Assemblage level monitoring of stream fishes: The relative efficiency of single-pass vs. double-pass electrofishing

The relative efficiency of single-pass electrofishing of longer stream sections vs. double-pass electrofishing of shorter reaches was evaluated in small (<5 m wide) wadable lowland streams in the Lake Balaton basin, Hungary. Two hundred meters long stream sections at 8 sites were divided into ten 20 m long sampling units each. These units were used to estimate the representativeness of species richness, species composition and relative abundance data at each level of sampling effort (single vs. double-pass, and number of sampling units pooled) using rarefaction and similarity-based approaches. Assemblage variables showed strong response to the length of the stream sampled (number of sampling units pooled). However, no differences were found between the single- and double-pass methods at any level of sample size for any assemblage variable. Estimates of species richness and species occurrence distributions required more sampling effort than estimates of species relative abundances, using any evaluation method. If a proxy estimate of sample representativeness cannot be obtained in the field, a minimum sampling of single-pass electrofishing of 100 m long sections may be necessary even in small wadable streams with low level of habitat and assemblage diversity to get a relatively unbiased picture on assemblage characteristics.     

Temporal dynamics of freshwater fish assemblages,their background and methods of quantifications—A synthesis

The temporal dynamics of biotic communities have been widely examined by ecologists. However, systematic reviews on how habitat features, sampling and data evaluation influence temporal patterns are rather sporadic. Here, we reviewed 307 peer-reviewed scientific articles to characterize the methods and the approaches, as well as to identify the knowledge gaps in the assessment of the temporal dynamics of freshwater fish assemblages with special regard to their stability patterns. The number of publications increased exponentially over decades. We revealed a highly uneven distribution of the studies among continents and ecosystems with a dominant number of papers derived from North America and Europe, and from lotic systems, especially. We also found large variability among studies even within similar habitat types in the examined spatial and temporal scales, sampling methods used, examined assemblage attributes, potential stressors and data analyses. Several knowledge gaps, such as the limited number of large-scale studies, the insufficient knowledge on the long-term dynamics of early life history stages and on trait-based assemblage organization, were highlighted. For enabling meaningful comparisons of fish assemblage dynamics in space and time, further developments in standardization procedures are needed across sampling and data evaluation possibilities. Publicly accessible long-term data sets with more details on sampling and environmental parameters would also be critically important to determine the effect of a variety of factors on the stability vs. variability of fish assemblages.