Why do we sometimes need to freeze fish before smoking?

[Wade]

When we are planning to cold smoke fish it is important to be aware that wild caught fish may contain parasites (e.g. Anisakis) that can cause illness in humans if ingested.

All commercial fish and fishery products are inspected at the fishery and any visible parasites removed before the fish is sold, and if the fish is sufficiently cooked before eating any parasites present will be killed. Research carried out by the Food Standards Agency (FSA) showed there is a negligible risk of parasites from farmed salmon.
Freezing is an additional way to protect the health of consumers as the freezing process will kill any parasites that may remain undetected.

Currently farmed Salmon, Atlantic halibut and rainbow trout are officially exempt from the requirement to freeze for parasite control, however the FSA recognise that other species of farmed fish also present a negligible risk and may be excluded in future guidelines.

A link to the current FSA requirements is at the bottom of the post however here are the important points and some additional explanations:

• Freezing certain fish and cephalopod molluscs is only required if they are to be eaten raw or lightly cooked. Fish that is brought to a temperature of 60 C or above for at least 1 minute is considered safe.
• Most farmed fish that are raised from embryo and fed on parasite free food are considered to be parasite free and do not need freezing - this includes commercially farmed salmon. This does not necessarily apply though to all fish caught from inland stocked fisheries.
• The freezing time required to kill any parasites is dependent on the temperature of the freezer
  It requires:
  - 15 hours at -35 C 
  - at least 24 hours at -20 C (most commercial freezers)
  - at least 4 weeks at -18 (most home freezers of *** or **** rating)
  - Fridge top freezing compartments or freezers of * or **  rating are insufficient as they do not reach sufficiently low temperatures.

When relying on freezing to control potential parasites in fish it is important to independently check the actual temperature in your freezer (not just rely on the thermostat setting). If your freezer has a "fast freeze" setting then this is a usually good to use as "fast freeze" will take most freezers down to ~-21 C. It is important though to check this before relying on it to achieve the necessary freezing temperature. 

FSA Freezing requirements for fishery products intended to be eaten raw or lightly cooked

[Guest homer]

if your worried about the parasites when smoking salmon..either ignore it and carry on as normal freezing the fish or look it up on youtube..sadly i done the latter last year and have not touched salmon smoked or cooked ever since?

[NicoWilson]

I had understood that there was an additional advantage to freezing food before smoking as the action of freezing the water in the food breaks down cell walls which makes it easier for the cure to "pull" the water out.

[Wade]

Often freezing is just something that gets done before curing because people have stored meat/fish in the freezer for a time before they decide to process it. Generally with meat and fish the act of freezing and thawing will result in an ~3% water loss due to cell damage caused by the freezing process. Whilst the freezing and thawing does release water it also breaks down the cell walls and ruptures the cells, effectively making the extracellular space larger and giving a greater space into which the salt from the cure will diffuse. This can result in a saltier tasting end product.
With unfrozen meats/fish it is osmosis that draws the water out of the cell causing the cells to dehydrate. Salt will still be able to diffuse into the flesh but as the extracellular space will be smaller less salt is taken up.

There will also be a slight degradation in texture caused by the cell damage of freezing and thawing. I use both fresh and frozen salmon when I cure and I certainly notice a difference in the texture of fresh vs pre-frozen salmon. Once the salmon has been fully cured and smoked though the final products are indistinguishable. As a general rule, the berst traditional smoked salmon is produced from the freshest fish.