Using Otolith Chemistry to Determine Early Life Environments and Movement of the Emerging Bigheaded Carp Population in Pools 16–19 of the Upper Mississippi River

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Abstract Bigheaded carp (Bighead Carp Hypophthalmichthys nobilis and Silver Carp H. molitrix) are invasive species in the USA and have spread throughout much of the Mississippi River basin. Population abundance upstream of Lock and Dam (LD) 19 (LD19) on the upper Mississippi River (UMR) has likely been limited by the high-head dam at this location, which restricts all upstream fish passage to the lock chamber. We measured otolith (lapillus) stable isotope composition and elemental microchemistry of 146 Silver Carp (n = 77 females, 69 males) and 141 Bighead Carp (n = 76 females, 65 males) to determine early life environments (ELEs) of adult bigheaded carp captured upstream of LD19 at the invasion front, in an area of intense management (Pools 16–19). Otolith oxygen isotope ratios (δ18O) and elemental ratios (Sr:Ca and Ba:Ca) were compared to values of isotope and elemental ratios in water from putative ELEs to assign the ELE for each fish. Most Bighead Carp (68.8%) and Silver Carp (54.1%) collected upstream of LD19 had otolith core signatures consistent with ELEs downstream of LD19. Nineteen percent of Bighead Carp and 34% of Silver Carp could not be classified. The sex ratios of bigheaded carp (Pools 17–19 combined) with otolith core signatures downstream of LD19 did not differ from 1:1. Our results, when compared to those of Whitledge et al. (2019), suggest low but stable recruitment above this population pinch-point dam in the UMR and suggest that targeting removal of bigheaded carp downstream of LD19 or inhibiting their movement upstream through the lock there might be effective as part of integrated control efforts.