Chapter 5: Factors Affecting Recruitment of Paddlefish: Hypotheses and Comparisons with Sturgeons
Dennis L. Scarnecchia, Jason D. Schooley, Steven O. McAdam, K. Michael Backes, Aaron Slominski, Youngtaik Lim, and David D. Fryda
Abstract.—Inadequate recruitment is a concern in many stocks of Paddlefish Polyodon spathula. Despite the importance of maintaining adequate recruitment, little understanding exists of specific recruitment factors and mechanisms. In this chapter we review and synthesize the results of the relatively few studies and observations of Paddlefish reproduction and recruitment and the factors potentially responsible for observed variations. Comparisons are made with studies on sturgeons. Although Paddlefish and sturgeon show many anatomical, life history, and behavioral similarities, some key features of Paddlefish differing from other Acipenseriform species are lifelong zooplanktivory, filter feeding, midwater foraging behaviors, and lack of armoring. Data from both sturgeon and Paddlefish show higher reproductive success associated with higher river discharge; greater Paddlefish reproductive success in some adfluvial populations has also been linked to increasing and high reservoir levels. Whereas several sturgeon studies suggest that recruitment (and year-class strength) are determined within 2–3 months of hatching, results from Paddlefish suggest that year-class strength may be determined later in their first year or in their second year. Observations and field results lead to the hypothesis that young Paddlefish must grow fast in their first and early second years of life to reach a size where they can successfully overwinter, avoid predation from piscivorous fishes and birds, and recruit. Support for this grow-fast-or-be-eaten hypothesis comes from several sources, both indirect (e.g., the relation between fish length and rostrum length) and direct (higher survival of larger fish released and higher lipids in age-0 fish in years of good recruitment). However, more research is needed to adequately test this hypothesis. Paddlefish rearing habitat changes include river-backwater and side-channel sedimentation, reservoir sedimentation and aging, and threats from increased native and nonnative predator populations. Managers of Paddlefish will need a more detailed understanding of the habitat conditions needed for successful recruitment in the wild.