With over-exploitation of coastal fisheries and rising disease rates of ocean populations, fish abundance levels have been dramatically decreasing. The outbreak, persistence, and eradication of infectious diseases are often dependent on the density of the host population. The growth of aquaculture has produced a large density of fish in fish farms, making them more susceptible to disease. They are often open to the surrounding ecosystem and therefore are exposed to wild fish populations as well. Krkošek (2010) explores the connection between outbreaks of parasites in wild salmon and density threshold in aquaculture growth. High densities lead to higher transmission rates of infectious diseases because of increased contact with infected individuals. Wild-farmed populations can exceed host density threshold from an influx of migrating individuals, increases in aquaculture production, or environmental changes such as global warming. In contrast, populations with lower densities show a much lower rate of susceptibility and slower rate of disease spread. Fisheries reduce abundance levels of wild populations, which should lead to a decrease in disease and parasite abundance as well. However when a predatory population declines, the prey population increases and therefore increases susceptibility of disease. —Lauren Lambert
Krkošek, M., 2010. Host density thresholds and disease control for fisheries and aquaculture. Aquaculture Environmental Interactions 1: 21–32.
Industrial aquaculture and number of fish farms are growing. They often have larger domesticated abundance than wild populations. These domesticated populations are concentrated by pens, nets, cages, rafts, or ropes that are open to the surrounding areas. Therefore any disease or parasites that arise in these populations will be open to the surrounding ecosystem and can be transmitted to wild populations. This is a major challenge for the aquaculture industry because of the concern for conservation of the wild marine ecosystem. The most common measure of disease persistence is known as R0, or the net reproductive value. If R0 >1, the parasite population can invade host population. If R0 <1, infections do not replace themselves and parasite populations will eventually die out. This is the process of disease eradication. Populations are susceptible to microparasites such as viruses, bacteria, and protists. Dynamics of microparasite diseases can be modeled by dividing host populations into categories determining status of infection (susceptible, infected, recovered). Individuals become infected by being exposed to others that are infected. Even if an individual recovers, over time it becomes more susceptible to disease and could become infected again. The number of infected individuals generated by a single individual can be determined by multiplying the average duration of infection period by the rate at which hosts become infected.
Macroparasites include helminths and arthropods such as intestinal worms and ectoparasitic copepods. Unlike microparasites, these must leave the initial host in order to complete their life cycle. They also produce sexually in or on a host. For example, sea lice reproduce on the surface tissue of fish. Models describing macroparasites must track free-living stages as well as number of parasites per host, per population. Birth and death rates, rate of infection of hosts, and host/parasite mortality rates are also determined.
Theory predicts that host density thresholds are important for host-parasite dynamics. For example, increases in sea lice in salmon were associated with small incremental changes. This is consistent with the theory in which a small increase in host density that crosses threshold triggers a sudden outbreak of disease. Threshold capacity could have been decreased by environmental conditions such as temperature, leading to an acceleration of an epidemic. Vaccination has contributed to the eradication of diseases in fish farms despite the large increase in fish production. Most developments of vaccines can only treat bacterial and viral diseases, and not parasitic ones.
Spatial scale of thresholds is dependent on physical characteristics that influence dispersion and survival of free-living parasites and infectious agents. Pathogens existing in marine environments are long lived and widely dispersed compared to those on land because they are able to be transported over long distances in a more stable condition of temperature and moisture. This is especially true in the case of wild marine populations that have hosts that are highly mobile or migratory which leads to spread of infection at a much more rapid rate. Outbreaks have been shown to follow the direction of currents, predicted using hydrodynamic modeling. With these migratory fish spreading disease to other populations, epidemics in fish farms can be spread over very large scales. However the infection is likely to decline with increased distance from the source population.
Parasitic life cycles are influenced by a variation of environmental factors such as temperature, moisture, and salinity. Global climate change can have a profound influence on these parameters. Developmental rates of parasites are highly dependent on the consistency of temperature. Knowledge of disease outbreaks and its possible influences has important implications for coastal fish farm planning. Minimizing transmission of pathogens among farm fish to wild hosts could increase the size of wild populations. This can be done by placing farms further away from wild fish migration routes and in locations that have low ocean tides and currents, to minimize rates of disease spreading. Selective breeding of fish that are resistant to disease can also provide the future populations with a higher survival rate. Increased vaccinations of fish, increased circulation of facilities, and maintaining lower density levels in these fish farms provides advances in disease control.