Chapter 7: Cryptic Mortality of Paddlefish
Phillip W. Bettoli, George D. Scholten, and Eric Ganus
Abstract.—Cryptic or unobserved fishing mortality in marine and freshwater fisheries has received much attention in recent decades and in this chapter we discuss various forms of cryptic mortality in Paddlefish Polyodon spathula. Specifically, we review the literature on catch-and-release (CAR) mortality in recreational sport fisheries, bycatch mortality in commercial fisheries, and wounding of Paddlefish entrained through dams or struck by recreational or commercial vessels. We also discuss opportunities to mitigate some forms of cryptic mortality. Indirect evidence suggests that CAR mortality of Paddlefish snagged in sport fisheries is minimal, probably because many of those fisheries occur when water temperatures are cool. Similarly, ample evidence points to minimal postrelease mortality of Paddlefish captured in commercial or research gill nets fished for short periods of time or overnight at cool (<11°C) water temperatures. Conversely, Paddlefish netted in warm (>16°C) waters experience high (35–71%) mortality rates. Chondrosteans exhibit depressed stress responses and faster recovery than teleosts and that trait may explain why Paddlefish often exhibit high postrelease survival after encountering fishing gear. Cryptic fishing mortality of Paddlefish arising from fishers targeting other commercial species has received scant attention. The spread of Asian carp Hypophthalmichthys spp. in the Mississippi River basin has prompted aggressive harvest strategies during which Paddlefish are often captured, which argues for increased scrutiny of Paddlefish bycatch in those fisheries. A final source of cryptic mortality of Paddlefish are mortal wounds resulting from boat and propeller strikes, and dam passage. Propeller strikes are a high-profile concern for endangered marine fish and sea mammals, but it is not clear whether this line of inquiry for Paddlefish warrants further study. Paddlefish in some locales experience high rates of amputated or damaged rostrums resulting from impacts with boats, or entrainment through turbines or passage through flood gates, and body condition is usually poorer in injured fish.