Long term trends do not indicate a recovery of salmonids despite signs of natural reproduction

Abstract

Abstract Healthy populations of salmonids are integral for the functioning of ecosystems and valuable for the socio-cultural well-being of humans. Past declines were attributed to anthropogenic stressors, raising concern about the sustainability of populations. Accordingly, many salmonids are listed on red lists and protected by national legislation. One region where salmonid populations are threatened is Northern Spain, the most southern distribution of both the Atlantic salmon (protected under the EU Habitat Directive) and the brown trout. Here, we collated ~ 700 biomonitoring samples of both species collected across 177 sites over a 10-year period (2010–2019) to describe ongoing trends in these species and to relate them to site characteristics and potential drivers. We showed that both species have declined substantially, with stocked individuals constituting the majority of both populations. Natural reproduction was almost entirely absent for the brown trout (< 1%), but present in the Atlantic salmon (~ 20%). Both species expressed distinct spatial patterns, likely related to their stocking. As such, the observed trends for both species illustrate that reproduction is almost entirely lacking, underlined by a lack of adult salmonids. As a result, we not only underline alarming conditions of both species, but also question the effectiveness of currently employed stocking. Given that both species are of increasing conservation concern, targeted restoration measures like dam removal and pollution reduction must be applied to generate self-sustaining populations. River basin specific analyses of constraints are required to detect limiting factors on which conservation actions can be taken to ultimately make stocking dispensable.