Turns out, there is a rather large market for merfolk bones on the black market, as components for magical rituals, alchemical concoctions, and not to mention the ever prevalent rumors that eating merfolk flesh will grant one immortality.
There's always some entrepreneur willing to help fill a market shortage. But it turns out hunting and killing merfolk is rather time consuming, dangerous, and expensive. Much easier to put an initial investment into catching a few live female specimens, inducing pregnancy, and harvesting the offspring. Harvested bones, organs, and flesh can be sold on black markets for a healthy proffit.
A large setup can guarantee as much as one unit per day, every day of the year. Breeding merfolk are often kept shackled in just enough water to prevent dehydration, often unable of any movement at all. Amputation of none-essential limbs such as arms and tails is not uncommon. Due to the tendency of merfolk to breed true, and the waste of maintaining a population of males on hand for the sole purpose of breeding, impregnation is often handled by guards, handlers, and paying clients.