Join us for the next part of the Salmon’s Tale! In this blogging series by DCRT trustee Chris Firth, follow Salmo the salmon smolt as he travels from the River Don’s headwaters to the low, meandering reaches of the river. Can you guess which species he meets along the wayfrom the descriptions (answers at the bottom)?
Salmo had reached Rotherham and was about to descend the weir in the centre of the town. He had become used to these walls across the river and normally he found them only a minor problem. He had also noticed that an increasing number of other smolts were building up at these locations and their presence was beginning to attract a number of large black birds which chased the smolts. As he descended further this was to become an increasing risk.
Below the weir the habitat varied little from that above but after a few miles things began to change. The water began flowing very slowly and became increasingly deep. In this new environment an increasing number of new fish species began to appear. Dominating the population were large shoals of silvery fish with vivid orange fins. These fish varied in size and seemed mainly concentrated close to the bottom. There were also smaller shoals of a brightly coloured fish with a dorsal fin that they raised like a sail when they became agitated. They were olive green in colour with darker bars extending down their bodies but it was their bright red fins which were the most striking.
Eventually he reached the weir at Thrybergh, this was by far the biggest walled structure that he had encountered and presented a more daunting prospect to descend. Around him were several hundred other smolts all milling around and their presence had attracted the attention of at least half a dozen of the large black predatory birds. The birds were amazingly agile swimmers and despite the agility of the smolts many were being captured. In his panic Salmo, followed many of his fellows who overcame their reluctance and dropped over the weir into the pool below.
Here the habitat changed once again and for several miles the river became a series of pools and shallows with a strong steady flow. The fish population resembled that that he had encountered between Rotherham and Sheffield although there were a few additions. The most common of these were large shoals of a mottled brown fish which, in shape resembled the barbel Salmo had met earlier in his journey. They had the same whiskers around their mouths but were all small like Salmo.
He had now reached the confluence with the River Dearne and again the conditions were changing. The flow reduced and the depth increased dramatically. He also noticed that it was becoming increasingly hard to breathe and the water was discoloured. Again, new species were appearing with one in particular very abundant. They were large deep bodied fish, brown or black in colour and were present in large shoals which seemed engaged in sucking up and blowing out the sediment from the river bottom. As Salmo watched them through the murky water, there was a sudden, deafening noise, reverberating and pounding through the river. The fish around him scattered towards the edges as a huge black, floating object came churning up the river stirring up the bottom and creating huge turbulence. For some time after the object passed it was almost impossible to see where he was and he became extremely disorientated. It took almost an hour for conditions to improve and for the shoals of fish to return back to their normal feeding activities.
Despite still feeling sickened and a little scared, Salmo continued his downstream journey. He reached another large weir at Sprotbrough but resting here was most uncomfortable. As he struggled to breathe he began to feel listless, and reluctant to continue but, eventually, passed over the weir into the pool below. Thank goodness! He instantly felt the benefit of the oxygenated water tumbling over the structure. Resting close to a tree which had fallen into the river, a huge torpedo like fish with a mouth filled with evil looking teeth rushed towards him. The fish was green in colour, almost three feet in length and missed grabbing him by no more than a few inches. Terrified he shot off as quickly as he could.
Many of the fish he had seen previously were even more abundant in this section, but there was one different to anything he had ever seen before. Long, thin and snake-like it wriggled along the bottom. The other fish ignored him as it buried into a dense weed bed alongside the bank.
Salmo was nearing the final obstacle on his downstream journey. He had reached Doncaster, the upstream limit of the rivers tidal influence. The final obstacle he had to contend with was a rock ramp fish pass made up of large rocks laid along a descending channel. This was much easier to deal with than the usual weirs and reminded him of the cascading water of the upper river where he once lived.
He had made it! All he had to do now was follow the rivers tidal flow down into the Humber Estuary. There would be hazards before he reached the sea, but his prospects were much improved. With good fortune, in two years’ time, he would be returning as a magnificent silver salmon which would retrace its steps back to the very same moorland section of river where he was born.
The shoal of silver fish with orange fins were roach, the predominant species in the River Don. These fish can grow up to 2.5kls but rarely exceed half this weight in the Don. They can form huge shoals and for this reason are popular with anglers.
The fish with the bright red fins and striped backs were Perch. This species can be extremely predatory feeding on the fry of other species. They have a large dorsal fin with sharp spines at the tips which they erect when threatened.
The large deep bodied brown/black fish were bream. This species has become increasingly common in the lower reaches of the river and form shoals which can sometimes reach several hundred fish.
The small mottled brown fish which resembled young barbel were gudgeon. These fish rarely grow to more than a few grams in weight but form huge shoals. They have a preference for gravelly sections of river bed where there is a steady flow.
The large torpedo shaped fish was a Pike. These vicious predators dominate the rivers food chain and can grow up to 20kg in weight. They feed predominantly on fish but will not hesitate to take young ducklings if the opportunity arises. They prefer to wait for prey near reed beds where they dart out to intercept their victims.
The large black floating object that Salmo encountered was a commercial barge carrying fuel oil. Commercial operations on the Don system have reduced dramatically over the last 40 years and now only one vessel still operates. When fully laden the keel of this craft almost touches the bed of the river. Fish populations adapt to passing vessels by seeking shelter close to the banks as they pass, however the craft’s movement causes large amounts of sediment to be disturbed.
During periods of very low flow the slower moving sections of river upstream of Sprotborough can suffer periods of low dissolved oxygen. This is due to a build up of nutrients which promote algal blooms. These minute plants take in oxygen during the night, depleting levels to the point where fish can be affected. Most coarse fish species tolerate these conditions, but salmon and trout require higher oxygen levels and can struggle. This was the situation Salmo experienced passing through this section.