Comparison of Striped Bass growth between the native and nonnative range through time

Objective The Striped Bass Morone saxatilis, native to eastern North America, was brought to the San Francisco Bay on the continent’s west coast in the late 19th century. Since then, the abundance of both native and nonnative populations has varied under changing fishing pressure and ecosystem conditions. The species has decreased in abundance in its nonnative range, while abundance in the native range has varied. Therefore, the objective of our study was to compare growth among and within nonnative and native ranges through time. Methods To meet our objective, we used long-term, fishery-independent length-at-age data (scale based) from the nonnative range in California (n = 234,248) and the native Chesapeake Bay in Maryland (n = 36,648), provided by each state’s natural resource agency. We first fitted age- and sex-specific generalized additive models and linear regressions to determine the relationship of length at age with time within each range. We used linear models, pairwise comparisons, and von Bertalanffy growth models fitted to pooled data sets and three distinct stock status time periods within each range to determine whether length at age and lifetime growth trajectories differed among and within ranges and whether results were robust to variable stock statuses. Results Striped Bass introduced into California were longer than their native counterparts in the Chesapeake Bay, generally at age 8 and younger for both sexes. Furthermore, within both ranges, the length at age changed through time. At ages up to and including age 7, native Striped Bass length decreased for both sexes, while nonnative fish length increased at age 3, followed by decreases through age 7, throughout the time series. At age 8, nonnative males and native females also exhibited declines in length. Conclusions Like other anadromous fishes, growth differed between native and nonnative ranges, with larger young Striped Bass observed in the nonnative range. Although testing the mechanisms that drive growth differences was beyond our scope of study, we hypothesized that rapid early growth in the nonnative range resulted from optimum estuarine water temperatures combined with ancestral adaptations. Similarly, we did not test the mechanisms driving decreased length at age through time. However, temporal shifts in length at age within the nonnative range were likely driven by elevated natural mortality, continual fishing pressure, and prey shifts, while temporal shifts in the native range were likely driven by increasing temperature, hypoxia, and disease, coupled with fisheries selection and prey shifts. These differences in Striped Bass growth among ranges and over time highlight the complex interplay of ecology, environment, and evolution on fish life history and have important implications for migration and management.