This absurd idea of using the rake marks as an evidence of unnatural aggression in cetaceans under human care is quite recent. In 2012 the Free Morgan Foundation was desperately fighting against the decision of the Dutch court to transport Morgan to Loro Parque. Despite Morgan was transferred on November 2011, the court case continued in Holland until 2014, when the Raad van State (Dutch Supreme Court) ruled that the transport of Morgan was absolutely lawful (and the only way for her to avoid euthanasia).
But, from the very beginning, Free Morgan Foundation was opposing to the transport and stubbornly requested to the Dutch authorities her release or her immediate transfer to a sanctuary (that didn’t exist in 2011 and still not existing nowadays). That’s the reason why in 2011 Free Morgan Foundation started a campaign against Loro Parque, trying to prove at any cost that Morgan was in a terrible situation in Orca Ocean. With that only goal in mind Ingrid Visser authored a non-scientific report  were she tried to depict the enormous suffering of Morgan at Loro Parque. That was the first time she described the rake marks as an evidence of aggression in captive settings, suggesting that wild killer whales were gentle giants that never bite each other. This was the beginning of the myth.
This misleading story telling was aimed just in destroying the reputation of Loro Parque, arguing that Morgan was in danger, but there is no sound science behind it. There is no scientific literature comparing the rake marks in wild and captive cetaceans, hence there is no way to elucidate if the aggression is enhanced under human care.
What is absolutely clear in the scientific literature is that rake marks are frequently found in wild cetaceans. The first scientific description of rake marks in wild killer whales is from 1978 when the first behavioural analysis of the species was published . If any catalogue of orca photo-identification is consulted, rake marks appear in almost every single individual  . Rake marks are so common in cetaceans that recently, Marley et al., used them to identify different levels of aggression in wild dolphins . In this scientific study it was demonstrated that 60% of the dolphins had rake marks, and the rest (40%) were usually young animals hence not likely to get involved in aggressive behaviours. In practice that demonstrates that any cetacean has rake marks, and it can be easily confirmed with a simple search in a scientific database, which provides examples of studies that use rake marks to describe and measure aggression in wild cetaceans      , and they are considered so common for the researchers that are even described as “natural marks” .
Ironically that’s not something that Ingrid Visser ignored when she wrote her report against Loro Parque in 2011, as she authored in 1998 a scientific paper describing prolific rake marks and collapsed dorsal fins in some orcas found in New Zealand . But Visser didn’t mention any of the scientific papers describing rake marks in wild cetaceans when she presented her report to the Dutch Court in 2012. In fact, when she discussed the appearance of the same kind of marks in Morgan on this report, she also forgot to mention her previous research on prolific rake marks in wild killer whales in New Zealand. That clearly indicates a lack of ethics, and proves without any doubt that sometimes even scientists prefer to prioritize their political goals over their scientific knowledge.
The most recent scientific paper on rake marks  evidence that aggression directed to wild killer by the members of their own pod occurs and varies with age, sex and ecotype. The authors found rake marks virtually in any studied killer whale in the northeastern Pacific population, demonstrating the fact that rake marks in killer whales are the natural consequence of social aggression. Hence, the appearance of rake marks in whales under human care should not be considered a sign of bad welfare, but the consequence of a natural behaviour.