It is safe to go back into the water again -- at least in Scandinavia . A warning over the weekend for male swimmers off the coast of Denmark and Sweden to protect their private parts because of a testicle-munching fish appears to have been a joke that got out of hand . After a Danish fisherman caught a South American pacu among his eels and perch this month , a professor at the Copenhagen Museum of Natural History told men to be careful because the fish sometimes mistake male reproductive organs for tree nuts , one of their favorite foods . `` Anyone choosing to bathe in the Oresund these days had best keep their swimsuits well tied , '' Professor Peter Rask Moller said in a Saturday news release about the fish discovery in the strait separating Denmark and Sweden . Wednesday , however , Moller said he was just joking and never meant for his warning to get so much publicity . `` We did say that we recommend men to keep their swimsuits tied up until we know if there are more pacus out there in our waters , '' Moller told CNN by e-mail . `` Of course , this is half a joke since it is very unlikely that you would actually meet one here and that it would bite you . It 's up to people themselves how careful they want to be . I 'll keep my shorts on , though . '' Finding a pacu , a close relative of the piranha normally found in South America , is unusual so far north . Moller said it was likely dumped in the strait by an amateur aquarium owner or fish farmer . William Fink , a piranha researcher at the University of Michigan who is also curator of fishes at the school 's Museum of Zoology , poured water on the pacu warning . For one thing , he said , pacus are vegetarian -- and there 's no record of them attacking a human . `` They 're fruit eaters . Those big crushing teeth they have is for crushing seeds , '' he said . Pacus tend to swim under fruiting trees during high water , waiting for the trees to drop their fruit , seeds and nuts , Fink said . The fish then swim to the surface to eat the tasty morsels . In fact , Fink said , that 's how fishermen catch them : by putting a piece of fruit on the end of a line and letting it float in the water . Pacus swim up to grab it and then get caught . `` The nuts that they 're eating , the fruits that they 're eating , are splashing down from above , and humans do n't act like that when they 're swimming , '' Fink said . And unlike piranhas , which have rigid , razor-sharp interlocking teeth , pacus have teeth that resemble human molars and fit together in a similar bite , Fink said . The pacus use those teeth to crush their food , not to rip it apart -- or off . He speculated that the pacu found in Denmark was probably released shortly before it was found because pacu species ca n't survive in either saltwater or chilly temperatures . `` It 's just too cold for pacus to be living there . They certainly would n't make it through the winter , '' or even past October , he said . CNN was among numerous news outlets around the world that reported the Danish news release over the weekend , focusing on the special warning for male swimmers . Moller said he only meant to publicize the surprise discovery of a pacu in the Baltic and to advise the public that the fish can be invasive and possibly dangerous . `` The story about the 'nuts ' was never meant to be the headline . But it certainly got people 's attention , '' Moller told CNN . `` I 'm sorry if it has caused you any trouble . It was a bit of a joke , but I still will keep my swimsuits tied up , and I will never swim in an aquarium with these fishes . ''
A Danish professor said a South American fish was found in Danish waters . He said the fish may attack male private parts , but now he says it was a joke . A fish expert says the fish are vegetarian and do n't swim in saltwater . There is no record of them attacking humans , the expert says .