A Bioenergetics Approach to Modeling Striped Bass and Threadfin Shad Predator‐Prey Dynamics in Lake Powell, Utah‐Arizona

Abstract

An understanding of the mechanisms driving the cyclic relationship between piscivorous sport fish and their food base is needed in systems where predator and prey are tightly coupled. To understand the dynamics between striped bass Morone saxatilis and threadfin shad Dorosoma petenense in Lake Powell, Utah-Arizona, we (1) synthesized and evaluated 20 years of historical data on temperature, diet, growth, and abundance of these fish, (2) collected similar data on a finer scale in 2003-2004, (3) used components of this data set to develop specific conversions between coarser historic data and present data, and (4) modeled striped bass and threadfin shad dynamics within a bioenergetics framework. We estimated the consumption of threadfin shad by age-0, subadult, and adult striped bass in Lake Powell from 1985 to 2003. During this period, threadfin shad abundance peaked at approximately 5-to 7-year intervals and striped bass growth, condition, and abundance corresponded closely to peaks in threadfin shad foraging. Individual consumption of threadfin shad by striped bass was highest for adults followed by subadult and age-0 fish; scaling individual consumption up to the population level resulted in highly variable subadult consumption that fluctuated from highs of about 1.2 million kg (18.4 kg/ha) to lows of about 0.3 million kg (4.6 kg/ha) on a 1-to 2-year cycle. Despite these fluctuations, the consumption of threadfin shad was dominated by subadult striped bass, which appeared to control threadfin shad numbers in all but the highest peak years of shad abundance. Based on bioenergetics output, striped bass demonstrate a type II functional feeding response; consumption rates reach an asymptote when the threadfin shad biomass index exceeds 10,000 kg/year. We demonstrate a modeling approach that allowed us to evaluate the large fluctuations in predator and prey populations, which often become evident only over long time periods. Our results increase our understanding of the Lake Powell ecosystem and highlight areas for future study.

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