The usual clupeid and gadoid strategy is to broadcast small buoyant eggs into the plankton and let currents disperse them. Atlantic herring instead produce demersal, adhesive eggs. A spawning shoal settles over coarse gravel, shell or stone and deposits eggs that stick to the substrate and to each other, building mats that can be several eggs thick.
That choice makes location matter. Herring spawning beds are specific, traditional patches of ground, and the fish return to them. Eggs stacked in thick layers can suffocate at the base, and the beds are exposed to dredging, seabed disturbance and localised predation by haddock and other bottom feeders that arrive to feed on them.
It also means a herring stock can be structured into subpopulations that spawn in different seasons and places within the same sea. That structure has been repeatedly under-appreciated in management, and mixed fisheries taking several spawning components at once have contributed to collapses. How much herring depend on learning spawning grounds from older fish, rather than finding them independently, is an open and important question.