Did you use water pan?
As you're probably aware, Norwegian mackerel is a very oily, buttery fish that adding water in the pan is totally unnecessary especially when smoked in large quantities. In this case, it's 28 pieces totalling roughly 25 lbs. It creates more than enough moisture of its own within the smoking chamber that adding water in the pan can be detrimental. The added steam/moisture from the water will make the charcoal work very hard to get up to temperature. The fish having a high fat content makes it very forgiving to smoke in terms of temperature fluctuations or being smoked/cooked too long. In short, the likelihood of drying it out is quite slim.