Climate Change May Cause Shifts in Growth and Instantaneous Natural Mortality of American Shad Throughout Their Native Range

Abstract

American Shad Alosa sapidissima is an anadromous species with populations ranging along the U.S. Atlantic coast. Past American Shad stock assessments have been data limited and estimating system-specific growth parameters or instantaneous natural mortality (M) was not possible. This precluded system-specific stock assessment and management due to reliance on these parameters for estimating other population dynamics (such as yield per recruit). Furthermore, climate-informed biological reference points remain a largely unaddressed need in American Shad stock assessment. Population abundance estimates of American Shad and other species often rely heavily on M derived from von Bertalanffy growth function (VBGF) parameters. Therefore, we developed Bayesian hierarchical models to estimate coastwide, regional, and system-specific VBGF parameters and M using data collected from 1982 to 2017. We tested predictive performance of models that included effects of various climate variables on VBGF parameters within these models. System-specific models were better supported than regional or coast-wide models. Mean asymptotic length (L ∞ ) decreased with increasing mean annual sea surface temperature (SST) and degree days (DD) experienced by fish during their lifetime. Although uncertain, K (Brody growth coefficient) decreased over the same range of lifetime SST and DD. Assuming no adaptation, we projected changes in VBGF parameters and M through 2099 using modeled SST from two climate projection scenarios (Representative Concentration Pathways 4.5 and 8.5). We predicted reduced growth under both scenarios, and M was projected to increase by about 0.10. It is unclear how reduced growth and increased mortality may influence population productivity or life history adaptation in the future, but our results may inform stock assessment models to assess those trade-offs.

What Have We Lost? Modeling Dam Impacts on American Shad Populations Through Their Native Range

“…Despite the fact that total available habitat was divided fairly evenly among eco-regions, the relative scope for population size is substantially greater in the NI, decreased in the SI and lowest in the SM ecoregion due to differences in life history. While this is intuitive based on the degree of iteroparity, growth rates, and maximum sizes (Gilligan-Lunda et al, 2021), it highlights the difference between relative and absolute losses of spawner potential due to dams. The SM eco-region's lost scope of 1.74 million fish is dwarfed by the loss of more than 13 million in the NI ecoregion, suggesting that parallel differences in fisheries potential also exist.…”

“…When considering the potential for all 164 rivers to support spawning runs of adult American shad, estimates of spawner potential are directly linked to estimates of available habitat (Figure 7). However, latitudinal variation in growth, maturation, and post-spawn survival (Leggett and Carscadden, 1978;Gilligan-Lunda et al, 2021) all influence the relative differences observed in population potential. On average, a square km of habitat results in 55,200 spawners in the NI eco-region but only 31,000 in the SI eco-region.…”

“…Early and influential assessments of these clinal variations in life history traits (i.e., Carscadden and Leggett, 1975;Leggett and Carscadden, 1978;Glebe and Leggett, 1981a) remain heavily relied upon by both researchers and managers. American shad are entirely semelparous (and more fecund) in the southern rivers with a lower length at age (Leggett and Carscadden, 1978;Gilligan-Lunda et al, 2021) than in northerly populations. North of the Cape Fear River in North Carolina, some level of repeat spawning is observed, and the proportion increases in rivers to the north.…”

“…The importance of understanding fish passage in light of both upstream and downstream efficiencies and survival was made evident. This approach was then applied to a series of rivers, range wide, based on informed habitat assessments (Gilligan-Lunda et al, 2021). Similar work by Barber et al (2018) used this population modeling approach to characterize the delivery of nutrient by another alosine, the alewife (Alosa pseudoharengus), by employing estimates of nutrient loss, delivery, and exodus.…”

Quantification of land–sea nutrient fluxes supplied by allis shad across the species’ range

“…Based on the existing literature, three life cycle processes, i.e. including growth at sea (Gilligan-Lunda et al 2021), survival of spawners , and survival of early-life stages in rivers , were linked to temperature to account for whole-life cycle impacts to global warming ). To assess the potential for shad to deliver nutrients across European river basins, an original nutrient routine providing estimates of nutrient imports and exports was designed and combined with GR3D.…”