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Trout in the Town Blog
29/10/2013 - 16:20
Had a great visit to and discussion around Shipley weir recently. Stephen Bottoms (who organised it all) has done a great blog post here:
23/10/2013 - 19:27
You can always have too much or too little of a good thing. When it comes to revetments (re-inforcements) of river-banks; there are a whole host of pitfalls.
On the one hand, many sections of river suffer from excessive grazing of the land surrounding them that leads to a dramatic reduction in the variety of bugs, plants as well as fish populations that can be supported. In addition, many rivers that run through towns and cities often pass through quite intensively used land upstream of urban reaches. The excessive inputs of fine silt and sand where bank-erosion is rampant and extensive often end up being accumulated in the engineered sections of channels in towns and cities. As well as causing maintenance problems,this can smother what may, otherwise, be perfectly good spawning gravel.
Conversely, where efforts to "lock" a river channel in one place are over-zealous; the result will be the strangulation of supply of spawning gravels and a variety of cobbles and other river-bed material. Each of the different diameters of gravels/cobbles/boulders that come from eroded banks form a unique and vital habitat either for particular species or particular stages within the lifecycle of a species.
So the optimum for biodiversity (and, consequently, the services that such biodiversity provides to society through flood risk attenuation, clean water that is more energy-efficient/cheaper to treat, protection against climate change in cities etc. etc. etc.) is somewhere in the middle.
When you add to this the fact that a large proportion of either ad-hoc or "one off" attempts to reinforce banks by installing a hard "skin" of some kind (perhaps concrete, corrugated sheet metal, rubble, boulders, gabion baskets etc.) often make the problem worse...then there seems to be a genuine need for short information pieces like the video above. Whilst at first sight, putting in a solid reinforcement along the face of a riverbank is totally logical; when the surrounding bank material is still as soft as ever - you really have to know what you are doing to make "hard" revetments work. The problem is that hard surfaces "reflect" or "bounce" fast current flows without taking any of the speed or "sting" out of them. When those currents career into the next soft bit of bank; they take away a huge chunk. To make matters worse, the angular surfaces of rocks or gabion baskets are brilliant at producing swirling whirlpool/eddying flows - especially during spates. These chew great big holes in soft bank material and are really good at eating the bank away so that flows can work their way BEHIND the revetment. Once this happens, the hard revetment actually squeeze and accelerate the flows between the soft bank and the inside of the reinforcement - this produces a much faster and much more dramatic rate of erosion than would exist if there was no revetment at all.
Contrast this with complex, bristly brash (especially thorn brash or coniferous brash which have impressive densities of twigs sticking out at a range of different angles)...this stuff actively absorbs and stops raging flood waters. The "braking" effect actually causes suspended sediment to drop out of the flood-water and accumulate amongst the branches. If grazing stock can be excluded, these rich river sediments can soon re-vegetate and start to knit together the banks. This is especially true if trees are allowed to re-colonise (as deep root structure then starts to perform a similar role to that of the initial brash installation. Even better than this, because such re-vegetated banks still allow a river to redistribute bank material; a sufficient supply of sediment is still maintained. So, even though you can always have too much of any good thing, using brash revetments can be a vital leg-up to re-establishing sections of healthy river-side vegetation.
They might even help to stop your fencing falling into the river and reduce your water-bills by reducing treatment costs.
Oh, and they are amazing cover for juvenile fish from feathered and finned predators!!
07/10/2013 - 19:20
The WTT borrowed the fabulous Emriver kit from Severn Rivers Trust during the 2013 CLA gamefair. It allowed us to set up a whole range of common river-channel scenarios in an accurate scaled-down simulation. Dredging was one of the things we modelled:
Dredging river-bed material is something that is of particular relevance to the urban/heavily-modified channel environment. As is so often the case with rivers, what seems like the obvious and correct thing to do can actually blow up in your face (or someone else's face several miles up or downstream!).
Just as intriguing are the potential knock-on impacts of ad-hoc dredging in rural environments in an effort to increase the capacity to drain land...
Nature abhors a vacuum and removing accumulated material from a river channel can have far-reaching and unintended consequences. The first principle effect is to increase the demand for eroded river-bed and river-bank material from upstream. This can dramatically increase the rate of erosion in upstream reaches.
It also rapidly leads to the re-filling of the dug-out channel... (back to square one, so you dredge again....)
The other major/worrying effect is the interruption of the transport of sediment downstream of the dredged reach. Whilst the bed material is being re-accumulated in the dredged area, there is far less material being supplied downstream. That "cutting off" of the sediment supply causes a net increase in the erosion downstream of the dredged reach as well. This is due to stopping the supply of material that would otherwise "patch up" and fill in eroded areas to produce a more stable dynamic equilibrium state. How many people who undertake dredging understand these processes? Is the move to "fast
-track" UK-landowners' ability to dredge their own streams with a much lower requirement for external assessment likely to create more problem than it solves?
In addition to all that, the video below might give an interesting perspective on dredging works for the purpose of flood-water conveyance in areas downstream of bridges and weirs in modified waterbodies...
19/09/2013 - 20:34
The Wild Trout Trust very gratefully benefited from the generous loan of Severn Rivers Trust's "Emriver" at this year's CLA gamefair. As well as proving to be a great way to engage game fair attendees of all ages, it provided a fantastic opportunity to accurately simulate many scenarios that we routinely encounter in our river habitat works. This first video introduces some of the basics - and shows the effect of two common habitat installation techniques: marginal brash and log flow-deflectors.
There will be a series of short videos that follow this first introductory piece. Each subsequent film will look at specific scenarios and model their outcomes - which are often highly unexpected unless you have quite a lot of existing experience with geomorphology...
23/07/2013 - 13:32
Met with Phil Williams last week to chat through the questions he had for me about the Trout in the Town project. Phil has now put his interview up on his website; and it is available to listen to below:
08/07/2013 - 12:38
Shaun Leonard gave a huge amount of masterclass training over both days of the Urban Conclave - enabling attendees to benefit from just a small part of his extensive experience in decoding the secrets of the lives (and sometimes deaths) of fish just using visual examination of small samples under a microscope.
Combining the Trout in the Town friendly competition monitoring methods with training in scale sampling (which does not harm the fish)provides anyone who cares about an urban (or rural!) river with a wonderful tool for understanding the fish populations in their river.
For instance, the picture at the top of this blog entry gives a clue to why one of the most commonly-heard myths about taking large fish for the table is completely wrong...
Just by way of explanation, the picture shows a trout scale under a microscope. The scale was taken from the fish (caught and released by the WTT's Gareth Pedley from the river Tweed) in the photograph below:
Although they are not like the rings in a tree trunk (i.e. one band for each year!) you can still often see periods of time where the growth is slower - and the groups of multiple rings are packed closer together. These more closely-packed groups of rings represent the slower growth during winter. Each small red dot on the top photograph indicates where those closely packed rings jump to more widely spaced rings (i.e. the change from slow winter to faster summer growth). Since trout spawn in winter, counting the number of winter periods out from the centre of the scale is a way of deducing its age.
Not only that, but sometimes the rigours of spawning cause the body of the fish to dig into its reserves of nutrients held in its body tissues. This reclaiming of nutrients from tissues can sometimes be seen in the scale rings (which are made of protein plates overlaid with calcium). When the normally concentric rings of trout scales "cut over" and cross rings of smaller diameter within a winter band - this indicates a period when the body of the fish has been forced to reclaim nutrients from its tissues due to spawning. There is an example of a "cutting over" spawning mark circled in red in the top picture.
In fact, when the original scale was examined with the benefit of being able to focus the microscope in and out (rather than the single fixed focus held for the snapshot above), spawning marks could be found in each of the fish's last 4 winters. Now, it is not possible to say whether the fish did not breed before that time - but what can be said is that a larger fish will produce significantly more eggs (or milt). For instance, workers on the Celtic Sea Trout project report that a single 20lb female sea trout can produce more offspring in a single spawning than seven finnock. As a broad generalisation, a female brown trout will produce roughly 900 eggs per pound of body weight.
Gareth's 6-lb fish from the Tweed was assigned an age of 8 years following scale reading. This means that - even if the fish had spawned in each winter since it matured, its huge body size over the last 3 to 4 years will mean that it has been making a much, much, greater contribution of offspring than it would have been able to as a recently-matured fish.
The calculated growth curve that was generated by combining Gareth's fish-length measurement with its scale-reading is shown below:
Imagine, therefore, killing this fish as a 3lb (5-year old) or a 4lb (6-year old) fish in either 2009 or 2010 using the logic that "it has done its breeding job already". We know for sure that the fish has spawned in its last four winters up to 2012 - and not just in 2009 and 2010. We also know that in 2011 and 2012 it was probably somewhere between 5.5 and 6lbs in weight - and consequently would have been capable of producing MANY MORE OFFSPRING IN THE LAST TWO YEARS compared to 2009 and 2010. So to have killed this fish would have been to removed more than half of its breeding contribution over those 4 years.
In a good case scenario - it would also have been making some breeding contributions in earlier years as well (without leaving visible spawning marks in the scale sample). Although these would have been less numerous, it also becomes retrospectively important - as it ultimately became a fish of 6lb (and now possibly more!). Not all fish have this potential, so it is good to have a specimen like this contributing multiple times to the gene-pool. After all, you don't shoot a Grand National winning racehorse after it has bred just once...Also, it is worth noting that this 6lb fish took 8 years to grow to that size. Taken together - where you have people routinely killing 2 - 3lb fish; you won't get nearly so many fish surviving beyond 4 and 5 years. This ruins your chances of catching 5 and 6lb fish...
The video below shows:
- Shaun training the Conclave attendees in SCALE SAMPLING (and measurement recording)
- Attendees catching fish (rod and line) and taking scale samples
- Taking those scale samples to a handy dining room table and reading them on the microscope (hooked up to TV screen)
- How to read those scales
- How to tell an original scale from a replacement scale
One fascinating outcome was that the scale reading showed that our original thought that we had 3 different year classes of fish - based on the lengths of 6"/one-year old, 9"/two-year old and 12"/three year old - was completely wrong!! In fact the 12" fish that we caught turned out to be a super, super fast growing 2-year old fish!! This, again shows the value of taking these measurements for your own fish - especially when people tend to make arguments of what constitutes a perfectly "takeable" fish for the table (or one which has already reached breeding age!)...
Watch, learn and enjoy!!