The relationship between stock and recruitment: are the assumptions valid?

The management of exploited fish populations is, at present, based on stock assessment methods which assume that spawning biomass is proportional to the reproductive potential of the stock. This implies that the survival rates of offspring do not substantially change with the age or size of the spawner. However, the commercial extinction of many fish stock worldwide has provoked concern about the techniques used in stock assessment. Is the contribution of different age classes to recruitment equal, irrespective of the age of the fish? Here we show that not only do older individuals contribute the largest amount of eggs to the reproductive potential of cod Gadus morhua, but that they also have the largest number of recruits likely to produce offspring with higher rates of survival. This important effect of population structure on recruitment should be taken into account in fisheries science and fisheries management.