In 1971, she founded the Seal Rehabilitation and Research Centre in Pieterburen, Groningen.

Later she was forced to leave the center as she strongly resisted against new, scientifically-based policies about seal care and the current employees of the center as well as the government did not longer want to cooperate with her.

In 2014 the people working at the Seal Rehabilitation Centre forced 't Hart to stop her activities at the centre, as she wouldn't accept the new scientifically based policy on treating injured seals and intervened too much with the people working there.

In the proceeding years after she left the Zeehondencreche Lenie and her colleagues have been subject in several investigations by the Dutch authorities for continuing to rehabilitate seals.

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