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Trout in the Town Blog
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!!
02/07/2013 - 19:47
Another of the MANY great talks that the participants in the 2013 Urban River Champions Conclave benefited from was Professor Lerner's account of how the Aire Rivers Trust has set up the plan for the restoration of Bradford Beck.
I felt that it was important to set up the whole Conclave by bracketing the subject of Resilience with two talks; the first of which was Phil Sheridan's deeply personal and incredibly inspirational dissection of the qualities of resilience that we encounter (and require) during each of our "lived experiences". The second key talk was very deliberately designed to expose the bare nuts and bolts of a highly structured series of practical solutions to many of the problems that Trout in the Town groups routinely encounter. In other words, the second talk offered one possible "route map" of how to turn the inspiration and personal resilience identified and instilled by Phil's talk into a series of effective an efficient actions.
This second "bracketing" talk was, of course, Professor Lerner's presentation. I've reproduced it in the video below so that both of the absolutely key expert presentations from the 2013 event are available as a reminder for the Conclave attendees - as well as for the benefit of anyone who may need their guidance. They make for a great "paired viewing".
Enjoy and, if you are in the process of caring for an urban river, take notes! :)
01/07/2013 - 09:49
Friends of Bradford Beck have taken a simple but profound step in replacing the series of signs that were first erected in the 1970s. The signs previously warned people to stay away from the dangerously polluted waters. Now that there have been significant strides made in improving water quality - these signs are misleading. They were just one more reason for local people to either overlook or actively avoid valuing and engaging with their local river corridor.
Local newspaper coverage here:
Telegraph and Argus
Local newspaper coverage here:
Telegraph and Argus