Biological, Chemical, and Physical Factors Influencing Young-of-Year Brook Trout Distribution and Abundance in Adirondack Lakes

Young-of-year (YOY) brook trout were captured by backpack electrofishing during late July-early August 2008-2010 in five Adirondack lakes varying in thermal stratification patterns and acid impairment.  Three different habitat types were sampled in each lake: tributaries, seeps (groundwater influenced areas), and lake shoreline areas (no groundwater influence).  Habitat and water quality information was collected at each electrofishing site to determine the chemical and physical factors influencing brook trout distribution and abundance.  Model sets were developed and compared using Akaike’s Information Criterion (AIC_c) to determine which physical, chemical, and biological (# of redds the year prior) factors best explained the presence/absence and abundance of YOY brook trout.  The proportion of YOY brook trout captured in each habitat type was consistent across years within each lake; however, the utilization of habitat type was not consistent between lakes and varied with habitat availability.  In four of the study lakes, the proportion of YOY brook trout captured was greatest in tributaries (range: 57 – 100%) followed by seep (range: 6 – 43%) and then lake shoreline habitat type (range: 0 – 11%).  In the lake with least amount of tributary habitat, the proportion of YOY brook trout captured in the seep habitat (range: 38 – 72%) was equal to or greater than the tributary habitat (range: 21 – 41%).  The model that best explained the presence/absence of YOY brook trout in tributaries included inorganic monomeric aluminum (Al_im), stream temperature, and topographic index value (a measure of groundwater discharge).  The model averaged inflection point of Al_im was 1.67 µmol L^-1.  Tributaries that contained Al_im concentrations above this value had less than a 50% probability of occupancy.  Al_im concentrations above 2.0 µmol L^-1 are generally recognized to negatively affect aquatic biota (Driscoll et al. 2001).  YOY brook trout densities were significantly greater (P < 0.001) in tributaries with Al_im concentrations below the inflection point.  This work confirms: 1) the importance of seep and tributary habitat, and 2) that elevated Al_im resulting from acid impairment restricts the distribution and abundance of YOY brook trout.