Inside the caves Finlayson and his colleagues found mussel shells and the bones of seals, dolphins, and fish mixed in with the remains of deer and other land mammals.
Many of the bones show signs of being cooked over a fire, and some have marks left by stone tools used to cleave off chunks of flesh.
Seafood remains are found throughout various layers in the caves, indicating that Neanderthals regularly exploited marine resources for tens of thousands of years.
"It seems to suggest that this wasn't a one-off, but that these guys were doing it on a regular basis," Finlayson said.
He and colleagues describe the findings online today in the journal Proceedings of the National Academy of Sciences.
Curtis Marean is an anthropologist at Arizona State University in Tempe who has found evidence that prehistoric humans were feasting on seafood in South Africa 165,000 years ago.
Marean said the new study clearly shows that Neanderthals occasionally ate seafood. But he is not convinced their exploitation of seafood was on par with that of early modern humans in Africa.
"I don't think there's enough evidence here to indicate that they are systematically being a coastal forager in the sense that we think of coastal foragers," he said.
In South Africa, Marean noted, scientists have found waste piles called shell middens that date back nearly a hundred thousand years. These piles contain several thousand pieces of shellfish discarded by humans.
By contrast, the Gibraltar caves yielded just 149 pieces of shellfish. Those pieces could be from a handful of mussels, Marean noted.
The differences in abundance could stem from different availabilities of seafood at the two sites, or in the abilities of the two species to actively forage for ocean food, he added.
To resolve the issue, Marean recommends a systematic comparison of Neanderthal and human seafood collection at sites with similar availability.
"Were Neanderthals [exploiting seafood] like we expect they would if they were modern? And if they weren't, then the question is: Why?" he said.
"We could be getting into something interesting there, for sure."
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