It's pretty obvious from the graphic below that fish farm diseases spread in the water from farm to farm. In inside waters, sounds and inlets, only one end of the channel is open. This makes tidal flow slower, in most cases, but even at speeds as low as 2- to 4-knots, that means over a tide change, which are roughly every six hours, that disease viruses/bacteria, are spread 12- to 24-nautical miles up to four times a day. This means 48 nautical miles per day in total because there are, typically, two floods and two ebb tides.
Little wonder that in the confined spaces of inside waters, that both upstream and downstream fish farms spread diseases/lice to one another rapidly.
To show that 2- to 4-knots is very conservative, as well as 48 nautical miles (54 miles or 87.1 km), in BC, Sechelt Narrows has currents exceeding 15 knots. Think how far a fish farm disease could be spread in areas with high tidal speeds.
Now, look at this graph from Norway:
It is from this press item: https://www.undercurrentnews.com/2018/03/08/study-predicts-spread-of-salmon-disease-in-norway/?utm_source=Undercurrent+News+Alerts&utm_campaign=e0855079a9-Europe_briefing_Mar_08_2018&utm_medium=email&utm_term=0_feb55e2e23-e0855079a9-92426209. From the contacts in it, you can find the paper, if you wish.
Here is what was done: "Norwegian University of Science and Technology (NTNU) researchers
monitored ocean currents in the Romsdalsfjord and in Nord-Trondelag
county in 2014. Data from the observations were then transferred to a
digital model, where researchers depicted virtual virus particles in
order to understand how the disease can spread from one fish farm to
Currents in Seymour Narrows, just south of the Broughton Archipelago, can reach 14 knots. Broughton, itself, is not as speedy, but not far from Seymour.
What else did the researchers in Norway say? This: "“If the ocean currents flow in a direction that results in one fish farm
spreading the virus to others, one possibility may be to kill the fish.
Farmers could also stop using multiple fish farms at the same time to
create small firebreaks in a production area," said Lars Gansel, an
associate professor at NTNU, noting research indicates firebreaks --
large designated coastal areas without fish farms such as Buholmsråsa in
Sor-Trondelag -- do work."
So, let's kill the Broughton Archipelago farmed fish, or phase them out to create a 'firebreak' zone for Fraser River salmon.