South East Rivers Trust (& the Wandle Trust)

Spawning Season for Salmonids

by Dr Chris Gardner


What are Salmonids?

In the UK, the salmonids we have are brown / sea trout [Salmo Trutta] and Atlantic salmon [Salmo salar]. Salmonid fishes are gravel spawners, depositing their eggs in redds ‘cut’ by spawning fish in the gravel bed of our rivers. This blog is all about their spawning behaviour and what can affect the success of spawning in our river network. 


What is a redd?

A redd is a ‘nest’ where fish bury their eggs to incubate and hatch with fry emerging in early spring. A redd consists of a hole (or pot) dug in the riverbed. Spawning fish achieve this by displacing gravel, while releasing their eggs. This “spawning action” results in the eggs being buried in the pile of gravel created. Eggs are held within the gravel for a number of weeks. In a pristine river environment, the gravel offers great protection while they develop. However in degraded rivers and catchments, eggs are vulnerable to suffocation by silt and sediment which is deposited between gravel particles from erosion upstream.

Figure 1. Salmon redd on the River Wylye. The deep hole or pot excavated by the spawning fish is clearly visible with the pile of gravel housing the incubating eggs immediately downstream at the ‘tail’ of the redd. This redd is classically positioned at the head of a riffle where intra-gravel water flow will be optimum.


Redds can be counted to give an indication of the amount of spawning taking place and the spawning distribution through the river system. This is usually carried out by walkover survey where redds are identified, and their location and size recorded. Redds are most visible soon after spawning, as a clean gravel patch on the riverbed (Figure 1.). Over time redds may fade and become harder to identify as they colour with age, especially after high flow events which may distort them and deposit sediment onto the clean gravel which can obscure them.


Fish with high standards

Gravel substrate has to be of a suitable quality for the successful development of eggs and emerging fry. Spawning gravels can be impacted by fine sediments. They block the spaces between the gravel particles reducing water flow through the redd, therefore limiting the dissolved oxygen available to the developing eggs. Hence, a negative relationship exists between fine sediment infiltration during the incubation period, and salmonid egg and embryo survival. During the construction of a redd, the spawning actions of the female fish (Figures 2 & 3) causes a substantial proportion of the fine sediment to be disturbed and washed away. This creates an appropriate incubating environment that allows for sufficient oxygen, thus improving the original condition of the spawning gravels.

However rainfall events, and the associated increase of erosion and sediment inputs into the river, can dramatically impact egg survival and therefore spawning success. Very wet winters can limit salmonid recruitment. 

Figure 2. A large hen salmon ‘cutting’ a redd on the River Nadder, Hampshire Avon.


Figure 3. Spawning brown trout on the Wandle showing the creation of a redd.


When do they spawn?

Salmonid spawning takes place within the period from mid-November to the end of January with a peak of activity in mid-late December. 

Figure 4. Shows how the initiation of spawning varies between types of trout and salmon.


Where in rivers do salmonids create redds? 

Fish will spawn extensively throughout river systems, selecting loose permeable gravel, often at the head of gravel riffles. Spawning habitats used by Atlantic salmon and brown trout overlap to some extent, but broadly speaking salmon tend to spawn in the main river and larger tributaries, whereas trout prefer smaller streams..

At the reach scale, both prefer the pool-riffle transition zone. Gravel spawning salmonids are known to seek out locations of high velocity, low turbulence and uni-directional flow for redd construction. They actively create these micro-habitat structures in places where flow dynamics maximise permeability of oxygen-rich water through the gravel, such as areas of constrained current (Figure 4) and riffle heads.

Figure 4. Large salmon redd positioned off the side of a fallen tree, benefiting from the condensed low turbulence, high velocity, uni-directional flow that maximises redd permeability and offers optimal conditions for incubating eggs.

Apportioning redds to species

Direct observation of fish in-channel (or dead on the bank!) can provide evidence of the species responsible. Fish will often sit on the redd for a few hours after spawning has taken place, allowing direct observations to made. However, in the absence of such sightings, redds can be broadly allocated to the most probable species, and historic local knowledge of river catchment is invaluable in this regard. This includes information regarding timing of spawning and redd size and with experience, it is possible to differentiate between brown trout, sea trout and salmon redds (Figure 5). 

Larger fish tend to spawn in areas of greater depth and water velocity. They are more able to move substrate made up of larger particles by way of more powerful tail thrusts, so can create redds in areas of larger gravel size. They can also maintain station in higher velocity currents which is highly beneficial as these stronger currents facilitate the downstream movement of particles from the cutting site much more effectively than low velocity currents.

Redd dimensions can reflect female size, however it is important to recognise that shape will also be partly governed by water velocity (under high water velocities, redds will be longer as material is carried further downstream). Redd width is therefore a better indicator of female size, than redd length. Thus, in many cases the width of the redd excavated can give an indication of the species responsible. For example, in southern English rivers, sea trout redds will usually be <110 cm wide. This is smaller than salmon redds which are >110 cm wide, but larger than wild brown trout redds which rarely exceed 40 cm in width.

Apportioning redds in this way is fairly subjective and larger brown trout redds can overlap with sea trout and even small salmon redds. Thus apportioning redds to species purely on size should be taken as an indication of species, rather than absolute. 

Figure 5. 

a. Trout redd on the River Ash, Hertfordshire. Brown trout redds are much smaller and especially shallower than redds made by larger salmon or sea trout. (Photo: Chris Reeds, Environment Agency).

b. A large sea trout sitting on a recently constructed redd in a New Forest stream.

c. A very large salmon redd on the edge of a channelised section of the Hampshire Avon in Salisbury town centre; the gravel tail measured over four metres in length.

Spawning distribution and genetics

The amount of spawning that occurs in different tributaries can be of particular interest. Migratory salmonids are known to return to and spawn in their natal river using olfaction (smell) to navigate. According to the ‘member-vagrant’ evolutionary hypothesis, habitats with distinct biotic and physical characteristics should contain locally adapted sub-populations. Atlantic salmon from the European rivers are known to be genetically distinct from those found on the east coast of north America. Further to this, genetic analysis has demonstrated locally discrete populations of Atlantic salmon to at least the tributary level in many rivers e.g. River Teno (Finland), River Varzuga (Russia) and the River Tamar (UK). Counting the number of redds can give a strong indication of tributary performance. This tributary level data is especially useful for assessing tributary performance and their relative contributions to the overall stock, highlighting areas in need of rehabilitation.


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