Many scientists have suggested potential causes of yellow perch variable recruitment, but the true mechanisms that drive fluctuations in young-of-year (YOY) yellow perch in Lake Erie remain unknown. We built EcoLE-FisH-YP, an ecological model of yellow perch population dynamics, building on a previous hydrodynamics and lower trophic level model of Lake Erie to identify factors that may influence annual fluctuations in Lake Erie yellow perch recruitment. Modeling the lake allowed us to manipulate biotic and abiotic mechanisms that previous researchers have identified as possible causes of yellow perch variable recruitment that cannot be tested empirically in the field. We incorporated the dynamics of YOY and adult perch, commercial fishing, sport fishing, and fishery management into the model –in addition to the physical, chemical, and lower trophic level components. We calibrated the model against observed data for 1997, and validated it for 1998 and 1999, except modeled recruitment, which was calibrated to 1999 observed data and validated to 2000 observed data. Model results suggest yellow perch eggs survived least (5.9-6.3%), followed by larvae (8.8-9.6%), and then juveniles (10.3-13.2%) in all years.
To investigate possible mechanisms of Lake Erie yellow perch variable recruitment, we manipulated single variables within the model and compared YOY survival and age-2 adult recruitment results with those of the original model. Increasing spring and early summer air temperatures (+2 °C) increased recruitment up to +489% in all areas of the lake, and decreasing them (-2 °C) reduced recruitment down to -45% in all areas of the lake in both 1997 and 1998 simulations. Of all model manipulations, this effected the greatest change in recruitment and may be most illustrative of mechanisms underlying yellow perch variable recruitment, as multiple life stages were negatively (cold temperatures) or positively (warm temperatures) impacted by changes in modeled Lake Erie spring water temperatures. When manipulating Lake Erie yellow perch fecundity in simulations of three real-life scenarios, increasing or decreasing the abundance of simulated adult perch to historical highs and lows greatly changed egg abundance (up to 24-fold), which disproportionately affected recruitment (up to 4-fold); changes in egg abundance and recruitment due to elongated and shortened winters led to similar, though diminished effects on recruitment (up to ±30%), and the institution of additional commercial fishery length limits effected minor changes in recruitment (up to ±6.4%). Doubling or eliminating high wind events (>8ms-1) increased and decreased percent egg survival, respectively (up to ±2.9%), which affected recruitment up to ±6.3%. Finally, increasing and decreasing zooplankton (YOY food) abundance by ±1 and ±2 orders of magnitude produced only small changes in juvenile survival rates, which increased or decreased YOY survival and age-2 recruitment by less than 4% in all lake areas.
Ultimately, multiple mortality/survival influences work in concert to produce strong and poor year classes of yellow perch. Thus, a combination of environmental conditions that are conducive to survival at multiple life stages may provide the best opportunity for strong year classes of Lake Erie yellow perch.