Mariculture is the branch of aquaculture that cultivates marine organisms either in the open ocean, an enclosed portion of the ocean, or tanks or ponds filled with seawater. Finfish (like flounder and whiting), shellfish (like prawns and oysters), and sea plants (like kelp and seaweed) are cultured in saltwater. Mariculture products are also used for cosmetics, jewelry -- such as cultured pearls -- and fish meal. Algaculture is the type of aquaculture that cultivates algae. Most algae harvested is either microalgae (phytoplankton, microphytes or planktonic algae) or macroalgae, commonly known as seaweed. Although macroalgae is used for a variety of commercial purposes, its size and cultivation needs make it hard to grow. Microalgae are easier to harvest on a large scale. To successfully harvest algae, an algae farm needs the right temperature range, light source and nutritional characteristics in the water source. Algae is most commonly cultivated in open-pond systems, such as ponds, pools and lakes. However, these systems don't allow for control of light or temperature. Yet, they're the most popular type of pond system, since they're cheaper to build and produce the highest yield of algae. On the other hand, closed-pond systems remedy some of the problems with the open-pond systems. Closed-pond systems are pools or ponds that are covered. Even though the closed-pond system allows more species to grow, it tends to be smaller in scale, so it produces a smaller crop. One variation of the closed-pond system is the photobioreactor, a system that incorporates a light source. For example, placing a greenhouse cover over a pond or pool creates a photobioreactor. Although nutrients must be brought into this type of system, it can produce high-yield crops. In fact, it can even produce excess crops, which could end up destroying the system. However, with proper care, photobioreactor systems produce successful results. Integrated multitrophic aquaculture (IMTA) is a more advanced system of aquaculture. In a multitrophic system, different species with various nutritional needs are combined into one system. IMTA uses the waste products of one species as feed or fertilizer for another species. For example, seaweed grows from the phosphorus and ammonia that fish and shrimp excrete. Shellfish feed on the solids that fish and shrimp produce. Although there are many different types and degrees of IMTA, the main principle behind the system is balance. An efficient IMTA is environmentally and economically sustainable because it recycles nutrients. This is a high-tech way to grow fish, but fish farming has been around for thousands of years. Next, you'll learn how fishmongers kept fish alive 2,000 years ago.