Which group of algae is known for causing foul odors or off-flavors in fish?

Blue-green algae, also known as cyanobacteria, are notorious for causing foul odors or off-flavors in fish. This group of algae can proliferate in nutrient-rich waters, especially during warm weather, leading to harmful algal blooms. These blooms produce various toxins and metabolites, some of which are responsible for undesirable tastes and smells in aquatic environments. When fish are exposed to these blooms, they may absorb these compounds, resulting in off-flavors that can render them unpalatable or unsafe for consumption.

The other groups of algae, while they can affect water quality and aquatic ecosystems, are not primarily known for producing the specific foul odors and off-flavors associated with blue-green algae. Thus, blue-green algae's unique metabolic pathways and ability to dominate nutrient-rich waters make them the correct answer in this context.