Spatial and Temporal Patterns in Brown Trout Growth in the Driftless Ecoregion

Understanding spatial and temporal patterns in growth is important for salmonid management. The Driftless Ecoregion of southeast Minnesota contains numerous groundwater-dominated streams with valuable Brown Trout Salmo trutta populations. Comparative growth assessments across space and time may inform and advance management. We compared Brown Trout mean back-calculated length-at-age and mean annual increment among eight streams in southeast Minnesota from 2010–2011; evaluated temporal trends in growth from 1991–1994, 2002–2003, and 2010–2011; and investigated associations between growth metrics and drainage area, groundwater input, and intraspecific density. Growth metrics were ontogenetically, spatially, and temporally variable from 2010–2011 and changed since the early 1990’s, yet relative growth differences were similar among streams. Drainage area, but not groundwater input, influenced spatial variability in growth metrics of age-2 Brown Trout. Age-1 growth was density-dependent in 2003 but became density-independent in 2010, whereas age-2 growth was unregulated by density in both years. Our results support the previous conclusion that common growth modifiers in other regions have at most a modest influence in southeast Minnesota streams and inform Brown Trout management relative to changes in climate and land use.