Landscape features determine brown trout population structure and recruitment dynamics

Abstract

Variation in brown trout (Salmo trutta L.) population recruitment and structure is related to migratory patterns, which should depend on ease of access to habitats providing increased opportunity for growth. We quantified the number of young of year (YOY) as a proportion of the total number of brown trout at 24 locations on 11 streams within the Taieri catchment, New Zealand, including back calculated growth rates and emergence dates from otoliths. Locations with high absolute and relative abundance of YOY fish were related to elevation and distance from the river mainstem (habitat used by migratory fish), fish density, and the interaction between invertebrate food biomass, distance and elevation. Hatch date and growth were not related to the proportion of YOY fish, though growth was negatively correlated to total fish density. We suggest landscape features play a large role in determining recruitment and population structure. Locations at lower elevations have a high YOY density, high competition and lower growth, likely prompting out‐migration. These conditions could be created by successful return migration and spawning of large fecund fish resulting in YOY densities exceeding the habitat carrying capacity. Environmental factors, such as food availability, also played a role in determining population structure. These results provide an example of how population structure and recruitment might be controlled by local conditions and access to high growth environments in wild populations of introduced brown trout across a catchment.

Straying of brown trout in the catchment of a large New Zealand river evaluated by otolith microchemistry

“…This behaviour has been shown previously in European anadromous brown trout, with a negative association existing between elevation and the density of sea‐run fish offspring (Bohlin et al, 2001). This pattern is consistent with the significant negative relationship between YoY trout abundance and elevation in the Taieri catchment (Jones et al, 2019). Additionally, this pattern is supported by findings of positive associations of elevation and distance with size and age of sea‐run spawning brown trout (L'Abée‐Lund, 1991), highlighting the migratory cost of reaching upland spawning areas by anadromous fish and the comparatively easy accessibility of Silverstream spawning grounds.…”

“…We assumed this section of the transect reflects the period of otolith formation whilst the YoY trout inhabited their natal stream (see Dodson et al, 2013), and hence should carry a trace element signature reflective of that stream. The assumption is based on the results of analyses of otolith microstructure completed by Dodson et al (2013) and results of YoY otolith analyses completed by Jones et al (2019) in the Taieri catchment. We avoided using the chemical composition of the area located near the core of the otolith, given this region may carry a maternal trace element signature (Gabrielsson et al, 2012).…”

Is recruitment of brown trout in a New Zealand river driven by parental spawning investment, density dependence, or environmental factors?

“…These predictions are based on the data on spawning investment of brown trout we collected along the model stream, combined with published strong stock-recruitment correlation (Ricker, 1975) described for European brown trout (Elliott, 1994;Nicola and Almodovar, 2002;Sánchez-Hernández еt al., 2016). Broadscale patterns in the wider Taieri River catchment suggest that diadromous fish with high reproductive potential dominate lower altitudes, whereas resident fish with low reproductive potential dominate headwaters (Kristensen et al, 2011;Jones, Akbaripasand, Nakagawa and Closs, 2019). We also predicted that the primary factors driving YoY trout population dynamics will change from downstream to upstream -with intra-specific competition (biotic) driving population dynamics in downstream reaches, and environmental factors (abiotic) driving populations in the headwaters.…”

“…Multiple studies have focused on the factors controlling stream-dwelling brown trout in a variety of geographical regions and habitats, but the results are often contradictory and generate more questions than answers (Lobón-Cerviá, Rasmussen and Mortensen, 2017). In New Zealand there are few studies of the population dynamics of brown trout (Allen, 1951;Kristensen and Closs, 2008a;Hayes, Olsen and Hay, 2010;Jones, Akbaripasand, Nakagawa and Closs, 2019), and one study has examined their migration dynamics (Holmes et al, 2014). The effects of natural reproduction, food abundance, floods and other environmental factors on trout population dynamics in New Zealand were analyzed by Hobbs (1940Hobbs ( , 1948, Allen (1951), Burnet (1959), Jowett (1989Jowett ( , 1990Jowett ( , 1995 and Hayes (1995), with the importance of different factors in driving brown trout populations in New Zealand being a source of long-standing debate (Huryn, 1996;Holmes et al, 2018).…”

“…Previous studies have determined that the broad scale spatial population dynamics of young-of-the-year (YoY) brown trout are closely related to the dominant life history traits of adults. Populations in river reaches accessible to large diadromous adults are characterized by high juvenile densities and unstable population dynamics (Kristensen et al, 2011;Jones, Akbaripasand, Nakagawa and Closs, 2019), whereas headwater populations are stable with relatively low YoY densities (Huryn, 1996;Kristensen and Closs, 2008a;Kristensen and Closs, 2008b;Jones, Akbaripasand, Nakagawa and Closs, 2019). However, the fine-scale temporal dynamics of YoY brown trout in sites with diadromous and resident life histories remain unknown.…”