Evaluating two sampling methodologies for shrimp density and biomass estimates in streams

Abstract

Freshwater shrimp can reach high biomasses, affecting ecosystem processes. It is important to define the most accurate methodology to estimate their densities and biomass. We studied two species of different sizes, Potimirim brasiliana Villalobos F., 1960 and Macrobrachium olfersii (Wiegmann, 1836), in three sites of a coastal stream in Ilha Grande, Rio de Janeiro, Brazil. We evaluated the efficiency of two infrequently used sampling methods: a depletion method using electrofishing, and a substrate method using benthic samplers. The methodologies provided different estimates, especially for M. olfersii, of density (electrofishing 1.27 ± 6.3 ind/m²; substrate sampling 0.1 ± 5.05 ind/m²) and biomass (electrofishing 1.66 ± 8.5 g/m²; substrate sampling 0.11 ± 1.16 g/m²). The median size of M. olfersii was higher for electrofishing, while the median size of P. brasiliana was higher for substrate sampling. Electrofishing is good at collecting bigger individuals that are not possible to catch with the substrate method and it samples a bigger area that includes many different microhabitats. The substrate method is cost-effective, especially for P. brasiliana, but by sampling a small area, its estimates can be highly affected by local heterogeneity. The substrate method is not recommended for M. olfersii, as it underestimated both its density and biomass.

Keywords:
Electrofishing; freshwater shrimp; Macrobrachium olfersii; Potimirim brasiliana; substrate sampling