Joe Winstanley
Senior Member & Supporter
Pete Reading stated;
“You can sex mature barbel by looking at the vent, near the anal fin; females have a fleshy protruding tubular vent, males have a single small hole flush with the skin, but the smaller the barbel the harder it is to see.
At one time Calverton selected the larger fish for some stockings, but as far as I am aware now stock the whole size range from that year class to ensure no sex selection occurs.
I do not think it likely that the sex ratio will vary much within stocked fish anyway, and have any real effect of spawning success.
Far more likely candidates for poor spawning success and recruitment are the obvious ones of poor habitat linked with low flows; barbel need plenty of clean gravel and clean water flowing over it to spawn successfully, plus refuge areas for fry to develop and over-winter in safely.
A combination of other negative factors, such as predation on fry by crays,algae choking eggs, unseasonal flood events, will all play a part in limiting populations.
There will not be a simple answer to what is a very complex question, but interesting to discuss all possibilities.
Hampshire Avon is full of tiny barbel this year, with baby fish from the last two years of spawning in great abundance, so something was right recently; be interesting to see how many survive in the next few years.â€
Personally, I think Pete’s comment above is one of the most interesting made on this thread.
There is no doubt that poor recruitment lies at the heart of the problem on some rivers. I seriously doubt that the aquaculture industry has anything to do with it and would be looking into other areas. Take agricultural practices for example. How does this affect some rivers and not others?
Taking the Trent as an example simply because it’s one of the UK rivers that is doing extremely well today. If you look at the type of agriculture adjacent to the Trent a huge proportion of it is not arable but pasture. This is due in part to the geology of the Trent valley which is extremely rich in gravels which makes for poor agricultural land. As such, the Trent does not suffer from extensive agricultural run off from arable land that other rivers do.
Many of the effects on rivers from agricultural practices include;
Land drainage
Agricultural drainage impacts on the water and sediments of the river system. Land drainage has been used since the mid 18th century to increase the suitability of land for cultivation. As a consequence, it has been suggested that in times of flooding or heavy rain, water levels have risen and fallen much faster (i.e. floods have become more flashy) as a result of drainage
Drainage and ditching lower the water table to enable agriculturally desirable plants to grow more productively. Water falling on the land is transported away more effectively. Thus, in drained catchments, river flow matches the rainfall profile more closely than in a natural catchment (although there will be a time-lag relating to catchment characteristics and the preceding soil saturation level). Drainage reduces the natural buffering capacity of the catchment against floods, so the magnitude and frequency of floods downstream may be increased. Conversely, as the natural 'sponge' effect of the land is reduced, drained land may be drier for longer. Lowering of the water table can be detrimental to wetland communities
Generally, field drains are not responsible for large sediment inputs, but do cause an accelerated throughput of water. They constitute a long lasting alteration of the natural flow regime and have become part of the 'natural' process of surface water run-off
Abstractions for irrigation
Unregulated water abstraction for crop spraying and irrigation reduces the flow left in the stream. This is particularly significant during periods of low flow when a given extraction rate takes a greater percentage of the total discharge. Aquatic flora and fauna will thus lose habitat, and pollution will be less diluted in periods of abstraction and low flow.
River engineering works
Piecemeal bank protection is often undertaken at sites where farmland is being eroded. Methods include dumping boulders, concrete and car bodies against the eroding bank. The use of such 'hard' material tends to alter instream flow dynamics, deflecting the main current and causing bank erosion immediately downstream. Traditionally, gabion baskets and rip-rap have been used to provide long-term bank protection. They are now widely questioned on environmental grounds as they are visually intrusive, reduce bankside habitat availability and alter natural instream and bankside processes. Other 'hard' bank protection methods include, the use of current deflectors (though this may cause erosion on the opposite bank), the use of a stone berm at the toe of the bank, and bank reprofiling to reduce bank slope Soft engineering involves planting and the use of geotextile filter layers, and aims to dissipate rather than deflect the river's energy. It is less obtrusive than 'hard' engineering. Good practices for river engineering are detailed in Hoey et al (1995).
Ploughing and crop cultivation
Soil erosion occurs when there is a combination of heavy rain or wind, and exposed soil. Erosion by water most commonly occurs where there are clear routes for the run-off to follow, such as plough furrows and tyre wheelings running up and down the slope. The impact of erosion upon the river system is greater during winter, when an increased sediment load due to exposed soil or high winds, and faster run-off due to a saturated or frozen substrate, results in the arrival of 'coffee coloured' sediment laden run-off in streams
Fields can be particularly vulnerable to soil erosion where crops such as potatoes, swedes and turnips have been grown in rows. Erosion can be serious where conditions have been wet at harvest and the ground between the rows is rutted. Run-off may then become concentrated between the rows. Soil may also be vulnerable to erosion after row crops are planted when there is a high percentage of bare ground. Crop rotation may lead to different levels of soil protection in different years, and ploughing-in of the crop residue after harvest will improve the organic matter content of the soil. Cultivation of steep slopes, and the compaction of land by heavy machinery also exacerbate erosion problems
Fields next to rivers should not be ploughed before or during winter when erosion is exacerbated by periods of heavy, prolonged rain and flooding. If a crop is sown in late summer or early autumn, e g. winter oil-seed rape or winter barley, there is a good chance that there will be an adequate crop cover to prevent serious winter erosion. Undersowing cereal crops with a grass mixture will mean that there is a good cover of vegetation over the winter after the crop is harvested, which will reduce the potential for erosion.
Direct inputs of sediment can be avoided by leaving buffer strips adjacent to watercourses. The appropriate width for the buffer strip depends upon the nature of the soils, the vegetation and the river. Treatment using fertilisers, insecticides, pesticides and herbicides must avoid buffer strips. Water margins can be an important wildlife habitat, and their value is increased if they are protected by a buffer strip. If there is no buffer strip, there is a danger that the margins will become just a nutrient and sediment sink, and that the diversity of plants will be reduced as the area becomes dominated by plants that can take advantage of these nutrients
Livestock
Trampling by cattle and sheep can compact the soil causing increased run-off. Livestock also trample and feed on vegetation, the root systems of which bind soil together. The destruction of vegetation reduces both the interception of rainfall by plants, and the resistance to run-off created by the plants themselves. This means that more of the rain falling on the land runs off into the watercourse immediately after it has fallen, increasing soil erosion and sediment transport. Heavy grazing of the riparian zone will affect the plant community, reduce shading for fish and perhaps reduce beneficial inputs to the stream
Livestock can break banks down by trampling. They can increase bed roughness by repeatedly crossing the stream at certain points (this locally increases flow height), and they disturb the bed, temporarily increasing suspended sediment concentrations Increased bed roughness and loose soil from trampling, increases the susceptibility of banks to erosion, particularly during flood flows. In addition, the sheer weight of livestock on a heavy saturated bank could cause slumping.
Of course rivers face many more threats with pollution being the instant killer if catastrophic pollutants enter rivers and streams. Other threats include endocrine disrupters. Studies in cells and laboratory animals have shown that EDs can cause adverse biological effects in animals, and low-level exposures may also cause similar effects in human beings. The term endocrine disruptoris often used as synonym for xenohormone although the latter can mean any naturally occurring or artificially produced compound showing hormone-like properties (usually binding to certain hormonal receptors). EDCs in the environment may also be related to reproductive and infertility problems in wildlife and bans and restrictions on their use has been associated with a reduction in health problems and the recovery of some wildlife populations.
When I was a SACG/SAA committee member, fellow committee member Phil Hackett championed a campaign against the use and effects of ED’s and I was one of Phil’s supporters that always thought the use of ED’s would come to haunt angling and wildlife eventually.
Regards,
Lee.
The Trent has an interesting and extremely diverse catchment, but it's not without it's problems though. The Upper Trent below Stoke suffers badly from nitrate and phosphate pollution but far less so down stream, no doubt aided by a substantial supply of clean water coming in from the Dove and Derwent. Perhaps the Soar to a lesser extent.
Overall the nitrate levels in the Trent are still considered too high and most of the catchment (Dove and Derwent apart) is designated as a surface water Nitrate Vulnerable Zones due to nitrate leaching from agricultural land.
PS - the SNH paper you have cut and pasted from is a well behind the curve in terms of the soil erosion risk and many of the issues in Scotland are different to those in lowland England - declining SOM for instance.
