Effects of climate-driven lake level decline on coarse woody habitat and fishes
Global climate change may influence freshwater fish populations through drought and changes in lake levels in addition to changes in other factors such as altered thermal or dissolved oxygen regimes. Littoral structures such as fallen trees (i.e., coarse woody habitat, CWH) can serve as critical fish habitat for prey production, refuge, and spawning. Yet, drought-driven lake level declines may eliminate these structures from littoral zones. A prolonged drought in northern Wisconsin, USA allowed us to test for the effects of lake level decline on CWH availability and the response of fish populations. We examined lake level; the corresponding reduction in CWH; the relative abundance of the dominant forage fish species, yellow perch (Perca flavescens); and the density and growth of the popular game fish and top piscivore species, largemouth bass (Micropterus salmoides), in Little Rock Lake South from 2001-2005 and 2007-2009. We used regression models to test for: 1) a relationship between lake level and submerged CWH available to fishes; and 2) the response of fishes to gradual, but sustained, losses in available CWH.
Lake level of Little Rock Lake South declined over 1.1 m and more than 75% of the previously submerged CWH was lost from the littoral zone during 2001-2009. The loss of CWH coincided with the disappearance of yellow perch and reduced growth of largemouth bass. Our analysis indicated that the probability of detecting yellow perch was <50% at a 22% reduction in CWH, <10% at a 37% reduction in CWH, and <1% at a 53% reduction in CWH. Growth rates of largemouth bass were density-dependent and generally increased with CWH. However, the relationship between growth rate and CWH varied across fish length. To evaluate this relationship, we simulated growth trajectories of largemouth bass under low and high CWH conditions. Our simulations suggested that growth asymptotes prior to largemouth bass attaining the minimum length limit in the region. Our work shows that climate change can affect fish populations by altering the availability of littoral habitat. Furthermore, it suggests that resilience of game fish populations could be increased by proactive management strategies.