Mortality at the early life stage

Ammodytes sp. matures at the age of 2-5 years and annually spawns 22,000-66,000 eggs per female. Of these eggs, if a total of two fishes (one male, one female) survive and mature to participate in spawning without dying of natural or fishing causes, then the Ammodytes resource will be maintained.

Based on the calculations from the number of spawned eggs, their survival rate is 0.0031 – 0.0089%, and the probability is as low as that of winning the lottery.

The survival rate is lower during the small and frail egg or larva/juvenile period than during the immature and adult fish stages. This is called “early reduction”. Furthermore, slight changes in environmental factors can have a large effect on early reduction. There are various theories as to why early reduction occurs. For example:

Starvation hypothesis: small larvae exhibit a slow swimming speed. Hence, low food densities can lead to hunger and death.

Transport hypothesis: Small larvae are plankton with poor swimming ability. Hence, they cannot swim to thermoclines (a layer of water in which its temperature and salinity change rapidly) and habitats that are easier to live in the way that nekton can. The survival rate would decrease if the larvae are carried away into the open ocean where they cannot survive due to abnormal ocean currents.

Water temperature hypothesis: Sudden drops in water temperature or high water temperatures result in mass death since they cannot escape.