Dear colleagues, I plan to measure fish abundance in rivers to assess the influence on habitat selection of diving birds. Use of biosonics dt-x sounds good, but it's too expensive. Would you kindly advise me? Thanks a lot.
The most common methods are angling and electrofishing.
Recent efforts to sample fish assemblages in deeper rivers by using multimesh gillnets developed for lotic environments (Fjälling et al. 2015), i.e. the Norden multimesh Stream Survey Net (NSSN).
https://www.researchgate.net/publication/282271055_Norden_multimesh_gill_net_for_fish_sampling_in_lotic_environments
Fish abundance or density is ideally expressed as numbers or biomass per area or volume of habitat sampled.
First option is to determine the absolute abundance. However, it is costly and time-consuming. It requires extensive data collection, such as a precise estimate of density at sampling sites and a probability-based array of those sites.
For many populations, indices of relative abundance are sufficient for assessment. I think that this is the best choice for you. Catch per unit effort (C/f or CPUE) is a relative abundance index, which is often directly related, though not always in a linear fashion, to absolute abundance. CPUE is more specifically expressed as numbers or biomass per unit effort (NPUE or BPUE).
You can find detailed description here:
https://www.researchgate.net/publication/292460108_Relative_abundance_and_catch_per_unit_effort .
https://www.fs.fed.us/rm/pubs_other/rmrs_2010_pope_k001.pdf
Precise relative abundance estimates should be collected from several areas (randomly distributed samples) in order to be representative of an entire system. In addition, the distribution of some species may need to be understood before undertaking relative abundance estimation, particularly for species that are rare or poorly sampled.
Abundance can be expressed separately for each species in the catch or summed over all species to represent the total fish community. Alternative abundance metrics may be calculated for groups of individuals according to e.g. size, age or functional traits as mentioned above.
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