In this article, we are going to talk of the last ingredient of a prey model diet: fish. Don’t remember what’s a prey model diet? Here’s the article on raw feeding.
I presented all the previous elements of a raw diet in these articles: meat, organs, raw meaty bones.

Why feeding fish?

The idea behind a prey model diet is to reconstruct what a cat eating a prey would eat. A prey being composed of meat, bones, and organs.
You may ask, but so why do we feed fish? The meat that can be found in a supermarket is rarely free-range and, therefore, it lacks omega 3 in comparison to wild prey. The fish is added to compensate for this lack of omega 3 in the (not free-range) meat.
The omega 3s are important as they help making the fur of your cat healthy, soft and shiny.
Small, oily fish are the best to feed, as they have the best ratio omega 3 to heavy metals. Fish is not mandatory: if you feed pastured-raised animals, the meat will already have enough omega 3s, and you won't need to feed fish. Alternatively, some replace fish with fish oil and vitamin D.

The risks of feeding fish?

Thiaminase

Some fish contain thiaminase. Thiaminase destroys thiamine, which is vitamin B1. Feeding too much raw fish may cause a deficiency in vitamin B1, possibly resulting in neurological problems.

Heavy Metals

Because of biomagnification, big fish (such as tuna) contain significant quantities of heavy metals like mercury, which are dangerous for your cat.

Freezing / Parasites

Fish needs to be frozen before being fed, in order to kill parasites such as the anisakis. The duration of the freezing depends on your freezer temperature. The EU rule for fish intended to be eaten raw is to freeze it:

at -20°C (-4°F): for at least 24 hours,
or at -35°C (-31°F): for at least 15 hours,
or in a household freezer (3 stars freezer or -18°C (0°F)) for at least 7 days.

So, if you don’t have access to a specialized freezer, always freeze the fish you buy for at least one week.

How to feed fish?

Quantity

Not everyone agrees on how much fish to feed:
Perfectly Rawsome: They recommend 1 oz per 35lbs of weight, corresponding to 28 grams for 16kg. So approximately 5-6 grams for a 3-4kg cat. Which is 5% of their diet.
Raw Feeding - Prey Model: They recommend the equivalent of one daily ration of fish every week, which correspond to 14-15% of fish daily.
Barf Asso : They recommend the equivalent of two daily rations of fish every week, which correspond to 28% of fish daily.
Raw feeding advice and support: They recommend 15% fish.
Cat Complety Raw and Proud: They recommend maximum 100g per kg, which is 10% of the diet.

Personally, I would recommend feeding 10% of small oily fish.

Which fish you can feed?

It is better to feed small fish, as they contain less heavy metals, and can be eaten whole. It is important to note that one common name for a fish can refer to several fish species. Some fish in the species may contain thiaminase, while some others may not.
Here is a list of fish you can feed and some indication on the presences of thiaminase for some of the species. For many of them, I couldn’t find any information on whether they contain thiaminase or not.

List fish you can feed:
Anchovies
Californian anchovy (Engraulis mordax) Thiaminase
European anchovy (Engraulis encrasicolus)
Japanese anchovy (Engraulis japonicus)
Peruvian anchovy (Engraulis ringens)

Mackerel
Atlantic mackerel (Scomber scombrus) No Thiaminase
Chub mackerel (Scomber japonicus) Thiaminase
Chilean jack mackerel (Trachurus murphyi)

Herring
Atlantic herring (Clupea harrengus) Thiaminase
Araucanian herring (Clupea bentincki)
Pacific herring (Clupea pallasii)

Sardines
European pilchard (Sardine pilchardus) Thiaminase
Californian pilchard (Sardinops caeruleus)
Japanese pilchard (Sardinops melanostictus)

Rainbow trouts
Rainbow trout (Oncorhynchus mykiss) No Thiaminase

Sprats
European Sprat (Sprattus sprattus) No Thiaminase
Black and Caspian Sea sprat (Clupeonella cultriventris)

Salmon
Atlantic Salmon (Salmo salar) No Thiaminase
Coho salmon (Oncorhynchus kisutch) No Thiaminase
Pink salmon (Oncorhynchus gorbuscha)
Chum salmon (Oncorhynchus keta)

Joel
Big-scale sand smelt (Atherina boyeri) No Thiaminase

Smelt
Pond smelt (Hypomesus olidus) No Thiaminase

How to serve the fish?

Little fish like the sardines, sprats, and anchovies can be fed to your cats whole. For bigger fish, such as rainbow trouts or mackerel you can cut them like in the picture below. As long as the fish is raw, the fishbones are not a problem.
For really big fish such as salmon, give only filets, as the fishbones are too big for a cat.

Variety

Like for the meat, it is best to vary the fish fed to your cat. In fact, every fish will have a different nutrients profile, especially in Omega 3 content. Moreover, it is important to vary between fish having thiaminase and fish not containing thiaminase, to avoid any thiamine deficiency.
Néline eats sardines, mackerels, herrings, and anchovies.

To conclude, fish provides omega 3s that may be lacking in a prey model diet based on not free-range meat. The daily ration should be composed of 10% small oily fish. In a future article, I will resume the different elements and their quantity in the prey model diet.

Sources

Freezing time European Legislation
Thiaminase in fish: Source 1, Source 2

You can find the content of this article in video (with English subtitles):

Fish in a raw diet