One of our Twitter followers (thanks @farmsalmon4ever) called our attention to a new sea lice study published Jan. 4, 2012 about sea lice epidemics.
The study, titled “Critical thresholds in sea lice epidemics: evidence, sensitivity and subcritical estimation” was published in the Proceedings of the Royal Society’s Biological Sciences journal and was written by Neil Frazer, Alexandra Morton and Martin Krkosek.
We decided to take a read through and see what it says.
Morton and Krkosek have published many studies together on sea lice; their work is the basis for much of the criticism of salmon farms when it comes to sea lice. In particular, they have tried to show that salmon farms amplify levels of sea lice, which are naturally present in the ocean.
The problem, they have tried to show, is that farmed salmon provide a place where sea lice can survive and increase their numbers at times of the year when lice levels would naturally drop. This, they then conclude, threatens passing juvenile wild salmon which have to contend with unnaturally high levels of sea lice around salmon farms.
But, as we asked in our last post, have sea lice from salmon farms harmed wild salmon?
After more than a decade of extensive research, scientists are divided, with the only real consensus being “maybe, but we’re not seeing it,” a conclusion which was recently reiterated by scientists at the Cohen Commission (see project 5).
While farms may increase the number of sea lice in a small area, scientists see no meaningful connection between farm lice levels and actual productivity of wild salmon stocks.
So where does that leave us? Again, in the world of “maybe, but we’re not seeing anything definitive.”
Reverse-engineering epidemic data
This new paper by Frazer, Morton and Krkosek is different than many of their previous sea lice studies together because it appears to be more of an attempt to develop a tool to better understand and predict sea lice levels than an attempt to prove anything.
The paper presents a mathematical model for predicting how many farmed fish should be allowed in an area before they reach a threshold and pose a risk of sparking a sea lice epidemic.
“Two observations motivate this paper,” they write. “One is that sea lice are seldom a problem in areas with low production even when lice are present on local wild hosts. The other is that lice are seldom a problem when sea-cage aquaculture is new to an area.”
It takes several farm production cycles and increasing density of fish to spark any notable increases in sea lice, they continue.
To prove this, the authors use the examples of a sea lice epidemic in New Brunswick in 1994 and an epidemic in the Broughton Archipelago in 2001, taking the numbers from the epidemic, reverse-engineering them and calculating the threshold beyond which stocking densities in salmon farms can provoke a sea lice epidemic.
There are problems with this. The study doesn’t have very much data for the Broughton Archipelago. “Lice data during subcritical stocking in the area are not available because monitoring of lice began after the epidemic emerged. However, assuming that the critical stocking threshold was exceeded in 2000-2002, the critical band is estimable.”
That is a BIG assumption to make. The authors are assuming that because there was an epidemic of sea lice in 2001, farms must have exceeded the critical stocking threshold for the region.
Frazer, Morton and Krkosek are assuming their theory is right based on completely circular reasoning.
- They believe there is a “critical threshold” for the amount of farmed salmon which can be in an area.
- They believe that beyond that threshold there will be sea lice epidemics.
- There was a sea lice epidemic in 2001.
- Therefore salmon farms crossed that “critical threshold.”
It’s fallacious reasoning. Using that logic, we could argue that:
- We believe UFOs are real.
- We believe UFOs leave crop circles.
- There was a crop circle in the farmer’s field this morning.
- Therefore UFOs are real.
Just like the UFO believer fails to consider that crop circles may have come from something other than a UFO, this paper fails to consider that a sea lice epidemic may have been prompted by something other than salmon farms.
The study also seems designed to reach the conclusions the authors were looking for.
“For the Broughton Archipelago, the Pacific Salmon Forum (PSF) recommends limiting Broughton Archipelago farmed salmon production to less than 18.5 kilotons per year,” it says.
How convenient that is just about exactly where the authors estimate the “critical threshold” production levels for the region.
A useful tool
Before anyone thinks we are trying to dismiss this study out-of-hand, we want to point out that it actually provides quite a useful tool. It considers water temperature and salinity, which can have a huge impact on sea lice abundance, and the model could be adjusted to help farmers forecast and plan production cycles.
“If changes in temperature and salinity could be forecasted, farmers could pre-emptively harvest or treat,” the authors write. “Also, locating farms in low-saline conditions may raise threshold values and prevent epidemics.
“Thus, if good records are kept while an aquaculture industry expands in a particular region, the critical stocking level can be estimated without ever experiencing an epidemic.”
This could be very useful for salmon farmers, who now have more than a decade’s worth of sea lice data as well as salinity and temperature data and would love to be able to keep sea lice levels as low as possible.
However the authors err by assuming their model is right.
Perhaps someone could take the decade of real-world data that is out there, plug it into this mathematical model and see if it actually lines up with any observed increases in sea lice levels on wild salmon. Then we would know if it’s right or not.
Perhaps the authors should have done that themselves.