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Fish Live in Trees

By Dave Southall

Dave Southall explains why, rather than trimming back the foliage, it's better to improve your casting skills


The only place fish were feeding in New Zealand's Mataura River system in January 2013 was under the willow trees!
The only place fish were feeding in New Zealand's Mataura River system in January 2013 was under the willow trees!

F­ish live in trees is a phrase often used by my friend John Shannon. As a retired EA fisheries officer, and now working for the East Yorkshire Rivers Trust, John has a wealth of knowledge about fishery management and, like me, believes that trees are a vital component of a healthy fishery. Unfortunately, all too many fishery managers and fishing club work parties seem to have the opposite opinion. “We want a tidy river where anglers can cast their flies without hooking up on overhanging branches and submerged snags” are the oft-heard comments from such folk. They seem to be unaware of the damage their ‘tidying’ does, and are not prepared to learn the casting skills required to cope with fisheries that have trees above and below the waterline.

Thornton Beck.

This controversial topic seems to generate highly polarised opinions. Here, I hope to convince at least some of the ‘chop down trees brigade’ that trees are good (except where they cast very dense shade).

So, why are trees so important to fish? Why is John’s statement that “fish live in trees” not the nonsense that it at first appears to be? Let’s first consider the advantages of bankside trees.

OXYGEN
• They provide shade which reduces the risk of high water temperatures during hot summers with low rainfall and sluggish flows. Trout, grayling and many of the invertebrates that they feed on are susceptible to high water temperatures, not only because it increases their metabolic rates (and therefore their demand for oxygen) but also because oxygen’s solubility in water decreases as the water temperature rises. Furthermore the BOD (biological oxygen demand) of bacterial breakdown of any organic pollution (from sewage outfalls, agricultural run-off, trout-farm effluent and natural sources) increases with temperature, further reducing the dissolved oxygen. In their book, The Trout, WE Frost and ME Brown (of the Fresh Water Biological Association) state that “Trout can live at temperatures above 20°C only if the water is fully saturated with oxygen” and that “The upper limit of temperature tolerance for brown trout (in tank experiments) seems to lie between 22.5 and 25.3°C”.

• Overhanging branches provide food in the form of terrestrial insects that fall into the river or lake: caterpillars, beetles, wood ants, etc. During my visit to New Zealand’s South Island in January 2013 fly hatches were virtually non-existent and the only significant window of opportunity I had to catch fish was around the middle of the day, under the overhanging willows. This was the only place fish could be found feeding, thanks to the numerous tiny willow grubs that were falling from the red galls that covered every leaf. In fact, Bob Wyatt, a past writer for FF&FT, wrote an article in a New Zealand magazine saying how hard the fishing had been in January 2013 on the Waikaia and Mataura Rivers and that fish could not be caught if they weren’t feeding, no matter how good the angler. If it had not been for the overhanging willow trees I would have had a very disappointing trip.

• Research has shown that on many streams and lakes a significant source of food for the aquatic food chain comes from leaf litter that has fallen into the water. The invertebrates may themselves eat the leaves, or decomposition may release methane that is used as a food source by methane bacteria; these are then consumed by Chironomid (midge) larvae. In some lakes it has been shown that over 60% of the carbon in Chironomid bodies is derived from methane.

• Overhanging trees provide cover from the eyes of avian predators, particularly the cormorants that are devastating so many waters in my area (North-East Yorkshire). They also disrupt the cormorants’ flight path which tends to discourage them in all but the harshest weather or when food is scarce because they’ve ‘cleaned out’ most of the more open local fisheries.

Left-to-right, clockwise: Willow grub galls, a willow grub and this willow grubber was foiled into taking the author's imitation.

Of course there has to be a balance. If there is too much shade aquatic plant growth may be restricted, which can limit the available food for aquatic invertebrates. In a scientific study that I carried out between 2006 and 2008 for the EA, Ryedale Anglers and North Yorks Moors National Park, selective removal of some of the old alder trees on a very heavily shaded section of the Rye near Helmsley resulted in a 45% increase in invertebrate numbers when compared with the unfelled control area.

So what about trees in the water as advocated by the Wild Trout Trust? Submerged trees (and their roots) are two other things that every fishery should have in abundance. I know that they snag flies and fish sometimes escape into them, but look at the advantages:

SANCTUARY
• They provide vital bolt-holes from predators. We have a plague of cormorants and increasing numbers of goosander, herons and otters. Fish need somewhere to escape. Unfortunately, grayling do not tend to seek cover but use the shoal mentality to evade predators (a poor strategy against cormorants), but wild brown trout will use any cover available. So, work parties and fishery managers, for heaven’s sake, don’t remove these vital sanctuaries!

• They provide shelter for fish during floods.

• They provide shelter, egg-laying sites and food for aquatic invertebrates.

• Trout often feed next to them so they provide feeding lies and places for the fly fisher to locate his or her prey. In the USA, anglers always target the ‘structure’ (debris dams, fallen trees and submerged tree-roots). Too many UK anglers view them as hazards to be avoided (sadly!)

• Trees and tree roots also stabilise banks against flood-erosion, reducing siltation problems.

Selective thinning work on the Dutchy Water in the traditional style.

CHALLENGE
One of my favourite small streams is Thornton Beck, on the North Yorks Moors. Totally neglected in its upper reaches, it’s full of woody debris and overhung with trees so low that they challenge the best of casters. It is no coincidence that it holds the biggest head of trout of any UK river I know. The fish have cool water, lots of food (aquatic and terrestrial) and most importantly lots of cover and suitable lies. I hope that the ‘chainsaw massacre brigade’ never, ever, visit this wonderful water.
Members of the Pickering Fishery Association, in conjunction with the East Yorkshire Rivers Trust, Wild Trout Trust and students from Bishop Burton College have done a lot of work selectively opening up the dense tree canopy and introducing woody debris into Pickering Beck’s Dutchy Water, for which we gained a Wild Trout Trust Award in 2013.

So, please, give me, and the fish, trees. I’d rather have challenging casting in a river (or lake) full of fish than easy casting in a water with few or no fish.


Dave Southall started fly fishing on the Yorkshire Rye and Driffield Canal in the 1960s, and later progressed to the South Midland Reservoirs. He has fished all over the world for a variety of species.

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