“If the burn was capable of holding that many fish then, what has changed that means that it can’t physically hold that many fish now? Yet another example of where the hatchery would be useful to bring a burn back to its previous levels of productivity? I’m afraid I don’t buy this tale of salmon / sea trout parr densities being sufficient – I’m convinced there must have been a whole heap more back in the day…..” Henry Taylor 2013.
Above is a quote from Henry’s comment on the recent Chabet Burn post on this blog. I responded at the time that I would provide a full reply once I’d gathered my thoughts.
First a few details about the Chabet Water (to use its proper name). The length from Glenconglass Farm to the Avon confluence is 6200m, with an estimated mean width of 3.5m the wetted area is 21,700m2, most of which is high quality habitat. We have no information on fish densities other than a paper record from 1991 when lots of trout and a few salmon were found in a quick presence/absence survey upstream of the culvert. We don’t know the upper limit of salmon penetration but experience elsewhere in the catchment shows that below 3m width trout tend to dominate over salmon. Nearby and similar quality burns such as the Blye/Lochy/Conglass support high densities of juvenile salmonids (based on our current and relatively recent standards), with salmon dominating in the lower reaches and trout in the upper.
I want to look at a few issues here, namely:
1) smolt production
2) marine survival and number of returning adults
3) number of spawners and smolt production
4) current juvenile densities
5) hatchery supplementation
1) What is the potential smolt output from the Chabet Water? We know from our smolt trapping in tthe Truim and Tromie, and research on other rivers that smolt production in highland Scotland is typically around 5/100m2, with fluctuation from year to year and from river to river. The Truim and Tromie produced about 7 salmon smolts/100m2 in 2012, a good figure but I’d suggest that a burn like the Chabet Water would be potentially capable of producing more (assumption based on the habitat quality, the presence of limestone in the catchment and fish densities from adjacent burns). For the sake of this illustration I’m going to assume that the smolt output from the Chabet is 9.2/100m2, only because that would result in a nice round smolt output of 2000 from this small burn. That seems a lot but combining salmon and sea trout smolts it is not unrealistic. Larger streams such as the Truim are dominated by salmon but a more even mix of salmon and trout smolts could be expected from a smaller burn such as the Chabet.
2) If we assume for a minute that all the smolts were salmon then at 30% marine survival the number of adults returning to the river would be 600. Back in the 1940s that number would have been hit hard by netting and angling, but there may still have been 3-400 adults returning to the burn, a number that could have produced the 100 foul hooked fish in a spate noted by the bailiff at the time.
In recent years the marine survival of salmon has generally been lower, ranging from 20-4% between 1981 and 2009. At 5% marine survival and the same smolt production we would only have 100 adult returning to the river. Current in river exploitation is very low so we might expect about 80-150 adults to return to the burn across the range of recent marine survival figures. We counted 35 redds in the Rothes Burn last December, a much smaller burn, so I wouldn’t be surprised to count 40-80 redds in the Chabet Water (job for December 2013).
Marine survival of sea trout is harder to track down but figures I’d seen in the past showed survival to be as high as 25% for those returning as finnock but consideraby lower for fish staying at sea for longer.
So if smolt production had remained stable then marine survival alone could explain the apparent big reduction in numbers of returning spawning fish. Of course the bailiffs report from 1941 may have been from an exceptionally good year?
3) Henry’s argument, and the same argument is made by many others, is that parr densities, and subsequent smolt production are nowhere near as high as they used to be, or potentially could be. So what actually happens when numbers of returning adults decline? Monitoring on the Girnock Burn, River Dee, provides one of the best long term data sets relevant to this question. In four decades from the mid 60s to the mid noughties the number of returning adults to the Girnock Burn decreased by 70%, from a mean of 84 down to 25. Over the same period the smolt output decreased by 35%, a much smaller decline, although still highly significant in fishery terms (figures from a paper by Gurney et al. published in 2008). As juvenile salmonids populations are controlled by density dependent factors the declines in numbers of spawning fish can be compensated for by improved survival rates of the remaining eggs/fry, and in the opposite case increases in adult numbers doesn’t necessarily mean more parr . Hopefully the figures from the Girnock above show that in actual fact parr densities are much more stable than many perceive them to be.
4) I spoke at length with a ghillie the other day including this subject and he stated that juvenile densities must have been 4-5 times higher in the past than they are now. I have looked at the long term trends (1997 to 2011) in parr densities from our own electrofishing surveys and with the proviso that the data weren’t collected for this specific purpose, nor has any statistical analysis been carried out, there does appears to have been a decline in mean salmon parr densities but it is in the order of 20%. That is a significant decline but it is not a collapse in the stock and we still have a well distributed and healthy salmon population throughout the entire accessible area of the Spey catchment. All the information and experience I can access indicates that that maximum parr densities nowadays are little different from those that would have existed at the peak of the salmon population. However, it is likely that more of the catchment now supports a lower parr density than existed when salmon runs were more abundant but that is not the same as saying parr densities must have been much higher.
5) It seems so obvous doesn’t it? Catches are going down, juvenile densities don’t look that good so lets put some hatchery reared on top of the wild ones. However, lots of recent research shows a negative impact of stocked fish on existing native stock with a further, more significant, negative impact on breeding success when they return to breed. Some of the papers I’ve read on this subject were on steelheads in the US but their ecology is similiar to our salmon. If the findings of those papers can be transferred to our situation we run the risk of making the situation worse by stocking. The ultimate scenario would be a downward spiral in fish numbers with an eventually total reliance on stocked fish. Hatcheries have a role in fisheries management but I’d argue that trying to enhance a reasonably healthy and robust fish population in an accessible part of the catchment is not an appropriate use.
One key question is what drives smolt output, the number of juveniles or the number of spawners? That depends on where you are on the stock recruitment curve:
With adult numbers to the right of the red line juvenile numbers are at maximum and more adults won’t result in increased production. Adult numbers to the left of the red line means there are not enough to fish to fully populate the available habitat and lack of juveniles will limit smolt production. Understanding where you are on this curve is important but difficult to ascertain. Knowing input and output figures would help but large flucuations in fry and parr densities can indicate that numbers of spawning fish are not consistently to the right of the red line. My guess is that with the early running fish we are likely to be around the red line, later running fish numbers may be better but we still have years when they are likely to be close to the red line. This is why in these days of relatively low marine survival, in a historical context, conservation of the stock is so essential.
the Chabet Water is potentially a very productive burn
marine survival alone explains most of the decline in adult numbers
its a myth that maximum parr densities were formerly much higher
salmon have a good strategy to cope with changes in population numbers
the fishery board employs a biologist to help formulate a coherent management policy on amongst other things, the hatchery operation
we need to really focus on understanding, managing and conserving the stock during a period of relatively low marine survival
We will survey the Chabet Water this summer and I’m sure that as in other burns in the area we will find a high density of juvenile fish. There will be a mixture of salmon and trout but then again I think the use of the term “of the salmon kind” in the 1940s bailiffs report hints that there were sea trout involved in the foul hooking incident.
To finish on a positive note the excellent smolt numbers from the Truim and Tromie last year suggest that these rivers are functioning well as “smolt factories”, maybe not at maximum output but producing very respectable smolt numbers nevertheless.