The continued decline of 1sw grilse and can we do anything in response?

It cannot have escaped anyone’s attention that summer and autumn grilse numbers have been in steep decline across the whole of Scotland for several years now. The West Coast rivers, being primarily grilse fisheries, are being particularly badly affected.  There will be a range of reasons for the decline – many most likely the result of global climate change (perhaps cyclical, perhaps not) – but one of these is likely to be the  rapidly expanding pelagic fish stock throughout the whole of the salmon’s 2000 mile journey in the North East Atlantic. It would appear that mackerel in particular have mushroomed in number and spread over the last 5 years or so and present a very stark competitive and predation risk to small surface-swimming salmon smolts in their first year at sea. It would also seem that if the smolts can survive this dense proliferation of predatory competition then the pelagic fish themselves become their prey – hence possibly why we see the early MSW salmon in such great condition when they return from the sea, albeit the fewer numbers that survive the competition in their first year at sea.

At the moment the pelagic predation and competition threat is a hypothesis (see link below for the summary paper) but we are compelled by the seeming evidence at ‘river level’ and we are working closely with its author (the leading marine biologist and pelagic/salmon expert Jens Christian Holst) to try and facilitate international cooperation to facilitate further research into this field. As a key part of Dr Holst’s hypothesis suggests that the relevant authorities are using outdated models to assess the pelagic stock and set the fishing quotas for these species, the end goal of the research (should the hypothesis be shown unequivocally to be based on fact) will be to attain much much larger quotas for fishing vessels to harvest mackerel in the NE Atlantic. This may also include a ‘safe passage’ corridor along the known path of migrating salmon smolts on the ocean conveyor belt on the continental shelf edge west of Hebrides all the way up to the feeding grounds in the Norwegian Sea. The threat of salmon by-catch by pelagic trawlers remains very real but it seems likely that the ecosystem-sized problem of mackerel competition probably dwarfs the potential numbers involved in anthropomorphic by-catch (the AST are currently undertaking a project using eDNA to try and put a number on the by-catch issue which should report at the end of the year).

http://www.pelagisk.net/media/fm/9By4XvjHKl.pdf

Meanwhile there will be a host of other factors, both in freshwater and the sea, that will be impacting salmon numbers. We are closely involved with the Atlantic Salmon Trust’s new ‘Likely Suspects’ project that will be attempting to put some numbers on these factors both in a local and national context. These figures will hopefully then inform where action and resources can be most effectively delivered. In the case of the Lochy such factors will inevitably include hydro dams, winter floods, bird and seal predation, forestry and fish farms. All of these areas are being addressed on a constant basis through close stakeholder dialogue and practical action, but it will help to perhaps be able to put some of these risks into an overall relative context and where we need to concentrate our efforts.

Meanwhile on the ground we are making sure we have a very clear picture of how the freshwater phase of the Lochy salmon’s life cycle is performing – A very detailed habitat survey of all spawning and juvenile habitat in the entire catchment, including all the main stems of the Lochy/Spean/Roy and Arkaig and all their tributaries, gives us a very clear idea of how many spawning salmon is takes to fill each area of the catchment. Obviously with these drastic falling numbers of returning grilse some of these spawning targets are not now being met. Generally speaking the higher catchment is performing better than lower down in the system, which is what one would expect when the bulk of the run appears in the spring and early summer. An annual electro fishing survey allows us to put numbers on this and shows us where the main gaps are in the catchment.

Using these data we are able to respond using our hatchery and reseed barren areas of juvenile habitat with summer fry. Furthermore, due to the extensive facilities and skill base at the hatchery, we are also able to safeguard the future juvenile populations by growing indigenous captive broodstock in the hatchery for each area of the catchment and then releasing eyed ova from these broodstocks into redds in the gravel each winter (in streams where no redds were counted the previous winter).

There is little doubt though that the recent declines seen in returning salmon and grilse to the Lochy lies primarily in their marine phase and we see the job of organisations such as the Atlantic Salmon Trust as critical to the future of the species. Meanwhile all we can do at a local level is maximise the healthy output of the river (which is essentially just a ‘smolt factory’) through habitat improvement and protection, controlled adult exploitation and targeted restocking to allow each river and tributary to reach its  juvenile carrying potential.

 

Jon Gibb, River Lochy Association. 12th August 2018.