In Canada, the Department of Fisheries and Oceans (DFO) monitors herring spawn events from the air, from boats and ships, and underwater by divers. “Test boats” equipped with underwater sonar are used to roughly estimate local school sizes and to extrapolate herring biomass in a given area. To determine the readiness of the school for spawning, sample herring are fished and their roe checked for their ripeness. The roe must be obtained just before spawning, to achieve the best price in the market. Based on the estimated numbers from the previous year, DFO decides how many tons of herring can be caught during the fishing season.
During spawn time DFO charters small aircrafts to survey and record the extent and intensity of spawn events as it is clearly visible from the air. Divers also assess spawning intensity in select spawn locations by looking at the coverage and number of layers of eggs on the spawning substrate.