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.

摘要 淡水虾类可形成较高生物量，对生态系统过程产生显著影响。确立用于精准估算其种群密度与生物量的最优方法，具有重要研究价值。本研究以巴西里约热内卢州格兰德岛某沿海溪流的3个采样点为研究区域，针对两种体型存在差异的虾类——巴西波氏虾（Potimirim brasiliana Villalobos F., 1960）和奥尔夫沼虾（Macrobrachium olfersii (Wiegmann, 1836)）展开调查，评估了两种较少被使用的采样方法的效率：一是采用电捕鱼法的去除采样法，二是使用底栖采样器的基质采样法。两种方法得到的估算结果存在显著差异，尤以奥尔夫沼虾为甚：其种群密度方面（电捕鱼法为1.27±6.3 个/平方米；基质采样法为0.1±5.05 个/平方米），生物量方面（电捕鱼法为1.66±8.5 克/平方米；基质采样法为0.11±1.16 克/平方米）。电捕鱼法采集到的奥尔夫沼虾体尺中位数更高，而基质采样法获取的巴西波氏虾体尺中位数更高。电捕鱼法能够采集到基质采样法无法捕获的大型个体，且其采样覆盖区域更广，涵盖多种不同的微生境。基质采样法具备成本效益优势，针对巴西波氏虾尤为如此，但由于其采样区域较小，估算结果极易受到局域生境异质性的影响。针对奥尔夫沼虾，不建议使用基质采样法，因为该方法会低估其种群密度与生物量。