Quantifying multiple pressure interactions affecting populations of a recreationally and commercially important freshwater fish

Biological Invasions Affect Resource Processing in Aquatic Ecosystems: The Invasive Amphipod Dikerogammarus villosus Impacts Detritus Processing through High Abundance Rather than Differential Response to Temperature

Anthropogenic stressors such as climate warming and invasive species and natural stressors such as parasites exert pressures that can interact to impact the function of ecosystems. This study investigated how these stressors interact to impact the vital ecosystem process of shredding by keystone species in temperate freshwater ecosystems. We compared metabolic rates and rates of shredding at a range of temperatures up to extreme levels, from 5 °C to 30 °C, between invasive and native amphipods that were unparasitised or parasitised by a common acanthocephalan, Echinorhynchus truttae. Shredding results were compared using the relative impact potential (RIP) metric to investigate how they impacted the scale with a numerical response. Although per capita shredding was higher for the native amphipod at all temperatures, the higher abundance of the invader led to higher relative impact scores; hence, the replacement of the native by the invasive amphipod is predicted to drive an increase in shredding. This could be interpreted as a positive effect on the ecosystem function, leading to a faster accumulation of amphipod biomass and a greater rate of fine particulate organic matter (FPOM) provisioning for the ecosystem. However, the high density of invaders compared with natives may lead to the exhaustion of the resource in sites with relatively low leaf detritus levels.

Revisiting the challenge: perspectives on Canada’s freshwater fisheries policies three decades after the Pearse Report

A seminal report by Peter H. Pearse (1988; Rising to the Challenge: A New Policy for Canada’s Freshwater Fisheries, Canadian Wildlife Federation, Ottawa) outlined 62 policy recommendations focused on the management of Canada’s inland fisheries. Over three decades later, freshwater ecosystems and inland fisheries in Canada are still facing similar challenges with many emerging ones that could not have been foreseen. Here, we reflect on the contemporary relevance of the Pearse Report and propose recommendations that policy makers should consider. Broadly, our recommendations are: (1) manage fishes, fisheries, and habitat using a holistic co-management framework, with clearly defined fishery jurisdictions and partnerships with Indigenous governments; (2) engage in transparent, inclusive, and agile research to support decision-making; (3) facilitate knowledge co-production, involving interdisciplinary projects with diverse groups of actors and sectors including Indigenous Peoples, anglers, policy makers, scientists/researchers, governments, and the public; (4) embrace technological advances to support freshwater fisheries stock assessment and management; and (5) align policy and management activities in Canada with global initiatives related to increasing the sustainability of inland fisheries. We advocate for an updated comprehensive report such as the Pearse Report to ensure that we embrace robust, inclusive, and sustainable management strategies and policies for Canada’s inland fisheries for the next 30 years. It is time to again rise to the challenge.

Not so robust: Robusta coffee production is highly sensitive to temperature

Coffea canephora (robusta coffee) is the most heat-tolerant and 'robust' coffee species and therefore considered more resistant to climate change than other types of coffee production. However, the optimum production range of robusta has never been quantified, with current estimates of its optimal mean annual temperature range (22-30°C) based solely on the climatic conditions of its native range in the Congo basin, Central Africa. Using 10 years of yield observations from 798 farms across South East Asia coupled with high-resolution precipitation and temperature data, we used hierarchical Bayesian modeling to quantify robusta's optimal temperature range for production. Our climate-based models explained yield variation well across the study area with a cross-validated mean R²  = .51. We demonstrate that robusta has an optimal temperature below 20.5°C (or a mean minimum/maximum of ≤16.2/24.1°C), which is markedly lower, by 1.5-9°C than current estimates. In the middle of robusta's currently assumed optimal range (mean annual temperatures over 25.1°C), coffee yields are 50% lower compared to the optimal mean of ≤20.5°C found here. During the growing season, every 1°C increase in mean minimum/maximum temperatures above 16.2/24.1°C corresponded to yield declines of ~14% or 350-460 kg/ha (95% credible interval). Our results suggest that robusta coffee is far more sensitive to temperature than previously thought. Current assessments, based on robusta having an optimal temperature range over 22°C, are likely overestimating its suitable production range and its ability to contribute to coffee production as temperatures increase under climate change. Robusta supplies 40% of the world's coffee, but its production potential could decline considerably as temperatures increase under climate change, jeopardizing a multi-billion dollar coffee industry and the livelihoods of millions of farmers.

The interactive effects of climate change and land use on boreal stream fish communities

Ongoing and projected climate change is likely to greatly alter co-occurring stressor mechanisms, yet these potential interactions remain poorly understood in natural freshwater systems worldwide. As the global biodiversity crisis deepens, successful conservation efforts will hinge on developing mechanistic multiple stressor frameworks that have been ground-truthed in natural systems containing complex species dynamics and ecological processes. Our study examined the combined and interacting effects of potential climate and land use stressors on boreal stream fishes using data from over 300 catchments across a broad 250,000 km² region. To characterize boreal fish community health, we examined four indicators including species richness, total catch per unit effort, the proportion of lithophilic spawners (fish sensitive to sedimentation), and the assemblage tolerance index which provides a measurement of the overall community tolerance to disturbance. Land use stressors included total anthropogenic land use area and linear disturbance at multiple watershed scales as well as two site-specific habitat degradation indicators (dissolved oxygen and the proportion of fine substrate). Overall community richness and productivity were not negatively related to land use changes indicating potential compensatory dynamics (e.g. where intolerant species are replaced with more tolerant species as habitat quality degrades). In contrast, we observed declines for sensitive species, including highly valued salmonids, that varied depending on interactions between local climate, land use, and stream type. Sensitive species declines were concentrated in regions experiencing increased land use and warming, whereas increases were observed in cooler regions consistent with a subsidy-stress response. In addition, lithophilic spawners declined in watersheds experiencing warmer and wetter conditions owing to potential indirect effects on spawning habitat quality. Results from our study provide novel insight into complex climate and land use interactions occurring across a broad, real-world landscape, and highlight the potential for amplified species declines under future warming and land use scenarios.

Production dynamics reveal hidden overharvest of inland recreational fisheries

Recreational fisheries are valued at $190B globally and constitute the predominant way in which people use wild fish stocks in developed countries, with inland systems contributing the main fraction of recreational fisheries. Although inland recreational fisheries are thought to be highly resilient and self-regulating, the rapid pace of environmental change is increasing the vulnerability of these fisheries to overharvest and collapse. Here we directly evaluate angler harvest relative to the biomass production of individual stocks for a major inland recreational fishery. Using an extensive 28-y dataset of the walleye (Sander vitreus) fisheries in northern Wisconsin, United States, we compare empirical biomass harvest (Y) and calculated production (P) and biomass (B) for 390 lake year combinations. Production overharvest occurs when harvest exceeds production in that year. Biomass and biomass turnover (P/B) declined by ∼30 and ∼20%, respectively, over time, while biomass harvest did not change, causing overharvest to increase. Our analysis revealed that ∼40% of populations were production-overharvested, a rate >10× higher than estimates based on population thresholds often used by fisheries managers. Our study highlights the need to adapt harvest to changes in production due to environmental change.