Projected lake sturgeon recovery in the Milwaukee River, Wisconsin, USA

Using a forward projecting population model, the timing and scale of Lake Sturgeon Acipenser fulvescens (LS) recovery and natural reproduction was estimated for the Milwaukee River in Wisconsin, USA for the years 2004-2048. LS, a widely distributed potamodromous Acipenseridae species in North America, have, like other sturgeon species, suffered population declines due to overharvest and other factors since the 1800s. LS recovery efforts were initiated in the early 1980s following the successful development of LS propagation techniques and stocking has become a widely utilized tool in LS recovery programs. Sturgeon recovery programs are long-term endeavors, with annual stocking usually planned for 25 plus years, along with population assessments and habitat improvements conducted over decades before project success can be verified. LS recovery activities on the Milwaukee River, a Lake Michigan tributary in southeast Wisconsin, began with barrier mitigation in 1997 and stocking in 2003. The modest size of the Milwaukee River, along with habitat improvements, and 17 years of stocking, provided an opportunity to model and predict LS population recovery trends. A forward projecting population model with an imbedded stochastic Ricker stock-recruitment relationship, adjusted for the estimated LS productive capacity of the Milwaukee River, estimated the timing and scale of LS recovery during 2004-2048 predicting a 6.5% annual average population rate of increase. Accelerated LS growth and maturity built into the model, due to goby consumption, resulted in projections of gravid adult males present in 2012, 9 years post initial stocking, and spawning adult females, and natural recruitment in 2019, 16 years post initial stocking. During the 2020 spawning period a group of adult-sized LS were observed in the Milwaukee River below an intermittent barrier on the river. The model predicted adult LS densities to grow from 221 in 2012 to 11,443 in 2048, and annual natural recruitment to grow steadily from 700 yearlings in 2020 to 3,800 yearlings/year during 2039-2049. The estimated number of yearling recruits per spawning (gravid) female dropped steadily from 20 in 2019 to five recruits per gravid female in 2048 as a function of the Ricker stock recruitment relationship built into the model. The model results indicate that significant LS recovery can possibly begin in a shorter time frame than originally expected on Great Lakes tributaries through systematic resolution of habitat issues and LS stocking. Results can also be used to help anticipate recovery trends, fine tune stocking and habitat strategies, and plan LS recovery assessments.

Automated summary

Utilizing a population model, the study predicted the recovery and natural reproduction of Lake Sturgeon in the Milwaukee River from 2004 to 2048, estimating an average annual population growth rate of 6.5%. Through stocking and habitat improvements, the research suggests significant Lake Sturgeon recovery can possibly begin in a shorter time frame than originally expected on Great Lakes tributaries.