Solving Scottish Salmon's Multi-Million Mortality Problem

Calling all mathematicians and number geniuses – Scottish Salmon Watch is inviting challengers for the inaugural 'Farmed Salmon Medal' (a fishy version of the Fields Medal).

Please help solve a problem which has vexed the public since 2012 (when numbers of dead salmon stopped being published after lobbying from the salmon farming industry). Last night's STV's evening news (30 September) reported on the "row" over mortality figures:

Pythagoras, Einstein, Tao and Matt Damon in the film 'Good Will Hunting' would all struggle to decipher the conundrum that is Scottish salmon's mortality problem.

Here's the seemingly simple question: how many farmed salmon die each year on Scottish salmon farms (including in the hatchery)?

Guesstimates of numbers of dead fish each year on Scotland's sea cage salmon farms range from 15-20 million (as calculated by Salmon & Trout Conservation Scotland – and reported in The Times in 2017).

To "up to 10 million" admitted by the Scottish Salmon Producers Organisation (as reported by The Sunday Herald in 2017):

During 2019, mass mortalities have reached such a critical mass that a lorry-load of diseased salmon spilled onto the A86 last month.

When calculating numbers of mortalities there's a number of factors muddying the waters:

1 – Numbers of dead farmed salmon are NOT reported after lobbying from the Scottish salmon farming industry deemed publication "commercially damaging"

As Rob Edwards reported in The Sunday Herald in 2013:

2 – Dead farmed salmon is reported by weight not number

In the absence of mortality numbers the data we have publicly available is solely weight of mortalities which is published via the 'Scotland's Aquaculture' web-site. We currently have data for the first six months of 2019 – including site specific details for all active salmon farms in Scotland:

Scotland's Aquaculture allows users to import mortality data into an Excel spreadsheet (CSV Export function) – giving the following data for mortality by weight:

2019 (up to the end of June): 8,821 tonnes

2018: 16,710 tonnes

2017: 25,736 tonnes

2016: 22,472 tonnes

2015: 18, 607 tonnes

2014: 16,167 tonnes

2013: 10,521 tonnes

For for first six months of 2019, here's the worst salmon farms in terms of monthly mortalities (by weight) as reported by Scotland's Aquaculture:

Mortality problems at The Scottish Salmon Company, Mowi and Grieg Seafood correlate with reported disease and welfare problems via the Scottish Government's Fish Health Inspectorate:

3 – The Scottish Government reports annual 'survival' in the sea not mortality (which in the absence of mortality data we can use as a surrogate)

Here's the survival data presented in the Scottish Fish Farm Production Survey 2018 published by the Scottish Government yesterday (30 September 2019):

The Scottish Government explained via a Freedom of Information reply in November 2018 that calculating the mortality was more complicated than simply using survival data.

4 – The last year where we have published data for mortality numbers is 2012 when 8.5 million farmed salmon died in sea cages

The Sunday Herald reported in 2013:

At least the above data gives us basic equations which you don't need to be Einstein to understand:

13,627 tonnes of dead salmon = 8.5 million fish

9,717 tonnes of dead salmon = 6.8 million fish

7,159 tonnes of dead salmon = 5.5 million fish

5 – The mortality rate in the hatchery and during the whole life-cycle for farmed salmon is unclear but must be over 40% (and potentially as high as 50%)

Here's data presented in the latest Scottish Fish Farm Production Survey 2018 published yesterday – including the information that between 63 and 70 million ova (salmon eggs) are laid down each year:

The published data also shows that between 38 million and 48 million smolts are put to sea each year:

Keeping things simple, if 70 million ova were laid down and only 38 million were put to sea then that's 32 million dead fish in the freshwater phase of production alone (i.e. a mortality rate in the hatchery of 45%)!

Mortality data published via the Scottish Government's Fish Health Inspectorate includes nearly 2 million dead salmon in five mass mortalities at hatcheries during 2019 including over 1.5 million at Mowi's Lochailort Recirculation Hatchery in March 2019 and another 89,000 in February 2019:

In October 2018, The Sunday Mail reported that 500,000 farmed salmon had died at Mowi's flagship hatchery at Inchmore – opened only a few months earlier by Scotland's Cabinet Secretary for Rural Economy, Fergus Ewing.

Read more online here

6 – The Scottish Government report 'Mortality Events' but these are only exceptional mass mortalities

As Scottish Salmon Watch reported last month:

Damning new data on mass mortalities and infectious diseases on Scotland's salmon farms was slipped out late last week (20 September) by the Scottish Government's Fish Health Inspectorate. Pancreas Disease, Cardiomyopathy Syndrome and Amoebic Gill Disease were cited as the biggest killers along with mortalities caused by mechanical and freshwater treatments (expect even more problems over the coming months as Mowi – Scotland's largest salmon farmer decimated by disease problems – predicts a "painful September and October for the salmon industry in Scotland").

Note that the mortality data published by the Scottish Government is NOT a comprehensive list and total numbers of dead farmed salmon have not been published since 2012 when the Scottish Salmon Producers Organisation successfully lobbied against disclosure arguing that publication would be commercially damaging.

7 – Around 70% of mortalities occur in the second six months of the year

As Scottish Salmon Watch warned last October:

Corin Smith knows only too well the bad smell coming from Scottish salmon farming – in 2018 he tracked mort trucks delivering tonnes of diseased salmon to a mort pit in North Uist. Watch the stomach-churning footage on the BBC One Show in September 2018 online here

In conclusion, here's Scottish Salmon Watch's back of a beer mat calculation for the number of mortalities on salmon farms in Scotland.

We know that 8,821 tonnes of mortalities in the sea were reported in the first six months of 2019 so we could expect another 20,000 tonnes of mortalities in the second six months (assuming that 30% of the mortalties occur in the first six months – and given Mowi's dire prediction of a "painful" September and October).

That could bring a total of ca. 29,000 tonnes of dead salmon in 2019 which would be a record (beating the 25,736 tonnes in 2017).

That represents thousands of truck loads but what does 29,000 tonnes of dead salmon equate with in terms of numbers?

So if we go back to the 2011 data when we know that 9,717 tonnes of dead salmon equated with 6.8 million fish then 29,000 tonnes would be ca. 20 million dead salmon in the sea phase of production.

Then you have to add ca. 30 million mortalities in the hatchery which gives ca. 50 million mortalities in Scottish salmon farming (from hatch to catch/cradle to grave) which represents a mortality rate of ca. 70%.

Can you do any better? Answers on the back of a beer mat please by the end of the week (and no breaking into the offices of the Scottish Salmon Producers Organisation or you will be disqualified for cheating).

The most comprehensive answer – and please provide all your working – will win a crate of Fish Tale Organic Amber Ale.