Climatide

Sesame Street watchers may remember a little song that started “One of these things is not like the others …” Well, that’s Atlantic mackerel.

NOAA FishWatch

Atlantic mackerel are moving in response to rising ocean temperatures.

In 2009, researchers at the Northeast Fisheries Science Center in Woods Hole, MA announced that they were seeing the effects of rising ocean temperatures on commercial fish stocks. Half of the 36 commercially fished species routinely surveyed by fisheries scientists along the east coast were moving north/northeast and into deeper offshore waters to stay within their temperature comfort zones. Mainstays of New England fishermen, including cod and haddock, were on that list. Atlantic mackerel was one of just three species doing something different: moving northeast, yes, but moving into shallower inshore waters.

Mackerel doing its own thing

A new in-depth study of Atlantic mackerel distributions over the past forty years confirms that the species shifts with water temperatures – seasonally, annually, and over the long term. In fact, mackerel appear to be one of the species most sensitive to temperature changes.

Mackerel prefer waters between 5ºC and 13ºC. Those temperatures used to be found primarily off the mid-Atlantic coast, in a relatively thin strip of water along the edge of the continental shelf, wedged between cold coastal waters and the warm Gulf Stream. But the shallow waters atop the continental shelf aren’t as cold as they used to be. Atlantic mackerel can now be found ranging across the continental shelf from Maryland to southern New England.

Dr. Jon Hare, Oceanography Branch Chief at the Northeast Fisheries Science Center and one of the authors of the new study, says one key difference between mackerel and species, like cod, that are moving offshore is where they live in the water column. Cod is what’s known as a groundfish; it lives on or near the ocean floor. The only way for it to adjust to rising water temperatures is to move to other near-bottom areas. But mackerel are what’s known as pelagic – they live up in the water column, in a very three-dimensional world where they can move up and down, as well as north, south, east and west. 200 feet deep, 300 feet deep … it’s all fine, as long as it’s the right temperature.

Here’s the catch

Mackerel’s move onto the continental shelf marks a significant expansion in geographic range for the species. Whether or not that’s a good thing depends on your perspective. The authors of the new study say the mackerel population may eventually grow to fill its new range. That could be good news for everybody. In the meantime, though, fishermen are faced with the same number of fish (a number that is still recovering from overfishing in the 1970′s) spread over a much larger area. That means it’s going to be harder to find and catch mackerel.

Different species differing responses to ocean warming also raise some intriguing questions for ecologists. Are predator and prey species moving together? Or are ocean food chains getting disconnected by climate change. Hare says those are questions very much on the minds of ocean scientists, but not questions for which we yet have answers.