Conducting a mackerel egg survey to determine the size of the mackerel spawning biomass: how do we do this?

Every three years, a mackerel egg survey is conducted in the Atlantic Ocean to determine the size of the mackerel spawning biomass. The data is used by ICES (International Council for the Exploration of the Sea) for annual stock estimates and catch advice. Wageningen Marine Research (WMR) conducts this mackerel egg survey in collaboration with other European institutes, partly in collaboration with the pelagic fishing sector. The theory is simple: count the number of eggs in the sea and divide this by the average number of eggs laid by a female mackerel. By doing this, you can estimate how many female mackerel are swimming in the ocean, and thus how large the spawning biomass of mackerel is. But how exactly do you do this and how do you recognise a mackerel egg?