Final answer:
The freshest fish and shellfish can be identified by the bright red color of their gills, which indicates high levels of hemoglobin. The location of the freshest fish varies by region and local fisheries. Most markets can special order certain types of fish, with availability often split between round and flat fish.
Step-by-step explanation:
Understanding Freshness in Fish and Shellfish:
To gauge which fish and shellfish are the freshest, one must look at several indicators, one of which includes the color of the gills. Fresh fish have gills that appear bright red because they are rich in oxygen-carrying hemoglobin, which is needed for the fish to breathe underwater. Fish intake water through their mouth and expel it through their gills, and the blood flows in the opposite direction to the water across the gill membranes, a process known as countercurrent exchange, to maximize oxygen absorption. Regarding the types of fish, it often varies from market to market whether they carry more round fish or flat fish. However, many markets are capable of special ordering fish that aren't normally carried, depending on their supply channels and customer demand. Similarly, the region where the freshest fish comes from largely depends on geographic location and the local fisheries. For example, salmon might be freshest when sourced from cold, northern waters, while snapper might be freshest when sourced from warmer, tropical waters. Lastly, when considering the sustainability and regulation of fishing practices, it's a complex issue that balances the immediate economic needs against long-term environmental and resource conservation. This is an important dialogue in ensuring the availability of fish and shellfish for future generations.