Stock Discrimination of Lake Sturgeon (Acipenser fulvescens) in the Lake Winnebago System Using Otolith and Fin Ray Microchemistry

Abstract

Stock discrimination is important in fisheries management and conservation for preserving individual stocks and thereby maintaining a robust population resilient to future changes. Genetic stock discrimination is widely used, but may not be applicable when fish spawning at different locations do not represent genetically distinct stocks or genetic stock discrimination studies provide conflicting results. When this is the case, chemical signatures in calcified structures might be used to discriminate among groups of fish that hatch or reside in different locations and might functionally be considered as separate stocks.

Otolith microchemistry provides a means of stock discrimination via analysis of trace elements in the otoliths of a fish. Otoliths can be used as a record of the life history of a fish and its exposure to various chemical environments over time. Alternatively, fin ray microchemistry may provide a non-lethal alternative to otolith microchemistry as the collection of fin rays does not require fish sacrifice. Non-lethal methods of stock discrimination are needed when dealing with an imperiled species or a species that is vulnerable to overharvest, as is the case with lake sturgeon Acipenser fulvescens.

The Lake Winnebago system (LWS) supports one of the largest self-sustaining populations of lake sturgeon in North America and provides for a spearing fishery that occurs in February each year. Lake sturgeon spawn in multiple tributaries to the LWS but the extent of recruitment occurring in each river and the relative contribution of these recruits to annual spearing harvest remain unknown. Verifying that larval lake sturgeon hatch at specific spawning locations within these tributaries is important to understanding where recruitment may or may not be occurring within LWS tributaries. Furthermore, all lake sturgeon harvested during the spearing season must be registered with the Wisconsin Department of Natural Resources (WDNR) at a designated station in the area where they were harvested, providing a unique opportunity to collect lake sturgeon otoliths and fin rays for potential use in stock discrimination. Thus, the objectives of my study were to determine if: 1) larval relative abundance varies among spawning locations within the LWS; 2) otolith microchemistry can be used to discriminate among age-0 lake sturgeon residing in rivers where spawning occurs; 3) chemical signatures in fin rays cluster fish in a similar manner to otolith signatures and 4) contribution to spearing harvest determined from microchemistry analyses varies among rivers.

Larval lake sturgeon were collected using D-frame nets deployed at known spawning sites throughout the LWS during post-hatch drift following spawning to assess the relative abundance of larvae among sites. Water samples were collected among various locations throughout the LWS in 2017 and again at the same sites in 2020 to determine whether water chemistry differed among rivers. Otoliths were collected from larval lake sturgeon, and both otoliths and fin rays were collected from larval lake sturgeon, age-0 sturgeon at the end of summer and from adult sturgeon harvested from the annual winter spearing season in the LWS. Otoliths and fin rays from age-0 fish were intended to provide baseline chemical signatures for rivers where lake sturgeon hatch and resided as age-0 fish. My intent was to assign river of origin for adult fish based on the chemical signatures observed within the core (innermost portion) of adult otoliths and fin rays. Water samples were analyzed via inductively coupled plasma-optical emission spectrometry. Calcified structures from lake sturgeon were analyzed via laser-ablation inductively-coupled-plasma mass spectrometry.

Larval lake sturgeon were collected at all spawning locations in the Wolf River drainage, but no larval lake sturgeon were captured during sampling in the Fox River at the Princeton spawning site. Relative larval abundance did not differ between sampling years nor among spawning sites. Principal components analysis of water chemistry showed similar chemical signatures within individual rivers between sampling years and suggested two clusters of chemical signatures, one that encompassed sampling sites in the Wolf River basin and one that encompassed sites in the Fox River basin. No age-0 lake sturgeon were captured during spotlight surveys conducted on the Fox River, and therefore I could not establish a baseline chemical signature for this river. Cluster analysis indicated multiple clusters best accounted for observed variation in the mean elemental ratios of otoliths and fin rays from juvenile lake sturgeon collected in the Wolf River and its tributaries. However, these clusters did not align with river of origin. The clusters included individuals from two or more rivers, indicating that chemical signatures in otoliths and fin rays would not allow for discrimination among lake sturgeon originating in the Wolf, Little Wolf, and Embarrass rivers.

Cluster analyses indicated that 2 clusters best accounted for the variation observed in chemical signatures observed in otoliths and paired fin rays removed from adult lake sturgeon harvested during the spearing season. However, adult otolith clusters do not align with the differences observed in water chemistry. For water chemistry, Mn:Ca, Ba:Ca, and Mg:Ca (in order of relative importance) were important in delineating between the two observed groups in water samples but Ba:Ca, Sr:Ca, and Mn:Ca appeared to be important in delineating between adult otolith clusters. Clusters were not consistent among adult calcified structures with only 46.3% of individual fish included in the same cluster when comparing cluster composition between otoliths and fin rays.

While water chemistry differed among the Wolf River drainage and the Fox River, I could not determine if otoliths and fin ray microchemistry would allow for discrimination among fish hatching in these two drainages because no juvenile lake sturgeon were collected from the Fox River. Additionally, the lack of larvae collected at Princeton suggests that the Fox River may not represent a significant source of lake sturgeon recruitment for the LWS. Microchemistry of juvenile otoliths and fin rays indicated that otolith microchemistry does not provide means to discriminate among juvenile lake sturgeon hatching in the Wolf River and its tributaries. Continued larval and juvenile sampling to confirm lack of hatching in the Fox River is warranted and efforts to determine why hatching success is relatively low in the Fox River could help inform future habitat management. Evaluating otolith chemical signatures in surrogate species might provide a means to determine if this approach can be used to discriminate among fish spawning in the Fox and Wolf rivers, but if the Fox River is not an important source of lake sturgeon recruitment this may be of little consequence. If the lake sturgeon population in the LWS is largely supported by recruitment from the Wolf River and its tributaries, another method will be needed to discriminate among fish spawning in these rivers given similarities in water and otolith chemistry.