How much spawn do fish produce?
Of course this varies with the species , but it appears to range between approximately 10% and 30% of the biomass (wet weight) of the fish. The percentage is also referred to as the “gonadosomatic index” (GSI). 20% may be a reasonably representative figure. Considering that 20% by weight of the average spawning fish stock is diverted to the zooplankton, the bulk of the nutrient transfer might start to look significantespecially if we consider the immense spawning stocks of fish that existed prior to human exploitation. For every 100 tons of living “spawning stock biomass” (SSB), approximately 20 tons will be annually converted to zooplankton. The fish that divert the highest proportion of their resources to this use are the biggest, oldest spawners. This is well known. The larger sized fish put less energy into somatic growth and divert a larger fraction of their intake into reproduction. Their own bodies growing only relatively slowly once they reach the larger sizes, the biggest spaw
