How Fish Use Rivers and How We Can Help Them
E-Book, Englisch, 130 Seiten
ISBN: 978-1-910456-40-8
Verlag: Old Pond Books
Format: EPUB
Kopierschutz: Adobe DRM (»Systemvoraussetzungen)
Dr Mark Everard is an academic (UWE Bristol), science advisor to a range of organisations, broadcaster and author with interests in fish and the wider aquatic environment
Autoren/Hrsg.
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Bottlenecks and Pressures
UNDERSTANDING RIVER HABITATS and the various ways in which fish depend on them throughout the year and at the different stages of their lives prepares us better to consider the ways in which contemporary rivers are compromised by current channel and catchment uses and modifications. Where there are multiple stresses on river systems, we can begin to understand how the impoverishment of habitats can cause major bottlenecks in the life cycles of fish and compromise their well-being. Once identified, these bottlenecks can then be considered priorities for management action. Naturalness
We have grown up with an image of rivers in their current state in the landscapes in which we live, and an assumption of what constitutes ‘naturalness’. However, we have also grown up in a post-industrial, densely populated and farmed world. Cityscapes substantially alter rivers not only within their bounds, but also through their influence on them downstream and upstream, in terms of flood diversion and other such management interventions. In the wider countryside, today’s familiar scenes of bucolic bliss may in fact be far from natural, given the scale on which the food sufficiency pressures of the post-Second World War era, the ongoing intensification of agriculture and the influence of subsidies have changed whole catchment landscapes. These pressures on land use have led to massive land drainage and reclamation schemes, the consequent breakdown of the natural connections of rivers with their floodplains, and the elimination of the wetland features that contribute to the biodiversity and health of river ecosystems. These, in turn, have resulted in vast changes to the timing and extremes, both high and low, of river flows. The increasing industrialisation of agriculture, with greater compaction of soils through the use of heavier machinery and tillage right up to river edges, also increases the erosion of soils and the siltation of rivers. Stock animals can, and commonly do, cause substantial damage to the marginal habitat, as well as directly polluting water and further exacerbating erosion. All of these pressures result in profound impacts, often unseen and underappreciated, on the river bed and marginal habitats. Compare a modern lowland river with an energetic stream that is hard to tame with conventional river engineering and/or land reclamation techniques, or where private ownership has averted the demand for such agricultural over-management. Such rivers are rare in the UK, but some reaches of Scottish salmon rivers, and a few in the north of England and in Wales that are in private estate ownership, contain remnants of braided channels. Fishing in the Himalayas, I see far more examples of rivers in a more natural braided state, for example the Brahmaputra River in the far north east of India. The braiding of rivers occurs where high flows each year carve out broad floodplains, the water finding its way downstream in a network of interlinked channels as the flows decline. This is a natural form of river in high-energy uplands. However, braided channels are also a natural river form in lowlands, where silt deposition and large woody debris sometimes naturally form temporary annual structures around which water pressures carve out new braided channels, which henceforth meander across the river corridor. As one well-documented example, the River Thames in Roman times ran through what is now greater London in multiple, interconnecting channels around a number of major islands, providing a rich resource of fish and waterfowl as well as a network of crossing points between islands, and offering navigability to the sea. In contrast, the Thames in London today comprises a single channel lacking in significant marginal habitat, up and down which the tide surges unabated by mid-channel structures. Comparing different rivers I fish, the Brahmaputra in north-east India is braided, running in multiple channels that change season by season. The Thames in London used to be like this up to Roman times, but is now constrained by hard banks, much as most of our rivers are now constrained and mainly disconnected from their floodplains We have become accustomed to the vast bulk of floodplain in rural landscapes, which formerly incorporated diverse wetlands and connected habitats supporting to the needs of all life stages of fish and other river life, having been drained, farmed and disconnected from the channel In urban landscapes, hard bank reinforcement is a common sight, compounding habitat loss in the river channel by further reducing the availability of marginal habitat, which is critical for feeding, refuge, breeding and nursery Even classic art from the nineteenth century portraying idyllic river scenes often depicts destructive forces. One such well-known example is John Constable’s 1819 painting, The Hay Wain, in which the wagon that gives the painting its title is being driven along the bed of a river, cottages and roads are built right up to the bank top, wooden-shuttered banks displace the natural riparian habitat, the floodplain is converted for agriculture and the banks are excessively trampled by stock! In fact, many classic English landscape paintings depict bucolic vistas in which rivers are disconnected from their floodplains, often with some sort of bank reinforcement or else bank trampling by horses or other stock, and the river channel is plied by heavy boats, blocked by weirs, mills or locks, or is otherwise in a condition that may be culturally meaningful yet is very far from natural. Our modern perception of rivers, even in such classically idyllic depictions, thus differs substantially from the truly natural environments within which fish evolved. And of course the amount of water that is abstracted for public and industrial supply and to irrigate crops, both from surface waters as well as from groundwater reserves that would naturally bolster river flows, has a massive influence on river levels, particularly during dry periods. With declining river flow and current velocity, rivers also possess less energy to purge silt from gravels and to carry out other important habitat-forming processes. All of these pressures on river channels, their adjacent floodplains and the connection to them, as well as inputs of silt and dissolved substances, and the effects of water abstraction from the catchment, impose significant changes on our rivers. And all of these changes compound the stress placed on the fish populations, which evolved naturally over far longer timescales than that of the modern human lifestyle. Loss of connectivity
When considering fish migration earlier in this guide, I noted that virtually all coarse fish species migrate. This includes both longer-distance migrations to locate a favourable spawning habitat and also lateral migrations across river channels into their margins, as well as into tributaries, ditches, drains and connected wetlands. The connectivity of river systems, then, is highly significant in allowing fish access to critical areas of habitat required for spawning, nursery, refuge, feeding and other needs. However, weirs and other barriers are common features of many of our rivers. Some of these weirs are a legacy from times when rivers were more commonly used for milling, navigation, ornamental landscapes or diverting flows to water meadows. The origins of many more extant weirs have become obscure over time. The purpose of a weir is to hold back water, but they also hold back flows of sediment and impede the passage of river organisms. Look upstream of a weir or a narrow bridge that constrains flow and you will see slack water running over muddy sediment and, commonly, thick beds of water plants in summer. Look downstream of that same weir or bridge and you will likely see a coarse gravel bed and the general widening of the river through the process of erosion. Part of the influence of a weir or other major flow obstruction is to slow flows, resulting in the deposition of suspended sediment particles from less energetic water and the accretion of sediment. By contrast, the faster-moving water below the constriction is starved of its burden of sediment, resulting in increased erosion of banks and the flushing of gravels. This is not necessarily a bad thing, as it often forms the kind of substrate and flow characteristics attractive to gravel-spawning species such as brown trout (Salmo trutta), barbel and chub, and provides oxygenated water that is attractive to many fish species in the warm, slacker flows of summer. However, in terms of the disruption of habitat-forming processes in the river system as well as blockage of the migration of fish, the influence of large or frequent impoundments can be of substantial detriment to fish stocks. Weirs are often culturally valued and familiar features, but this one on the Bristol Avon near Malmesbury presents an impassable barrier to trout, eels, dace and...
