Insights into the 2015 Invasion of Mississippi River Tributaries in Arkansas By Young-of-Year Silver Carp (Hypophthalmichthys molitrix)

On August 6, 2015 innumerable young-of-year Silver Carp (Hypophthalmichthys molitrix) were observed actively swimming up the lower White River approximately 16 river kilometers upstream from the Mississippi River confluence.  Sixty-six individuals were opportunistically sampled using a dip net. Mean TL was 59 mm (Range = 50 – 71 mm).  Additionally, 29 young-of-year Silver Carp were opportunistically sampled from the St. Francis River on 14 August 2015 approximately 21 river kilometers upstream of the Mississippi River confluence using a dip net.  Mean TL was 51 mm (Range = 40 – 62 mm).  A subsample was aged using lapilli otoliths to estimate date of spawn; mean age was 31d for the White River (Range = 26 – 37d) and 29.5d for the St. Francis River (Range = 28 – 33d).   Water level in the Mississippi River was rising during the estimated date of spawn, while flow was minimal in the lower White and St. Francis rivers. We hypothesize increased summer flow in the Mississippi River triggered spawning with subsequent dispersal into tributaries, perhaps attracted by increased tributary flow. Understanding river conditions surrounding invasion may provide management opportunities for this species as it attempts to recruit into tributaries of the lower Mississippi River.