Big River bluegill: Combining vital rates and long-term monitoring to understand population dynamics in large rivers

Long-term monitoring indicates bluegill catch rates are relatively stable in some reaches of the Upper Mississippi River and highly variable in others, whereas in the Illinois River, catch rates have decreased. A lack of age structure information precludes understanding population processes responsible for patterns in catch rates. To build a better understanding of why catch rates have changed over time, we integrated short-term age structure information with long-term monitoring data to quantify and assess spatial patterns in bluegill population dynamics across six study reaches of these two rivers. Specifically, we estimated and compared age and size structure, growth, maturity, mortality, and recruitment. We used quantile regressions to apply age estimates to long-term data for investigating trends in age-based catch rates reflective of recruitment (age-1 catch rates), mortality (using age-1 and age-2+ catch rates), and spawning stock (age-2+). Our findings indicate trends in bluegill age-2+ catch rates increased and then stabilized across upstream study reaches, but dynamic rates, size structure, and age at maturity varied among reaches. Bluegill populations in downstream study reaches had low maximum size, early maturation, low mean age, low proportional stock density, declining recruitment, and declining age-2+ catch rates. An insufficient number of bluegills were collected from the study reach furthest downstream to adequately quantify dynamic rates. Our results support life history theory in that bluegill respond to unstable environmental conditions through life history adaptations. These findings show how integrating periodic age structure information with long-term monitoring can enhance population assessments.