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Cochran, PA, RJ Wagner, EJ Golden, 2014. The feeding behavior of a South American parasitic catfish (Ochmacanthus alternus). Journal of Freshwater Ecology doi: 10.1080/02705060.2014.967816.

Abstract
Ochmacanthus alternus is a small South American parasitic catfish. Previous work, primarily based on examination of gut contents collected in the field, has indicated that it feeds primarily on mucus. The objective of this study was to conduct laboratory experiments on feeding behavior, including assessments of host size selectivity and location of attacks on hosts. Ochmacanthus alternus fed by making contact with a fish and pivoting from side to side, apparently scraping mucus. Feeding attacks were of short duration, with a mean of 2.5 seconds, but longer attacks (up to 29 seconds) occurred earlier in feeding trials. Large hosts (standard length: 75–96 mm) were attacked significantly more often than small hosts (standard length: 29–51 mm), regardless of whether or not expected frequencies of attack were adjusted to reflect host surface area. Attacks tended to be directed anteriorly on the host but were terminated posteriorly. The catfish seemed to be attracted to sources of physical disturbance in the water, such as those created by hosts struggling to terminate attacks by other catfish. This tended to result in attacks occurring in distinct bouts. We conclude that O. alternus is capable of host size and site selectivity, at least under the conditions tested in the laboratory.

From the above I get the impresion Ochmacanthus alternus keeps in the water, does not enter the host fish, and is thereforee comparable with a scale eater.

This makes me wonder, why is it called parasitic. It is basically a predator which does not kill its prey, but which feeds from it while it is still alive.

In fact, by selecting larger prey, it tries to prevent the prey (or host) from dying.

Are all predators which keep their prey alive seen as parasites?

Bas Pels wrote:This makes me wonder, why is it called parasitic. It is basically a predator which does not kill its prey, but which feeds from it while it is still alive
....Are all predators which keep their prey alive seen as parasites?

Essentially, yes. A parasite is any organism that enters into a "long-term relationship" with another organism, wherein the parasite benefits from the relationship and the other species is harmed by the relationship.

In this case, the fish would be classified as an ectoparasite (like a mosquito, flea, or tick) because the parasite does not enter the host's body. Parasites which enter into their host's body and live within the host are classified as endoparasites.

In principle, this fish can feed off its target (its prey or host), then leave and return later to feed more (if it chooses to attack the same fish again), just like the mosquito.

Speaking of food, is it lunch time yet?
-Eric

Thank you

@ lunchtime, it must be lunchtime somewhere on the world