Responding to the general warming of the northwest Atlantic Ocean over the last 40 years, a significant number of fish stocks have shifted to the north and deeper in an apparent effort to find optimal water temperature conditions, a study by NOAA researchers shows.

And some shifts are taking commercially important stocks toward the outer border of the 200-mile U.S. exclusive economic zone, the study's authors say.

Published in the Oct. 30 edition of Marine Ecology Progress Series, the study of 36 stocks, including mainstays of the New England and Mid-Atlantic fisheries, reported that "there were clear poleward shifts consistent with warming" in 17 stocks, while four stocks appeared to shifted southward.

In addition to the 17 stocks that moved northward, 17 stocks — not all the same ones — were also found to be centered in deeper water. Both shifts were attributed to the search for optimal water temperatures.

Twenty-four of the 36 stocks examined had shifted their epicenters consistent with warming, according to the study, which is titled "Changing spacial distribution of fish stocks in relation to climate and population size on the Northeast United States continental shelf."

Janet Nye, a post-doctoral researcher at the National Oceanic and Atmospheric Administration's Northeast Fisheries Science Center laboratory at Woods Hole, said different studies have found the average overall water temperature in the Northwest Atlantic to have risen by one-fourth degree Celsius since 1982, and 1 degree since the 1960s.

In a telephone interview yesterday, Nye described such shifts — despite seeming trivial — to be significant.

"During the last 40 years, many familiar species have been shifting to the north where ocean waters are cooler, or staying in the same general area but moving into deeper waters than where they traditionally have been found," Nye said. "They all seem to be adapting to changing temperatures and finding places where their chances of survival as a population are greater."

Nye and coauthors Jason Link, Jonathan Hare and William Overholtz of the science center at Woods Hole selected the 36 species to study because they were consistently caught in high numbers in the center's annual spring bottom trawl survey.

Shelley Dawicki, a spokeswoman for the science center, said the study used trawl survey data. Because the surveys "are broad enough not to miss the fish," the study should not undercut the validity of the surveys, which are used to develop biomass estimates over the same area.

"The fact that we see responses in many species consistent with what you would expect with warming, but in different types of species that have experienced different historical fishing pressure, suggests that we are already witnessing the response of fish to a warming scenario," Nye said. "The community structure is changing from cool-water to warm-water fish species. These trends will likely continue."

Stocks that moved north include alewife, shad, northern and southern hake, Georges Bank cod, other hake, halibut, yellowtail flounder from southern New England waters, and winter and windowpane flounder.

Stocks that moved deeper in the water column include herring, shad, Gulf of Maine cod, hake, halibut and mackerel.

Stocks that moved south include Gulf of Maine cod and northern winter flounder.

Stocks that moved up in the water column include southern silver hake, spotted hake and mackerel.

While consumers will find familiar fish species at their local fish markets for the foreseeable future, fishermen may have to travel farther to catch some species until eventually it will not be economical.

"Consumers in the Northeast, for example, may eventually start seeing less familiar species like Atlantic croaker at local markets and on restaurant menus as southern and Mid-Atlantic species move northward into New England waters," Nye said. "The fish appear to be adapting to a changing environment, and people will as well over the next few decades."