When a friend and I were harvesting mussels off the coast of California one season, we were very cognizant of the time of year—late April. Both of us had heard that you’re not supposed to eat shellfish in months without an R, namely, the summer months of May, June, July, and August. We harvested the mussels and had a nice dinner, but even if we had collected the mussels a few weeks later, in May, would it have made a difference? Should we really avoid shellfish during certain months?
The problem with eating filter feeders like oysters, mussels, clams, and scallops during the summer has less to do with a letter and more to do with algal blooms. Commonly known as red tide because of the color it can turn the water, scientists now call the rapid increase in toxin-containing organisms a HAB, or harmful algal bloom. According to the National Oceanic and Atmospheric Administration, HABs occur in almost every coastal state and they can cause waters to turn brown, red, green, or have no color at all. While only a few HABs have toxins that are poisonous to people and animals, when they do occur, the toxins can accumulate in shellfish and cause poisoning when ingested.
Although these blooms can theoretically happen at any time during the year, they are associated with warmer water temperatures, as happens during the summer months. During blooms, most state and local agencies close beaches and restrict harvesting of shellfish, so regardless of the month, if you pay attention to local warnings and closures, you should be safe from eating bad clams.
Still, most cases of shellfish poisoning occur in recreational harvesters, not the commercially available stuff. That’s because most commercial shellfish has to go through testing, so even if there is a HAB in your area, you can feel pretty assured that the store, restaurant, or fishmonger you buy shellfish from in June is toxin-free. Either they have imported their shellfish from another area, harvested from a toxin-free area, or their product has been tested and is safe.
Another supposed reason for avoiding oysters during the summer is that this is the season when they spawn. Although it made good conservation to avoid consuming wild oysters for this reason, farmed oysters have a variety of species, and not all of them spawn during summertime.
If you’re harvesting wild shellfish, pay attention to closures and check with your local fish and wildlife management to ensure there are no toxic blooms. If you’re getting your shellfish from a respectable fishmonger, you can eat them year round.
Updated June 16, 2010