Preservation in formalin variably affects egg size parameters of Walleye

Fish egg studies provide insights into reproductive health, hatchery success, and population dynamics. Metrics for assessing egg quality, such as diameter, weight, and energetic content, are challenging to accurately measure because eggs degrade rapidly, and eggs are often preserved with formalin. Research is limited and has shown mixed results regarding effects of formalin preservation on egg diameter. No data exist on the influence of formalin preservation on oil droplet diameter. Egg and oil droplet diameters can be indicators of egg nutritional condition and subsequent larval survival and dynamics. We analyzed the diameter of Walleye Sander vitreus egg and oil droplets before and after formalin preservation from one Wisconsin and five Minnesota waterbodies in 2023. Preservation in 10% formalin altered egg and oil droplet diameters. The preservation effect differed depending on the initial egg or oil droplet diameter. Larger and smaller eggs decreased and increased in diameter following storage in formalin, respectively. Preserved eggs were more challenging to measure than fresh eggs due to changes in morphology, burst oil droplets, and increased opacity. Even though the preservation effects were small (5–12%), we recommend that studies involving precise egg measurements avoid formalin storage. The error introduced by formalin preservation was sufficient to prevent detection of small changes, particularly in the oil droplet.