Brown shrimp (Farfantepenaeus aztecus) and white shrimp (Litopenaeus setiferus) are two of the most abundant species of penaeid shrimp in the U.S. However, their population dynamics are not clearly understood. A previous study demonstrated some associations between the density of shrimp and the density of their fish predators. However, this association only explained a small part of the variation in shrimp density. Although both species exhibit an annual lifecycle, abundances of shrimp in two consecutive years are not strongly associated with each other. These two facts suggest some environmental factors are strongly regulating their population dynamics. Here, the random forest analysis was applied to 37 years of monthly time series data of shrimp collected from eight major bays along the Texas coast using bag seine (targeting post-larvae) and trawl net (targeting juveniles) and environmental variables.
Results/Conclusions
Results show that post-larval brown shrimp and white shrimp were significantly affected by temperature when they settle or immediately before they settle in estuaries. Post-larval brown shrimp density peaked in spring, and their density was high when the the water temperature in winter was low. On the other hand, post-larval white shrimp density peaked in fall, and their density was high when the water temperature in summer was high. Although this may be simply suggesting the existence of phenology, the temperature was a better indicator of high density than months of the year in the analysis, suggesting the existence of temperature effect. Brown shrimp juvenile density was explained by their post-larval density suggesting there is no strong environmental regulation between post-larval and juvenile stages. Although white shrimp juvenile density was also explained well by their post-larval density in the northern bays, this relationship was not significant in the southern bays. In fact, in these bays, high settlement density of post-larvae frequently led to subsequent low juvenile density. This suggests there is strong environmental filtering between post-larval and juvenile stages of white shrimp in these bays.