Climate Impacts on the Hudson

In the last 70 years water temperatures in the Hudson water have increased 1.7°F, and as warmer water holds less oxygen, this has stressed many of our river species. Warming affects the level of metabolism and activity in the fish, and some of our more oxygen sensitive species have suffered as temperatures have warmed. This summer we saw the effects on one regionally important herring, the Atlantic menhaden. Menhaden travel in schools, swimming tightly packed for protection and moving quickly with opened mouths to capture plankton in the water. The combination of dense schooling and high swim rates adds stress to their metabolisms in warm oxygen-depleted water. These factors combined in July and early August to leave menhaden gasping for oxygen as they moved through this section of the estuary. You may have notice some washed up on the shore, as some of them died from the low oxygen levels.

Menhaden are still part of our Hudson River system, but for a few of the more than 230 fish species that have been identified in the estuary, the water is already too warm. Rainbow smelt, previously part of our Hudson fish network in late winter and early spring, have adjusted their range to areas further north where there are cooler waters. Similarly the Atlantic tomcod have been seen in reduced numbers in the estuary. Known as the ‘frostfish’ from their habit of spawning in the winter and often under ice cover, it is unclear how long we can expect to see this species in the Hudson. As in any situation, with a warming of the water there will be winners and there will be losers in the affected species, but we can be certain that the complement of fish will be altered.

These are visible signs of climate impacts, but there are other less visible signs, like the impact on the triggering of spawning migration for some of our iconic Hudson species. The Hudson River estuary is a critical part of the spawning cycle in several marine species like Atlantic sturgeon, American shad, and striped bass. These species move along the Eastern seaboard, their entry to the estuary triggered by specific water temperature needs for the success of their offspring. Adjustments in the timing of spawning can alter food source availability for fish in their first year of life, making these runs less successful. This would have a ripple effect through the entire food web.

Because the Hudson is an integrated system there are also impacts in other parts of the system. Increased water temperature can lead to increased growth in bacteria, changing the current balance in the river. Upriver the shallow embayments along the rivers edge have already experienced a few incidents of harmful algal blooms of cyanobacteria. These blooms have been linked with increased water temperatures in these shallow, slower moving sections of the estuary. The early outbreaks alert us to the need to increase our under- standing of the many factors that are part of this complicated system. We are only beginning to understand all the climate implications for the river. It is important that we continue to monitor the Hudson.