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It is a common misconception that grayling have the same requirements of trout.  Grayling have specific habitat requirements for different life-stages, these habitat requirements can be divided into three categories:


Grayling zones

  • Dead zone; marginal areas with the slowest flow rate
  • Transition zone; between the dead zone and main channel with an intermediate flow rate
  • Main channel; generally the middle of the stream with the fastest flow rate


Habitat Requirements


The geographic variables of a river such geology, altitude, gradient, topography and climate dictate it's suitability to support grayling, irrespective of in-stream habitat availability. Huet (1949) described rivers according to the type of physical, chemical and biological qualities associated with suitable habitat condition for different adult fish species.


The grayling zone is characterised by rivers and larger, more rapidly flowing streams. The gradient is usually less than that of trout water with riffles and rapids being separated by pools. Trout and grayling inhabit the more rapid stretches whereas the lentic waters in between are occupied by cyprinids such as barbel and chub. The gradient in the grayling zone is typically 5.7 m km-1.



Great Ouse habitatGrayling prefer a good sequence of pool, riffle glide and run and prefer spawning substrate with a diameter between 2-8 cm, such as coarse gravel. The in-stream distribution of grayling varies between day and night (diurnal variation). As juveniles develop, they can withstand stronger currents and will migrate during the day from the dead zone to the main channel. Once the grayling have attained 60 mm or more in length, they make this migration from the marginal habitats.


At night all size classes of grayling can be observed in the dead zone. This change in habitat is characterised by a shift in depth from deep to shallow water at night for all size classes. As previously mentioned, local habitat use is limited by the ability of the grayling to hold station against velocity of the water.

Chemical and Temperature Requirements


Generally, the water quality requirements of grayling are similar to brown trout, cool, well oxygenated waters. The optimum pH of water is around pH 7, with a range of pH 6.5 - 9.0. Grayling can survive at pH 5 but are more susceptible to illness. Grayling have oxygen requirements of 7 mg l -1 at temperature 18-20 oC, below 4 mg l -1 oxygen at temperatures >25 oC all grayling die. Larval grayling can tolerate between oxygen levels as low as 1.4 mg l -1 at 8 oC and 1.8 mg l -1 at 20 oC. Successful embryonic and larval development can occur when interstitial oxygen concentrations are at 10 mg l -1. Temperature: the lower critical limit of grayling is 4 oC and the upper limit is between 18 - 25 oC, a temperature of 26.6 oC being the upper lethal limit.

Susceptibility to Pollution


STW outfallAlthough the environmental requirements of grayling are broader than those for brown trout, grayling succumb to pollution and higher temperatures quicker than trout. This is because the organ of detoxification, the liver, is smaller in grayling as a percentage of the total body weight, than that of trout and coarse fish. Consequently, grayling are far less tolerant of high levels of organic pollutants than trout.


Pollutants which can be lethal to grayling include organochlorines, organomercury, acid rain resulting in acidification, PCBs, DDEs, high levels of suspended solids, free sulphides and high organic inputs resulting in high BOD levels.

Flow Requirements


Measuring flow @ SheffieldThe timing of low and high flows events are critical to ensure that flow conditions are appropriate for each of the grayling's life stage. Timing and frequency of high flows (Freshets) can be important for initiating certain behaviour in Salmonids, specifically the spawning migrations. Flow regulation and compensation release schemes rarely consider the impacts on the life history of grayling, and as with habitat management, there is sometimes a belief that flow management for trout or salmon will also be appropriate for grayling. This may not always be the case particularly given their difference in spawning season and habits. Trout or salmon are usually the target organisms for setting flow levels even though grayling may be the dominate species.



GS Symposium Speakers

GRT Funded MSc, PhD Studies

Two degree project interim reports were among presentations at the most recent Grayling Society symposium. Both studies are currently funded by the GRT and both operate on a premise that alongside the intrinsic value of grayling as a game fish, their survival challenges provide early indication of problems that are or will likely become problems for other salmonids.


Stephen Gregory (Game and Wildlife Conservation Trust) described an MSc study plan for statistical mining of the existing Wylye Study data, questioning the effect of extreme climate events on grayling population dynamics. He emphasized that the GWCT now leads all processing aspects of the 30-year Wylye Grayling Study (WGS) dataset - the longest and most complete in Europe...possibly in the world.


Vanessa Huml's (Manchester Metropolitan University) PhD study is titled Assessing adaptive genetic variation for effective management and conservation of European grayling. Read her description of planned work, noting reference to new sequencing technology and reference to the four U.K. genetically distinct groups identified in the earlier genetic census funded by the GRT.


The two studies both look at grayling population health/stability under extant environmental conditions but the doctoral work extends inquiry to genetic proclivity for survival ('evolvability').


Both investigators will submit detailed results for publication here after review in their respective peer literature.