Prey Supply and Predator Demand in a Reservoir of the Southeastern United States

Abstract

We estimated annual prey supply and predator demand distributions for the fish assemblage of Norris Reservoir, Tennessee, to assess potential prey deficiencies. Prey supply was defined as the surplus biomass that could be removed without affecting future prey generations and was limited to cohorts consumed by predators. Demand was represented by the annual consumption by the predator community (piscivorous fishes) and was estimated with bioenergetics models. Our demand estimates were conservative because predators may have greater demand for prey than what they actually consume. Together, clupeids (gizzard shad Dorosoma cepedianum, threadfin shad D. petenense, and alewife Alosa pseudoharengus) and Lepomis spp. accounted for 80% of overall prey fishes consumed. Median annual demand for clupeids and Lepomis spp. combined was 271 kg/ha (90% confidence interval = 183–552 kg/ha) from August 1996 to July 1997 and 182 kg/ha (92–509 kg/ha) from August 1997 to July 1998. Median supply during these times was 848 kg/ha (103–18,989 kg/ha) and 413 kg/ha (317–540 kg/ha). Dividing supply values by demand values yielded supply–demand ratio distributions with medians of 2.9:1 (0.3:1–67.5:1) and 2.3:1 (0.8:1–4.8:1) for the two periods. Using a historical area–density data set (coves sampled with rotenone; N = 22 years), we found the median annual supply to be 167 kg/ha (49–982 kg/ha), demand to be 109 kg/ha (51–239 kg/ha), and the supply–demand ratio to be 1.5:1 (0.4:1–10.4:1). Historical values of the supply–demand ratio were less than 1:1 (i.e., supply was insufficient to meet demand) 32% of the time. The latter estimate is minimal because the threshold supply–demand ratio (the ratio below which predators are forage limited) was probably greater than 1:1, as predators cannot capture all prey with 100% efficiency and our definition of demand was conservative. We suggest adjusting demand to equal the median historical supply.