1.5 Water Quality

The aquaculture water can have salinity ranges from freshwater to brackish to marine. The sources of water used in aquaculture are categorized into those three mentioned. In its publications, the FAO presents the salinity of freshwater as < 0.5 ‰ (0.5 g of NaCl/kg of water), and the salinity of brackish waters (0.5‰ to < 30‰) is in between the freshwater and full seawater at 30‰ (30 g of NaCl/kg of water). Brackish wells can supply saline water, but the water ion quality needs to be analyzed. Various ion replacement salts can be supplemented to supply any deficient levels to replicate the natural brackish waters. The species reared in a system is adaptable to the salinity of the source water. The water and its qualities are more thoroughly covered in Chapter 2.

Freshwater production is considered “inland” aquaculture production in industry publications. The production facility is sited near the water source that will best serve the species to be cultured. Secondary considerations in facility site selection include proximity to feed source, slaughter, processing, climate, and markets.

Marine culture, on the other hand, became sizable with net pens being sited in coves and other weather-protected sites along the coastline. Suitable sites have suitable water qualities (temperature, salinity) and tidal activity as well as accessibility to on-shore support services (e.g., feed, processing, maintenance services, and climate). Currently, there is a large interest growing in land-based facilities to raise both marine and brackish species that are higher-value species.

Freshwater

Freshwater aquaculture involves native and introduced. These systems mimic or provide some type of inland water characteristic such as thermal, chemical, and flow rates. Freshwater species are categorized by the temperature range needed to successfully live, grow, and reproduce. There are coldwater, coolwater, and warmwater fish. The temperature ranges given here are not universally used by every management agency, nor do the temperature ranges fully depict the precise temperatures for each species in the category. Some temperatures shown are maximum or minimum survivable temperatures. The preferred temperatures are shown in most cases.

• Coldwater fish species complete their life cycle in water less than 70 °F. The preferred temperature range for these fishes is between 50 and 65 °F. Commonly grown coldwater species are rainbow trout and landlocked Atlantic salmon.
• Coolwater fish species thrive and reproduce in water temperatures under 80 °F but warmer than 60 °F. The preferred temperature range is 65 to 70 °F for these fishes. A few examples of coolwater fish species grown for aquaculture are northern pike, walleye, and muskellunge.
• Warmwater fish species thrive and reproduce in temperatures warmer than 80 °F. These fishes thrive best between 70 and 85 °F. Basses, sunfishes, and catfishes are a few groups of fish that like warmwater habitats. Tilapia are common food fish reared mostly indoors to provide the warmer temperature. The common carp also tolerates warm temperatures and has the most tons of production harvested on an annual global basis.

The designation of species as a cold, cool, or warm species is not an absolute as, depending on the culture site, different temperature conditions can exist seasonally. One example is ice-covered rivers or lakes in the winter to tepid water temperatures in the summer.

Brackish Water

The specific gravity for this range of salinities is between 5% (1.0004 g/kg) and 30% (1.0226 g/kg). There are fish that move between saltwater and waters with lower salinity that are near the freshwater. Fish that adapt to multiple levels of salinities are called euryhaline. To the reader with interest in freshwater aquarium fishes, species of the molly (i.e., Poecilia spp.) appear to be capable of living in most salinities.^[1] There are species that spend part of their life cycle in freshwater and the remainder in marine waters. These fish are classified by the direction in which they migrate as either anadromous or catadromous fish. They are discussed later in Chapter 3. The list of species cultured in brackish water conditions is numerous. Salmonid species are frequently cultivated in net pens anchored in estuaries where freshwater and saltwater blend.

Other commercially reared fishes that reside well in brackish water are barramundi and several of the seabream. While there are numerous stocks of cod, the Baltic cod successfully spawn in the brackish water of their native habitat.[footnote] Nissling, A., & Westin, L. (1997). Salinity requirements for successful spawning of Baltic and Belt Sea cod and the potential for cod stock interactions in the Baltic Sea. Marine Ecology Progress Series, 152, 261-271.[/footnote] It is noticeable that the temperature ranges for species differ. Cod can utilize colder water temperatures than Mediterranean species such as the gilthead seabream (Sparus aurata). The gilthead seabream dies at or below 41 °F and drastically reduces eating around 60 °F.

Marine Water

Marine aquaculture rears fish in saltwater conditions using either sea-pen facilities or near-shore and even on-shore facilities to allow pumping the seawater into the facility. As noted for the freshwater fish, there are temperature preferences in marine species, as well. Several salmon species thrive in cold saltwater. Seawater near freezing is survivable for salmonid species, but no growth is accomplished due to the slowed metabolic functions and susceptibility to disease increases. Extending the temperature range to tropical would include many species such as pompano (Trachinotus ovatus), orange-spotted grouper (Epinephelus coioides), and coral trout (Plectropomus leopardus).