Fish definitely presents a challenge for the keen composter.
It’s something I’ve become well aware of, as when I am not gardening or composting I am often out fishing, either on the salt marshes by my house or on the small boat I have a half share in.
I personally find fishing a great way to provide food to go along with the vegetables from my garden – at the same time avoiding the harms caused by nets used by commercial fishermen and the damage caused by the use of antibiotics in fish farmed on the sea.
However, I typically fish on a weekend, which causes a problem, as our rubbish is picked up on a Friday. The choice is either leave fish guts tied up in a bag for a week where it will stink like few other foods can, or compost it.
The idea of composting fish is attractive because fish is generally considered good for the soil*. However, it also has an incredible ability to attract pests. In fact, if you have ever tried the old tip of burying a fish head under a young tomato plant, you may also have experienced the despondency of seeing a hole where your tomato plant used to be.
Burying smellier items in hot compost can usually do the job. However, when I previously tried this with fish guts, it ended up stinking the whole garden out.
Now I think I have cracked it. Here’s what I did with the guts from a large fish a couple of weeks ago:
- Placed the fish guts or fish in a container and cover it with sawdust.
- Buried it in hot compost inside an enclosed compost bin.
- Cover with another thick layer of sawdust.
While I think the heat is important in breaking down the fish quickly, the sawdust does seem to be the key – in fact I am currently trialing some (accidentally!) defrosted bait fish in a bucket of sawdust to see if sawdust alone helps stop that awful smell.
Be cautious with tumblers
A quick search, while I wrote this, showed that some other sources recommend using compost tumblers.
While I haven’t tried composting fish yet, I personally wouldn’t try this unless I had a completely waterproof tumbler.
The one I have is not waterproof, despite being highly rated, and would be completely unsuitable for composting fish in a rainy climate.
It’s highly likely that with my (poor) tumbler, anaerobic composting conditions would set in and the fish would start sending off foul smells.
*Fish has been used for centuries to improve the soil. It’s also considered beneficial by many gardeners.
Studies seem to back this up, 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.”
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