shrubs this weekend!

But I just wanted to say I had great results when I made fish hydrosolate (spelling?) a little while ago. I blended 5 large fish carcasses into liquid (wife unhappy), added sugar and about 10 lacto-baccilius (again, spelling) pills and let it sit for a month. It was a great success. Although I am not sure about the mechanics of pulverizing 50 gallons of fish maybe you wouldn't have to? Chop finely and wait longer?

-Mr. Thread D. Railer

As a child, my chore after going on the "half-day charter fishing boat trip" was to bury the inedible fish parts around all the ornamental shrubs. So, maybe direct burial is a better plan? At large scale maybe its a cut with a plow, fill trench, cover?

Already been said, but I'll emphasize. LOTS of brown material (wood chips/leaves/paper) is needed to balance the pile with meat/fish/fats.
Mixing, mixing, mixing. If you have big clumps of fish, you will have more smell than if it's evenly distributed, and if you can reduce the part size (chopping/grinding) that will help with mixing evenly.
One final thing, cover the freshly mixed pile with aged compost, as a "filter"... the smell will be less. You will have to repeat this each turning, and therefore need a supply of aged stuff to work with. Alternately, you could cover with wood chips, again, after each turning.

going back thousands of years native Americans would plant a fish along with corn squash and bean seeds. fps is one of the best natural fertilizers there is.period
you will probably want to mix those fish scraps with something like wood chips, leaves and cover with soil or sand, that will prevent flies and rodents being attracted.
from what I understand one of the main reasons that the forests in western states are so lush is thanks to the bears and other animals feeding on fish in the rivers, dragging them all about and pooping all over the place.
minerals that come from fish are not so common in nature and do wonders for building healthy soil

We just bury ours near trees and shrubs. Bury them a couple of feet deep and don't let the dogs watch. I try to dump the water I use to rinse out the bucket somewhere else to distract them.

Geoff Lawton advocates putting a carcass of some sort in the center of a compost pile for the unique biological communities that help supercharge the compost at its start.

I had an 800lb+ grizzly bear in my garden this spring.  It was happily and noisily munching dandelions and clovers that were so deep that it didn't see me and the breeze was blowing my scent on a slightly askew angle.  I was crouched a few beds over prepping a bed and the bear must have come into the garden while I was crawling about.  I got up with my pail full to dump it into the wheelbarrow with a bang and up pops this head about the width of my shoulders.  

At any rate, I don't put any carcasses in my compost.  I've got enough attractants just with my weeds!

I also suspect that this bear was responsible for digging out (and I suspect lapping up thousands of red wiggler worms) out of one of my bins a few weeks earlier.

If you intend to compost fish without a bokashi ferment intermediary, I would strongly suggest sawdust and biochar.  Fish is extremely wet when it degrades, and it is very high in nitrogen, so you need a lot of carbon and spongy material to absorb it's liquid and stench.

A bear can smell a ripe plumb tree from miles away.

I don't recommend composting fish in bear country without a properly constructed electric fence and other 'bear aware; skills and assets.

I grew up on the North Coast are of British Columbia where the smell of rotting salmon on the river edges was common.  It is... unforgettable, and... almost nauseating in its power.  Try to get it off your boots, or in the case of my youth, runnng shoes! -Good luck.

That said, I think fish composting is very worth the effort, if you can do it safely and effectively.  The body of a fish is an incredible source of nutrients.

Gray Henon wrote:We've had good luck composting mammal butchering scraps in 55 gal drums using wood chips to absorb the stink.  Gave it a try with some carp carcasses.  Holy stink!  Anyone else compost fish?  How do you do it?

I just bury fish carcasses by the bucketful directly in my garden beds, or, when opportune, bury them under new trees I'm planting, or dig a hole for where I know I want a tree to be.

My daughter burried her gold fish next to the rose bush. Full ceremony. Thinking it's not the same. 😁

However, I started thinking about adding fishtank water to my garden.

When I clean my fish tank, I add the spent filters to my compost barrel. They don't compost very well. I pull the filter out before adding the compost to my beds. But they are covered with green fish poo and other fishy good stuff. The charcoal doesn't hurt anything. The water dreggs go into beds and pots of herbs and everything seems happy for it.

So.... mini fish cycling...LOL

I have good luck soaking carcasses in water with lots of lactic acid bacteria added before composting, kind of a quick bokashi. For what it’s worth (though it’s not composting per se), there’s a great natural farming method called Fish Amino Acids where carcasses and offal are mixed with equal weight of brown or raw sugar, and inoculated with indigenous microorganisms then left to ferment for 6-12 months, mixing as needed. Leaves you with a fantastic and shelf-stable foliar feed or can be watered in, though if you’re running through tons of fish bodies the space and time might not be worth it. But for me it’s a great way to use half a dozen or so sea bass bodies, yields me a few quarts of FAA which is more than enough for my 1.5acres.

My father has a large plastic barrel with a sealable screw down lid with some water in it.

He throws all his fish guts, heads, and other parts into the barrel and puts the lid back on.

When its full, he tops it up with water, seals it, and leaves it in a shady out of the way place for maybe a year.

Eventually it breaks down into a fish emulsion that doesn't stink to high heaven, and he uses that as a liquid fertilizer on his garden. It's incredible stuff.

Gray Henon wrote:We've had good luck composting mammal butchering scraps in 55 gal drums using wood chips to absorb the stink.  Gave it a try with some carp carcasses.  Holy stink!  Anyone else compost fish?  How do you do it?

I compost my catfish carcasses directly in the garden. I am constantly expanding and rotating my garden, so I will simply bury the fish carcasses about a foot under the soil using post hole diggers. I usually have about 8 inches of mulch over these areas too. I use landscape flags to mark where I have already buried fish. This area will always remain fallow for the next season before I plant it. After skipping a season, the only thing remaining are the pectoral fin spikes, and the dorsal fin spike attached to its vertebra. Be careful because I have found them with the bottom of my feet many times. There is very little smell, unless a critter get into it.

Steve Mendez wrote:I took part in a Rainbow Trout mort composting project using a 3 bin system built from pallet wood at the College of Southern Idaho Fish Hatchery in the early 90s. Each chamber was approx. 1 cubic meter. We kept daily records of the weight of the morts added. Each day's layer of morts was covered by about 10 cm of moldy straw.
It was interesting to see how bin #1 would fill to halfway in fairly short order and then take much longer to fill all the way up from there. The pile would really start heating up once it was half way and it would just kind of collapse on itself and start to decrease in volume even as we added morts and straw. Temps of 60c to 68c (140f to 154f) in the center of the bin were quite common once the system got going.
The smell could be intense if the morts weren't covered sufficiently with straw.
By the time the material made it through bin #2 there was no evidence that it was composed of about 80% dead fish by weight.
There was always a waiting list of people who wanted the black earthy smelling compost from bin #3 once it cooled down.
The CSI Fish Hatchery still composts the morts, 25 years after the research project ended.

Fascinating - I had a dream about doing something with the river kill we have around here back this past Fall... Could you tell me more about that? When did you transfer bin 1 to bin 2, what happened in bin 2, etc.. thanks!

When bin #1 was full, bin #3 would be forked out into a small trailer. Bin #2 would then be forked into #3 and then #1 would be forked into #2. Then the whole process would start over. We kept daily records of weight of morts added and daily temps using a dial thermometer with an 18" probe. #2 was generally about 20 c cooler than # 1 and #3 would be about 20 c cooler than #2.
The front of the bins have removable horizontal slats so that the composting material can be forked straight in from one bin to the next without much heavy lifting. We started out with moldy straw that had been the growing media from a commercial mushroom farm. Early on we had some pretty nice mushroom harvests from # 2 and #3. After a couple years we switched to old bedding straw from the Vet Tech program on campus because it was much closer to the fish hatchery, the mushroom harvests ended after that.
We would often mix the #3 compost in the trailer with dried fish manure before giving it away to local gardeners. It was very popular stuff, probably still is.

Thanks! That sounds great!  I doubt I’ll ever get around to it, but it sure seems like there’s a massive amount of fish kill waste around here .... of course, nature eats it right up, but if ever I needed a source of ferts, it seems a little Korean natural farming and dead fish could do the job.  Thanks for your story.

It's been almost two weeks since I buried the carp carcasses in charcoal and capped them with mulch.  No stink!  I haven't noticed any smell at all!  What a difference compare to just mulch.  I opened one of the barrels and there was a wave of warmth that came out.  

I haven't tried to compost fish leftovers in earnest outside of an occasional piece put in the core of an existing hot pile. If I had a substantial amount and was worried about smell, I'd be tempted to trench compost it in a space where I was planning on growing some hungry growies that could take advantage of the nutrition boost.

Gray Henon wrote:We've had good luck composting mammal butchering scraps in 55 gal drums using wood chips to absorb the stink.  Gave it a try with some carp carcasses.  Holy stink!  Anyone else compost fish?  How do you do it?

If you can bury it (deep!) immediately, there is a plus. If not, hope racoon, rats etc. don't come at night to unbury it... Or cross your fingers they won't come and deal with the stench as best you can.
Odors, a putrid as can be, do not hurt. I heard that Native Americans used to bury fish under corn. I don't know how that turned out, but it should add *some* fertilizer.

Cécile Stelzer Johnson wrote: I heard that Native Americans used to bury fish under corn. I don't know how that turned out, but it should add *some* fertilizer.

I heard the Native Americans learned to do that from the European settlers. Prior to that, if soil fertility dropped, the Natives moved to a new location. Since they grew corn as a polyculture, it was less of an issue. Usually, easy firewood was getting scarce by that point also, so moving allowed trees to recover also.

Jay Angler wrote:

Cécile Stelzer Johnson wrote: I heard that Native Americans used to bury fish under corn. I don't know how that turned out, but it should add *some* fertilizer.

I heard the Native Americans learned to do that from the European settlers. Prior to that, if soil fertility dropped, the Natives moved to a new location. Since they grew corn as a polyculture, it was less of an issue. Usually, easy firewood was getting scarce by that point also, so moving allowed trees to recover also.

I was curious about that, so I asked which cultures have used fish as fertilizer, and then, immediately, I knew: to use fish as fertilizer, fish has to be abundant and close. This is what I got:
Various cultures throughout history have used fish as fertilizer to improve soil health, most notably Indigenous North Americans, who buried fish to nourish crops like corn. Other cultures include ancient Egyptians (using Nile fish), ancient Romans, and traditional Indonesian farmers (rice-fish culture).
Key Historical Uses of Fish Fertilizer:
Indigenous North American Peoples: Often cited in lore (e.g., Squanto teaching settlers), they used fish, such as menhaden, to fertilize corn, beans, and squash by burying them under mounds.
Ancient Egyptians: Utilized fish carcasses and liquids from the Nile River.
Ancient Romans: Used fish waste to fertilize crops.
Japanese (Edo Period): Used commercial fish-based fertilizers derived from by-products of oil extraction.
Indonesian Farmers: Employed "rice-fish culture," where fish in flooded rice fields acted as both pest control and fertilizer.
Modern Applications: Fish emulsion is used today, developed from techniques dating back centuries.
Fish are used as fertilizer because they are rich in nitrogen, phosphorus, and potassium, providing necessary nutrients for plant growth.

I've only composted a little at a time (less than a kilo), mixed with the usual vegetable peels, coffee grounds and chicken litter. Both cooked scraps (bones, some skin) and raw heads, guts etc seem to disappear within a couple of days if the temperature is ok (except really big bones, vertebrae and collar bones ? from bought cod). It doesn't appear to change the smell of the compost at all. The chicken litter has a lot of wood shavings in it, and if the stuff from the kitchen is very moist I add some paper and dry leaves when they are available. This is in a insulated bin, not a pile though - based on previous experience, the household doesn't produce enough material for a pile to heat up noticeably.

I have also buried fish parts and whole fish (excessively bony bycatch, for example) in the garden. Whether by luck or not, this has never resulted in foxes or badgers digging, unlike chicken guts...

It seems that burying fish scraps or non-food fish for fertilizer has been traditional in Norway too, apparently especially for planting potatoes - although this may be because that was virtually the only thing grown in areas where soil was poor and labour cheap. I haven't seen any real research on this though, so how widespread the practice was or how far back it goes I don't know. It is sort of a semi-common anecdote usually referring to some time between the late 1800's and the war.

When I put fish or other bones in my compost, I first mix them in with the other kitchen scraps. Then put on the compost heap and cover more, like leaves, small branches, a.a.

My compost heap is in the back yard, at a distance of about 8 meter (almost the same as 8 yards) from my back door. It's often said that fish and meat scraps in the compost attract rats, but I never saw a rat in my garden.
I did have some mice in my apartment, but I think they came from under te floor. After I stuffed the holes (from heating pipes) with steel-wool I never saw the mice again.
There might be mice (a different species, not the house-mouse) in the garden, but I didn't see them. Maybe they are hiding because there are many cats in the neighbourhood, some of which frequently visit my garden.

I did make 'fish emulsion' a few times. That's very smelly, made of dead fish in water in a bucket. It's a good fertilizer in the garden.

Fish decomposition is supposed to smell, so there's no getting around that. There may be things we can do to lessen the stench if we understand the process of decomposition better.
I was curious as to how long we can expect the stench and what makes it worse (or better for our garden), so I asked:
A decomposing fish typically smells bad for a few days to several weeks, depending on temperature, size, and location. In hot, humid conditions, a fish can decompose and smell for only a few days to a week, while in cooler or enclosed areas, the odor can persist for several weeks.

Key factors affecting the duration of the odor include:
Temperature and Humidity: Warmth and moisture accelerate decomposition and the release of odor-causing bacteria, shortening the overall duration of the stench, USDA (.gov).
Size: Larger fish create more organic matter to decompose, resulting in a longer-lasting, more intense odor than smaller fish.
Location: If a fish dies in an area with low airflow, the smell can linger longer.
Final Decomposition: The smell persists until the fish is entirely decomposed or dried out.  

So if the weather is hot and humid, it will decompose faster, but the stench will be pretty strong. If not, the stench will last... and last... and last!
The size and amount seems to be the factor that's easiest to influence : chop it finely and mix it with other stuff and it will decomp faster. The airflow can be influenced too. If you have a fan on, you won't suffer as long, or conversely, you can bury the fish pretty deep where you won't smell it, but if and when you turn that pile, that fish may still reminds itself to you!
Mention was made of critters visiting. That's where a bin is useful as it can prevent them from 'investigating'. A tight wire fence (1") is probably the best as it will keep critters (racoons) out while not restricting airflow too much. Buried in the garden, it can be unearthed by critters. Having cats around may prevent rats and mice, even our field mice, although they typically don't have rotting fish on their diet. But if cats to the investigating, we may not be better off.
If only the fishmeal they sell in stores wasn't so expensive!...