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Despite Myriad Challenges, River Herring Numbers Rising

Keith Carver
Ospreys and striped bass are among those who stand to benefit from efforts to rebuild river herring populations.

May 21st is World Fish Migration Day. Yes, that’s a real thing, and groups around Cape Cod have already begun participating by doing what many of them do every spring – gathering to count and learn about river herring as they try to make their way from the ocean up to fresh water to spawn.

There are some eighty herring runs in Massachusetts, with a number on Cape Cod. Biologists estimate that one hundred fifty years ago, each run may have supported a million fish. Today, it’s a fraction of that number. There’s no one culprit at which to point the finger, more like a dozen.

New England’s river herring (there are actually two species – alewives and blue-backs) face incredible challenges. During their oceanic phase, they are caught in large numbers by commercial fishermen targeting altogether different species. Nutrient pollution and resulting algal blooms can produce deadly conditions in near-shore waters. Rising water temperatures are rendering certain areas uninhabitable, and also exacerbate the effects of nutrient run-off. That’s all before the fish get to a river.

There, the effects of climate change are again an issue; extremes in precipitation (or lack thereof) can translate into water levels being either two low or two high when herring need to navigate a river.

And, finally, there are the rivers, themselves. Many have been blocked by roads, dammed for small-scale hydropower, or diverted through cranberry bogs. There are fewer trees shading rivers, and less dead wood in the rivers, leaving herring far more exposed to avian predators.

Given that litany, it’s not the river herring’s decline that seems unbelievable, but rather, the fact that there are any of them left at all. And yet, there are. In fact, the statewide trend in Massachusetts appears to be a slight increase in river herring numbers in recent years.

Here, again, there’s no one clear reason. Incidental ocean harvesting has been restricted, as has harvesting in rivers. Significant effort is going into reducing nutrient pollution. And many communities are working to remove obstructions and return rivers to a more natural state.

The benefits could extend much further than just the herring themselves. River ecosystems, of course, are critical to many species. But herring hold a special place near the bottom of multiple food chains. Striped bass and ospreys – not to mention humans – are just a few of the species that stand to gain from efforts to restore herring populations and the rivers upon which they rely.


  • Brad Chase, Senior Fisheries Biologist with the Massachusetts Division of Marine Fisheries
  • Linda Deegan, Senior Scientist at the Marine Biological Laboratory’s Ecosystems Center and a Professor in the Brown University – MBL Joint Program
  • Christopher Neill, who is a senior scientist at the Marine Biological Laboratory’s Ecosystems Center