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In risk analysis, a hazard is generally specified by describing what might go wrong and how this might happen (Ahl et al. 1993). A hazard refers not only to the magnitude of an adverse effect but also to the likelihood of the adverse effect occurring (Müller-Graf et al. 2012). Hazard identification is important for revealing the factors that may favour the establishment of a disease and/or potential pathogen threat, or otherwise detrimental for fish welfare. Biological pathogens are recognised as hazard in aquaculture by Bondad-Reantaso et al. (2008). A broad range of factors can be taken into consideration as long as they are associated with disease occurrence, i.e. they are hazards.
Table 17.2 List of potential hazards for aquatic animal health in aquaponics
table thead tr class="header" th/th th Hazard identification /th th Hazard specification /th /tr /thead tbody tr class="odd" td rowspan=9 Abiotic/td td pH /td td Too high/too low/rapid change /td /tr tr class="even" td Water temperature /td td Too high/too low/rapid change /td /tr tr class="odd" td Suspended solids /td td Too high /td /tr tr class="even" td Dissolved oxygen content /td td Too low /td /tr tr class="odd" td Carbon dioxide content /td td Too high /td /tr tr class="even" td Ammonia content /td td Too high, pH dependent /td /tr tr class="odd" td Nitrite content /td td Too high /td /tr tr class="even" td Nitrate content /td td Extremely high /td /tr tr class="odd" td Metal content /td td Too high, pH dependent /td /tr tr class="even" td rowspan=2 Biotic/td td Stocking density /td td Too high/too low /td /tr tr class="odd" td Biofouling /td td/td /tr tr class="even" td rowspan=3 Feeding/td td Nutrients by the fish species /td td Surplus/deficiency /td /tr tr class="odd" td Feeding frequency /td td Inadequate/improper feeding /td /tr tr class="even" td Dietary toxins /td td/td /tr tr class="odd" td/td td Feed additives /td td Unsuitable growth promoters /td /tr tr class="even" td rowspan=6 Management/td td Aquaponic system design /td td Poor system design /td /tr tr class="odd" td Fish species /td td Unsuitable for aquaponics /td /tr tr class="even" td Operational issues (water circulation, biofilter, mechanical) /td td/td /tr tr class="odd" td Chemotherapeutants use /td td Threat for the microbial balance /td /tr tr class="even" td Staff hygiene /td td/td /tr tr class="odd" td Biosecurity /td td/td /tr tr class="even" td rowspan=3 Welfare/td td Stressors /td td Too high /td /tr tr class="odd" td Allostatic load /td td High /td /tr tr class="even" td Rearing conditions /td td Suboptimal /td /tr tr class="odd" td rowspan=3 Diseases/td td Nutritional diseases /td td/td /tr tr class="even" td Environmental diseases /td td/td /tr tr class="odd" td Infectious diseases /td td/td /tr /tbody /table
The sustainability of aquaponics is linked with a variety of factors, including system design, fish feed and faeces features, fish welfare and elimination of pathogens from the system (Palm et al. 2014a, b). Goddek (2016) reported that aquaponic systems are characterized by a wide range of microflora as fish and biofiltration exist in the same water mass. Since a great variety of microflora exists in aquaponic practices, the occurrence of pathogens and risks for human health should also be considered in order to guarantee food safety. In terms of sustainability of aquaponic systems, pathogen elimination to prevent losses due to diseases may be a challenging factor when aquatic animal production is intensified.
The use of chemotherapeutants in aquaculture to fight pathogens presents a number of potential hazards and risks to production systems, the environment and human health (Bondad-Reantaso and Subasinghe 2008) (Table 17.2).
To eliminate hazards, the fish rearing and plant cultivation phases should be considered separately. The biggest risks in fish rearing are related to water quality, fish density, feeding quality and quantity and disease (Yavuzcan Yildiz et al. 2017). Depending on the species of fish reared, the level of risk can increase if the species is not appropriate for the conditions of the particular system. For example, potassium is often supplemented in aquaponic systems to promote plant growth, but results in reduced performance in hybrid striped bass. Normally, freshwater and high-density culture-tolerant species are utilized in aquaponics. The most common species of fish in commercial systems are Tilapia and ornamental fish. Channel catfish, largemouth bass, crappies, rainbow trout, pacu, common carp, koi carp, goldfish, Asian sea bass (or barramundi) and Murray cod are among the species that have been trialled (Rakocy et al. 2006). Tilapia, a warm-water species, highly tolerant of fluctuating water parameters (pH, temperature, oxygen and dissolved solids), is the species largely reared in most commercial aquaponic systems in North America and elsewhere. The results of a recent online survey, based on answers from 257 respondents, showed that Tilapia is reared in 69% of aquaponic plants (Love et al. 2015). Tilapia presents an economic interest in some markets but not in others. In the same survey (Love et al. 2015), other species utilized were ornamental fish (43%), catfish (25%), other aquatic animals (18%), perch (16%), bluegill (15%), trout (10%) and bass (7%). One of the major weaknesses in aquaponic systems is the management of water quality to meet the requirements of the tank-reared fish, while cultivated crops are treated as the second step of the process. Fish require water with appropriate parameters for oxygen, carbon dioxide, ammonia, nitrate, nitrite, pH, chlorine and others. A high level of suspended solids can affect the health status of fish (Yavuzcan Yildiz et al. 2017), provoking damages to gill structure, such as the epithelium lifting, hyperplasia in the pillar system and reduction of epithelial volume (Au et al. 2004). Fish stocking density and feeding (feeding rate and volume, feed composition and characteristics) affect the digestion processes and metabolic activities of fish and, accordingly, the catabolites, total dissolved solids (TDS) and waste by-products (faeces and uneaten feed) in the rearing water. The basic principle on which the aquaponic system is based is the utilization of catabolites in water for plant growth. Aquaponic systems require 16 essential nutrients and all these macro- and micronutrients must be balanced for optimal plant growth. An excess of one nutrient can negatively affect the bioavailability of others (Rakocy et al. 2006). Therefore, the continuous monitoring of water parameters is essential to maintain water quality appropriate for fish and crop growth and to maximize the benefits of the process. Reduced water exchange and low crop growth rate can create toxic nutrient concentrations in water for fish and crops. On the other hand, the addition of some micronutrients (Fesup+2/sup, Mnsup+2/sup, Cusup+2/sup, Bsup+3/sup and Mosup+6/sup), normally scarce in water where fish are reared, is essential to adequately sustain crop production. In comparison to hydroponic culture, crops in aquaponic systems require lower levels of total dissolved solid (TDS, 200—400 ppm) or EC (0.3—0.6 mmhos/cm) and require, like fish, a high level of dissolved oxygen in water (Rakocy et al. 2006) for root respiration.