Whole in the Water
Well-Known Member
An interesting article that provides an good overview.
Link to full paper here: http://www.livingoceans.org/sites/d...- Dr. Dill Final Report - October 30 2017.pdf
The Risks of Open Net Pen Salmon Farms to Wild Pacific Salmon: Summary of Scientific Findings: A report prepared for Wild Salmon Forever by Lawrence M. Dill, PhD FRSC,
Professor Emeritus, Simon Fraser University
30 October, 2017
Executive Summary
Concern about the potentially harmful interactions between wild Pacific salmon and
farmed salmon contained in open net pens has been a longstanding issue in British
Columbia and elsewhere. Here I review recent scientific findings relevant to this debate.
My principal findings are as follows:
1. Because of the large numbers of farmed Atlantic salmon in close proximity in open
net pens (ONPs), lice, viral and other pathogen populations can grow to very large
sizes, shedding millions of infective stages (lice) or copies into the local environment
outside the farm, where they can infect wild fish. In addition, conditions inside the
farms are exactly those which evolutionary theory predicts will lead to selection for
increased pathogen virulence, i.e., an increased negative effect on its host, and there
is evidence that this has happened in aquaculture facilities. Therefore, what comes
out of ONPs can be much more dangerous to wild salmon than the pathogens that
the wild salmon may have passed to the farmed Atlantic salmon in the first place.
2. The risk to wild salmon from sea lice produced in Open Net Pens (ONPs) is
unambiguous and substantial. Lice have been shown to reduce productivity of both
wild pink and coho salmon populations in the Broughton Archipelago, and there is
no reason to think they are not having similar effects elsewhere on the BC coast.
The mechanisms by which lice impact individual survival are well understood, and
these individual and population level effects have been found consistently
throughout the world and are supported by large-scale experiments.
3. Piscine orthoreovirus (PRV) and the disease it causes (Heart and Skeletal Muscle
Inflammation or HSMI) have recently been confirmed on a BC salmon farm. The
virus has been implicated in the heavy pre-spawning mortality of Fraser River
sockeye salmon. Additionally, it has been shown that productivity of these stocks
depends in part on the number of Atlantic salmon in the ONPs that the smolts pass
on their northward migration to the open ocean. While we do not know what it is
about the farms that underlies this latter relationship, pathogen transmission remains
the most likely explanation. It is tempting to speculate that PRV may be involved
but we don’t yet know the source of the PRV with certainty.
4. A number of other viruses and disease-causing organisms (bacteria, myxozoans and
microsporideans) are known to be present in ONPs. The risk they present to wild
Pacific salmon is currently unknown, but could be substantial. There is evidence that
some can be passed to wild salmon with harmful effect, but we cannot say with
certainty that any wild salmon population has declined because of them.
5. Lice (and to an extent, viruses) have been shown to affect the vulnerability of wild
salmon to other mortality agents, including starvation and predation. Even if these
pathogens do not kill the fish directly, infected fish are likely to be rapidly removed
from the population by a predator, making the business of proving that a given
agent causes widespread wild salmon mortality and population decline a very
difficult task.
6. As a result of these indirect effects, the impact of parasites and viruses on wild
salmon depends on environmental factors such as water temperature and
competition with other species. The less benign the environment, the greater the
impact to be expected.
7. Apparently healthy fish in the ONPs may still be fighting infection and releasing
viral particles into the waters surrounding the farm, where they can infect wild fish.
Therefore the fact that only a small percentage of farmed salmon die of a given
disease greatly underestimates the risk they present to wild salmon.
8. Lice impacts on wild salmon can be mitigated by appropriate control strategies on
the farms, particularly the timing of parasiticide treatment. Although there is concern
that lice may evolve resistance to SLICE and other chemicals used to control them,
a large wild fish population may help to maintain the efficacy of SLICE and delay
the evolution of resistance, meaning that the preservation of healthy wild salmon
populations is in the salmon farmers’ self interest.
9. The evidence of risk to wild salmon is sufficient that the precautionary principle
should be invoked, and Governments should mandate and support the aquaculture
industry’s move from ONPs to land-based closed containment production systems.
