Indexing habitat opportunity for juvenile anadromous fishes in tidal-fluvial wetland systems

Abstract

In intertidal wetlands, habitat access by out-migrating juvenile anadromous fishes, such as salmon (Oncorhynchus spp.), is influenced by both physical and biological factors. Physical limits are set by inundation patterns and water quality parameters, while biological bounds are determined by species-specific migration timing and seasonal residency behaviors. We developed a new metric called the fish habitat opportunity index to quantify the total time (h) and volume-time (m3-h) per calendar year that salmon could access a given wetland, based on periods of inundation (volume) and seasonal migration period (time). The basic assumption of the index is that the volume of water inundating a restored wetland and the amount of time the wetland is inundated are both positively related to ecological benefits to juvenile salmon in terms of foraging success, growth, condition, and refuge from predators. We tested this index using data collected at a reconnected tidal freshwater wetland area in the lower Columbia River and estuary during 2006–2009. We found the wetland to be accessible to different salmon species for different lengths of time and periods of the year. On average, Chum Salmon (O. keta) had remarkably consistent opportunities to use wetland marsh habitat over the four years sampled, while opportunities were more variable for Chinook Salmon (O. tshawytscha), and Coho Salmon (O. kisutch). Inundation varied nonlinearly with tidal height. Fish had access to the productive marsh edge for approximately 40–50% of the time during out-migration periods. The fish habitat opportunity index is applicable to wetland systems throughout the tidal-fluvial continuum. It provides a metric for assessing fish habitat opportunity that is more realistic than simple wetted-area calculations or species presence/absence data. Most importantly, the fish habitat opportunity index affords a method for a priori comparison among potential restoration sites to help resource managers predict the likely relative effects on habitat opportunity from restoration aimed at benefiting juvenile anadromous fishes. Maximizing the fish habitat opportunity index should positively affect fish populations that use tidal-fluvial wetlands as vital habitats in their life cycle.