A method to address the non-random spatial distribution of on-board observer data to map landings and discards

Abstract

Migration is a ubiquitous life history trait with profound evolutionary and ecological consequences. Recent developments in telemetry and genomics, when combined, can bring significant insights on the migratory ecology of non-model organisms in the wild. Here, we used this integrative approach to document dispersal, gene flow and potential for local adaptation in anadromous Arctic Char from six rivers in the Canadian Arctic. Acoustic telemetry data from 124 tracked individuals indicated asymmetric dispersal, with a large proportion of fish (72%) tagged in three different rivers migrating up the same short river in the fall. Population genomics data from 6,136 SNP markers revealed weak, albeit significant, population differentiation (average pairwise FST = 0.011) and asymmetric dispersal was also revealed by population assignments. Approximate Bayesian Computation simulations suggested the presence of asymmetric gene flow, although in the opposite direction to that observed from the telemetry data, suggesting that dispersal does not necessarily lead to gene flow. These observations suggested that Arctic Char home to their natal river to spawn, but may overwinter in rivers with the shortest migratory route to minimize the costs of migration in non-breeding years. Genome scans and genetic-environment associations identified 90 outlier markers putatively under selection, 23 of which were in or near a gene. Of these, at least four were involved in muscle and cardiac function, consistent with the hypothesis that migratory harshness could drive local adaptation. Our study illustrates the power of integrating genomics and telemetry to study migrations in non-model organisms in logistically challenging environments such as the Arctic.

Use of avoidance behaviours to reduce the economic impacts of the EU Landing Obligation: the case study of a mixed trawl fishery

“…Given that discards might be overestimated for species with a discarded proportion larger than 0.9, an alternative method of calculation was used as in Pointin et al (2018).…”

“…Based on the main choke species identified from the previous section, the spatial and temporal distribution of landings and discards were explored for the métiers most likely to be impacted by the LO. In doing so, the nested grid method was used to map landings and discards over the period 2011-2016 using the R software (Pointin et al, 2018). This method adjusts the size of each grid cell as a function of the number of observations therein: small cell sizes are used in areas with many observations, and vice versa.…”

“…From the geographical coordinates of each FO observed from 2011 to 2016, the nested grids were constructed based on an iterative process of cell division: starting with a coarse regular grid, each cell was divided one or several times providing that the number of FOs therein was larger than a maximum threshold; each sub-cell with a number of FOs smaller than a minimum threshold was associated with low precision estimates. For each cell size, a maximum and minimum FO threshold were determined with a level of precision set to 0.35 (for more details, see Pointin et al, 2018). To map landings and discards in each grid cell, total landings and discards were then estimated over the whole study area from 2011 to 2016.…”

“…To map landings and discards in each grid cell, total landings and discards were then estimated over the whole study area from 2011 to 2016. They were finally distributed proportionally in each cell depending on local (i.e., per cell) estimated proportions computed from the observed FOs (for more details, see Pointin et al, 2018).…”

“…In this study, direct impacts of the LO implementation on economic results are first assessed. Depending on the expected economic incentives, avoidance behaviours are then identified using a mapping method based on nested grids (Pointin et al, 2018). The spatial and temporal distribution of landings and discards is thus explored with a particular focus on choke species, for which a reduction in bycatch is most likely to increase net profits.…”