Disease in fisheries and aquaculture

Infectious marine diseases have profound impacts on fisheries and aquaculture through their effects on growth, fecundity, mortality, and marketability. Economic losses have motivated research to minimize the negative impacts of disease on these industries. However, this relationship is reciprocal, as fishing and aquaculture can shape disease transmission. The effects of fisheries and aquaculture on disease are scale dependent, with different outcomes at the population, metapopulation, community, and ecosystem levels. Management approaches are limited in fisheries, and intense in aquaculture, sometimes with undesirable impacts on wild species. Management needs can be particularly intense in hatcheries, where stocks are sensitive and kept at high densities. Increased interest in microbiome–disease interactions are opening up new opportunities to manage marine diseases in aquaculture. Solutions for marine diseases in fisheries and aquaculture may ultimately improve human health by reducing exposure to pathogens and increasing nutrient quality, but could negatively impact human health through exposure to antibiotics and other chemicals used to treat parasites.

Bacteriophage can drive virulence in marine pathogens

The roles of prophages in disease have mainly considered human pathogens, while their role in marine pathogens has only recently been considered. This chapter reviews the relevant literature on what is known of prophages in marine ecosystems and provides a meta-analysis of the abundance and function of prophages in marine pathogenic bacteria. According to these results, bacterial pathogens in marine environments contain a significantly higher abundance of prophage DNA in their genomes than host-associated bacteria, which are non-pathogenic. The authors also surveyed the genetic content of the prophages that were associated with known pathogens and compared their functions to non-pathogens. Their findings suggest that horizontally acquired prophage-encoded DNA may play a large role in the ecology and evolution of marine diseases

Disease ecology in marine conservation and management

While disease is a part of all natural systems, emerging marine diseases are on the rise and many are exacerbated by anthropogenic stressors. Marine and terrestrial environments are fundamentally different, requiring a suite of new approaches to understanding and managing the host–pathogen–environment relationship. Promising strategies include establishing marine protected areas, developing forecasting tools, and using natural ecosystem filters to control pathogens. Aquaculture is one measurable avenue by which natural systems come into direct contact with managed systems, often with negative consequences. This chapter presents examples where pathogens, invasive species, and degraded water quality are associated with impacts on adjacent natural systems. While effective regulatory procedures exist, international transport presents a challenge to implementation and needs special attention. Ecological restoration, a growing management science, would benefit from consideration of disease processes, such as genotyping to determine differences in natural resistance that could be used to guide selective breeding efforts.

Invasions can drive marine disease dynamics

Over half the world’s human population lives near the coast, with diverse impacts on the structure and function of coastal ecosystems, including the introduction of parasites that result from shipborne trade, aquaculture, and other human-aided dispersal. The scale of these activities has accelerated through time, expanding the potential for new introductions and subsequent impacts in coastal systems. However, the extent, dynamics, and impacts of marine parasite invasions are relatively unexplored compared to free-living organisms. This chapter (1) advances a framework to consider which parasites are most likely to invade, specifically considering diverse life-history traits, (2) reviews the current baseline knowledge for transfer mechanisms and the history of marine invasions, and (3) considers the ecological and evolutionary implications of parasite invasion. While recent advances have aided our understanding of the intersection of disease and invasion ecology, a closer look at the smallest disease-causing organisms will open new avenues for understanding the full scope of parasite invasions and their role in emerging diseases.

Pollution can drive marine diseases

Humans pollute the marine environment biologically, chemically, and physically, which can potentially drive or facilitate the emergence, proliferation, or impact of disease. This chapter synthesizes what is known about the effects of biological (e.g., wastewater), chemical (e.g., pharmaceuticals), and physical (e.g., sound/light) pollution on marine disease dynamics. The presence of these pollutants has been found to alter disease prevalence, increase host susceptibility to infection, and alter the spread and host range of different diseases. Despite the importance of the marine environment as a primary food source for humans, many complexities linking disease ecology and pollution are yet to be explored. Future investigation of these connections would benefit from an integrated approach using experimental, environmental, molecular, and pathological methods.

Climate change can drive marine diseases

As an ultimate driver of marine ecosystem processes, climate change is expected to influence proximate disease drivers in marine systems. The observable effects of climate change, including changes in temperature, hypoxia, CO2 accumulation, precipitation, and storm and cyclone frequencies and intensities, may directly act as proximate drivers of marine disease, especially in poikilotherms. These climate-driven changes are expected to result in the active and passive movement of pathogens and hosts into previously naïve geographical areas, thereby disrupting the long-evolved, stable host–pathogen relationships. Additionally, large-scale ecological changes stemming from climate change are expected to impact pathogen virulence and host susceptibilities. These real and anticipated changes present evolving challenges for resource managers who are charged with managing stochastic marine diseases in a constantly changing environment.

Disease can shape marine ecosystems

This chapter reviews how marine ecosystems respond to parasites. Evidence from several marine ecosystems shows that parasites can wield control over ecosystem structure, function, and dynamics by regulating host density and phenotype. Like predators, parasites can generate or modify trophic cascades, regulate important foundational species and ecosystem engineers, and mediate species coexistence by affecting competitive outcomes. Sometimes the parasites have clear positive impacts within ecosystems, such as increasing species diversity or maintaining ecosystem stability. Other times, parasites may have destabilizing effects that signal an ecosystem out of balance. But it is now clear that some (but not all) parasites can have strong and, at times, predictable effects, and should thus be incorporated into food web and ecosystem models

Modelling marine diseases

The unique characteristics of marine ecosystems have pushed investigators to refine well-tested and widely applied epidemiological modeling methods to understand marine disease dynamics. This chapter begins by reviewing models used to quantify within-host parasite dynamics in open marine ecosystems where infection is near universal. These models are powerful tools for quantifying how diseases respond to changing environmental conditions and, when reliable environmental data are available, can forecast marine disease risks into the future. This chapter then describes epidemiological models that consider transmission processes and parasite life histories unique to marine systems, and then incorporates disease processes in fisheries assessment models. Finally, because disease dynamics vary across local host populations, this chapter concludes by overviewing ocean circulation models and their use in understanding parasite dispersal and spread in marine ecosystems.

Diagnosing marine diseases

Infectious disease concerns are paramount when considering the health of the oceans and seas of the world. Understanding the ecology of disease in marine environments requires knowledge of diagnostic principles and techniques. Morphologic and molecular approaches exist that allow for the detection of infectious agents from marine life and from the marine environment. However, detection of infection may not be the equivalent of a diagnosis of disease. Disease determination requires recognition of anatomic, biochemical, and molecular features that are characteristic of the disease state and that identify pathogenic organisms. Disease investigations in marine scenarios can be complex and may engage concurrently a wide variety of techniques including microbiological culture and isolation, histotechnological procedures performed on arrays of tissue samples, immunohistochemical methodologies, and nucleic acid-based techniques that make use of genetic, genomic, transcriptomic, and metagenomic data. Effective use of these techniques requires knowledge of their capabilities and limitations so that appropriate selection, proper application, and accurate interpretation can be made.

Future directions for marine disease research

Marine Disease Ecology synthesizes current work on disease in marine systems. Although we have learned a great deal about marine diseases, basic surveillance is difficult, disease mechanisms are complex, and general ecological principles are illusive. Fortunately, technology is no longer what thwarts our capabilities to sample and analyze individual samples for potential parasites, pathogens, or opportunists. Increasingly sophisticated advancements in remote sensing, molecular biology, computer technology, and data analytics open new avenues to study marine diseases. With such tools in place, we now need better surveillance, a quantitatively savvy workforce, and new visions that will produce clarity in how to understand, prevent, or mitigate marine diseases. Although we are making great strides, more holistic approaches can increase our reach in disease monitoring and disentangle the contextual interactions among hosts, their disease-causing agents, and the environment. Such new approaches will also allow us to look into the past, rediscover what we missed, and use this information to predict, and ideally prevent, the next outbreak in the ocean.