Redd spotting

This article first appeared in the WTT Autumn 2018 newsletter, but as it is spawning time again we thought it was worth repeating! 

WTT’s Conservation Officer in the north, Gareth Pedley, outlines what to look for when trying to spot trout (and salmon) redds

In the UK, trout spawning generally occurs between October and January, triggered by shortening day length and decreasing temperature. The exact timing varies slightly by geographical location, as incubation takes longer in colder water and it is vital that the fry hatch out in time for the better feeding that Spring offers. There’s also a genetic component to when, and where, trout will spawn.

A trout will seek out areas of raised gravel and accelerating flow, towards the tail of a pool (or sometimes riffle), with gravel of 10 – 40mm (20 – 60+mm for larger trout/​sea trout) in which to cut her nest, technically called a redd. Areas of upwelling flow are ideal, but not vital; the main requirement is simply that flows will naturally keep the redd area free from finer material (silt etc) to allow a good flow-through of water. 

A hen fish will often test the flow and riverbed with her fins to identify suitable areas. If the location is suitable, she will cut a redd, turning on her side and using her tail to dig a pot in the river bed, 15 – 30cm deep (Fig. 1). 

Hen fish on redd
Figure 1. Hen sea trout on her side, cutting a redd. A cock fish is waiting with her, keen for an opportunity to spawn; he will actively defend that position, although other fish may also try to sneak in and fertilise the eggs as they are released (photo: Peter Henriksson).

The hen then lays her eggs into that pot, over which the cock fish will release his milt (sperm); she’ll move upstream slightly, and again use her tail to cover the eggs with gravel. This whole affair takes anything from hours up to a few days, with a single hen fish often laying in several redds. 

The freshly-created redds in Figure 2 are easily identifiable, as lighter areas free from algae and fine sediment. Older redds are less obvious as the algae and sediment quickly re-accumulates. That is when recognising the characteristic, clam shell-like shape left in the river bed becomes important (Fig. 3). 

The redd in Figure 3 may end up so high and dry that the eggs laid within it will die through dehydration, frost or a lack of water flow-through to oxygenate them. These sub-standard sites may be used when prime spots are in short supply.

Sea trout redds 1
Figures 2 a & b. There are so many freshly-cut sea trout redds in this example that it is useful to first look at the original photo (above) then the annotated photo (below): at least ten redds are easily identifiable.
Redd spotting 2
Small trout redd
Figure 3 A redd created by a smaller trout. Note how finer material is carried furthest downstream and much of the larger material remains at the upstream end (right hand of shot), creating silt-free gravels and a flow-through of water to oxygenate the eggs.

This as an abbreviated version of a paper for one of our redd identification workshops. If you are interested in further information or even attending such a workshop, please contact your local WTT Conservation Officer.