Biology

When out fishing it is always beneficial to understand and know something about the physiology and habits of our prey. The sea trout is an anadromous fish meaning that it migrates up rivers from the sea to breed in fresh water. Other fish that have this ability are the Salmon, Thwaite Shad and Flounder. During this period the sea trout ceases to feed. They become solely reliant on the fats stored in their bodies. If they enter freshwater a long time before spawning then they can reabsorb their scales and use the energy stored within.

Once in the river system they travel until they reach a point where they decide to rest prior to spawning. At this time if they have only been in the river a short time then the angler has the best chance of catching them. Once they have been in the river three weeks to a month they progressively become harder to catch. Often a fish will only strike at the lure because it ‘reminds’ them of food they were feeding on in the sea. This is where using the sunk lure that resembles a sand eel can often prove very effective. When the temperature drops (typically November or December) to a level for them to spawn they will then move up river to areas of gravel and shingle. Here they pair up and the female begins to cut a redd. The female moves onto her side and using her tail forces water down onto the gravel causing it to lift and be carried downstream. This creates a hollow in the gravel that the male and female use to lay their eggs. The male moves alongside the female and vibrates his body stimulating the female to release eggs. As she does the male releases sperm (milt) into the water and fertilisation takes place. The eggs are carried into the gravel by the flow of water. The female then covers the eggs over with gravel by using the same technique as to cut her redd.

Some eggs will be lost due to the flow of the river or by the action of small fish removing the eggs before the female has a chance of covering the eggs.The eggs are left alone to fend for themselves whilst the adults return to sea. Development occurs over the next one to three months and are dependent upon water temperature and food availability. The hatchling is now known as an Alevin and has a yolk sac that contains enough food and energy for approximately one month. Once this sac is exhausted the alevin is then termed a fry and begins to seek food. As the fry grow it develops camouflage to hide it from predators. The fish when it reaches 4 inches in length is now termed a parr and is hard to distinguish between a brown trout and a sea trout parr. Its diet is now small insects, nymphs etc.

During this time it grows and becomes a smolt after approximately three years in the river. Smoltation occurs when the scales become silvery and the fish undergoes physiological changes to its gill structure to prepare it for the journey to sea. The fish will make use of early summer floods to help with its transit downstream. Once at sea the fish feed rapidly and its body weight builds. Once the fish has reached a pound or more in weight it may re-enter the river system becoming known as ‘Peel’, ‘Herling’ or ‘Finnock’. Some fish do not return until they have spent 3 or more years at sea returning as the large sea trout of two to twenty pounds in weight. The fish does not always spawn each time it enters the river.