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Feeding


The availability of food (prey items) plays an important role in the growth and density of grayling populations. The available food must be suitable for all life stages of grayling in order for them to survive, irrespective of whether the river has the required environmental preferences (temperature, flow, habitat).

 

As grayling mature, the habitat they prefer changes as does their preferred food. Larvae forage pelagically and feed on drift in the upper layers of the river. Juveniles, migrate away from the river margins and into the main channel where they hold a benthic feeding station within 5 cm of the river bed and catch drifting invertebrates. Grayling remain feeding in the upper layers until they reach a length of 15 cm. At these greater sizes/ages grayling begin bottom feeding on prey such as Ceratopogonidae (biting midge larvae) and Sialis (alderfly) larvae, although they still frequently rise to the surface into intercept drifting prey.

 

The prey consumed by grayling changes with age. Larvae grayling feed mainly on chironomid larvae (non-biting midget larvae) or (dipteran (true fly) larvae) irrespective of diversity of invertebrates available. Juveniles exhibit greater diversity in the consumed prey items, although copepods, oligocaetes, chironomid larvae and pupae still constitute more than 90% of prey ingested.  Other prey items include Simulium (blackfly) and Ithytrichia lamellaris (a caddis fly) larvae.

 

Gammarus a prefered prey item of adult graylingOlder grayling (aged >1+) consumed mostly Gammarus (freshwater shrimp) in addition to prey eaten by juvenile fish (aged 0+). Generally adult grayling become predominately bottom feeding with increasing age and the contribution of aerial prey to their diet falls correspondingly.

 

Grayling may take terrestrial prey when aquatic prey is scarce, fish eggs may be consumed opportunistically.

 

 

The size of prey items consumed generally increases with fish size, and in turn gape size (the size of the grayling’s mouth). The shift in habitat with life stages is closely related to changes in foraging strategy. Feeding usually begins on first emergence to surface water. As the fish grows, the mouth-gape diameter enlarges enabling the grayling to consume a greater variety of larger prey items. Feeding rate is related to temperature and oxygen levels, which in turn affect grayling activity.

 

Feeding activity peaks at dawn and dusk, although grayling feed during the day, they will seldom feed at all during the night.


 

 

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.