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

doi:10.21203/rs.3.rs-2436991/v1

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

https://doi.org/10.21203/rs.3.rs-2436991/v1

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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.

migration

fisheries

recruitment

salmonids

long term monitoring

Atlantic salmon

Brown trout

North Iberian rivers

population trends

environmental pressures

Freshwater ecosystems cover only ~ 0.8% of the planet’s surface while accounting for a large percentage of the global biodiversity, including ~ 18.000 fish species (Zhongming et al., 2021). Due to the manifold stressors affecting freshwater ecosystems (Davis, 2003; Stendera et al., 2012), declines in freshwater biodiversity have exceeded those of terrestrial or marine ecosystems (Dudgeon et al., 2006). Frequent stressors of riverine biodiversity are hydromorphological alterations, invasive species, pollution, or (among others) the effects of intensive agricultural activities. Borsuk et al. (2006) for instance showed that suboptimal hydromorphological conditions (such as riverbed depth and width, diverse bank structure, shading, appropriate substrate, etc.) can be the most important and ubiquitous stress factors that have impacts of sufficient magnitude to explain currently declining fish populations. Lange et al. (2014), however, showed that agriculture intensity primarily impacted fish population densities, underlining the complex nonlinear interactions across multiple stressors (Harvey & Railsback, 2007).

Observed declines in many fish populations lead to marked changes in the socio-economic services provided by freshwater ecosystems and recreational fisheries in particular (Arlinghaus & Cooke, 2009). Despite few notable exceptions (see e.g., Nicola et al., 2018), the majority of studies investigating changes in the abundance of socio-economically important fish species focused on specific locations rather than changes at the catchment scape or entirety of the river network (see e.g., Willmes et al., 2018; Walters et al., 2019). This is especially important, as streams are interconnected networks (Fergus et al., 2017) and many fish species (especially migratory ones) are subject to high spatial variability and different environmental conditions (Jobling et al., 2010; Pusey et al., 2011). Two species that have been signalled as ecologically and socio-economically important (on local as well as national scales) are the brown trout Salmo trutta fario and the Atlantic salmon Salmo salar (Jonsson et al., 1995; Knapp et al., 2013, Criddle & Shimizu, 2014), which often co-occur in freshwater streams across Europe (Lobón-Cerviá, 2009; Perrier et al., 2011). Both species are relevant environmental quality indicators and umbrella species for conservation targets due to their species-specific reliance on specific hydromorphological conditions and functioning river connectivity (Armstrong et al., 2003; Newson & Large, 2006). Further, they are of great importance from cultural, historic, and ethnographic points of view as wild populations sustain recreational fisheries, regulated by the regional fisheries council, providing considerable economic value (Liu et al., 2011).

Although these species are protected by the EU Habitat Directive (Atlantic salmon) or several national laws (brown trout; Doadrio, 2001), they have declined significantly in recent decades (De Leániz & Martinez, 1988; Lebón-Cerviá, 2009; Barquín et al., 2012), proposedly due to changes in climatic conditions and habitat linkages (Mills et al., 2013), overfishing (Buchsbaum et al., 2005) and the use of pesticides (Fairchild et al., 1999; Almodóvar & Nicola, 2004) – affecting both species to a different degree (Lobón-cerviá, 2009; Nicola et al., 2018). On the other hand, the removal of weirs and thus, partitioned connectivity among stream sections and spawning grounds in tributaries, are vital for the recovery of Atlantic salmon populations (De Leaniz, 2008; Wolter, 2015). Previously abundant populations of either species showed severe declines most prominently expressed in diminished recruitment (De Leaniz, 2008). Recently, Atlantic salmon status was updated in IUCN from “Least Concern” to “Vulnerable” while brown trout remained at “Least Concern” according to the latest version IUCN (Freyhof, 2011; World Conservation Monitoring Center, 2022). Both Atlantic salmon and brown trout are protected under national Spanish laws and considered “key species” for conservation efforts at regional levels (i.e., in Cantabria, Spain; See Fig. 1). Yet, they are merely categorised as indeterminate, i.e., a taxa known to be either "Endangered", "Vulnerable", or "Rare" by the Spanish Red List of Endangered Species and the IUCN (Comité Español de la UICN y Fundación Naturaleza y Hombre, 2019).

In Europe, the southern limit of the Atlantic salmon distribution is Portugal, which is within the southern band of the salmonid family's latitudinal range (including brown trout). This makes the documented declines in especially Atlantic salmon (Quinn, et al., 2006; Chaput, 2012) particularly noteworthy (Horreo et al., 2011; Saavedra-Nieves et al., 2021). To assist the recovery of Atlantic salmon and brown trout populations in Northern Spain (De Leániz & Martinez, 1988) – and as a means to maintain economically relevant trout fisheries (Baer & Brinker, 2010) – individuals of both species have continuously been stocked since the early 1980s (Ayllon et al., 2006; Bacon et al., 2015) albeit uncertain outcomes for long term conservation targets for natural recruitment (George et al., 2009).

Lobón-Cerviá (2009) used data from 1984–2008 and suggested that localised brown trout populations in the Cantabrian corridor were recovering ‘naturally’ in 2004–2006 to population sizes comparable to those of the mid 1980s, amidst showing a decline in 2008. Considering recent long-term data analyses revealed the severe effects of climatic stressors for brown trout (Svenning et al., 2022), Pacific Salmon (Cline et al., 2019) and Atlantic Salmon (Nicola et al., 2018), it is possible that the recovery postulated in 2004–2006 was an artefact created by intensive stocking as indicated by recent literature, indicating that sustainable improvement has not been lasting (Saavedra-Nieves et al., 2021). Yet, it was neither tested with real data whether this postulated increase was a localised or temporally isolated incidence, driven by climate or environmental change, nor were the direct effects of long-term stocking on the conservation of natural populations investigated. Using a 10-year period (2010–2019) of annual stream monitoring data of Atlantic salmon and brown trout populations in Northern Spain with a considerable spatial resolution covering 704 sampling events and supplemented with a selection of climatic and environmental variables, we investigated and aimed to explain trends in both protected species in space and over time. We (i) analysed temporal population trends in these two species, (ii) related these trends to site characteristics and potential drivers and (iii) identified if recruitment is naturally occurring or if recruits originate mainly from artificial stockings. Due to their respective requirements (Armstrong et al., 2003) and the considerable stress induced by anthropogenic factors and climate change (Nicola et al., 2018; Svenning et al., 2022), we specifically hypothesised that (1) a sustainable improvement in either species’ natural population – despite past and ongoing stocking efforts - has not been lasting, as both populations of both species have likely declined, and (2) eventually occurring recruitment success of either Atlantic salmon and/or brown trout will differ.

2.1. Data collection

Data was generated by electrofishing conducted by the company Ecohydros s.r.l. as part of the annual fish stocks monitoring of the Fisheries Agency of the Cantabrian Autonomic Government between 2010 and 2019. The dataset covers 177 sampled sites of the river network of the entire region of Cantabria, Spain (Fig. 1) sampled annually within the same three-month season (summer). To correct for the high variability of sampling years (ranging from 1 to 10 years with the majority of sites being sampled less than 4 times) and thus, also to correct for oversampled years (i.e., sites sampled in 2010, 2018 and 2019), sites sampled in these years only were not included, retaining a total of 78 sites. Thirty-one of which were sampled annually (Supplementary Table S1; TS2). The surveys followed a multi-pass method for density estimates (Hedger et al., 2013; 2018), controlling for the reach length. Typically, two or three passes were performed, after which no further individuals were obtained. Sampling was conducted using two electrofishing gears: (i) backpack gear (Hans Grassl ELT 60 II) producing an electric power of 1.3 kW and pulsed current (PDC) of 300 / 500 V, conducted by three technicians; (ii) bankside gear (Hans Grassl EL65II GI) generating 13 kW of electric power applied in wider and deeper streams; conducted by a crew of four members. With both approaches, the electric output was a 300 to 600 V PDC, in the frequency range of 80–100 Hz, with the fishing following the same multi-pass sampling method. All captured individuals were identified to the species level, sized (cm; fork length) and weighed (g; wet weight). Stocked individuals were identified using the ‘splitted’ adipose fin, which was missing in stocked individuals. Based on the individual weight of all fish caught within a sampled site, the biomass (in Kg/100m) of each site’s catch was estimated. Using the sampled reach’s length and the number of individuals caught (i.e., total abundance), the density (in Ind/100m) of caught fish within each site was calculated.

A multiparameter waterproof meter (HANNA® instruments HI9829) was used to measure physic-chemical parameters (water temperature; conductivity; oxygen content; pH). Because measurements were not available from all samplings throughout the entire period missing for ~ 30% of all measurements and because these can be subject to considerable local variation (Bae et al., 2016), further the supplemented site-specific climatic data (i.e., modelled air temperature and precipitation with a 0.1° resolution). These were extracted from the E-OBS gridded dataset, developed by the European Climate Assessment and Database; ECAandD, www.ecad.eu; Cornes et al., 2018) using the coordinates from all sampling sites and the respective date of the sampling event. Using the daily average temperature and precipitation from each site, we computed the average monthly temperature and precipitation from the 12 months prior to the sampling conducted at each respective site.

2.2. Analyses of observed trends

In a first step – in order to obtain an overview on identifiable trends – non-parametric Mann-Kendall trend tests with the mmkh function from the modifiedmk R package were used (Patakamuri et al., 2020) to identify increasing or decreasing monotonic trends in observed abundances (total number of individuals caught per site), biomass (in Kg/100m) and density (in Ind/100m) of the overall caught individuals, young-of-the-year, i.e., freshly spawned individuals (age class 0+) and ‘fingerlings’ (age class 1+) of Atlantic salmon and brown trout over the studied period (2010–2019). In addition, site-specific Mann-Kendall trend tests were performed to investigate the spatial distribution of trends. Adult individuals of the Atlantic salmon were excluded due to their scarcity and migratory pattern that did not overlap with the sampling period.

In addition, Generalized Additive Models (GAMs) implemented in the mgcv R package (Wood, 2015) were used to investigate changes in the relative abundance of stocked individuals of each species, as well as trends in biomass and density of both Atlantic salmon and brown trout. Thus, each individual model consisted of the specific response variable (relative abundance of stocked individuals, overall abundance, biomass, and density of age classes 0 + and 1 + of each species) and the explanatory variables: ‘year’, ‘latitude’, ‘longitude’, ‘elevation’ to investigate spatial and temporal patterns. To further investigate drivers of the response variables we included predictor which could modulate the trends, the respective model was expanded by including modelled average of temperature (in C°) and precipitation (in mm/day) 12 months prior to sampling to infer effects of climate and the North Atlantic Oscillation (NAO). The irregular fluctuation of atmospheric pressure over the North Atlantic Ocean that has been shown as a strong effect on winter weather in Europe, we included NAO to investigate the relationship between the discharge in late winter/early spring and the recruitment (Lobón-Cerviá, 2009). Following Lorenzo-Lacruz et al. (2011), the index was calculated as the average of the indexes for December in one year, and January, February, and March of the following year. The NAO index data was obtained from the Climate Research Unit website (http://www.cru.uea.ac.uk/cru/data/nao/nao.dat). Further, a common smooth spline for the predictors ‘year’ ‘elevation’, ‘temperature’, ‘precipitation’, ‘latitude’ (north-south gradient), ‘longitude’ was used and ‘NAO’ as nonlinear responses are expected, and the penalised regression terms ‘river ID’ (i.e., river sampled) and ‘sampling effort’ (sampled sites per year) as conventional random effects to minimise the effects of variability or randomness in the sampling and to correct for overdispersion. For proportional data (i.e., the relative abundance of stocked individuals), beta regressions (with a log link) were incorporated as data followed a binomial distribution (Bates et al., 2014). For abundance, biomass, and density estimates, (i.e., count data), negative binomial (with log link to the predictor) family were used as appropriate for data when the residual variance is found to be larger than the mean (in ecological data commonly known as over-dispersion; White & Bennetts, 1996; Wood, 2017).

All predictors were checked to ensure that variance inflations were lower than four, to exclude the possibility of collinearity, and that all predictors were unaffected by nonlinear correlations or dependencies (concurvity; Marra & Wood, 2011). Further, the overdispersion was checked by the conditional distribution implied by the model using the countreg R package (Kleiber & Zeileis, 2017). All analyses were performed in R version 4.0.2 (R Core Team, 2020).

2.3. Site characteristics and selected environmental data

Comprehensive abiotic data for the region examined were not available. Thus, the trends in environmental condition were evaluated in relation to the presence of the two monitored fish species using abiotic measurements (measured water temperature, modelled air temperature and precipitation, pH, conductivity, oxygen levels, and sampling effort) and obtained biotic measurement (density, biomass, abundance) during the fish monitoring. These were analysed using a space-for-time approach (Picket, 1989), as parameters were not continuously measured, which in contrast to long-term data analysis can be at risk of biases related to shifts in samplings (Pilotto et al., 2020). The advantage of utilising ‘local’ snapshot data from spatially separated sites that are collated and analysed based on a regional and temporal gradient can serve as a usable proxy for predicting trends similar to time-series (Picket, 1989; Buyantuyev et al., 2012). Data from adult Atlantic salmon was excluded from this, as there was only one individual caught over the ten years period. In addition, GAMs were used to investigate broader climatic trends in the studied region by analysing modelled air temperature and precipitation over the period 1980–2020, focusing on the period 2010–2019 to infer climatic effects.

3.1 Trends in Atlantic salmon and brown trout

The applied Mann-Kendall trend test identified significant declines in the abundance (i.e., total number of caught individuals), biomass, and density of overall, 0+, and 1 + individuals of the brown trout (with exception for the biomass of 0 + individuals). The ratio of stocked individuals declined for the overall number of caught individuals of the Atlantic salmon, driven by 1 + individuals (Fig. 2; Supplementary Table S3). No clear spatial pattern in site-specific trends of either species was identifiable (see Figure S2). Mature individuals of the adult Atlantic salmon were not included as only 1 adult specimen was caught.

3.2 Trends in the stocking of Atlantic salmon and brown trout

The 95.5% of 0 + and 61.3% of 1 + individuals of the Atlantic salmon were stocked, as identified by the missing adipose fin. In total, stocked Atlantic salmon comprised 79.2% of the overall sampled individuals, whereas the percentage of stocked individuals of the brown trout was 98.2% (0+: 99.3%; 1+: 97.0%; Supplementary Table S2). The percentage of stocked individuals of the Atlantic salmon was consistent over time and across the south-north gradient, albeit showing a decrease of 1 + Atlantic salmon towards recent years. A similarly consistent stocking ratio was observed over the elevation grant, while 0 + Atlantic salmon were only found until ~ 550 masl (Fig. 3). Albeit a slight decrease in stocked 1 + brown trout in 2012–2016 and a latitude of 43.4°N, the ratio of stocked 1 + brown trout increased with elevation (Fig. 3).

3.1. Drivers of observed trends

The observed abundances of 1 + individuals of the Atlantic salmon increased over time (peaking in 2014), whereas both 0 + and 1 + individuals were found to decrease towards the south, i.e., inland, peaking between ~ 200–550 masl, afterwards steeply declining (all p < 0.05). A positive NAO index and higher temperatures positively predicted the abundance of 1 + individuals, whereas precipitation predicted 0 + individuals (all p < 0.05; Supplementary Table S4). The abundances of Brown trout (both 0 + and 1 + individuals) were found to increase over time (both p < 0.001) and further inland towards higher elevated regions (all p < 0.05), being most abundant between ~ 43.0°N and 43.2°N. An increase in NAO and precipitation positively predicted the abundance of 0 + individuals (p < 0.05; Supplementary Table S4).

The GAMs further identified that ‘year’, North Atlantic Oscillations (‘NAO index’), and latitudinal gradient (‘Latitude-Longitude’; i.e., the North-South gradient) had significant effects (all p < 0.001, except for the ‘NAO index’ on Atlantic salmon 0 + density p < 0.05) on all age classes of both species except 0 + of salmonids (Supplementary Table S4). Biomass and density of 1 + Atlantic salmon decreased over time, peaking in the beginning in 2010 and 2014, while both 0 + and 1 + brown trout expressed an initial increase and subsequent decline in this period. Elevation had a significant effect on both brown trout age classes as well as age class 1 + of the Atlantic salmon (all p < 0.001). Biomass and density of age class 1 + were identified as bell-shaped for Atlantic salmon, with the highest values occurring at an elevation of ~ 500 masl, whereas the highest biomass and density of 1 + brown trout occurred at ~ 200 masl and ~ 850 masl respectively. Biomass and density of age class 0 + from both species were stable over time, but with higher densities of 0 + brown trout occurring also at ~ 850 masl. The North-South gradient indicated the highest biomass and density of age class 0 + from Atlantic salmon to occur above 43.4°N. Lastly, an increasingly positive (NAO-index) positively affected biomass and density of age class 1 + Atlantic salmon and both age classes of brown trout (Fig. 4). Longitude was identified as non-significant or significant at the 0.05 level (which should be interpreted with caution as GAMs yield estimates, not exact values). For class 1 + Atlantic salmon, stocking ratio was a significant predictor for both biomass and the occurring individuals (Supplementary Table S4).

3.3 Site characteristics and selected environmental data

Among the abiotic variables sampled during the fish monitoring, the applied Mann-Kendall trend test identified that only dissolved oxygen concentration decreased significantly over the investigated period in our space-for-time approach (Fig. 4), while water and air temperature, pH, water conductivity, and sampling effort did not change notably (Supplementary Table S5). Concomitantly, temperature and precipitation collated from all studied sites over the period 1980–2020 changed significantly over time (both p > 0.05), temperature increasing significantly by 0.09°C and precipitation decreasing by respectively 0.02mm. In 2010–2019 (both p > 0.05), temperature increased by 0.72°C whereas precipitation decreased by 0.48mm (Supplementary Table S6).

The ongoing decline of salmonid species is a conservation issue of striking global importance due to their importance in recreational and commercial fisheries, as well as cultural practices being iconical species (Nicola et al., 2018). As such, the observed declining trends in all age classes from brown trout and age class 1 + of Atlantic salmon are worrisome, yet not surprising. It should be noted, however, that for adult Atlantic salmon, the low number of caught individuals is likely related to the migratory period being in fall rather than in summer when the sampling was conducted. Only one individual was caught over the entire period, not allowing for any conclusion, especially considering that not all adult individuals immigrate into rivers (Metcalfe & Thorpe 1992). Further, almost 80% of the collected juvenile salmon were identified as stocked individuals, suggesting low reproduction and sea return rates. Amidst the higher availability of data, brown trout declined similarly. Yet, without information on stocking pressure, it remains difficult to evaluate the observed declines. Although declining trends were expected, they mirror forecasts predicting their decline and even disappearance, as it could be expected that below a certain threshold, a recovery of populations is unlikely (Hutchings, 2015).

4.1. The mixed signals of trends

Although fishing was not banned and angling pressure, the presence of dams, and water quality impoverishment have remained constant until the first decade of the 21st century, localized populations were described to have recovered ‘naturally’ in a very short period (in 2004–2006) to population sizes comparable to those of the mid 1980s (Lobón-Cerviá, 2009). However, albeit the lack of an appropriate baseline, the data indicated a contrasting trend over one decade in terms of declining density and biomass trends for both species. While this could be linked to an observed decrease in observed density of stocked individuals of the Atlantic salmon over time (of both age classes), the dominance of stocked brown trout did not decrease. Concomitantly, we found no significant trend in either species spatially (see Supplementary Figure S2), not regarding the stocking density over time (Supplementary Figure S3), substantiating our assumption that stocking efforts remained constant. Yet, with only ~ 20% of mostly 1 + juveniles of the Atlantic salmon not having been stocked and thus, no sufficient recovery being identifiable in an increasing presence of natural reproduction in 1 + Atlantic salmon, the substantial natural reproduction is only occurring in the case of the Atlantic salmon but not the brown trout (Sahashi et al., 2015; Iida et al., 2018). Thus, the decline in the stocking ratio noted in 1 + Atlantic salmon can, however, not be unequivocally attributed to an increase in natural reproduction, especially considering that wild adult population abundance has likely declined over the same period. Surprisingly, the percentage of stocked 0 + Atlantic salmon decreased when compared to 1 + individuals, suggesting a higher mortality in stocked individuals as stocked individuals were likely less fit (Schill et al., 2017).

An increasing spatio-temporal variability in recruitment (i.e., the process of small, young fish transitioning to an older, larger life stage) and its success were strongly linked to increased variability in stream discharge and stream-bed quality (Burkhardt-Holm, 2008). In turn, recruitment as well as stocking efforts appear to be the major determinants of year-class strength and hence, of population size (Lobón-Cerviá, 2009) since the ratio of naturally spawned to stocked recruits (particularly age class 1 + of the Atlantic salmon) increased over the duration and towards the end of the study period. This resulted in about ⅓ of Atlantic salmon parr being naturally recruited and a higher share of naturally spawned individuals of the Atlantic salmon mostly occurred between 200 and 500m asl. While this suggests that intense stocking may mostly occur in this range, it also suggests that intensive stocking may negatively affect natural recruitment (Sahashi et al., 2015) and can limit the distribution of both species spatially, as localised presences may be particularly affected (Aprahamian et al., 2003).

Furthermore, although information on the actual stocking effort was unavailable, it remains questionable if intense stocking is of interest for the conservation of natural salmonid populations and biodiversity in general. This is because salmonids are capable food-web manipulators, with smaller individuals predating invertebrates, and fish occurring more frequently with increased size (Radke et al., 2003), and high numbers of stocked individuals ultimately altering nutrient cycles and affecting community compositions (Martin et al., 1998; Maucieri et al., 2019). While there is concern that stocked individuals cause genetic pollution (Palmé et al., 2012) and are not as competitive as wild populations (Weber & Fausch, 2003), other studies’ findings indicate long-term caveats and drawbacks on recruitment (due to elevated mortality rates among native and stocked individuals; Ayllon et al., 2006; Almodóvar et al., 2020) and that often, no stocked individual returns as adult in the successive years (Saltveit, 2006; Finnegan & Stevens, 2008). As such, the data shows that conservation efforts should consider that any stocking practice will ultimately induce a high degree of competition that can inversely alter community compositions (Kennedy & Strange, 1986) and cause top down and bottom-up effects within trophic webs, decreasing the survival rate of naturally recruited individuals (Lynam et al., 2017).

4.2. Site characteristics and selected environmental data

Data limitation and the resulting interpretation are incessant issues in long-term studies (Picket, 1989; Ellner & Turchin, 1995), in particular for the conservation of protected species like salmonids (Niemelä et al., 2005). Long-term salmonid studies with sufficient spatial and temporal resolution are rare (but see Niemelä et al., 2006; Nicola et al., 2018). This is also true in this study, as abiotic measurements were not available throughout the entire study period and every individual sampling point and therefore, could not be included into the applied GAMs without a considerable loss of statistical power. The observed trends, however, do not explain declines in salmonids. Environmental degradation and inadequate hydromorphological conditions (O'Briain et al., 2019), concomitant anthropogenic activities and impoundments, may nonetheless have created an interruption of continuum processes as described by the ‘serial discontinuity concept of lotic ecosystems’ (Ward & Stanford, 1983) affecting in particular the Atlantic salmon. The results further indicate a decline in precipitation (which can be interpreted as a proxy for runoff) and an incline in temperature – both particularly congruent with the studied period – being indicative of a worsening of habitat suitability. Considering past declines and stress due to climate change (Isaak et al., 2012), the observed decline in available oxygen – arguably driven by climatic changes – poses another substantial threat for salmonids, not projecting a favourable situation for salmonid species (Whitmore et al., 1960).

Amidst the particular cases of migration obstacles eliminations (https://amber.international; AMBER Consortium, 2020) and changing environmental policies (e.g., as implemented in the EU Water Framework Directive or the EU Biodiversity Strategy), latitude and elevation significantly affected both species’ juveniles. In addition, Atlantic salmon in northern Spain occur mostly at lower altitudes and in a condensed spatial range while brown trout covered a greater range with a limitation in elevation (Barquin et al., 2012). Indeed, it may be reasoned that the further south a mature salmon migrates, the more dams and weirs must be overcome – especially considering the even spatial distribution of barriers (e.g., dams or weirs) still in place in Cantabria (Supplementary Figure S1). The effect of such barriers is subsequently emphasised by the frequency of barrier occurrences increasing the further south and the higher a stream extends into the Cantabria mainland (Rodeles et al., 2017), concomitantly substantiating the effect of latitude and elevation as a barrier. This is especially true for the anadrome Atlantic salmon, as underlined by the observed elevation effect. Yet, as previously shown (see e.g., Jonsson & Jonsson 2004; Friedland et al., 2009), an increase in the NAO-index positively affected both species (in terms of biomass and density), adding to the importance of this abiotic factor in management and monitoring. However, the identified correlation with the NAO-index in this study could be explained by the NAO-index systematically impacting the environmental conditions in the rivers, which again – albeit existing stocking pressure – could impact the catchability during electrofishing (Sætre et al., 1999; Ottersen et al., 2001).

Regarding the first hypothesis, however, the results presented here indicate that there are no clear differences in the effects of environmental changes between both species. Nonetheless, the available data suggests that both species are likely to be affected differently due to the variability found across both species’ spatial distribution by environmental variables. This is further emphasised by their specific hydromorphological requirements (Armstrong et al., 2003).

4.3. Implications

Both species showed decreasing trends despite extensive stocking efforts in Northern Spain (Almodóvar et al., 2020). These trends can only partially be explained with environmental changes (Fairchild et al., 1999; Isaak et al., 2012; Mills et al., 2013), suggesting that factors not recorded in our study, such as overexploitation, pollution, or hydromorphological degradation, could affect both species substantially; albeit possibly to a different extent. This is particularly worrying as both species are of high conservation but also recreational interest. The brown trout in particular is an important angling species with high socio-economic value, creating a complex legislative situation that makes effective management and conservation difficult (Almodóvar et al., 2002). Exemplary, it should be noted that past declines in brown trout and Atlantic salmon accelerated recent declines in the European freshwater pearl mussel Margaritifera margaritifera which relies on either species for its reproduction (Geist et al., 2006; Filipsson et al., 2018), but seemingly persists in the study area (Perea et al., 2022). However, having observed an increase in the share of naturally spawned individuals towards the end of the study, raises hope for a potential future recovery at least for the Atlantic salmon.

As a direct response to documented declines and concomitant to the declared aim of the EU Water Framework Directive to re-establish longitudinal connectivity and ecological quality (Council of the European Commission, 2000), the removal of barriers (e.g., weirs and dams) and the construction of fish passages was already initiated (Branco et al., 2014). The EU biodiversity strategy (https://ec.europa.eu/environment/strategy/biodiversity-strategy-2030_en) or projects such as AMBER (amber.international) target these impoundments that create an interruption of continuum processes (Belletti et al., 2020) and greatly detriment any recovery of salmonids, especially populations of the Atlantic salmon (Hill et al., 2019). Such efforts should be continued in alliance with the targets of Annex II of the Habitat Directive and the designation of Special Areas of Conservation, and Annex V that requires management plans for the exploitation of the species listed (Council of the European Communities, 1992).

Related efforts to repopulate rivers with the Atlantic salmon by conservation stocking of age class 0 + or age class 1 + have mostly failed to achieve any detectable amelioration (Salveit, 2006; Bacon et al., 2015), while recolonization often occurred without anthropogenic efforts following an improvement in river connectivity and habitat conditions (Kiffney et al., 2009; Perrier et al., 2010; Griffiths et al., 2011). When aiming to recover or possibly sustainably harvest native populations in the future, they must be promoted more extensively. This is, because the current conditions of either species are best reflected by decreasing trends which cannot be compensated with stocking alone. Therefore, it is important to prioritise rivers for conservation measures (Verspoor et al., 2008) and consequently use a variety of existing conservation and management tools. Considering that the Atlantic salmon is a migratory species makes the construction of fishways and elimination of migratory barriers the prime targets for management (Hill et al., 2019). This is not the case for Brown trout, making the restoration of its habitats crucial (Stewart et al., 2009), also benefitting the Atlantic salmon as habitat requirements of year classes of both species overlap (Armstrong et al., 2003). Regarding the study area, several measures have been applied quite recently (See Goldenberg-Vilar et al., 2022). These include aside from the stocking efforts of the Regional Government of Cantabria (Spain), the demolition of barriers and construction of fish passage devices in the frame of the National River Restoration Strategy of the Ministry for Ecological Transition and the Demographic Challenge (MITECO). This was done across the whole study area throughout the studies duration. Among these actions, initiatives of the hydrographic and regional administrations further include weir demolitions to restore the ecological connectivity in different rivers of Cantabria for salmon and trout to ensure reproductive migration under the umbrella of the DIVAQUA Life project (https://lifedivaqua.com/en/; Goldenberg-Vilar et al., 2022). The aim of the DIVAQUA Life project is to restore and improve the ecological and functional processes (ecological connectivity, hydrological regime, sediment transport, etc.) that occur in the aquatic ecosystems of the Natura 2000 network sites within the DIVAQUA area (Branco et al., 2022). In another outstanding initiative, hydroelectric facilities have constructed a fish lift and several fish ladders for the salmon recolonization of the Nansa River upstream of a 20 height dam (Hernández-Romero et al., 2022). In addition, long-term monitoring which has been conducted and will continue in the future will reflect possible changes, as naturally recruited Atlantic salmons have been captured during the sampling in the Nansa River, upstream of the mentioned dam for the first time, which reflects a first positive outcome of applied measures (Bravo-Córdoba et al., 2022; Hernández-Romero et al., 2022). As the longitudinal connectivity is being recovered, the habitat for salmonid reproduction and feeding should be improved through environmental flow management, guaranteeing both minimum flow rates as well as magnitude and timing of extreme flows during the most sensitive bio periods (King et al., 2016).

This study however also suggests that even though stocking might maintain the appearance of healthy populations, it may fail at promoting reproduction and could hinder sustaining natural populations (Sahashi et al., 2015; Almodóvar et al., 2020), thereby diminishing the potential of native populations to recover (Lorenzen et al., 2012; Hutchings, 2015). Here, genomic resources could select specimens from the same genetic lineage that are competitive to increase the effectiveness of stocking and help manage the density and biomass of both species (Waples et al., 2020).

It should nevertheless be noted that using uncalibrated electrofishing data to analyse temporal trends has previously been criticized for its limitations and biased capture probabilities (Honkanen et al., 2018; Glover et al., 2019). However, abundance-based information can be valuable for stakeholders and governments. As such, the current monitoring of salmonid populations should consider individual basins to detect those limiting factors on which action must be taken: improving habitat continuity and hydromorphological management at critical points to increase the potential useful spawning and recruitment habitat. Ultimately, the monitoring of the repercussions of this type of actions on the dynamics of salmonids will provide clues for the optimisation of habitat management plans to change the current negative trends.

Acknowledgements

Data on fish stocks have been provided by the Department of Rural Development, Farming, Fisheries, Food and Environment of the Regional Government of Cantabria (Spain), which explicitly approved the publication of the underlying data. This study was supported by the Grant Agency of the University of South Bohemia, project number 065/2022/Z. PJH received funding from the EU Horizon 2020 project eLTER PLUS (Grand Agreement No 871128).

Conflict of interest

The authors declare no conflict of interest.

Data availability statement

The data that support the findings of this study are available in the supporting information of the article or from the corresponding author upon reasonable request.

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

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Abstract

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1. Introduction

2. Methods

2.1. Data collection

2.2. Analyses of observed trends

2.3. Site characteristics and selected environmental data

3. Results

3.1 Trends in Atlantic salmon and brown trout

3.2 Trends in the stocking of Atlantic salmon and brown trout

3.1. Drivers of observed trends

3.3 Site characteristics and selected environmental data

4. Discussion

4.1. The mixed signals of trends

4.2. Site characteristics and selected environmental data

4.3. Implications

Declarations

References

Additional Declarations

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