Otolith Microstructure

Otolith Microstructure

Natal origin and growth rate of lake whitefish (Coregonus clupeaformis) larvae in the lower waters of Green Bay and Lake Michigan

The University of Wisconsin-Green Bay                                                                                                                                              January 2017 to May 2019

In an effort to develop tools to improve our understanding of the early life history and population dynamics of Lake Whitefish, this study validated the use of otolith microchemistry and daily age estimation of larvae during drift. Factors influencing the survival of larvae have significant impacts on year class strength and thus ultimately the metapopulation structure of Lake Whitefish. The ability to trace individuals back to their natal location will help managers identify spawning sites contributing significantly to the population. This may in turn help managers understand why while many of the other Great Lakes Lake Whitefish stocks are declining, the Green Bay population is growing.

To this end, otolith microchemistry was applied to larvae collected from three of the four major Green Bay tributaries (Fox, Oconto, Peshtigo and Menominee Rivers) and an open water location in Green Bay. It was found that of the 67 larvae analyzed 53 could be correctly reclassified as either originating from the tributaries or open water Green Bay. To determine if it was possible to reclassify individuals to their correct natal site a Linear Discriminant Function Analysis (LDFA) that included each sampling location was performed. This LDFA produced an average reclassification success of 53.7% with site-specific reclassification success ranging from 69.6% to 44.4%. The reclassification of individuals to their natal sites was primarily due to differences in the ratio of Barium and Strontium to Calcium, which is created by underlying differences in water chemistry directly dictated by regional geology.

These results were within the range found in other Great Lakes otolith microchemistry studies and currently offers better delineation for these populations than what is possible with genetics. It is likely the anthropomorphic associated stock collapse in the 1930’s and 40’s caused the loss of genetic diversity throughout the Great Lakes Lake Whitefish populations. For this reason, the ability of otolith microchemistry to act as a natural tag may improve stock delineation, by taking advantage of heterogeneous water chemistry created by regional differences in geology. With refinement, this method should be applicable to the cores of adult Lake Whitefish otoliths, helping managers identify spawning locations contributing the most individuals to the harvestable population.

Due to the importance of larval production to year-class strength in Lake Whitefish, it is important to understand how differences in natal environment affect the growth and survival of larvae. To identify the techniques capable of accurately producing site-specific growth rates, the second chapter of this thesis focused on validating the methods best able to estimate age in larval Lake Whitefish. Daily circuli counts have been successfully used to estimate daily age in other coregonids, and here it was found that 81 of 140 sets of otoliths could be aged within +/- 5 days of the known age. Difficulty in accurately reading circuli originated from artifacts in the otoliths, such as sub-daily increments and/or faint or missing circuli, which were both impacted by prenatal environment. Stress caused by the lack of food produced a slower otolith crystallization rate, exacerbating the effects of these features leading to lower accuracy of age estimates. Larvae are unlikely to experience continual starvation and cold water temperatures in natural setting, most likely making their otolith formation more similar to individuals reared in ideal conditions. It was possible to age individuals from this treatment group within +/- 5 days of the known daily age approximately 63% of the time. Perhaps a viable alternative to circuli counts would be the use of sagittal otolith surface area. Although, natal environment did affect the strength of the relationship with known daily age, for the Fed treatment group otolith surface area was able to account for 80.6% of the variation within the data. In addition to having a stronger relationship than circuli counts, otolith surface area requires less preparation and man-hours. Either of these methods could be applied to wild-caught larval to produce site-specific growth rates.

Combining daily age estimation with otolith microchemistry will allow managers to trace an individual back to their natal site and help to identify how site-specific differences in natal environment affect growth rate. This will in turn help managers to identify the natal locations that require protection and guide restoration activities to improve natal environments for this ecologically and economically significant species. The research presented in this thesis validates the usefulness of these new methods for the study of Lake Whitefish and with further refinement otolith microchemistry and larval daily age estimation could become important tools for the management of Lake Whitefish throughout the Great Lakes.