Large numbers of dead fish are being spotted along the Hudson River, in areas from New York Harbor north to Haverstraw Bay. Riverkeeper is receiving calls from observers with questions and concerns, from Manhattan, Riverdale, Yonkers, and elsewhere.

The widespread deaths of Atlantic menhaden, and possibly other species, are most likely the result of prolonged heat and lack of rain, combined with other factors, which reduce levels of dissolved oxygen that the fish need to survive.

Importantly, it’s a symptom of a compromised and fragile ecosystem in the river and harbor. It’s yet another warning about our need to restore the river to health and balance in the face of climate change.

During these long, sunny days in June and early July, sunlight is overheating the river. Periods of cool or cloudy weather are scarce. Warmer water holds less oxygen than colder water, due to the kinetic energy of the water molecules.

Low oxygen (hypoxia) or no oxygen (anoxia) can also occur in water bodies when excess organic materials, such as large algal blooms, are decomposed by microorganisms. During this decomposition process, oxygen in the water is consumed.

Ultimately, oxygen levels can become so low that fish suffocate.

The river’s underlying health also plays a role.

Sewage pollution and fertilizer act as “nutrients” that fertilize phytoplankton and spur algae growth. Scientists have determined that the Hudson is the most “nutrient rich” estuary on earth. Sewage treatment plants take out some nutrients, but not all.

Compounding this imbalance is the devastating loss of aquatic life in the Hudson and Harbor that consume algae. According to the Billion Oyster Project, New York Harbor once had 344 square miles of oyster beds and reefs. The destruction and loss of these beds killed off these filter feeders.

So the river starts from a compromised position, with over-nutrification from sewage and fertilizer.

Later this summer, when we get a series of rainstorms or cloudy days, we’ll see lower water temperatures and less algae growth, and we’re likely to see a reduction of fish mortality as oxygen returns to adequate levels.

But we should consider this yet another warning that we need to restore the baseline health of the Hudson and New York Harbor in the face of climate change and ever-increasing global water temperatures.

Fifty years from now, or 20, or even 10 years from now, sunny, hot months like this past June will have even greater consequences, because baseline water temperatures will be that much higher.

The warm, sunny weather and low dissolved oxygen have tipped the river beyond its ability to support some aquatic life. Seeing this, we need to do what we can, now, to restore the river to balance.

Please use your voice to help restore the river. Here are some actions you can take.