Pallid sturgeon size structure, condition, and growth in the Missouri River Basin

We synthesized wild and stocked pallid sturgeon Scaphirhynchus albus capture data collected in four recovery areas of the Missouri River during 1998-2007, providing the first basin-wide analysis of size structure, growth, and condition. Proportional size distribution (PSD) values ranged from 20 to 33 and were indicative of past stockings given the continued lack of natural recruitment. A new weight-length regression derived from 2268 captured wild (8%), hatchery-stocked (75%), and unknown origin (16%) pallid sturgeon had a significantly lower slope and intercept from a previously published model that used only 214 wild fish and a truncated size range. Relative condition (Kn) declined after stocking throughout the basin but stabilized at 0.94 within 3 years. Spatially, Kn of juvenile pallid sturgeon (330-629 mm) was generally highest in the reaches of the Missouri River with large tributaries. In accordance with the latitudinal counter gradient growth hypothesis, similar absolute growth increments (both length and weight) of tagged age-1 pallid sturgeon in the Upper and Lower Missouri River indicated upstream fish grew at faster rates given the 1.3 fold difference in growing season length. From North to South, von Bertalanffy growth coefficients (k) showed a latitudinal increase while L∞ decreased, providing additional support of the latitudinal counter gradient growth hypothesis. However, growth rates of tagged juvenile pallid sturgeon aged 2-6 years were highest at the two most downstream reaches showing an increased influence of growing season length. In the Missouri River, pallid sturgeon (≤ ages 1-9 or 10 years) exhibited linear growth in the two most upstream reaches (Montana and North Dakota), exhibited logistic growth in the inter-reservoir reach in South Dakota/Nebraska while a power function best described growth downstream of the lowermost dam (Nebraska, Iowa, Kansas, and Missouri). Differing growth models among reaches highlights the confounding affect of habitat fragmentation by dams in the Missouri River on growth as well as potential latitudinal affects on sexual maturation. Relative condition and growth of pallid sturgeon appears adequate throughout the Missouri River indicating success of past stocking efforts to forestall extirpation.