Scallops are most commonly harvested using scallop dredges or bottom trawls. They also become stuck in traps, pots, trawls, and dredges. The crew used a variety of dredges and trawls to collect biological samples. It also operates dredges, tugboats and patrol boats of height. From the 1890s Otago rivers were dredged for gold, using New Zealand-developed floating dredges. The use of sluices, dredges, or other machinery is forbidden. The first widely used auger dredges were designed in the 1980s. Some dredges also self-offload using drag buckets and conveyors. Mining dredges were imported from America to work the placer gold of the river. Clam-specific dredges can utilize hydraulic injection to target deeper into the sand. Dredges have been classified as suction or mechanical. Since the construction method of hydraulic fill was chosen, four electric dredges were built. Some large suction dredges ( & ) are used in commercial production throughout the world. Dredges and in some cases mine shafts were used. However, much equipment (such as locomotives, dredges and other floating equipment) was still serviceable.