Macroinvertebrate biomass estimates in Cantabrian streams and relationship with brown trout (Salmo trutta) populations

In this work, the average biomass of 80 relatively common families of benthic macroinvertebrates in the streams of the Biscay region (Basque Country, northern Iberian Peninsula) was estimated. The macroinvertebrates were collected using a kick net and preserved alive in cool aerated stream water. The fresh weight, dry weight, and ash-free dry weight (AFDW) were obtained for each family. The biomass values obtained were applied to more than a hundred macroinvertebrate samples collected from 1997 to 2006 in 17 stretches of 14 rivers inhabited by brown trout. The total and partial biomass of each sample was estimated. Significant differences in the density (F = 2.91; df = 16, 100; P < 0.001) and biomass (F = 6.52; df = 16, 100; P < 0.001) of the macroinvertebrates occurred among the stations, with up to a 11-fold and 17-fold range, respectively. The brown trout population size was positively correlated to both the macroinvertebrate abundance and biomass. By age classes, this relationship was significant for only the 1+ and 2+ trout. The AFDW metric is recommended because it represents the real organic biomass value that joins the trophic food chain, avoiding an overestimation of the energy contribution of taxa with shells or cases. The Gammaridae had higher values of AFDW than expected, due to their high content of organic matter. A positive selection of gammarids by the trout was also observed in some cases.