Abstract
The species-specific feeding method of the haddock is aimed primarily at the entire fraction of small to medium-sized endobenthos, and secondarily at ophiuroids. This food has a low nutritional value and gives stomach contents that are highly particulate and rich in hard fragments.
The stomach of the haddock is adapted to an exceptional degree for powerful contraction to decompose the stomach contents by crushing and grinding. This process is made possible by the particulate nature of the stomach contents. The grinding process makes a higher feeding rate possible, to compensate for the low nutritional value of the food. The feeding rate is probably high, and fairly constant during feeding periods. The haddock feeds heavily on ophiuroids and echinoids largely because these form its most effective grinding elements. The adaptations of the stomach for the grinding process involve a low capacity for expansion and a high pH of the digestive juice. Gastric parasites are uncommon, probably because of the grinding process.
The minimum prey size was determined mainly by the gill raker dimensions. The requirements of the grinding process generally do not permit as large prey as the swallowing capacity, and so usually seem to determine the maximum prey size. The prey size and the feeding depth in the sediment increased with fish size. The long-term food diversity was low, and lowest in fishes 20–30 cm long. The temporal variations in food composition were extremely small.
The combination of its special feeding method and digestive grinding process makes the haddock more specialized for utilization of the small and medium-sized endobenthos than any other littoral teleost of the Northeast Atlantic. Among the marine teleosts of the world studied hitherto, the closest relatives of the haddock in terms of feeding method are found in the Cheilodactylidae, but a gastric grinding process like that of the haddock is not yet known in any other marine teleost.