A10. Redd counting

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Atlantic salmon Salmo salar redd and individual – Allier river @ LOGRAMI

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1. Objectives

Redds are nests made in gravel, consisting of a depression dug by a fish for egg deposition (and then filled) and associated gravel mounds (biologyonline.com).

Although some sedentary species’ redds can be visible, the spawning ground counting method is mainly used for anadromous species which spawn in the upstream part of the river basin. This includes Atlantic salmon (Cowx and Fraser, 2003), lamprey, trout and Sea trout (Harvey and Cowx, 2003). The monitoring objectives include:

  • Counting the number of redds in an area.
  • Deducing the rough number of genitors in the area (with bias).
  • Comparing the effective number of redds to the theorical number, related to the surface of suitable habitat.
  • Monitor the movement of migratory populations.
  • Identify inter-annual variation.

Shad have different spawning behaviour compared to other migratory fish. This will be discussed separately.

2. Method summary

Atlantic salmon Salmo salar redd from bank – Allier river @ LOGRAMI

Redd counting consists of surveying a river reach by walking along the banks or by observing the river using a kayak, or other floating equipment. This allows surveyors to get close to the spawning area and make an accurate count of the number of nests. Aerial surveying using a helicopter is another method, which can cover larger areas that may be inaccessible relatively quickly. This can also be achieved using drones which have the advantage of being cheaper, however, their use should comply with licencing requirements. The survey area is decided upon according to the hydromorphology of the river and the target species. Some river features are recorded, including the closest bank, size of the redd, water depth in the redd and water velocity.

Salmonid redds are characterised by their oval shape and bright appearance compared to that of the surrounding substrate which is duller. They are made by the female who moves the sediment downstream with her tail. The gravels and rocks exposed by this process are clear of algae and biofilm and appear as an brighter spot on the riverbed, compared to undisturbed areas. On the riverbed, the redd makes a depression upstream and a flare pile of gravel and rocks slightly downstream, which protects the eggs laid beneath. The salmon redd is positioned between the slow flowing part of the river (pool) and riffle, or only in the riffle section. Salmon redd sizes vary between 0.8 m2 and 5 m², depending on fish size and water velocity (Bach et al., 2011). It is almost impossible to distinguish Atlantic salmon redd from Sea trout redd.

Sea Lamprey (Petromyzon marinus) redds usually occur in shallow streams with currents >40 cm/s (Bruslé and Quiniard, 2001). Coarse gravel and cobbles are overturned and disturbed by the male to make a semi-circular depression 50 – 60 cm deep and 1 – 2 m in diameter. It also makes a clear stain in the riverbed when newly dug. The spawning period and the redd shape do not correspond with any other migratory fish, so misidentification with other species is rare. Spawning time across the UK can be highly variable (March to mid-July each year) (Harvey and Cowx, 2003), so any field survey must be preceded by local research.

River lamprey Lampetra fluviatilis redds are harder to see because of their size (20 – 40 cm in diameter), but are still visible, especially when a lot of fish spawn on the same area of riverbed. The nests are characterised (at least in the River Derwent) by clean gravel and sand caused by overturned and disturbed material. This contrasts with biofilm and epilithic algae outside of the redd (Jang and Lucas, 2005).

Allis Shad Alosa alosa and Twaite shad Alosa fallax do not make redds but lay their eggs after performing a particular nocturnal behaviour called ‘bubbles’. This is audible to humans and can be recorded by receivers. A lot of Alosa can reproduce simultaneously, producing a lot of bubbles. So far no standardized method has been used to deduce the genitor number by only observing bubbles, but work is in progress. This method can only provide data on the presence/absence of genitor.  

3. Advantages

  • Can provide valuable information on spawning ground distribution and make links with environmental variables.
  • Estimate the genitor density and number (not for all mentioned species).
  • Alosa alosa and Alosa fallax ‘bubble’ behaviour can be recorded by in-situ recorder.

4. Disadvantages

Atlantic salmon Salmo salar redd from bank – Allier river @ LOGRAMI

4.1 Salmonids

  • It may be difficult to find good weather conditions for counting during the spawning period of salmon and sea trout.
  • Potential bias as some redds can be abandoned before use or used by several females (Taggart et al., 2001). The number of redds may not provide accurate estimates of the number of adults or juvenile recruitment (i.e. birth rate).
  • Bias when Salmon spawning grounds are shared with Sea trout.

4.2 Lampreys

  • Lampetra fluviatilis redds can be very dense and males can father multiple redds (1 female for 6 males). Without marked recapture study, the number and density of genitors is impossible to evaluate.
  • Unlike other species of anadromous fish, river and sea lamprey do not exhibit complete fidelity to their natal river (Bergstedt & Seelye, 1995; Waldman et al., 2008; Swink & Johnson, 2014 in Hansen et al., 2016).
  • Due to inter-annual variations in run size and lack of fidelity for some species, assessments should be done over several years to establish the degree of variation in annual run size.

4.3 Allis Shad

  • Reproduction behaviour can be missed easily.
  • No redd to count.

5. Recommendation for method application

Atlantic salmon Salmo salar redd, from helicopter – Allier river @ LOGRAMI

* Define the area to survey by selecting the suitable habitat in advance. This depends on the species and its spawning ground requirements, e.g. hydrology and water temperature both important factors for Allis shad spawning.

  • Favour good observation conditions during the sampling (low water level, turbidity, sun etc.)
  • Use polarized sunglasses.
  • Do not estimate the number of genitors in tricky situations (mix between Atlantic salmon and sea trout) or for River lamprey.

6. Costs

Depends on the material used (e.g. kayak). 2 or 3 people are required, especially for health and safety requirements. Costs can be very high for a helicopter survey.

7. References

  1. Bach, J. M. et al. (2011) Rapport d’activité 2010 : Recueil de données biologiques Poissons Migrateurs de la Loire, LOGRAMI.
  2. Bruslé, J. and Quignard, J. P. (2001). Biologie des poissons d'eau douce européens. Lavoisier.
  3. Cowx, I. G. and Fraser, D. (2003) ‘Monitoring the Atlantic Salmon: Salmo Salar’, Conserving Natura 2000 Rivers Monitoring Series, (7), p. 35. Available at: http://publications.naturalengland.org.uk/publication/113031.
  4. Hansen, M. J. et al. (2016) ‘Population ecology of the sea lamprey (Petromyzon marinus) as an invasive species in the Laurentian Great Lakes and an imperiled species in Europe’, Reviews in Fish Biology and Fisheries. Springer International Publishing, 26(3), pp. 509–535. doi: 10.1007/s11160-016-9440-3.
  5. Harvey, J. and Cowx, I. (2003) ‘Monitoring the river, brook and sea lamprey, Lampetra fluviatilis, L. planeriand Petromyzon marinus’, Conserving Natura 2000 Rivers Ecology Series, 5(5), p. 35.
  6. Jang, M. H. and Lucas, M. C. (2005) ‘Reproductive ecology of the river lamprey’, Journal of Fish Biology, 66(2), pp. 499–512. doi: 10.1111/j.0022-1112.2005.00618.x.
  7. Taggart, J. B. et al. (2001) ‘Spawning success in Atlantic salmon (Salmo salar L.): A long-term DNA profiling-based study conducted in a natural stream’, Molecular Ecology, 10(4), pp. 1047–1060. doi: 10.1046/j.1365-294X.2001.01254.x.