I notice that Atlantic Mackerel (Scombrus scombrus) is endemically affected by parasites. In one study I read that the researchers found that over 70% of the mackerel sampled were infected with parasites.
Is there any evidence that the diseased state of the mackerel population is possibly due to loss of natural predation? The major predators of mackerel are tuna and cod, both heavily fished by humans. At one point cod was fished nearly to extinction and is still considered endangered. Since cod will typically catch the laggards and least fit of their prey fish, like mackerel, I would expect that over time this predation would tend to eliminate sick or unfit fish. Without this natural predator, perhaps it is leading to sickly conditions in mackerel?
Is there any basis for this thinking?