Demographic Properties of Largemouth Bass in the Chesapeake Bay Watershed

Largemouth bass Micropterus salmoides is a widely distributed, apex predator of inland streams and ponds of North America.  It has been introduced to many new ecosystems because of its popularity as a sport and food fish.  Managing the number or minimum size that can be harvested is often required for protecting these populations from extirpation.  The Chesapeake Bay watershed (Maryland, U.S.A.) offers a wide range of suitable habitats for largemouth bass, but is also encumbered by locally high levels of nitrogenous waste.  While habitat conditions of most tidal rivers support largemouth bass populations, there are only two highly popular fisheries (Potomac River and upper Chesapeake Bay) that support hundreds of largemouth bass tournaments each year.  Here, I determined if and which properties of largemouth bass demography: 1) differed between heavily fished and less fished drainages; and 2) varied with interannual and intrannual variation in habitat conditions.  Demographic properties were measured for 4–12 years for isolated populations in tidal fresh regions of 7 Chesapeake Bay drainages.  Demographic variation among populations and over time was mainly described by variation in abundance, longevity of individuals (i.e., size structure), and somatic growth.  For two heavily fished drainages, populations were larger and more variable in size (1999 – 2010) relative to other drainages.  The size of these populations has likely led to their popularity in the fishery.  More largemouth bass were caught in drainages with water that was warm (annual average water temperature > 16 °C) and had low levels of nitrogen (annual average total nitrogen < 0.85 mg/L).  Longevity did not differ between heavily fished populations and less fished populations.  Instead, longevity varied among years and tended to be lowest when intrannual variation in total phosphorus and total suspended solids varied more than 8% and 15.2%, respectively.  More stable water quality or clarity conditions may promote greater longevity or lower mortality rates for the population.  Changes in nutrient availability may explain significant increases in average somatic growth since 1999, but such relationships were not robust.  Patterns of growth did not differ among drainages, despite fishing intensity.  Concentrations of nitrogen or phosphorus in waterways can affect the structure of plankton communities, biomass of benthi-planktivorous fishes, and prey availability of both juvenile and adult largemouth bass.  Elucidation of these ecosystem-level interactions may improve management of the fishery by directing mitigation efforts, such as reducing nitrogenous waste.