Abstract
It is hard to find a definition of gill health in the literature although there is a lot of information on changes to gill structure as a result of infectious and non-infectious challenge. How these changes relate to overall fish health is sometimes not clear. Interaction between the gill, the fish, and a range of anticipated changes in the environment will have a currently unknown effect on marine health and aquaculture production. To a degree, fish will likely be able to ameliorate certain changes, such as compensating for slightly elevated carbon dioxide; however, these actions may come at the cost of compromising other functions such as osmoregulation. Compensation will also depend on gill epithelial health and other environmental factors like external nitrogen and ammonia sources which can rise depending on the direction future culture and levels of eutrophication take. Fish can also remodel gill structure in response to salinity, hypoxia, or acidification but it appears that increased temperatures may be associated with increased pathology observable in the gill, and certain fishes may be more susceptible to change. There is a need for more targeted research into climate change-specific gill physiology and a need to recognise gill health as being a key component of food security and not just fish health.
Highlights
Eutrophication will reduce the ability of zooplankton to control algae because it leads to cyanobacteria dominance, which is harder to feed on and digest (Moss et al, 2011), and together, climate change and eutrophication will limit the ability of zooplankton to control algae, reinforcing the potential for harmful blooms that have already been demonstrated as an emerging problem in gill health and in fish health more generally
In order to properly interpret results from histopathological evaluations, the literature shows that a healthy condition is not characterized by the complete absence of any histologically apparent traits, and an apparently healthy gill may display moderate alterations, such as minor structural disorders or mild inflammatory reactions (Bernet et al, 2004), prompting the question of what does a healthy gill look like? In addition to assessing the gill’s normal or abnormal physical function, physiological function of the gills is going to be a prime concern for fish production in a changing environment, especially in light of the four very important roles that the fish gill serves: gas exchange, osmoregulation, acid–base regulation, and nitrogenous waste excretion
It is understood that eutrophication can have many effects in the anticipated future, but high environmental loading with nitrogenous compounds, such as ammonia and ammonium, will affect osmoregulation, which would be expected to be exacerbated by gill tissue lesions
Summary
Gill diseases represent a significant challenge in marine and freshwater finfish aquaculture globally (Iva et al, 1999; Adams and Nowak, 2003; Teruo et al, 2005; Griffin et al, 2009; Lovy et al, 2011; Mitchell and Rodger, 2011; Kim et al, 2016; Oldham et al, 2016; Bloecher et al, 2018), and may involve infectious or non-infectious agents (Rodger et al, 2011; Shi et al, 2015). The projected increase of the average temperature could warm oceans to depths of at least 3000 m, which may have direct and indirect effects on aquatic organisms, such as their metabolism and migration to other geographical areas, as well as the formation of harmful algal blooms that could form marine biotoxins and contaminate, for example, mussels used for food production (Miraglia et al, 2009). By 2100, the concentration of atmospheric carbon dioxide is expected to exceed 500 parts per million with global temperatures expected to rise by at least 2◦C (Hoegh-Guldberg et al, 2007) The authors report these values significantly exceed those of at least the past 420,000 years during which most extant marine organisms evolved. What effect this will have on gill health and subsequently on fish health remains to be seen
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