Contrasting Demographic Responses to Size-Selective Harvesting among Neighbouring Wild Fish Populations

1. Sustainable harvesting of wild populations relies on evidence-based knowledge to predict harvesting outcomes for species and the ecosystems they inhabit. Although harvesting may elicit compensatory density-dependence, it is generally size-selective, which induces additional pressures that are challenging to forecast. Furthermore, responses to harvest may be population-specific and whether generalizable patterns exist remains unclear. 2. Taking advantage of Parks Canada's mandate to remove introduced brook trout Salvelinus fontinalis to restore alpine lakes in Canadian parks, we experimentally applied standardized size-selective harvesting rates (the largest similar to 64% annually) for three consecutive summers in five populations with different initial size structures. Four unharvested populations were used as controls. 3. At reduced densities, harvested and control populations exhibited similar density-dependent increases in specific growth, juvenile survival and earlier maturation. However, size-selective harvesting simultaneously induced changes to size and age structure that contrasted among harvested populations. Average body length decreased in three of five harvested populations, whereas it tended to increase in control populations over the 3 years. We also detected contrasting, population-specific changes in body length variability and ultimately in length- and age-at-harvest in harvested populations but not controls. 4. Overall, populations with smaller, more homogeneous body sizes, and living at high densities were most resilient to size-selective harvesting, exhibiting the smallest change in size-at-age. In contrast, large-bodied populations exhibited more substantial size-structure changes following selective harvesting: large-bodied populations experienced either stabilizing or disruptive pressures, when initial length variability was high or low, respectively. 5. Synthesis and application. Our results show that within species, size-selective harvesting inherently leads to more risk and uncertainty when harvesting populations with larger and more varied body sizes than smaller-bodied populations with less range in body size. Our study supports prioritizing regulations that protect harvested populations with larger and more varied body sizes. Such a management strategy would reduce the likelihood of eliciting unpredictable or undesirable demographic changes to fish populations with these attributes.