Decreasing trophic efficiency in cool-water aquaculture ponds: size-selective predation removes large prey

Abstract

Maximizing young-of-year (YOY) ¢sh production in an aquaculture setting depends on matching predatory demand with prey availability. With a size-selective YOY ¢sh species (saugeye: Sander vitreus Mitchell females  S. canadense Gri⁄th & Smith males) supplied with natural zooplankton prey (Bosmina sp. Baird), selective removal of larger individuals may decrease prey fecundity. However, increased nutrient fertilization may also ameliorate the top-down e¡ects of ¢sh predation. We tested these interactions in outdoor earthen production ponds (ca. 4000 m 2 ; n 512) by measuring Bosmina sp. size at ¢rst reproduction (SFR), maximum size (MAX) and neonate size (NEO) in ponds that varied in YOY saugeye densities (18^50 saugeye m À 3 ) and also di¡ered in phosphorus maintenance levels (either 20 or 30 mg PO 4 -P L À 1 ). We found that SFR decreased by 8% [from 0.298 mm AE 0.007 (mean AE 1SE) to 0.275 mm AE 0.005], MAX decreased by 11% (from 0.367 mm AE 0.009 to 0.328 mm AE 0.009) and NEO decreased by 5% (0.198 mm AE 0.004 to 0.189 mm AE 0.003) over the range of saugeye densities, and that SFR increased by 4% (from 0.279 mm AE 0.004 to 0.290 mm AE 0.003) and MAX increased by 3% (from 0.336 mm AE 0.004 to 0.347 AE 0.004) with increased fertilization. Further, prey o¡spring lengths strongly related to mother lengths and lengths di¡ered from early to late in the production season. These results indicate that multiple factors a¡ect prey sizes and emphasize that the removal of large prey individuals by size-selective YOY predators may decrease trophic e⁄ciency, ultimately decreasing ¢sh production.