Estimating whole-lake fish catch per unit effort

The European standard for gillnetsampling to characterize lake fish communities stratifies sampling effort (i.e., number of nets) within depth strata. Nets to sample benthic habitats are randomly distributed throughout the lake within each depth strata. Pelagic nets are also stratified by depth, but are set only at the deepest point of the lake. Multiple authors have suggested that this design under-represents pelagic habitats, resulting in estimates of whole-lake CPUE and community composition which are disproportionately influenced by ecological conditions of littoral and benthic habitats. To address this issue, researchers have proposed estimating whole-lake CPUE by weighting the catch rate in each depth-compartment by the proportion of the volume of the lake contributed by the compartment. Our study aimed to assess the effectiveness of volume-weighting by applying it to fish communities sampled according to the European standard (CEN), and by a second whole-lake gillnetting protocol (VERT), which prescribes additional fishing effort in pelagic habitats. We assume that convergence between the protocols indicates that volume-weighting provides a more accurate estimate of whole-lake catch rate and community composition. Our results indicate that volume-weighting improves agreement between the protocols for whole-lake total CPUE, estimated proportion of perch and roach and the overall fish community composition. Discrepancies between the protocols remaining after volume-weighting maybe because sampling under the CEN protocol overlooks horizontal variation in pelagic fish communities. Analyses based on multiple pelagic-set VERT nets identified gradients in the density and biomass of pelagic fish communities in almost half the lakes that corresponded with the depth of water at net-setting location and distance along the length of a lake. Additional CEN pelagic sampling effort allocated across water depths and distributed throughout the lake would therefore help to reconcile differences between the sampling protocols and, in combination with volume-weighting, converge on a more accurate estimate of whole-lake fish communities.

Viscous-Flow Approach to In Situ Infiltration and In Vitro Saturated Hydraulic Conductivity Determination

Infiltration is dominantly gravity driven, and a viscous-flow approach was developed. Laminar film flow equilibrates gravity with the viscous force and a constant flow velocity evolves during a period lasting 3/2 times the duration of a constant input rate, qS. Film thickness F and the specific contact area L of the film per unit soil volume are the key parameters. Sprinkler irrigation produced in situ time series of volumetric water contents, θ(z,t), as determined with TDR probes. The wetting front velocity v and the time series of the mobile water content, w(z,t) were deduced from θ(z,t). In vitro steady flow in a core of saturated soil provided volume flux density, q(z,t), and flow velocity, v, as determined from a heat front velocity. The F and L parameters of the in situ and the in vitro experiments were compared. The macropore-flow restriction states that, for a particular permeable medium, the specific contact area L must be independent from qS i.e., dL/dqS = 0. If true, then the relationship of qS ∝ v3/2 could scale a wide range of input rates 0 ≤ qS ≤ saturated hydraulic conductivity, Ksat, into a permeable medium, and kinematic-wave theory would become a versatile tool to deal with non-equilibrium flow. The viscous-flow approach is based on hydromechanical principles similar to Darcy’s law, but currently it is not suited to deduce flow properties from specified individual spatial structures of permeable media.