Microplastics (MP), defined as plastic particles < 5 mm in one dimension, are emerging contaminants that threaten aquatic organisms. The impacts of MP on fish are shown to vary across studies, influenced by factors such as fish species, type and size of microplastics, and exposure levels and routes. The objective of this study was to investigate the effects of high-density polyethylene microplastics (HDPE-MP) at sizes of 45 µm and 125 µm on juvenile yellow perch, an ecologically important species in the Laurentian Great Lakes and favored for human consumption. A five-week feeding trial was conducted to test four diets containing either 1% or 2% of each size of HDPE-MP, alongside a control diet without added HDPE-MP. Each treatment contains three replicates with 75 fish (30-day post hatch, 49 mg/fish) per replicate raised in an indoor flow-through water system. Water temperatures was maintained between 20-22°C. Results showed that fish survival was unaffected by the MP contaminated diets under the current conditions. However, weight gain decreased with increasing levels of HDPE-MP in the test diets, and fish fed 45 µm HDPE-MP showed lower growth compared to those fed 125 µm at the same concentration. Fasting blood glucose levels were higher in fish consuming the 125 µm HDPE-MP compared to those on the control diet. Prior to heat shock, blood glucose levels were similar across all treatments. Heat shock significantly elevated blood glucose levels in fish across all diets. This preliminary study suggests that smaller MP negatively impact fish growth more significantly when compared to larger MP, while larger MP may disrupt blood glucose homeostasis and heat shock responses. The underlying mechanisms warrant further investigation.