Atlantic menhaden are often characterized in the popular press and by others as aquatic vacuum cleaners, that swim the Chesapeake Bay waters and systematically consume large blooms of phytoplankton; this makes the water cleaner. It has even been said that if menhaden were not harvested from the Bay’s waters, their water cleaning capability would help combat the now persistent hypoxia (low oxygen) zone in Chesapeake Bay.
A new study by the Virginia Institute of Marine Science
says otherwise. In fact, the study says “. . . filter feeding by Atlantic menhaden has little net effect on overall water quality in Chesapeake Bay". You could say the same of other coastal waters in the United States.
Environmental and recreational angler organizations have opposed menhaden fishing for many years, stating publically that the Atlantic menhaden population is overfished. These groups latched on to this notion of filtering the water of phytoplankton by menhaden as a way to support their claims of overfishing.
Prior to the VIMS study, other studies had arrived at this same conclusion, including a study by Friedland (2006) who measured the spacing in the menhaden’s filtering structure and concluded that as young-of-the-year (juvenile) fish, menhaden filter out and consume measureable amounts of phytoplankton. However, as the fish reach age 1 and older, most of the phytoplankton is too small to be retained in their gill filaments and are passed back into the water.
Another study, conducted by Oviatt (1972), measured excretion by menhaden after feeding and found that about 60% of the food they consume is excreted as ammonia, a nitrogenous product that enhances continued phytoplankton growth and spread. In fact, that study found that significant amounts of ammonia could be measured as much as two miles behind a school of feeding menhaden. The current study concurs with Oviatt’s findings.
Dr. Rob Latour, one of the study’s authors stated "Far more nitrogen is coming into the Bay than what is possible for menhaden to remove, even if all they ate was phytoplankton." Another of the study’s authors, Dr. Mark Brush, stated "Based on our results as well as ecosystem modeling simulations, menhaden do not appear to represent a significant mechanism for removing nutrient inputs to the Bay, at least not at the scale of the entire Chesapeake." Patrick Lynch, the lead author, stated "So much nitrogen is being put into the Bay that they (menhaden) are likely only able to remove a small fraction of the total." We would like to believe that a force of nature such as menhaden could be harnessed to help us clean up the mess that has been made in our coastal waters; however, we must face the reality that the clean-up can only be accomplished by a dedicated effort to reduce the amount of nitrogenous runoff we humans create.