Emily Collins – Conservation Student Intern
Salmon and Trout Lifecycle
When I first arrived at the National Park and learnt about how Freshwater Pearl Mussels in the River Esk depend on salmon and trout (both salmonids) in the early stages of life, I got a little confused. I thought that the mussels remained on the gills when the fish went out to sea and couldn’t understand how the mussels would survive this transition from freshwater to saltwater. It was only when I read about the salmonid life cycle that I realised I was wrong.
Salmon and trout hatch in the spring as alevins which remain in the gravel and feed off their attached yolk sacs. Once the sac has disappeared they start foraging for food around the hatch-site and are known as fry. Only when they leave that part of the river to defend their own territories are they known as parr – these can be identified by the fingerprint markings on their side.
Parr defend the same area of river for two to five years and it’s at this stage in their lifecycle that they interact with the mussels by hosting them in their gills till the mussels drop off. Once mature, the parr then undergo a process called smolting when they turn a silvery colour, their fins darken and the parr markings disappear. They also undergo changes internally which allow them to adapt to survival in saltwater. Importantly they manage a process called osmoregulation which involves excreting excess salt and guarding against loss of water to maintain a healthy ion balance.
Hormonal changes guide the fish downstream and out to sea where they feed on sand eels, krill, herring and crustaceans. At this point in their lifecycle they grow very rapidly and build up layers of fat in order to store energy for the journey back up the Esk. After one to four years, between April and November, they return to freshwater and take on the challenging counter-current swim to their natal site where they themselves were born.
This will be the final journey for the majority of the females. All of their energy is invested in producing eggs and reaching the spawning site so that by the time they have dug a hole and released eggs for the male to fertilise, they have very little energy left to make it back to the sea. In fact, just 5-10% of Atlantic Salmon survive their first spawning.
Against all the odds fish do spawn, however we want to shorten the odds and influence the probability.