During which season can mussels from the Pacific coast become poisonous?

Mussels from the Pacific coast can become poisonous during any season due to the presence of harmful algal blooms, which can occur at various times throughout the year. These blooms produce toxins that can accumulate in shellfish, including mussels, making them unsafe for human consumption. The occurrence of harmful algal blooms is largely influenced by environmental factors such as temperature, nutrient levels, and ocean conditions, which can change seasonally.

Therefore, no specific season can be singled out as the only time when mussels can become toxic, because the risk is present year-round depending on these environmental factors. This makes the answer encompassing all seasons the most accurate representation of the situation regarding mussel toxicity.