Habitat Use and Movement of Sub-Adult Lake Sturgeon in the Lower Wolf River, Wisconsin

Abstract

Lake Sturgeon Acipenser fulvescens populations have experienced precipitous declines over their native range due to high exploitation and habitat fragmentation. Because of these declines, harvest on many Lake Sturgeon populations was banned during the twentieth century and the need for research to aid in rebuilding populations was recognized. Previous research has largely focused on adult Lake Sturgeon, including their habitat use and movement. However, there has been little research on sub-adult Lake Sturgeon. Since Lake Sturgeon do not mature until 12 to 27 years of age, changes in recruitment may go undetected for relatively long periods of time, if only the adult portion of the population is monitored in standard surveys. Habitat use and movement of sub-adult Lake Sturgeon is largely unknown for most systems. Establishing patterns of habitat use and movement at this life stage could assist managers in capturing sub-adult fish, which would provide a better understanding of recruitment. Sampling sub-adult sturgeon can provide a more immediate and accurate method for assessing the effectiveness of management actions on Lake Sturgeon recruitment. This monitoring is especially important for exploited populations, where changes to harvest management could be implemented if recruitment declines and increases can be detected before cohorts reach adulthood. The Lake Winnebago System Lake Sturgeon population supports an annual spear fishery with average annual harvests of approximately 1,400 fish from a population of adults that is estimated at approximately 42,000 fish. Little information on sub-adult iv Lake Sturgeon in the Lake Winnebago System is available and standardized sampling has not targeted these fish in the past. Fishery managers are interested in determining habitat use of sub-adult Lake Sturgeon to aid in developing recruitment surveys to better understand the population at this life stage. While sub-adult Lake Sturgeon likely occupy multiple habitats in the Lake Winnebago System, the first phase of this research focused on sub-adult Lake Sturgeon in the lower Wolf River. The objectives of my study were to determine if: 1) numbers of sub-adult Lake Sturgeon in the lower Wolf River are sufficient to justify sampling this portion of the Lake Winnebago system as part of a basin-wide recruitment survey; 2) linear home range or movements of sub-adult Lake Sturgeon in the lower Wolf River varies in relation to season, sex, or total length (TL) category (small < 96.0 cm TL; large ≥ 96.0 cm TL) and 3) sub-adult Lake Sturgeon selectively occupy certain habitats in the lower Wolf River in terms of substrate and channel morphology. A total of eighteen sub-adult Lake Sturgeon were captured on the lower Wolf River during fall 2013 and 2014 using various sampling techniques. A total of 618.5 hours were invested in attempting to capture these fish. Sub-adult Lake Sturgeon were surgically implanted with radio-transmitters and released back into the river near capture locations. Relocation of fish was attempted every two weeks over the two years of the study, except for winter, when relocation attempts occurred once per month. Latitude, longitude, and substrate type were recorded at each relocation. Data collected were used to determine overall, annual, and seasonal linear home range sizes for each fish. Side-scan sonar was used to collect images of the lower Wolf River that were uploaded into ArcGIS™. Substrates were identified with color-coded polygons that corresponded to different substrate types to create a substrate map of the entire study area. This map was used in conjunction with fish relocations to determine substrate use. Linear home ranges did not differ in relation to sex or TL category, but did vary among seasons. The majority of movement for sub-adult Lake Sturgeon occurred in spring, which is similar to trends observed in previous studies of adult fish. Additionally, fish usually exhibited limited movement during winter (linear home range < 0.5 rkm). Selection ratios indicated that sub-adult Lake Sturgeon were not selecting for any substrate or channel morphology type. Although some sub-adult Lake Sturgeon do occupy the lower Wolf River, I conclude that sufficient numbers of sub-adult Lake Sturgeon do not reside in the river to justify extensive sampling as part of a recruitment index survey. This information is important because sampling effort is typically limited by cost and logistics and my study suggests this effort may be better expended in other locations within the system. However, additional work is needed to determine where the largest concentration of subadult Lake Sturgeon reside within the Winnebago System, as there is still interest in developing a method for sampling sub-adult Lake Sturgeon to monitor recruitment trends before fish reach adulthood.