Seismic change: One of the often-quoted facts about wild salmon is that during the 1980’s twenty to twenty five percent of migrating salmon smolts returned to Scottish rivers to breed. Now the figure is less than five percent. Writing for the British Ecological Society, Dr Stuart Middlemas, an ecologist with Marine Scotland Science, suggests that a number of studies have implicated climate change as being behind recent changes in growth and survival of salmon at sea but he also says that the underlying mechanisms are unclear. He says that changes in the distribution of predators or prey could be one example as to why salmon are failing to return. Such a complex relationship may be one reason why there has been little attempt to better understand what is happening at sea. It is also why the wild fish sector continue to focus on aquaculture as it is right on their doorstep. The wild fish sector always says that only they understand wild salmon and therefore only they can arrive at a solution. The continued declines might suggest otherwise. Perhaps a fresh perspective on the problem may be all that is required
The fall in numbers of returning salmon over the last forty years led me to pose a question to the scientists at the Freshwater Fisheries Laboratory at Pitlochry. Prior to the 1980s were there even more salmon returning to Scottish rivers or had the numbers been around 20-25% over many years? The answer was that they didn’t know, but they suspected that the decline was a recent occurrence. Whilst it is unlikely that one single factor is to blame for the declines, could there be something that has had a greater impact than, say, changes to food supply. Skinny grilse were highlighted at one time as an example of disrupted food supply, but there has been little mention of them since.
I therefore wondered whether something else that had never been considered before had brought about change around the early 1980s that might have had a wider and longer-term impact on wild salmon. As it happens there was one big change around then that might have had some impact on the migration route of wild salmon. That was the advent of North Sea oil and gas.
Like most people, I never gave oil and gas a thought in relation to wild fish that is until one of my correspondents drew my attention to an MMO course that is regularly held in Inverness. At first, I thought MMO referred to the Marine Management Organisation, but it actually referred to a course to become a Marine Mammal Observer. This is a job I knew nothing about but MMO’s main role is to work on geological survey ships to provide warnings of the presence of whales, dolphins and seals in the sea around the survey area. These warnings ensure that any impact on these mammals is minimal. The need for more and more people to undertake this role is because of the way that the surveys are increasingly being conducted, Since the 1970’s the majority of surveying for the oil and gas industry is undertaken with a device called a seismic airgun.
Seismic airguns are charged with high pressure air at around 2000 psi and then discharged to create a sound wave that penetrates deep into the seabed. It also spreads across the surrounding sea. In some cases, the noise can be detected up to 2,500 miles away. Depending on the type of survey being undertaken, the airgun is towed behind the survey vessel for months a time, surveying up and down predetermined lines covering large areas. Airguns are typically discharged every ten seconds, however, in most cases, one airgun is insufficient to conduct effective surveys so the tow may be made up of an array of up to sixty airguns discharging together or in groups. The noise impact can be extremely significant.
Even though new oil fields are generally not coming online, the survey work continues day in day out for most of the year to monitor underground reserves, but the peak period is usually late spring and summer when several surveys can be running at the same time across vast areas. The combined effect must be extremely significant.
There has been quite a lot of work conducted on the impact of seismic surveying on marine mammals and this is why survey vessels are now obliged to carry MMO’s to warn of the presence of such whales and dolphins. Under certain circumstances. the MMO can stop the survey work progressing if marine mammals have entered the survey area. In addition, the survey work cannot restart until the animals have moved away. Every encounter is recorded and sent for analysis to ensure that surveying has the least possible impact on these marine mammals.
Of course, it is not just marine mammals that are impacted by seismic airguns. Some studies have looked at the impact on fish and invertebrates. In the case of fish, seismic surveys have been found to damage the ears of fish at distances from around 500 metres to several kilometres. In one trial, no recovery was detected in the fish’s hearing two months after exposure. In addition, to actual damage, the behaviour of marine fish also was observed to change depending on species and the level of exposure. Such changes included swimming at deeper depths, milling around in compact schools, ‘freezing’ or becoming more active.
In areas where commercial fisheries operate, the catch has fallen by 40-80% and decreased abundance has been observed in cod, haddock, herring and rockfish stocks.
In addition to seismic airgun, other survey work involves electromagnetic surveys using electric and magnetic fields to map deposits under the seabed.
Both seismic airguns and electromagnetic surveys have significant impacts on a variety of animals. I have looked to see whether any research has been conducted on potential impacts on migratory fish such as salmon. So far, I have found only hints that there could be a problem, but it seems that this is because no-one has ever looked at the possibility of such impacts. One reason is because everyone is too busy blaming salmon farming to even consider what is happening at sea. Surveying for oil and gas does not even feature on the list of threats to wild salmon being considered by government.
I suspect that repeated and significant undersea noise or electromagnetic surveying could disrupt salmon’s migration. It is possible that the fish are so impacted that they lose all their bearings and consequently fail to return to freshwater at all? Surveys differ all the time which might explain why the decline has been so protracted. One year, there may be little impact, the next, the opposite. After forty years of North Sea oil, it maybe only now that the combined effects of this long-term survey work is becoming apparent to those working and fishing in Scottish rivers.
What is most interesting about these surveys is that concern for marine mammals led to the requirement for MMO’s to be on board survey vessels during the surveys. Perhaps, similar concern for migratory fish might lead to a ban on surveying when fish are migrating back to Scottish rivers to breed.
To hear an example of the firing of a seismic airgun, please click on play below. This audio represents just one firing of a seismic airgun whereas multiple airguns are repeatedly fired every ten seconds, sometimes for months at a time. The clip is just sixteen seconds long. Just imagine what the impact would be on a migratory salmon!
Additional comment: I have been investigating the possible impacts of seismic airguns on wild salmon for some months. Although this commentary was written during the week, BBC News website coincidentally reported on February 28th about a joint study from Ireland and Canada about the impacts these airguns are having on whales. The rules as to mitigation of the impacts of these guns varies from country to country and not everywhere may use MMO’s on board as they do in the UK. The study found a huge decrease in whale sightings whilst the surveys were in progress. Could this be further indication about the possible impacts on wild salmon during their migration.