Likely Suspects Framework:
An Evidence-based Approach to Support Salmon Management
The Likely Suspects Framework: Understanding Salmon Mortality
The death of a salmon in one location at one time might be attributable to a certain cause (e.g. low oxygen, predation, disease) , but the pattern of events and prevailing conditions that led up to the event taking place are critical to understand if we want to be able to predict and manage our salmon in the future. The focus of the Likely Suspects Framework is to understand the mechanisms behind the causes of salmon mortality. If a salmon is killed by a larger fish predator during its outward migration towards ocean feeding grounds it is “marine mortality” , but the conditions that helped to “create” this may be controlled by a combination of factors. For example :
The distribution of the predators and migrating salmon overlaps more perhaps due to the presence of abundant shoals of small pelagic fish that are the preferred prey species attracting predators.
The predator can select and capture the salmon more easily as it may be smaller, slower or weaker than other prey species as a result of poor freshwater growing conditions, disease or parasites.
The abundance of potential salmon predators is increased as a result of improved marine productivity in areas surrounding salmon migration routes.
Salmon anti-predator behaviour is compromised, or ineffective due to exposure to contaminants, , disease, parasites or poor growth in the freshwater stages.
Unless we recognise and fully understand the underlying mechanisms that create, and determine the outcome of interactions between salmon and elements of its ecosystem, we run the risk of continuing ineffective management approaches, while salmon populations continue their decline.
High level Likely Suspects Framework conceptual framework (version 1)
This figure presents a high level conceptual overview of the main physical and ecological drivers and components influencing Atlantic salmon survival throughout a simplified life-cycle. It provides the background organisational structure for the Likely Suspects Framework (LSF) project goal of providing an adaptive ecosystem management system to improve the resilience of salmon populations and the dependent fisheries. Building from this conceptual framework the LSF project will proceed to assess the evidence, acquire data (see The Central Data Resource), and instigate research to identify, allocate and partition candidate mortality factors. For further information on see Partitioning the Lifecycle: Domains and Key Mortality Questions: Hypotheses. Preliminary numerical allocations of partitioned mortality will be added, and the framework refined as specific hypotheses are tested. Using an evidence-based approach to populate and expand areas of the framework will identify new and emerging risks for salmon, vulnerabilities in the management system, and suggest appropriate mitigation strategies.
Components in the figure
Central to the conceptual framework are two groupings of salmon management drivers representing the need to understand impacts of climate change on the salmon and wider ecosystem, and a multiple-species ecosystem-based approach to determine strategies, stock assessment and forecasting. In order to address the management drivers, targeted strategies will be added to a more detailed model or models and linked to domains or life-stages, with a further tier of associated management actions focused on maintaining and enhancing salmon stocks.
For climate change, three main management drivers applicable to the generalised life-stages (freshwater, estuarine and ocean grouping) are included. For the Ecosystem-based management drivers, several will likely apply at multiple points in the salmon life-cycle but their delivery mechanisms will vary depending upon domain and life-stage. The management drivers include restoring wild salmon stocks and protecting the wider ecosystem, whilst highlighting the importance of promoting resilience in a changing world. Managing “carry-over effects” is a key driver that links freshwater and marine phases of the life-cycle, and acknowledges that conditions experienced by salmon in one domain can be fundamentally important in determining the survival chances in subsequent domains. For example, variation in freshwater rearing conditions may result in smolts migrating out in sub-optimal condition that subsequently leads to higher rates of marine mortality when faced with challenges.
This conceptual framework will be the central component for the LSF project, establishing the basis to build an understanding of the drivers of salmon mortality, and provide the evidence-base upon which to build adaptive management strategies.
Progression and development through the life cycle includes time spent in various freshwater habitats (within the green box) and migration through transitional estuarine habitats (merged colours) to coastal and open ocean habitats (within the blue box). The main times and places occupied by salmon during the course of their life-cycle are indicated as ecosystem domains (grey boxes) with some (i.e. estuary and river domains) utilised by several life stages, for different activities (i.e. feeding, migration). The figure does not aim to represent the length of residency or possible temporal overlaps between life-stages in these domains.
Salmon growth and survival as it transitions between domains is dependent upon the individual meeting its nutritional and other physiological requirements, whilst avoiding detrimental stochastic events, and evading predators. Whether these conditions are met and an individual survives will be controlled largely by physical (environmental) and ecological (trophic) factors interacting across various scales and acting both directly and indirectly on salmon. These factors may in combination determine the survival outcome at a particular place and time, or contribute to an outcome at a later life stage by influencing feeding opportunities, energetics or physiological status at an earlier stage, leaving fish less capable of transitioning.
A hierarchy of environmental controls that act across varying scales determine the availability of primary production and base energy inputs for both the freshwater and marine ecosystems that support salmon. These controlling conditions are represented in the model by blue boxes and the base energy inputs in green and gold. Shifts in basin scale conditions variously affect elements of the salmon’s physical, nutritional and ecological needs, and responses can differ as changes in controlling conditions cascade down amongst the various domains.
Fundamental ecological processes impacting salmon growth and survival are depicted as interactions amongst forage, competitors and predators. These are represented in the framework as six “clusters” of interlinked boxes, illustrating coupling and responsiveness to variations in environmental conditions at various key points in the salmon life-cycle. The species composition, abundance, and dynamics within each box in a cluster are not represented, and these will vary amongst clusters, and locations. Certain co-existing species may have positive and negative ecological interactions with salmon depending upon their life stage and size in relation to salmon occupying any given domain. For example emerging trout fry may be an important seasonal forage for salmon parr in an upland stream, but then adult trout may prey on salmon smolts as they migrate downriver. Similarly, herring fry may provide an important component of post-smolt salmon diets during their first few months at sea, but older herring may also be competing for prey and may attract marine predators that also opportunistically consume salmon. Such complexities in trophic interactions cannot be adequately represented in this framework, but will be examined as the Likely Suspects Framework project develops.
For all Likely Suspect Framework enquiries please contact colin@atlanticsalmontrust.org