In the classic 1948 film noir "Key Largo," gangster Johnny Rocco dines on what was then a unique subtropical cuisine: pompano and champagne.

But in the 64 years since actor Edward G. Robinson so famously licked his chops over the meal, pompano have ceased to be a species found only in southern waters. In fact, it's now conceivable that a southeastern Connecticut fisherman could serve such a meal from his own Long Island Sound catch.

"We're getting some strange fish we haven't seen before, and some that we used to get only occasionally we're seeing more often," said Jon Hillyer, owner of Hillyer's Tackle Shop in Waterford, Tuesday. "A lot of guys are catching them by mistake while they're fishing for bluefish or porgies, and some of them we can't even identify. And some of the things we used to see, like tomcod and smelt, we don't get any more."

The shifting diversity of fish species in Long Island Sound is the subject of a study published last month in the journal "Marine and Coastal Fisheries: Dynamics, Management and Ecosystem Science." Authors Peter Auster, research professor emeritus of marine science at the Avery Point campus of the University of Connecticut in Groton, and Penny Howell, fisheries biologist with the state Department of Energy and Environmental Protection, wrote that the changes correspond with an overall trend of warmer water in the Sound that is making the estuary more welcoming to southern species and less so for those that like colder waters.

"The strongest shifts are in the spring," said Auster, who is also senior research scientist with Sea Research Foundation, the parent organization of Mystic Aquarium. "The underlying 'so what' here is that this is consistent with the predictions of climate change models, and that we're seeing these kinds of shifts in a mid-latitude estuary."

The study used fish-count data from 1984 to 2008 from DEEP's trawl surveys, which are done annually from research vessels based at the agency's marine headquarters in Old Lyme, and water temperature data dating to 1976.

In 1976, average spring water temperature was recorded at about 43 degrees; by 2008, it was almost three degrees warmer. Autumn temperatures followed a similar trend: an average of 65.7 degrees in 1976, and 68.5 degrees 32 years later. Between those two dates, an overall gradual warming pattern was found. This summer, the temperature in the Sound got so warm that Unit 2 at the Millstone Power Station in Waterford had to shut down, because the cooling water it uses from Long Island Sound exceeded 75 degrees, the maximum plant specifications allow.

The temperature difference might not seem significant, Auster said, but a few degrees can make a difference, and extremes factored within the average would have the greatest impacts on fish. Also, subtle temperature changes can affect fish spawning success, growth rates and vulnerability to predators, he added.

The study, he said, is a reminder that climate change effects are not just being seen on coral reefs in the tropics and polar bears in the Arctic, but also in areas with more moderate climates like the Sound. It looked at species that are valued by commercial and recreational fishermen as well as those that are not, finding for both groups that warm water species are increasing while cold water ones are decreasing.

Cold water species being seen with declining numbers in the spring and fall include three kinds of flounder - windowpane, winter and fourspot - as well as Atlantic herring, red hake and little skate. Moving in to replace them are increasing numbers of warm water species such as scup, butterfish, summer flounder and black seabass. Some of these are species that historically have been found in the Sound, but had been more abundant farther south.

The trawl surveys are even turning up some subtropical species such as African pompano, Atlantic moonfish, grey triggerfish and banded rudderfish, two types Hillyer said his customers have caught recently.

Karen Westerberg, owner of AW Marina in New London, said her customers are reporting frequent catches of species that favor warmer temperatures, such as weakfish, black sea bass, northern kingfish and spot, as well as a lot of fish they can't identify.

"They're not sure what it is, whether there's a minimum size, and whether they're edible," she said. "They're not sure what to do with them."

One message of the study, Auster and Howell said, is the challenge being presented to fisheries regulators by the dynamic conditions. Rules for seasons, size and catch limits will need to be reviewed and revised often in response to the rapidly changing mix of species, they said.

"It's a whole new realm, a moving target," Howell said.

To understand how best to regulate species historically more prevalent in waters to the south, such as spot and black sea bass, DEEP likely will turn to colleagues in states to the south, just as colleagues to the north might seek out their advice for species new to their waters.

"Spot are in our top 30 species for the first time, and in the South, they're losing abundance," Howell said.

Seahorses are even finding their way into Long Island Sound, she noted.

"This is a cycle that is not going back to original conditions," she said. "Things are changing. The diversity is changing. For an estuary where the animals are notoriously well adapted to variable environments, to see this kind of shift in such a short period of time, that really says something about what's happening."

j.benson@theday.com