The other day I had a walk up one of the burns identified in the electrofishing report as suffering from an apparent decline in the salmon parr density. Spawning time is a critical time of year when there is a chance to try and assess the number of fish spawning. Obviously too few spawning fish will result in low parr numbers so I mapped each redd carefully using the GPS.
I started at a bridge where the banks of the burn were heavily wooded and the surounding landuse agricultural. The general habitat quality looked excellent with mixed fry and parr habitat.
There had been some recent bank reinforcement works undertaken by the farmer but that wasn’t the source of all the sand that smothered much of the burn bed. Extensive sand deposits will reduce the carrying capacity for parr due to lack of cover. The abundance of invertebrates such as mayflies and stoneflies may also be reduced if the spaces between the stones are buried under sand.
Redds were few and far between in this lower section, no surprise really as there were very few areas suitable for spawning. I had walked 1.8km before I found what I considered to be the first salmon redd. There were a few trout redds in the lower reaches, 19 in the lower 1.8km. As the woodland thinned the character of the burn changed and the first big beds of spawning gravel appeared. Doubts about whether I was looking at salmon or multiple trout redds were soon dispelled when a hen salmon appeared on one redd.
Sometimes it is difficult to say what is a salmon or sea trout redd but as I walked further the redds were clearly all salmon due to their size and depth.
I have been fortunate to have received training in redd identification and counting from the best but you hardly needed to be an expert to spot redds in this burn. However if there has been little change in the gravel colour when it has been turned over by the spawning fish then you have to study the structure of the river bed carefully to identify redds. On my first training day the term “hens eggs” was used. Mystified at first the description was perfect. The hen fish excavates the gravel to form a depression for the eggs working until only the larger stones remain, looking just like a clutch of eggs in a nest at the upstream end of the redd. She then deposits the eggs in amongst these larger stones where they nestle in securely. Her last act is to cover the eggs with more gravel. “Hens eggs” are often the sign of an incomplete redd.
This part of the burn flows through a huge flat floodplain and the gradient was just right for deposits of spawning gravel.
Further upstream there were what looked like relatively recent dumps of gravel; there were many redds in the channels flowing through this area.
Most of the burn was quite natural looking but even here it had been straighened at some point in the past. Still it had recovered to an extent and there were some big redds in this area.
I was seeing plenty redds in this part and recording them in the GPS was becoming tedious but it is worth it for the maps that can be produced. The occasional pair or single fish were present but most of the fish had spawned and left. I walked about 6.5km recording 46 salmon redds and 24 trout redds. This will be an underestimate as I was only recording 100% certain redds. Redd counts from the 90s shows that between 25 to 83 redds were recorded during three counts, although there is a note to say that the 25 redds were recorded during poor counting conditions. I’m not sure whether the 90s records were for salmon only or whether they included sea trout; must try and find that out. However given lower marine survival a decline in the redd count is not unexpected. Similar drops in total numbers of spawning fish have been recorded in other upland Scottish tributaries, e.g. the Girnock on the Dee, but studies there shows that smolt outputs remained relatively constant during the period when adult numbers declined.
Towards the top of my walk I came across a big eroding bank. Points like this will be the source of much of the sand and gravel seen in the burn. A look on Google Earth shows that there are extensive areas of erosion further upstream. On my way back to the pickup I met the farmer and had a long chat. He said that in 2007 there was a deluge and a massive quantity of gravel was washed downstream, raising the burn bed by 3′ in places. I’m pretty sure the sand deposits seen in the lower end were from these natural events, as were the abundant spawning gravels in the middle reaches. Spawning gravel is the lifeblood of a salmon river but it is often unequally distributed. Thankfully the Spey catchment seems to be well blessed with this important asset. So although the sand seen in the lower reaches may be having a negative effect the upside is the tremendous spawning gravels further upstream.
Lower parr numbers in this burn was the reason for the investigation but could my chat with the farmer have shed some light on the subject? Prior to about 2007 our monitoring sites both supported high salmon parr densities but there was a marked decline from about that time. I wasn’t there when we surveyed the burn this year so I don’t know the exact position of our electrofishing sites but I know I walked past the lower site. From what I saw there was no shortage of spawning salmon in the middle reaches of the burn and the fry from that area would be expected to populate the entire burn downstream. Maybe the recent reduction in parr numbers at our monitoring sites shows a reduction in carrying capacity due to the inundation of sand and gravel that happened in 2007, and in many subsequent spates since. The lesson for me here is that habitat is dynamic, not static, and that not all areas of the catchment will be in an optimum state suitable for maximum juvenile fish production all the time.
The farmer also mentioned that when he was a lad they would see salmon sprachling (may not be the correct spelling; I’ve never seen it written before, but a good Scots term for carrying on all over the place!) about in the upper burn at spawning time well beyond the end of my walk. I’ll have to have a look further upstream soon.