Paul Watson of Sea Shepherd Conservation Society must remain in custody while German authorities weigh a request to have him extradited to Costa Rica, his lawyer said after the hearing.
It's unclear how long the process will take, Oliver Wallasch said.
"My client is shocked," he told reporters.
Watson alluded to the proceedings on Twitter, saying he "might have to stay one more night until we clear this up..."
The ship captain is wanted in Costa Rica for allegedly endangering a fishing boat in 2002. He was arrested Saturday at Frankfurt Airport on an international arrest warrant issued by Costa Rica, a spokesman for the local prosecutors' office said.
"He is alleged to have used a ship to intimidate another vessel and put its crew at risk in 2002," said the spokesman, Guenter Wittig.
Watson is currently in temporary custody and a judge will decide later whether to formally place him in detention pending extradition, Wittig said.
A spokesman for the Department of Foreign Affairs and International Trade said Ottawa "stands ready to provide consular services to a Canadian citizen who has been detained in Frankfurt, Germany."
"Consular officials are in contact with local authorities to gather additional information," Ian Trites said in an email Monday.
Sea Shepherd said Watson was filming a documentary at the time of the alleged incident.
It took place in Guatemalan waters, when the U.S.-based group said it encountered an illegal shark finning operation run by a Costa Rican ship, the Varadero.
Sea Shepherd said in a statement Sunday that it told the Varadero's crew to stop and head to port to be prosecuted. The crew accused Sea Shepherd of trying to kill them.
The group was formed in 1977 and has had a controversial history.
It sends vessels to confront the Japanese fleet each year, trying to block them from firing harpoons at whales.
Its tactics have drawn praise from supporters and vehement attacks from critics.
According to Sea Shepherd, Watson is being assisted in jail by European Parliament vice-president Daniel Cohn-Bendit and European deputy Jose Bove.
_ With files from The Associated PressSuggest a correction