When considering mortality levels in salmon farming, it should be remembered that Atlantic salmon have a very different reproductive strategy to the mammals or birds in agriculture. A sheep might produce twin lambs once a year for perhaps eight years. A chicken could produce 250 eggs per year, but only one per day. A female salmon, however, can produce up to 10,000 eggs at one time, over 1,000 eggs per kilo of her bodyweight.
The strategy is designed to cope with the very high mortality rate at all stages of the natural lifecycle of the fish, as evidenced by the very low percentage of wild salmon which naturally survive from egg to adulthood. Scottish Government data from the survey traps on River Dee tributaries (on Scotland's east coast) show that, even of the fish which survive from hatching to going to sea, the number returning to breed is below 2%.
As submitted to the Scottish Parliament REC committee in November 2020 as evidence, the annual average survival rate achieved for post-smolt farmed Scottish salmon is 85.5%, or 17 out of every 20 farmed salmon (14.5% mortality).
Did you know? The Scottish salmon farming sector is the only UK farming sector to publish monthly stock mortality rates.
This figure is calculated using the monthly sector wide and openly published figures which detail the rate of survival of growing salmon at the marine stage. Over the course of a year the average monthly survival rate was 98.7%, resulting in a loss of 1.3% each month. This equates to a compounded 14.5% annual mortality.
Annual losses is a much more accurate (and comparable) figure than the losses from production which makes a broad assumption on when smolts are introduced to the marine environment and also when fish are harvested. While there is a broad output target weight for many fish at harvest, different markets require different specifications of fish and that therefore impacts how long fish will be in the water for.
Where data is available for other species – cattle, sheep, pigs etc. – the annual losses can be slightly higher for Atlantic salmon but not too dissimilar compared to livestock raised in their natural environments (hill sheep for example).