Image techniques in turbid rivers: A ten-year assessment of cyprinid stocks composition and size

Riverine fisheries frequently face complex challenges along with the limitations of costly and yet poor data. This is the case of the middle to low reaches of many basins with turbid waters, where large stocks of cyprinids perform wide spawning migrations. These reaches support both commercial and recreational fisheries frequently exploiting stocks of unknown size. There, habitat complexity, turbid waters, and large moving populations hinder the use of non-extractive image techniques (video and photography) for species identification and stock assessment. So far, such techniques are therefore restricted to clear freshwaters and marine environments. In this study, image techniques and sampling methods are used to monitor large riverine stocks of two barbel species (Luciobarbus sp.) over ten years, while maintaining low economic costs and human effort. The techniques take advantage of the behaviors of the fish relative to a weir and a fish pass while migrating upstream to their spawning grounds. For that, meristic and morphological traits taken from individuals simultaneously captured and photographed were used to identify the taxa and validate this method. The subset of traits still reliable in the images was used to identify free-ranging leaping individuals from a ten-year span photographic database (2007-2016) and determine the annual relative abundance of every taxon. Video images taken at the fishway in dry and humid years were used to estimate absolute stock size in contrasting hydrologic years (2014-2016). Finally, estimates of migrating barbel stocks and their size across hydrologic year variation were obtained through sampling methods and combination of the aforementioned data. The pros and cons of the sampling methods and image techniques used are discussed relative to their applicability to the monitoring and management of cyprinid riverine fisheries in turbid reaches.