Egg Hatchability and Egg Thiamine Concentrations in Lake Trout from the Northern Refuge of Lake Michigan

Abstract

The Northern Refuge of Lake Michigan was established in 1985 under the auspices of the Great Lakes Fishery Commission as part of an intensely managed effort to restore a self‐sustaining Lake Trout Salvelinus namaycush population to the lake. Despite the stocking of millions of Lake Trout into the refuge and survival of these stocked fish to adulthood, signs of natural recruitment by Lake Trout in the refuge have been very sparse. To ascertain whether low egg viability or poor fry survival could be limiting natural recruitment by Lake Trout in the refuge, hatching rates of eggs taken from Lake Trout caught in the Northern Refuge during October 2019 were determined in the laboratory. Moreover, survival rates of the hatched‐out fry were determined along with the thiamine concentrations in the fry. In addition, thiamine concentrations in eggs of ovulating Lake Trout caught in the Northern Refuge during 2019–2021 were determined. Egg hatching rate in the laboratory averaged 93%. Survival rate of Lake Trout fry up to 5 weeks after hatching averaged 97%. Mean (SE) thiamine concentrations in eggs of Lake Trout caught in 2019, 2020, and 2021 were 19.1 (3.6), 5.9 (1.3), and 6.2 (0.4) nmol/g, respectively; these levels are well above the threshold concentration of 2.3 nmol/g, above which the likelihood of experiencing mortality due to thiamine deficiency complex (TDC) is low. Thiamine concentrations in the Lake Trout fry at the end of the laboratory experiment averaged 11.2 nmol/g, and abnormal behavior in the fry was not observed. We conclude that TDC does not appear to represent a major impediment to Lake Trout rehabilitation in the Northern Refuge at present, although TDC may have contributed to the lack of detectable natural recruitment in the past.

Lake Trout population dynamics in the Northern Refuge of Lake Michigan: Importance of stocking rate

“…These early signs of natural recruitment appeared to be related to mitigation of the major impediments. The negative effects of Alewives on natural reproduction were likely too great to allow for successful natural recruitment of Lake Trout in the refuge during the 1990s (Madenjian and Desorcie 2010; Madenjian et al 2022). However, the strength of these negative effects has been reduced since the early 2000s via declining adult Alewife abundance and increasing levels of thiamine concentrations in Lake Trout eggs (Madenjian et al 2022).…”

“…The negative effects of Alewives on natural reproduction were likely too great to allow for successful natural recruitment of Lake Trout in the refuge during the 1990s (Madenjian and Desorcie 2010; Madenjian et al 2022). However, the strength of these negative effects has been reduced since the early 2000s via declining adult Alewife abundance and increasing levels of thiamine concentrations in Lake Trout eggs (Madenjian et al 2022). Lake Trout spawner abundance exceeded 17 fish • 305 m −1 • day −1 , the threshold value identified by Selgeby et al (1995) for sustaining natural recruitment of Lake Trout, throughout the 1990s on Boulder Reef and Gull Island Reef, yet natural recruitment was not detected.…”

“…Abundance of adult Alewives in Lake Michigan was at record‐low levels during 2014–2021 (Madenjian 2022; Warner et al 2022), which would be expected to further facilitate natural recruitment of Lake Trout. In addition, the observed thiamine concentrations in eggs of Lake Trout from the Northern Refuge during 2019–2021 indicated that TDC is currently not an impediment to Lake Trout rehabilitation in the refuge (Madenjian et al 2022).…”

“…Sea Lamprey wounding rate on Lake Trout in the Northern Refuge was lower during 2015–2021 than during any other time since 1992. Adult Alewives in Lake Michigan have been at record‐low abundance since 2014 (Madenjian 2022; Warner et al 2022), and Lake Trout natural recruitment in the refuge does not appear to be restricted by TDC (Madenjian et al 2022). Finally, early signs of natural recruitment of Lake Trout have become evident in 2021.…”