Fish Telemetry at a Wastewater Treatment Plant
Acoustic telemetry analysis of fish habitat use near a wastewater treatment plant outfall in southern Sweden
Acoustic telemetry analysis of fish habitat use near a wastewater treatment plant outfall in southern Sweden
This project examines fish residency patterns and habitat use near a wastewater treatment plant (WWTP) outfall in southern Sweden. This analysis uses acoustic telemetry data to quantify species-specific residency and spatial preferences in relation to the outfall and surrounding sites. Once residency patterns are calculated, the results are compared across species, locations, and months to identify significant differences in habitat use. The analysis focuses on four species: common bream (Abramis brama), northern pike (Esox lucius), zander (Sander lucioperca), and common rudd (Scardinius erythrophthalmus), and identifies their proximity to the WWTP outfall relative to upstream and downstream sites. In addition, environmental variables such as water temperature are incorporated to assess whether they explain observed spatial patterns. The workflow outputs species-specific residency metrics, statistical comparisons of site preference, and temporal patterns of habitat use, providing insight into how WWTP discharge may influence fish behavior in natural systems.
1) Does the monthly mean water temperature range vary upstream, downstream, and at a WWTP outfall in a southern Swedish river system?
2) Do fish residency patterns differ between species and between the outfall, upstream, and downstream sites?
The study area was located in the lower portion of Arboga River at the outfall of Arboga Vatten och Avlopp wastewater treatment plant in Västmanland County, Sweden (59.399° N, 15.8831° E, WGS84). 13 acoustic receivers were placed across upstream, downstream, and outfall sites at a WWTP outfall to detect tagged fish between May and October 2023 (Fig. 1).
79 fish from multiple species were captured, surgically implanted with transmitters, and released at their capture locations. Detection data was analyzed in R to calculate species-specific residency using a weighted residency index (RI). Statistical models (GLMMs) were used to assess differences in residency across species, sites, and months, while additional analyses evaluated whether water temperature influenced habitat use.
Each species showed different spatial preferences in habitat use near the wastewater treatment plant (WWTP) outfall. Northern pike consistently showed a strong preference for the outfall across all months. In contrast, rudd preferred upstream habitats and avoided downstream areas, while bream showed moderate use of the outfall but lower presence downstream. Zander showed no consistent site preference. Overall, fish were most frequently detected at the outfall. Water temperature differences between sites were minimal and only significant in June, indicating that temperature was not the primary driver of fish habitat selection.
Monthly water temperatures exhibited clear seasonal variation throughout the study period. Mean temperatures peaked in June and gradually declined through late summer and autumn, providing environmental context for temporal changes in fish movement and habitat use across the telemetry array (Fig. 2).
Spatial variability in daily water temperature range across upstream, downstream, and outfall sites remained relatively limited during most of the study period. Significant site-level differences were detected only during June, indicating that localized thermal variation was not the primary driver of the observed spatial habitat-use patterns (Fig. 3).
Species exhibited substantial differences in overall residency within the telemetry array. Rudd maintained the highest long-term occupancy across the study area, while zander showed the lowest overall residency and spatial persistence. Northern pike and bream displayed intermediate detection durations, reflecting moderate long-term use of the monitored river system (Fig. 4).
Geospatial analysis of weighted residency index (RI) values identified strong species-specific spatial habitat-use patterns across the study area. Northern pike consistently exhibited elevated habitat use near the wastewater outfall throughout the monitoring period, indicating prolonged occupancy and repeated use of the outfall environment. In contrast, rudd showed stronger upstream spatial preference patterns and limited downstream residency (Fig. 5).
Bream demonstrated moderate spatial association with the outfall while maintaining relatively low downstream occupancy. Zander displayed comparatively weak site fidelity overall, with less pronounced spatial concentration at any individual monitoring location (Fig. 5). These results indicate that fish responses to wastewater-associated environments were highly species dependent rather than uniform across the aquatic community.
Spatial comparisons between monitoring locations further highlighted contrasting habitat-use strategies among species. Northern pike showed consistently higher weighted residency values at the outfall relative to upstream and downstream sites across multiple months, suggesting concentrated use of wastewater-associated habitat conditions.
Rudd exhibited the opposite spatial pattern, with the strongest habitat use occurring at upstream sites and substantially lower downstream occupancy. Bream generally showed reduced downstream residency relative to the outfall, while zander displayed comparatively diffuse spatial use patterns and weaker location-specific habitat preference overall.
Together, these spatial patterns demonstrate that wastewater discharge influenced habitat use differently across species, contributing to localized ecological heterogeneity within the river system.
Receiver detection efficiency varied across the telemetry array throughout the study period. Despite temporal fluctuations in individual receiver performance, redundancy within the monitoring design maintained reliable spatial coverage of the outfall region and supported consistent detection of tagged fish movements across sites (Fig. 7).
Detection activity varied substantially throughout the monitoring period. Total daily detections were highest during September, indicating increased fish presence and movement within the study area during late summer and early autumn. Seasonal variation in detection frequency suggests temporal shifts in habitat use and spatial occupancy across the telemetry array (Fig. 8).
Spatial analysis of telemetry detections revealed clear species-specific responses to the wastewater treatment plant outfall rather than a uniform ecological pattern across the fish community. Northern pike consistently concentrated habitat use near the outfall, suggesting that wastewater-associated environments may provide favorable ecological conditions such as increased prey availability, stable environmental conditions, or localized habitat resources. In contrast, rudd exhibited stronger upstream spatial preference patterns and limited downstream occupancy, indicating that species responded differently to environmental conditions associated with the discharge zone.
Thermal variation between monitoring sites remained limited during most of the study period, suggesting that water temperature alone did not explain the observed spatial distributions. Instead, additional environmental drivers associated with wastewater discharge — including nutrient availability, altered water chemistry, or habitat structure — likely contributed to the localized habitat-use patterns observed across species.
These findings demonstrate the importance of incorporating spatial behavior and species-specific ecological responses into environmental monitoring and aquatic ecosystem management. Telemetry-based spatial analysis provides valuable insight into how wastewater infrastructure may reshape habitat use and ecological dynamics within freshwater systems.
This study demonstrates that wastewater treatment plant outfalls can substantially influence fish habitat use and spatial occupancy patterns, although responses vary considerably among species. Northern pike consistently concentrated habitat use near the outfall environment, while other species showed avoidance or comparatively weak spatial association with wastewater-affected habitats.
The results highlight how wastewater discharge can contribute to localized ecological heterogeneity and reshape spatial behavior within freshwater ecosystems. Integrating telemetry-based spatial analysis into environmental assessment frameworks can improve understanding of how anthropogenic infrastructure influences aquatic habitat use, species interactions, and ecosystem dynamics.
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