A systematic review of approaches to assess fish health responses to anthropogenic threats in freshwater ecosystems

Abstract

Anthropogenic threats such as water infrastructure, land-use changes, overexploitation of fishes and other biological resources, invasive species and climate change present formidable challenges to freshwater biodiversity. Historically, management of fish and fishery species has largely been based on studies of population- and community-level dynamics; however, the emerging field of conservation physiology promotes the assessment of individual fish health as a key management tool. Fish health is highly sensitive to environmental disturbances and is also a fundamental driver of fitness, with implications for population dynamics such as recruitment and resilience. However, the mechanistic links between particular anthropogenic disturbances and changes in fish health, or impact pathways, are diverse and complex. The diversity of ways in which fish health can be measured also presents a challenge for researchers deciding on methods to employ in studies seeking to understand the impact of these threats. In this review, we aim to provide an understanding of the pathway through which anthropogenic threats in freshwater ecosystems impact fish health and the ways in which fish health components impacted by anthropogenic threats can be assessed. We employ a quantitative systematic approach to a corpus of papers related to fish health in freshwater and utilize a framework that summarizes the impact pathway of anthropogenic threats through environmental alterations and impact mechanisms that cause a response in fish health. We found that land-use changes were the most prolific anthropogenic threat, with a range of different health metrics being suitable for assessing the impact of this threat. Almost all anthropogenic threats impacted fish health through two or more impact pathways. A robust understanding of the impact pathways of anthropogenic threats and the fish health metrics that are sensitive to these threats is crucial for fisheries managers seeking to undertake targeted management of freshwater ecosystems.

Keywords: Anthropogenic threats; conservation physiology; health indicators.

Figure 1

A conceptual model showing the sequential steps through which stressors impact fish health, fitness and population dynamics. ‘Stressor’ refers to anthropogenic threats and their resulting environmental alterations and impact mechanisms (described further on) that operate in freshwater environments. ‘Fish Health’ refers to health components that make up the primary, secondary and tertiary stress responses. ‘Fitness’ refers to parameters including but not limited to reproductive capacity, migratory capacity and survival. ‘Population dynamics’ refers to factors such as recruitment, population resilience and biomass. The letters ‘A’, ‘B’ and ‘C’ highlight the links between these steps.

Figure 2

A conceptual diagram depicting our framework of an impact pathway, linking anthropogenic threats to environmental alterations, impact mechanisms and fish health responses. The anthropogenic threat ‘Water Infrastructure’ is used as an example, which leads to flow alterations resulting in changed food availability, impacting fish condition as a tertiary stress response.

Figure 3

A global map showing the number of papers examining impacts of anthropogenic disturbances on fish health in freshwater systems published by each country. Grey polygons represent no papers published.

Figure 4

The frequency of occurrence of health indicators in our corpus from the years 1972 to 2021 (a), the number of studies that were conducted in an artificial, field, field/laboratory or laboratory setting (numbers above bars represent number of papers per study type) (b), the frequency of occurrence of anthropogenic threats in our corpus from 1972 to 2021 (c), and the number of occurrences of each health indicator used within the corpus (numbers above bars indicate number of occurrences per health indicator) (d). Tertiary stress responses are marked with a ‘T’ in panels (a) and (d). For panels (a) and (c), some papers within the corpus used multiple health indicators, meaning the number of occurrences shown in this figure is greater than the number of papers within the corpus.

Figure 5

An alluvial diagram highlighting a network of four stratum: anthropogenic threats, environmental alterations, impact mechanisms and health indicators (tertiary stress responses denoted with T). The relative size of each category within each stratum is based on transformed (square root) occurrence counts to aid readability and is therefore not to scale. The plot is read either from left to right or right to left, with each stratum connected by ‘flows’, forming an 'impact pathway' across the four stratum. The proportion of each category within a stratum is provided numerically ‘n’.

Figure 6

Six alluvial diagram subsets, one for each anthropogenic threat category from the main alluvial diagram in Fig. 5: (a) water infrastructure, (b) overexploitation, (c) land-use changes, (d) invasive species, (e) climate change and (f) other. Each subset depicts the last two stratum, impact mechanisms and health indicators, of the main alluvial diagram in order to highlight the final stage of the impact pathway of each anthropogenic threat. Impact mechanism abbreviations are as follows (from top to bottom): CFA = changed food availability, CMP = competition, CPT = capture, CWQ = changed water quality, FS = fish screen, ICC = Increased chemical contaminants, II = Increased infections, MHC = morphological habitat characteristics, NL = noise/light, P = predation, R = radiation and TS = thermal stress. Health indicator abbreviations are as follows (from top to bottom): BEH = behaviour, EA = external abnormalities, GI = genetic indices, LF = lipids and fats, O = other, OI = organosomatic indices, PD = parasites and disease, PSR = primary stress response, SSR = secondary stress response and WBM = whole-body morphology.