Fishery, life-history change, and population dynamics of mantis shrimp in Tokyo Bay

Background/Question/Methods

Life-history theory predicts that when fisheries-induced size selection causes the survivability of large individuals to decline, the average growth rate is reduced and the size-at-maturation becomes smaller. These changes may often reduce biomass and also population size, since most species have higher batch fecundities at larger body sizes. Therefore, many papers encouraged fisheries to develop monitoring and management strategies incorporating these changes. However, the fact that also spawning seasons often differ with spawners’ body size has not been well discussed. Here we show a possible example of a population dynamics affected by the shift of the peak of the spawning season, in addition to the decrease of average body length and size-at-maturation. In mantis shrimp, Oratosquilla oratoria, in Tokyo Bay, individuals smaller than the minimum salable body-size and therefore discarded when caught are currently the major spawners. The maturity of females in this body-size class was 0% in 1950s and less than 10% in 1980s whereas it increased up to 59% in 2000s. Their spawning peak (summer) is later than the traditional season (spring) in which larger, salable individuals spawn. Recently, newly-settled juveniles have not been observed in the season and area where dissolved oxygen concentration is at a suffocation level (i.e., hypoxic) and these time and area match to those of expected settlement for spring cohort. Juveniles start to appear in the expected settlement season for summer-cohort. Therefore we expect the spawning of small individuals in new, late-shifted season has resulted in avoiding hypoxia and allowed for higher survivability, increased the number of successful recruits, and ameliorated the population decrease. To investigate this theory, we developed a mathematical model which described annual population sizes of 6 stages: age-0 individuals that were born in (1) spring and (2) summer; age 1 individuals that are (3) small, non-salable and (4) large, salable; and older individuals that are (5) small, non-salable and (6) large, salable.

Results/Conclusions

Computer simulations assuming no shifts of size-at-maturity and spawning-season peak showed larger decrease of population size than the actual data. In addition, simulations assuming no seasonal difference in larval survivability and settlement success showed increase. Thus the simulations suggested positive contributions of the life-history change and spawning-season shift to population dynamics. Simulations also showed that reducing fishing mortality on small individuals sustains the population effectively due to the later spawning peak for small individuals.