Daily, Incremental Changes in Incubation Temperature Alter Metabolism and Hatchling Phenotype of Developing Lake Whitefish
Authors: Eme John , Mueller Casey A. , Manzon Richard G. , Somers Christopher M. , Boreham Douglas R. , Wilson Joanna Y. 
Published: November 2023

Temperature has pervasive effects on poikilotherm physiology, and embryos and newly hatched/born animals are especially vulnerable to temperature shifts. Lake whitefish (Coregonus clupeaformis) are a North American, freshwater lake species with lengthy (80–150 day, 2–8 °C) overwintering embryonic development. Stenothermic, cold‐water embryos may be particularly susceptible to thermal shifts caused by anthropogenic thermal effluent and climate change. We incubated whitefish embryos in petri dishes (N=2000, 6–10 replicates each treatment) at constant treatments of 2.0±0.1°C, 5.0±0.1°C, and 8.0±0.1°C (mean±SD), and in a variable temperature treatment (VT; N=500, each treatment). In the VT, a daily 2°C temperature change (2°C–4°C‐6°C–8°C and back from 8°C‐6°C‐4°C‐2°C) occurred throughout embryonic incubation (mean 5.0±0.3°C). Mortality was quantified as number of hatchlings÷starting embryo number, time to hatch (days post fertilization, dpf) was recorded, and hatchling yolk‐free dry mass was measured. Oxygen consumption rate (VO2 ; N=8–10, each treatment) was measured at 60% of embryonic development and in hatchlings within 3 days post hatching. VO2 was measured for the four treatments (2°C, 5°C, 8°C, VT) at each constant temperature of 2°C, 5°C, and 8°C to determine if V.O2 of VT embryos resembled the VO2 of other treatments. VO2 was measured at 2°C, 5°C, and 8°C after exposing embryos/hatchlings from each treatment to a daily 2°C temperature change, if necessary, until embryos/hatchlings reached the measurement temperature; embryos/hatchlings remained at the measurement temperature for a 24 hr acclimation period, and VO2 was measured. Mortality was significantly affected by treatment (KWRS, P<0.001, Steel‐Dwass post hoc), with 8°C (14.8%±17.6) and VT hatchlings (83.2%±0.07) having a significantly reduced mortality compared to 2°C (88.4%±0.02) and 5°C (90.5%±0.04) hatchlings. Time to hatch (dpf) was significantly different across all treatments (1‐way ANOVA, P<0.001, Duncan post hoc) ‐ 8°C (68±2), VT (111±4), 5°C (116±4), 2°C (170±3). Likewise, hatchling yolk‐free dry mass (mg) was significantly different across all treatments (1‐way ANOVA, P<0.001, Duncan post hoc) ‐ 8°C (0.66±0.08), VT (0.97±0.06), 5°C (1.07±0.03), 2°C (1.36±0.04). VO2 was significantly affected by the interaction between treatment and measurement temperature (2, 5, 8 °C) at 60% of embryonic development and for hatchlings (2‐way ANOVA, P<0.01), and hatchling VT whitefish had a significantly different VO2 compared to 5°C measured at 2, 5 or 8 °C (1‐way ANOVA, P<0.001, Duncan post hoc). This study demonstrates that variable thermal embryonic incubation produces fewer (increased mortality), smaller embryos that hatch earlier. The plasticity of VO2 for this stenothermic‐incubating fish species under variable incubation conditions could represent a metabolic cost to fluctuating thermal incubation conditions.Support or Funding InformationJ.E. and C.A.M. were supported by a MITACS Accelerate grant to D.R.B and J.Y.W. Funding was provided by Bruce Power, Inc. and a Collaborative Research and Development Grant from the Natural Sciences and Engineering Research Council of Canada to J.Y.W., R.G.M., and C.M.S.