Composting tips, advice and science.

Freshly caught fish in a bucket.

How to Compost Fish: No Smell, 3 Simple Steps

Fish guts and remains can present a challenge for the amateur composter. 

It’s something I’ve become well aware of, as when I am not gardening or composting I am often fishing.

Fishing on the estuary close to my house.

It’s a great way to provide protein to go with the veg from the garden – and it’s far less damaging to the environment than farming or trawling for fish. 

It’s true that fish waste can be more difficult to compost than other materials due its rapid decomposition and strong odor. But with the right approach, you can turn this nutrient-rich material into an excellent fertilizer for your garden—without the smell or pests.

Why Compost Fish?

Avoid odor while waiting for rubbish collections

If you like eating fish, you’ll know just how quickly it starts to smell!

The smell can quickly affect your kitchen – and when placed outside it can attract rats. 

I find this a big problem, as I usually go fishing on the weekend – but my local rubbish collection is on a Friday.

Composting fish, if done correctly, can eliminate odor and related pest problems that brings. 

Improve the soil without buying expensive amendments

Like any other organic material, fish will decompose into compost, helping to benefit the soil. In fact, if you’re a keen gardener there’s a chance you already use fish as part of the popular fertiliser fish, blood and bone. 

Studies into fish as a fertiliser are limited but positive, although fish waste or manure is often mixed with other substances before being used. 

For example, research by López-Mosquera et al showed that fish, when composted with seaweed, has a high NPK ratio, while another study by Radziemska et al found that fish waste was “non-phytotoxic, mature, stable, and suitable for use in agriculture.”

Challenges of composting fish

The challenge of composting fish is down to the smell. More than most compost materials, fish deteriorates rapidly, creating an awful smell in the process. 

Like animal products, it’s also very effective at attracting pests. I’ve tried the internet tip of burying a fish head under a young tomato plant, only to find a hole where the tomato plant was the next day. 

Burying smellier items in hot compost can usually do the job. However, I initially had some failures here, which led to a foul odor in my garden. 

How to compost fish safely

After much experimentation, I’ve arrived at a simple three-step process that removes or eliminates odor. 

Step 1: Mix Remains With Sawdust

First, remove the unwanted fish parts, then mix them with sawdust. The key reason for adding sawdust is to minimise odor, but it has the added benefit of adding carbon to balance out the high nitrogen in the fish remains. 

Step 2: Add The Fish To The Compost

Bury the fish remains in the compost. Aim for at least 6 inches deep, but deeper if possible. 

Step 3: Cover The Compost With Sawdust

Cover with another thick layer of sawdust, which will help trap any remaining odors. I prefer to use my enclosed compost bin for this process, as it has a smaller top surface area than my pallet bins. 

As an experiment, I tried covering fish remains with sawdust in a bucket. It still smelled foul the next day, so digging it in is very important. 

In addition, it’s important to follow good composting guidelines to ensure your compost will quickly break down the fish. You can find out how to do that in our guide to hot composting

What else can you do with fish remains?

If composting is not an option, you can instead dig fish remains directly into the soil. 

The two things to do here to minimise the risk of smell are:

  1. Dig it deep to reduce the chances of odor making it to the surface and attracting vermin.
  2. If you can’t dig it deep, try covering the soil where it has been with a metal grid to prevent larger animals from tunnelling down.  

As I use a No-Dig approach to gardening, though, my preference is to compost fish guts if my compost bin is hot enough. 

What if my compost is not ready yet?

If your compost is not currently hot, you can always freeze the fish remains until your compost heap is ready. 

How to use fish blood in the garden

Guts and remains are not the only by-product of fish – you’ll sometimes get a lot of blood when gutting the fish. Blood is a high nitrogen soil amendment and is often sold in a dried form, but you don’t have to dry it first.

Instead, simply dilute the blood with water and apply to the soil. Don’t forget to dilute the blood, as its high nitrogen levels could damage plants. 

I often use it to feed hungry plants like leeks and courgettes. In contrast to fish remains, I haven’t found this attracts pests. 

Perhaps not surprisingly, there are few studies into the use of fish blood as a fertilizer, but it’s worth noting that blood itself has been highly valued as a fertiliser for millennia

Be cautious with tumblers

Tumblers are sometimes recommended for composting fish. 

Based on my own experience with tumblers, I wouldn’t personally experiment with this unless you had a completely waterproof, odor-proof tumbler.

Many cheaper tumblers (such as the one I have!) are not waterproof and are therefore completely unsuitable for composting fish in a rainy climate. 

With these devices, anaerobic composting conditions would likely set in and the fish would start sending off foul smells. 

If you do want to experiment, try using sawdust or another dry, high carbon material to absorb odors and provide a source of carbon.

Conclusion

Composting fish is not for the beginner, and it’s important to first master the basics of composting. If you’re not quite there yet, it’s simpler just to dig the fish (deep!) into your soil. 

However, if your compost heap or bin is hot, effective fish composting is a great way to rapidly break down a waste material into something that will help your soil and your plants for months to follow.

Why not give it a try?

FAQs

Can you compost all parts of the fish, including bones and skin?

Yes, all parts of the fish, including bones, skin, and guts, can be composted. Bones may take longer to decompose – but I’ve never found fish bones left over when I’ve turn the compost over. 

How deep should I bury fish in my compost to avoid attracting pests?

Aim to bury fish remains at least 6 inches deep in the compost pile, but ideally deeper. This helps mask the smell and deters pests like rats and raccoons.

What should I do if my compost bin isn’t hot enough?

If your compost isn’t hot, freeze the fish remains until the bin is ready, or consider burying the remains directly in the soil, ensuring they are well-covered to avoid odors and pests.

Can fish compost be used on all plants?

Fish compost is ideal for plants that like high nitrogen soils. However, avoid using it on young seedlings, as the high nitrogen content may be too strong for delicate roots.

How do I prevent fish compost from smelling?

Mix fish remains with high-carbon materials like sawdust or shredded leaves and bury them deep in the compost. Always maintain a proper balance of carbon and nitrogen to reduce odors. 

 How long does it take for fish to decompose in compost?

Answer: The decomposition time depends on your composting method. In a well-maintained hot compost system, fish remains can break down in a few short weeks.  

What is the best carbon material to use with fish remains?

Sawdust works particularly well, but you can also use shredded newspaper, straw, or dry leaves to balance the high nitrogen content in fish.

10. Can I compost seafood like shrimp shells or crab carcasses the same way as fish?

Yes, shrimp shells, crab carcasses, and other seafood can be composted similarly to fish. Ensure harder materials such as crab shells are broken into smaller pieces and mixed with high-carbon materials to reduce odor.

Does fish compost attract pests or insects?

When properly buried and covered with sawdust or other carbon-rich materials, fish compost should not attract pests. Keeping the compost bin sealed will further prevent issues.

What can I do with fish compost once it’s ready?

Once fully decomposed, fish compost can be used to enrich soil in flower beds, vegetable gardens, and around fruit trees. It’s especially beneficial for nitrogen-hungry plants like tomatoes and corn.

Sources

Callis, S: Another Short History of Composting International Compost Awareness Week 

López-Mosquera, M. E., Fernández-Lema, E., Villares, R., Corral, R., Alonso, B., & Blanco, C. (2011). Composting fish waste and seaweed to produce a fertilizer for use in organic agriculture. Procedia Environmental Sciences, 9, 113-117. https://doi.org/10.1016/j.proenv.2011.11.018

Radziemska, M., Vaverková, M.D., Adamcová, D. et al. Valorization of Fish Waste Compost as a Fertilizer for Agricultural Use. Waste Biomass Valor 10, 2537–2545 (2019). https://doi.org/10.1007/s12649-018-0288-8

Ahuja, I., Dauksas, E., Remme, J. F., Richardsen, R., & Løes, A.-K. (2020). Fish and fish waste-based fertilizers in organic farming – With status in Norway: A review. Waste Management, 115, 95–112. https://doi.org/10.1016/j.wasman.2020.07.025

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