Final answer:
Fresh fish should have firm and elastic flesh to be considered of high quality and free from contamination; signs such as a strong odor, soft texture, or discoloration are indicative of spoilage or poor handling.
Step-by-step explanation:
To ensure that fresh fish is of the highest quality and free from contamination, it should have firm and elastic flesh. This indicates that the fish is fresh and has been properly handled and stored. A strong odor, soft and mushy texture, or discoloration on the skin are all signs that the fish may be of poor quality or contaminated. When testing for quality, there are other factors that might be considered, such as the absence of a fishy odor, which becomes more noticeable when 10% KOH is added, a pH greater than 4.5, or the presence of clue cells. Additionally, understanding the biology of the fish, such as the mucus that reduces drag and aids in osmoregulation, can provide insight into why certain textures and qualities are important for assessing freshness. However, for the purpose of quality control upon receiving fresh fish, the firmness and elasticity of the flesh are the most relevant indicators of freshness.