Size-Based Mortality Caps as Thresholds for Managing Hybrid Striped Bass in Kansas Reservoirs

Total annual mortality and mortality caps (maximum mortality thresholds) were estimated for hybrid striped bass (striped bass Morone saxatilis x white bass M. chrysops) populations sampled in six large Kansas reservoirs spanning 1995-2001; all reservoirs lacked a minimum length limit but had a 2/d creel limit. Total annual mortality of age-1 and older hybrid striped bass was estimated using a weighted catch curve. Mortality caps were modeled by varying growth rates and management objectives. Mortality rates that approach the cap signal the need for increased monitoring to determine what additional management action should be taken (e.g., harvest regulations to reduce mortality) or to reevaluate the size objective (targeted mean length of harvested fish) for the fishery. Estimated mortality caps were then compared with observed growth and mortality rates. Total annual mortality rates for all reservoirs varied from 22% to 52% (mean = 38%; SE = 4.9). All but two reservoirs had total annual mortality rates greater than 40%. Analysis of mortality caps indicated that when the objective was to maintain the mean length of harvested hybrid striped bass at current levels, total annual mortality was lower than the mortality cap for most reservoirs if a 381 mm total length (TL) minimum length limit (i.e., preferred length) were to be imposed. A management objective of 500 mm TL for the mean length of harvested hybrid striped bass was realistic for all reservoirs modeled with a 457-mm-TL length limit (special regulation available to managers) but only for two reservoirs with a 381-mm-TL length limit. Likewise, only two of the study reservoirs could support a management objective of 550 mm TL (herein defined as trophy length) for mean length of harvested hybrid striped bass when modeled with a 457-mm-TL length limit. Our analysis illustrates the potential of mortality caps for monitoring and establishing realistic management goals for hybrid striped bass fisheries.