1. Understanding the effects of density-dependent and density-independent factors on recruitment is often inhibited by difficulties quantifying their relative contributions in highly variable recruitment data. Use of data-driven statistical methods with data that include one or more extreme recruitment events could help overcome these difficulties. 2. Juvenile Atlantic salmon and trout abundances in Wales have declined over the last 2 decades, and 2016 was a notably poor recruitment year in rivers around southern Europe, including England and Wales. The 2016 recruitment crash coincided with extreme winter weather conditions, leading to speculation that unusually warm temperatures and high flows adversely affect salmonid recruitment and caused the 2016 crash, although this remains untested. 3. We developed data-driven statistical models to: (1) describe juvenile salmonid recruitment from density-dependent and density-independent factors; and (2) assess whether the density-independent factors probably contributed to the 2016 salmon recruitment crash. We compiled salmon and trout young-of-year juvenile abundances from electrofishing surveys, egg deposition estimates and river flow and air temperature data from 2001-2017 for seven Welsh rivers, broadly representative of rivers around Wales. We used river flow and air temperature data to derive ecologically and behaviourally meaningful density-independent explanatory variables. 4. Salmonid recruitment in Wales was best described using density-dependent and density-independent factors, especially for salmon: after accounting for a concave relationship with egg deposition, salmon juvenile abundance was reduced under (1) warmer spawning temperatures that might inhibit spawning, and (2) higher flood frequencies during pre-emergence and emergence that might washout eggs or alevins. Results were less clear for trout, perhaps because they are behaviourally more plastic. 5. Our findings provide empirical support for general and predictable effects of temperature and flow during spawning and emergence on salmonid-especially salmon-recruitment in Wales. Furthermore, we suggest that the 2016 salmon recruitment crash was caused-in part-by particularly inclement spawning and
Summary Understanding of the winter ecology of stream salmonids is biased by research conducted in northern temperate and boreal regions dominated by hard rock geology. Such systems are driven by highly dynamic surface‐flow regimes and tend to be physically diverse, nutrient poor and influenced by ice. This study investigated how the behaviour of brown trout, Salmo trutta, inhabiting a stable groundwater‐fed, productive and comparatively warm southern English chalk stream differs from that described for other systems, and how this is translated to performance, measured as growth. Physical characteristics were mapped, and high‐resolution temperature data collected using a spatial array of data loggers installed throughout the study reach during the winter. A combination of passive integrated transponder and radio telemetry was used to monitor distribution, density, and movement of trout. Micro‐archival data storage tags inserted in some individuals provided information on temperature regimes experienced. Growth performance was calculated for recaptured fish. Trout density was positively related to depth and there was no evidence that temperature influenced microhabitat selection. Three patterns of movement were observed. Over three‐quarters of tracked fish exhibited high site fidelity and tended to remain in a single focal position throughout the study. Fourteen per cent of trout exploited more than one distinct location, while the remainder were detected at multiple locations and showed no preference for any one. Trout exhibited regular daily activity patterns and highly periodic local movements at dusk and dawn and tended to experience positive growth performance during periods that included winter. This study challenges the conventional view of salmonid winter ecology, which is biased towards populations that inhabit hard rock surface‐flow dominated rivers that experience the influence of ice. Despite inhabiting a distinctly different winter habitat template than more commonly studied populations, trout occupying a hydrologically stable and productive chalk stream exhibited behaviours similar to those described for elsewhere, yet performed considerably better.
Retention of both rigid and new soft standard size visible implant (VI) tags in brown troutSalmo trutta after 6 months varied between 42 and 97% for different batches, with no evidence of increased retention of the new type of tag. The 6 month retention of visible implant elastomer tags (VIE) also varied but with a mean of 96%. The VIE retention gradually declined with time up to 42 months.A recent development in fish marking has been the use of tags which are embedded in transparent tissue and obviate many problems of traditional external tags (Farooqi et al., 1995). The usefulness of such tags, however, depends on their retention, but for some of these tags this has not been widely tested, especially in field conditions as opposed to captive fishes. The present study, carried out over several years, investigated the retention of three types of tag in wild brown trout Salmo trutta L. in the River Piddle, a chalkstream in Dorset, England, and assessed their suitability for survival and movement experiments. The tags were rigid and soft visible implant (VI) and visible implant elastomer (VIE) tags (North-west Marine Technology Inc., Shaw Island, WA, U.S.A.).The VI tags are coloured slivers of plastic printed with unique alphanumeric codes which are supplied in two sizes, standard (2Á5 Â 1Á0 mm) and large (3Á5 Â 1Á5 mm). There are two types, a rigid plastic type and a newer soft plastic type. The tag is injected with a supplied injector into the transparent eyelid tissue, immediately behind the eye. The VIE is a fluorescent coloured liquid polymer which is mixed with a catalyst and injected into transparent tissue (e.g. eyelids and webbing between fin rays) with a syringe and it cures to a rubbery consistency after some hours. While the VI tag is intended for identification of individual fish, the VIE is generally used for batch marking.
Predicting fish responses to modified flow regimes is becoming central to fisheries management. In this study we present an agent-based model (ABM) to predict the growth and distribution of young-of-the-year (YOY) and one-year-old (1+) Atlantic salmon and brown trout in response to flow change during summer. A field study of a real population during both natural and low flow conditions provided the simulation environment and validation patterns. Virtual fish were realistic both in terms of bioenergetics and feeding. We tested alternative movement rules to replicate observed patterns of body mass, growth rates, stretch distribution and patch occupancy patterns. Notably, there was no calibration of the model. Virtual fish prioritising consumption rates before predator avoidance replicated observed growth and distribution patterns better than a purely maximising consumption rule. Stream conditions of low predation and harsh winters provide ecological justification for the selection of this behaviour during summer months. Overall, the model was able to predict distribution and growth patterns well across both natural and low flow regimes. The model can be used to support management of salmonids by predicting population responses to predicted flow impacts and associated habitat change.
Globally, freshwaters are the most degraded and threatened of all ecosystems. In northern temperate regions, beaver (Castor spp.) reintroductions are increasingly used as a low-cost and self-sustaining means to restore river corridors. River modifications by beavers can increase availability of suitable habitat for fish, including salmonids. This study investigated the response of a population of brown trout (Salmo trutta) to reintroduced Eurasian beaver (Castor fiber) habitat modifications in northern Scotland. The field site comprised two streams entering a common loch; one modified by beavers, the other unaltered. Electrofishing and PIT telemetry surveys indicated abundance of post-young-of-the-year (post-YOY) trout was higher in the modified stream. Considering juvenile year groups (YOY and post-YOY) combined, abundance and density varied with year and season. In the modified stream, fork length and mass were greater, there was a greater variety of age classes, and mean growth was positive during all seasons. Beavers had profound effects on the local brown trout population that promoted higher abundances of larger size classes. This study provides important insight into the possible future effect of beavers on freshwater ecosystems.
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