Fundamentals of Aquatic Veterinary Medicine. Группа авторов

have been banned in most countries due to their highly toxic nature. They are toxic to most animals, have a high level of bioaccumulation, and persist in the environment for long periods of time (Fulton et al., 2013).

      Organophosphates have a lower level of bioaccumulation and a shorter half‐life in the environment. They may be used as a treatment for several fish parasites, but can be easily overdosed. Some fish are more sensitive to organophosphates than others. If using organophosphates for treatment of disease, always determine the species sensitivity.

      Carbamates are frequently used for agriculture and residential insect control. They also have minimal bioaccumulation and environmental persistence. They are less toxic to fish than organophosphates, but still can cause chronic toxicity if overused around an aquasystem (Fulton et al., 2013).

      Pyrethroids and nicotinoids are also commonly used in the environment. Nicotinoids cause irreversible block of the post synaptic nicotinoid receptors. Pyrethroids cause damage to the sodium channel in the neurons leading to hyperexcitability, tremors, ataxia and paralysis (Sabra and Mehana, 2015).

      There are many other types of insecticides to consider. Although most will not persist in the environment more than one or two weeks, if used consistently, they may lead to chronic disease. This may appear as decreased growth, low reproduction, and slow increase in mortality.

      1.5.3.3 Herbicides

      There are many different classes of herbicides. Most herbicides do not directly affect fish; however, different species of fish respond to varied levels and classes of herbicides. Studies performed on forestry herbicides in regard to fish development showed no evidence of toxicity in zebra fish to picloram, clopyralid, imazapic, glyphosate, imazapyr, and triclopyr (Stehr et al., 2009). However, studies evaluating Cyprinus carpio and rohu showed an increased mortality and histologic changes in the kidney, liver, and gills when exposed to glyphosate at 86 mg/l. This same study showed increased mortality in response to the use of the salt formulation of 2, 4‐D at 100 mg/l and paraquat at 26 mg/l.

      Atrazine has been shown to affect the hypothalamus–pituitary–gonadal axis in vertebrates. One study has shown that this herbicide negatively affects the egg production of fathead minnows (Tillitt, 2010).

      Pretilachlor is frequently used in rice fields. Studies performed on Clarias batrachus exposed to this herbicide had significant behavioral changes, including decreased feeding and increased buccal activity (Soni, 2018).

      At high levels, triazine has been shown to cause acute toxicity in fish. This herbicide can also cause negative affects regarding reproduction and reproductive development (Hostovsky et al., 2014).

      When diagnosing possible toxic exposure in a fish population, herbicides should be considered if they have been used in the water or in the area nearby due to potential runoff. When developing a farm management program that includes herbicides, it is important to consider the following:

       The sensitivity of the species you are working with.

       The potential exposure to wild fish in the area and their sensitivity.

       The bioaccumulation of the herbicide directly into the water or in the runoff.

       The half‐life of the chemical if it is used in an unpopulated pond.

      1.5.3.4 Harmful Algal Blooms

      Micro‐ and macroalgae have been implicated in millions of dollars of loss in aquaculture over the past decade (Hallegraeff et al., 2017) and large numbers of deaths of aquatic animals in the wild and in aquariums. These harmful algal blooms have recently increased in our environment due to increased migration, climate change, CO₂ levels, and nutrients in the water. Harmful algal blooms are caused by microalgae species including diatoms, cyanobacteria, raphidophytes, prymnesiophytes, pelagophytes, and silicoflagellates (Landsberg, 2002). The most common freshwater algal blooms are caused by cyanobacteria.

      Increased water traffic has allowed for migration of algal blooms from continent to continent, allowing the microalgae to find the most conducive environment for growth. Changes in water temperature, pH, and nutrient upwelling have allowed many of these microalgae species to bloom in new areas where animals are not adapted to their presence (Anderson et al., 2012). Climate change has also affected the percentage of toxic compared with nontoxic strains such as Microcystis and a decreased overwintering period as seen in Cylindrospermopsis (Wood et al., 2015).

      The effects of harmful algal blooms are very difficult to anticipate due to the variety of microalgae, the toxins that each species contain, and the sensitivity of aquatic animals to those toxins. Each species of microalgae has a different environmental niche. Some are benthic and require limited sunlight. Others must remain near the surface. There are some microalgae that can adjust their buoyancy to move to the most effective area where both photosynthesis and nutrient absorption are greatest. Many adaptations of microalgae allow for them to have an advantage over other algae. These adaptations also tend to give the nontoxic forms of the species less of an advantage over the toxic forms.

      The toxins of algal blooms include hepatotoxins, neurotoxins, and dermatotoxins. Each bloom may occur without the presence of toxins in the water and the toxins may only be found in the water before the mortality is detected. This adds to the difficulty in the diagnosis of algal bloom toxicity. The Centers for Disease Control and Prevention list the majority of toxins that are known at this time (Roberts et al., 2020). A more complete explanation of the individual events that have shown proven effects of the toxins in aquatic animals can be found in Jan Landsberg’s (2002) article “The effects of harmful algal blooms on aquatic organisms”.

      Cyanobacteria can cause fresh water algal blooms. They are found in ponds, lakes and estuaries. When fresh water incursion occurs and nutrient levels are high, these blooms can reach marine life on the coast. Cyanobacteria most commonly produce microcystins, anatoxins, and cylindrospermopsin. Most aquatic species appear to be resistant to anatoxins and cylindrospermopsin. There have been bird deaths with anatoxin‐a(s) and a possible alligator mortality with cylindrospermopsin. Microcystins have been shown to cause both acute disease and chronic disease. The route of uptake is through the gastrointestinal tract, gills, and skin. There have been many field observations and laboratory experiments with fish regarding microcystin‐LR. These studies have shown that microcystins affect the leukocytes, liver enzymes, growth rate, and ionic stability. The histopathological changes are dose dependent and include degenerative changes to the kidney, liver, and gills (Malbrouck, 2009).

Fish appear to be more resistant than mammals to many neurotoxins, such as brevetoxins, domoic acid, and saxitoxins. These toxins bioaccumulate in fish and cause high mortality rates in birds and marine mammals. Saxitoxins cause paralytic shellfish poisoning (PSP) in mammals due to sodium channel blockage. There are 21 derivatives of saxitoxins involved with PSP. Recent studies show a direct effect on mollusks causing increased mortality and poor growth. Herring exposed to Alexandrium tamarense show increased mortality due to asphyxiation (Landsberg, 2002). Brevetoxins include nine types of neurotoxins, several of which are found in sea spray and act as a respiratory irritant. The neurotoxins alter membrane properties in the neurologic cells. Manatee deaths caused by brevetoxins is postulated to be caused by the ingestion of tunicates found on the surface of sea grass and through inhalation when traveling through large blooms (Landsberg, 2002).

      Domoic acid is an excitatory neurotransmitter binding glutamate receptors. There are some copepods that are very sensitive to domoic acid, others are less sensitive and act as a vector. Domoic acid has not been identified in fish mortality events, but it has caused mortalities in brown pelicans after ingestion of anchovies (Lansberg, 2002).

      Harmful algal blooms can also cause a reduction in the available oxygen during respiration, exaggerating the toxic effects. Many of the species involved in these blooms are more tolerant to the low oxygen levels and will thrive on the nutrients created by the increased mortality of other organisms. Harmful