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Utilization of Floodplains by Salmon

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Utilization of Floodplains by Salmon C S A S S C C S Canadian Science Advisory Secretariat Secrétariat canadien de consultation scientifique Research Document 2002/007 Document de recherche 2002/007 Not to be cited without Ne pas citer sans permission of the authors * autorisation des auteurs * Floodplains, flooding, and salmon Examen des plaines d’inondation, des rearing habitats in British Columbia: A inondations et des habitats d’alevinage review du saumon en Colombie-Britannique Tom G. Brown Pacific Biological Station Fisheries and Oceans Canada 3190 Hammond Bay Road Nanaimo, B.C. V9T 6N7 * This series documents the scientific basis for the * La présente série documente les bases scientifiques evaluation of fisheries resources in Canada. As des évaluations des ressources halieutiques du Canada. such, it addresses the issues of the day in the time Elle traite des problèmes courants selon les échéanciers frames required and the documents it contains are dictés. Les documents qu’elle contient ne doivent pas not intended as definitive statements on the subjects être considérés comme des énoncés définitifs sur les addressed but rather as progress reports on ongoing sujets traités, mais plutôt comme des rapports d’étape investigations. sur les études en cours. Research documents are produced in the official Les documents de recherche sont publiés dans la language in which they are provided to the langue officielle utilisée dans le manuscrit envoyé au Secretariat. Secrétariat. This document is available on the Internet at: Ce document est disponible sur l’Internet à: http://www.dfo-mpo.gc.ca/csas/ ISSN 1499-3848 (Printed) © Her Majesty the Queen in Right of Canada, 2002 © Sa majesté la Reine, Chef du Canada, 2002 2 Abstract The purpose of this review paper was to examine the relationships between floodplains, flooding, and juvenile salmon habitats. A wide range of topics were explored, these include; defining and characterizing fish habitats and floodplains; explaining flooding and hydrological processes; and outlining fish behaviours, diets, and ecology. The potential impacts of forestry, agriculture, and urban development on floodplain processes, fish stranding, and fish habitats were explored. This includes a short discussion on chemicals and pollutants that enter fish habitats as a result of human activities on floodplains. In developing an understanding of our current knowledge of floodplain habitats and in identifying significant knowledge gaps, over 500 references were cited. This review was focused on British Columbia’s floodplains but does contain references to research done elsewhere. Social values exceed environmental concerns on a developed floodplain and few viable options remain. A major flood can cause loss of life and tremendous property damage. Thus, humans tend to view floods with fear. From an ecological perspective flooding is a natural event and an integral part of salmonid life cycles. When a river is separated from it’s floodplain and held to a permanent course by dykes, considerable loss of salmonid habitat will eventually result. We can not maintain natural flooding patterns or regain many of the historic floodplain fish habitats, once floodplains have been urbanized. We can only protect the remaining critical habitats from development. On an undeveloped floodplain, consideration can be given to limiting activities to those that are consistent with maintaining natural systems. It is important that we maintain the natural hydrograph, permit the flooding of floodplains, support the natural avulsion of a river channel, and protect wetlands. From an ecological prospective, floods are natural and are important in maintaining the health of the river, riparian zone, and floodplain. Fish and invertebrates are adapted to seasonal flooding. Periods of high water may serve as a queue for migration or an opportunity to move into and exploit different habitats. Floods create new channels and a succession of new habitats while eliminating others. Floods clean the substrate and alter the species composition of the riparian communities. The wetlands associated with floodplains support the rearing of juvenile salmon. Floodplains provide habitat for juvenile salmonids in the form of seasonal wetlands, temporary tributaries, off-channel ponds, sloughs, flood-channels and seasonal estuarine drainages. Natural floodplains reduce the heights of floods, storing floodwaters in wetlands, and distributing the floodwaters over a wide area. They also filter storm waters, trapping sediments, nutrients, and removing pollutants. Floodplains are a major source and processor of litter. When compared to lentic habitats, these seasonal habitats support a different mix of invertebrates, usually have more modified water temperatures, and may have different water quality concerns. Many of these habitats support higher densities of juvenile salmon and have higher grow rates than main channel habitats. Coastal and interior floodplain habitats are used by a number of regionally important salmon species. These fish appear to have adapted behaviours that enable them to successfully exploit seasonally flooded lands. Human activities such as forestry, agriculture, and urban development can affect salmonid floodplain habitats. Based on this review, some major gaps in our knowledge and concepts we should consider when examining floodplains are listed below. 3 Résumé Cet article de synthèse porte sur l’examen des relations entre les plaines d’inondation, les inondations et les habitats du saumon juvénile. Une vaste gamme de sujets sont abordés, tels que la définition et la caractérisation des habitats du poisson et des plaines d’inondation, l’explication des processus d’inondation et des processus hydrologiques et un survol des comportements, des régimes alimentaires et de l’écologie du poisson. Les incidences potentielles de la foresterie, de l’agriculture et du développement urbain sur les processus des plaines d’inondation, le poisson et ses habitats sont examinés, incluant une courte discussion sur les produits chimiques et les polluants introduits dans les habitats du poisson suite aux activités humaines menées dans les plaines d’inondation. Plus de 500 études ont été consultées afin d’établir l’état de nos connaissances sur les habitats des plaines d’inondation et d’y identifier les lacunes importantes. Bien que cet examen visait les plaines d’inondation de la Colombie-Britannique, des références sur des recherches menées ailleurs sont aussi présentées. Les valeurs sociales l’emportent sur les préoccupations environnementales à l’endroit des plaines d’inondation. Peu d’options viables sont donc disponibles. Comme les graves inondations peuvent causer des pertes de vie et des dommages gigantesques aux biens, nous avons tendance à les craindre. Du point de vue écologique, une inondation est un événement naturel qui fait partie intégrante des cycles de vie des salmonidés. Lorsqu’une rivière est isolée de sa plaine d’inondation et confinée à un tracé permanent par des digues, une perte importante d’habitat des salmonidés en résultera éventuellement. Il est impossible de maintenir les régimes d’inondation naturels ou de recouvrer nombre des habitats du poisson autrefois présents dans les plaines d’inondation une fois celles-ci urbanisées. Tout ce que nous pouvons faire, c’est protéger les habitats essentiels restants du développement. Dans une plaine d’inondation non aménagée, on peut songer à limiter les activités à celles qui favoriseront la pérennité des systèmes naturels. Il est important de conserver l’hydrographie naturelle, de ne pas empêcher les inondations, de favoriser l’avulsion naturelle et de protéger les terres humides. Du point de vue écologique, les inondations sont un événement naturel et important pour le maintien en bon état d’un cours d’eau, de sa zone riveraine et de sa plaine d’inondation. Les poissons et les invertébrés sont adaptés aux inondations saisonnières. Ainsi, les périodes de crue peuvent leur servir de signal pour la migration ou leur offrir l’opportunité de pénétrer dans différents habitats et de les exploiter. Les inondations créent de nouveaux chenaux et une panoplie de nouveaux habitats, tout en en détruisant d’autres. En outre, elles nettoient le substrat et modifient la composition taxinomique des communautés riveraines. Les terres humides caractéristiques des plaines d’inondations sont essentielles aux activités d’alevinage des salmonidés juvéniles. Ces plaines leur offrent des habitats sous forme de terres humides saisonnières, d’affluents temporaires, d’étangs hors chenal, de faux chenaux, de chenaux de hautes eaux et de plans d’eau estuariens saisonniers. Les plaines d’inondation naturelles réduisent le niveau des inondations, emmagasinant les eaux de crue dans les terres humides et les distribuant sur une vaste étendue. Elles filtrent aussi les eaux pluviales, piégeant les sédiments et les nutriments et enlevant les polluants. Les plaines d’inondation sont une source majeure et un important transformateur de litière. Comparativement aux habitats lénitiques, ces habitats saisonniers abritent un mélange différent d’invertébrés, offrent généralement des températures de l’eau davantage modifiées et ne souffrent pas des mêmes problèmes de qualité de l’eau. Un grand nombre de ces habitats accueillent des densités plus élevées de saumons juvéniles et permettent des taux de croissance plus élevés que ceux retrouvés dans les habitats du chenal principal. 4 Diverses espèces de saumon importantes au niveau régional utilisent les habitats des plaines
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