Institutional constraints and ecological limits: an interdisciplinary approach to native fish conservation in Colorado's South Platte River basin

Abstract

Conservation in Highly Managed River Systems. Conservation efforts in over-allocated river basins are frequently complicated by the tension between ecological needs and established legal frameworks. The South Platte River Basin in Colorado exemplifies this challenge, operating as a complex socio-ecological system defined by competition over scarce water resources. Water governance in the region is structured by the rigid prior appropriation doctrine, a system characterized by fixed rules and seniority-based water rights. While this rigidity provides predictability for rights holders, it can impede adaptive management required to address complex environmental problems. This dissertation addresses the challenge of managing native fish through two steps that aim to integrate both social and ecological experiments. The first utilized a collaborative mapping process to visualize water professionals' mental models, internal representations of system cause-and-effect relationships, to identify shared priorities for adaptive management. The second quantified the physiological tolerances of the native Plains Topminnow (Fundulus sciadicus) and the invasive Western Mosquitofish (Gambusia affinis) to determine the specific stressors associated with management-induced environmental changes, such as temperature and salinity shifts. Chapter 1: Cognitive Barriers and Solutions. The analysis of water professionals' mental models confirmed that rigid laws fundamentally structure how actors perceive their ability to influence the system. Fuzzy cognitive mapping (FCM) analysis revealed structural differences between sectoral mental models. For example, the agricultural sector produced complex maps characterized by a high number of components (averaging 22 concepts and 70 connections), yet these models contained zero components classified as primary drivers. This structure represents the constraint imposed by the seniority-based water law, suggesting that agricultural actors perceive their capacity to initiate change within the existing legal framework as highly limited. This finding confirms that the rigid laws of the prior appropriation doctrine contribute to a perceived lack of agency, reinforcing sectoral reliance on existing water rights rather than collaborative innovation. Scenario analyses projected conflicting outcomes under high-stress futures (Hot Growth), particularly concerning salinity. Agricultural models predicted salinity reductions, directly opposing environmental models, which anticipated sharp increases. This contradiction highlights a divergence in understanding regarding the contribution of irrigation return flows to systemwide salinity. A lack of shared problem definition can be a significant barrier to developing collaborative solutions. Despite this divergence, the collective FCM sensitivity analysis identified increasing water supply as a dual-leverage intervention that was projected to increase native fish while reducing salinity. This identification was externally corroborated by stakeholder suggestions for coordinated water interventions, such as "additional groundwater recharge" and "better coordination among water users to avoid dry ups". This outcome suggests that creating new water supply sources, rather than reallocating existing, contested senior rights, provides a pathway for consensus within the prior appropriation system. Chapter 2: Salinity and Thermal Stress. To evaluate the ecological value of proposed interventions, the second study quantified the tolerances of the native Plains Topminnow (PTM) and the invasive Western Mosquitofish (MSQ) to two key stressors associated with water management in the basin: salinity and temperature. Experiments demonstrated that current river salinity levels, resulting from water reuse practices, are unlikely to be the primary cause of native fish decline. Both PTM and MSQ exhibited high survival rates (greater than 95%) across the entire salinity gradient tested (0–2400 mg/L), which encompasses the maximum concentrations observed in the South Platte River. This finding suggests that interventions aimed solely at reducing salinity may not address the species' declines. In contrast, Critical Thermal Maximum (CTM) analysis identified a difference in thermal tolerance between the species. This identified thermal stress as a greater threat than salinity (Figure 8). The invasive MSQ maintained a significantly higher thermal tolerance (CTM of 35°C) compared to the native PTM (CTM of 32.5°C). While average summer temperatures are typically below these thresholds, water diversions frequently create "dry-up" points. These events fragment habitats into isolated pools where water temperatures can spike to lethal levels (38–40°C). The 2.5°C thermal buffer held by MSQ provides a competitive advantage in these high-stress environments. This ecological outcome supports the hypothesis that the invasive species' success stems primarily from its greater thermal tolerance, allowing it to succeed in thermally stressed and fragmented habitats created by rigid water management practices. Furthermore, PTM's intraspecific aggression under the crowded, warm conditions of isolated pools may compound the disadvantages imposed by reduced connectivity. An Interdisciplinary Approach to Native Fish Conservation. Success is achieved by identifying actions that are feasible within institutional constraints (like coordinated water supply enhancements) and leveraging them to benefit high-priority conservation initiatives. Water managers, operating under the institutional constraints of prior appropriation, showed a consensus on enhancing water supply coordination to avoid "dry-up" events. The physiological data confirmed that this action would provide the most significant benefit to native fish by directly mitigating acute thermal stress. The collaborative mapping process projected benefits from maintaining flow and reducing salinity, while the physiological data confirms that thermal mitigation is a critical benefit for PTM. This thermal mitigation can be achieved through restoring flow connectivity and maintaining a critical volume for thermal refugia. This interdisciplinary approach provides a quantitatively supported solution that works within the constraints of prior appropriation. This study advances the theory of adaptive governance by demonstrating conservation success in institutionally rigid systems, such as those governed by prior appropriation, hinges on achieving a shared understanding of the system's mechanics. The use of collaborative visualization tools, like FCM, is crucial because it makes implicit sectoral constraints (e.g., the agricultural sector's perceived lack of agency) and divergent understandings (e.g., the conflict over salinity sources) explicit. This process allows actors to visualize what might otherwise appear as intractable legal or resource conflict into an opportunity for coordinated problem-solving. For native fish conservation in managed arid river basins globally, this interdisciplinary methodology offers a roadmap. The process is twofold: first, understand the human system by using collaborative mapping tools to identify sectoral constraints and any shared, feasible interventions. Second, ground-truth these proposals with targeted ecological research to pinpoint the precise mechanism of native species decline, whether it be thermal stress, water quality, or competition. For instance, in another basin where stakeholders agree on "environmental flows" but disagree on their purpose, physiological studies could determine if the priority should be maintaining spawning cues, preventing stranding, or as in the South Platte providing thermal refugia through river connectivity. This ensures that scarce resources such as water are committed toward actions that address the most binding ecological constraint.