Habitat - Wetlands
Date Published: October 2004
Number of Pages: 90
Author(s): Julie Henning
Palustrine wetlands are prominent habitats of Pacific Northwest coastal floodplains. The importance of these flooded wetlands (e.g. marshes, beaver ponds, remnant oxbows) to fish has not been well documented historically. The seasonal influence of the hydrological regimen across the floodplain is critical (Bayley 1995; Poff et al. 1997; Michener and Haeuber 1998) and maintains biological and physical diversity. Based on observations in relatively undisturbed river-floodplains, bigger floods inundate larger areas and make greater quantities of food and shelter available for fish and other aquatic and semi-aquatic organisms as floodwaters flow over riverbanks (Welcomme 1979). This flood process maintains high production due to the decomposition of organic matter and nutrients, which can result in high yields of fish (Bayley 1991; 1995).
The river-floodplain connection has been greatly reduced in most temperate systems because humans have substantially modified the rivers’ peak flows. In addition, many river channels and associated floodplains have been isolated from one another (Junk et al. 1989). Examples of actions that have affected the hydrologic pattern include: channelizing and armoring river segments, dredging, forest clearing, diking, and ditching and draining wetlands. Awareness of the value of wetland habitats has gradually increased and wetland creation, restoration, and functional enhancement have become important tools for compensating wetland acreage and functional losses (Ratti et al. 2001). However, fish are rarely the focus of these wetland projects.
Wetland restoration and enhancement activities have become an integral component of nation-wide conservation efforts under such federal authorities as Natural Resources Conservation Service (NRCS) Wetland Reserve Program and U.S. Fish and Wildlife Services North American Wetland Conservation Act. These federal agency programs have restored and created over 1.5 million acres of wetlands in the United States since the initiation of the 1990 Farm Bill (NRCS 2004).
In floodplains, restoration projects with water control structures are often intended to conserve and restore wetland habitat characteristics including floodplain function, hydrologic connectivity, and critical habitat for migratory wildlife and threatened and endangered semi-aquatic and aquatic wildlife and plant species. Wetland restoration projects often include a mechanism to retard the rate of water drainage. This is often accomplished by blocking drainage ditches or installing water control structures to retain water within the wetland floodplain area. Also, decreasing the rate of drainage can facilitate longer connectivity between the river and floodplain. A water control structure can be manipulated to vary water depths in the wetland throughout the year. In some cases, the structure includes a culvert connected to a half round riser where boards are placed to control the wetland hydrology (Figure 2). Water control structures can increase wetland water storage and return water to the river more slowly, long after water levels recede in the river channel. Moreover, if properly constructed and operated such outlets can maintain connectivity between the wetland and river. In this study, wetlands that contain water control structures are referred to as regulated wetlands and non-regulated sites are those wetlands without water control structures.
Many wetland research studies have focused on the importance of wetland habitat for invertebrates, waterfowl, and amphibian populations (Kaminski and Prince 1981; Murkin et al. 1982; Safran et al. 1997). Most wetland restoration and enhancement projects are wildlife oriented and are rarely designed specifically to benefit fish populations. In fact, many wetland projects could be detrimental to fish (i.e. entrapment, predation, and water quality limiting). However, in a floodplain environment a wide variety of fish species, including salmonids, have the potential of accessing and benefiting from seasonal wetlands. Floodplain wetlands may serve directly as important rearing habitat (i.e. feeding, refuge) and indirectly as a source of primary and secondary production for the main river channel. Significant information gaps remain in our understanding of the use of floodplain wetlands by fishes, particularly salmonids, in temperate systems. This is related to the uncertainties of river-floodplain relationships, the diversity of wetland habitats in floodplains, and the sampling problems associated with seasonal habitats, which can have extreme environmental variability (Sommer et al. 2001).
In the Pacific Northwest, the degree to which floodplains support fish remains poorly understood. Numerous studies have shown the significance of off-channel habitat and beaver ponds to juvenile coho salmon Oncorhychus kisutch (Bustard and Narver 1975; Brown and Hartman 1988; Swales and Levings 1989). Off-channel habitats are channels that have formed by the channeling of runoff through swales created by the migration of the mainstem stream (Peterson and Reid 1984). These channels and associated ponds are productive habitat for overwintering fish and contain a hydrologic connection to the river during the winter (Peterson and Reid 1984). In Carnation Creek watershed (a drainage in Vancouver, B.C.), a floodplain area of about 50 hectares in a given location, 15-25% of the total smolt yield were captured in off-channel sites, and most of those fish spent the winter in off-channels devoid of standing water during summer (Brown and Hartman 1988). However, these studies may not be as applicable when inferred to large river-floodplain habitats because most of these studies were done in headwater areas of small watersheds (10-300 km2 river drainage) that contain narrow floodplains, low discharges, and spring fed ponds. The Chehalis River floodplain has seasonal wetlands that have infrequent surface water connections to the river, are temporarily flooded for brief periods, and are primarily rain-fed.
Federal listings of threatened and endangered fish stocks, specifically Pacific salmon, underscore the need to understand the effects of regulated wetlands on such populations. Specific questions pertaining to how salmon respond to regulated wetlands include identifying the degree of utilization, growth, survival and outmigration. These responses have major management implications for the successful creation and restoration of wetland habitats to benefit and/or to avoid detrimental effects to fish. The continued development of such wetland projects is dependent on new information about the overall consequences of these restoration projects on salmonid growth and survival.
The goal of this study was to broaden our understanding of the role of regulated floodplain wetlands in the Pacific Northwest as rearing (i.e. feeding, refugia) habitat for fishes. This was accomplished by comparing six wetlands in the Chehalis River (Washington State) floodplain. Two wetlands are regulated and four are non-regulated. Two non-regulated wetlands, N1 and N2, were selected based on their proximity and similarity to pre-impoundment conditions of the regulated sites (R1 and R2). The other two non-regulated sites, alternative sites A1 and A2, are a seasonal off-channel and a remnant oxbow permanent pond with a beaver dam. The alternative sites were added to compare with existing fish habitat literature. Detailed descriptions of each study site are in chapter 2. Given the potential that fish are accessing floodplain wetlands, specific objectives of the study were:
- Compare relative fish abundance, especially salmon, between regulated, non-regulated, and alternative wetland sites.
- Determine fish community characteristics such as species richness, diversity, and ratio of native to non-native fishes in regulated and non-regulated wetlands and examine fish population metrics (condition index and fork length) over the duration of the study.
- Examine the duration of fish access in and out of sampled wetlands.
- Characterize physical parameters (e.g. water temperature and dissolved oxygen concentration) of regulated, non-regulated, and alternative wetlands and associate characteristics with juvenile salmon populations.
- Compare relative amphibian abundances between regulated, non-regulated, and alternative wetlands.
Henning, Julie 2004. An Evaluation of Fish and Amphibian Use of Restored and Natural Floodplain Wetlands. Final Report EPA Grant CD-97024901-1. Washington Department of Fish and Wildlife, Olympia, Washington, USA. 81 p.