especially when used to catch large species of fish (like herring or mackerel) that shoal tightly together. When used to catch fish that shoal together with other species, or when used in parallel with fish aggregating devices, the percentage of by-catch greatly increases. Use of purse seines is regulated by many countries; in Sri Lanka, for example, using this type of net within of the shore is illegal. However, they can be used in the deep sea, after obtaining permission from authorities. Purse seine fishing can have negative impacts on fish stocks because it can involve the bycatch of non-target species and it can put too much pressure on fish stocks. Power block The power block is a mechanized pulley used on some seiners to haul in the nets. According to the UN Food and Agriculture Organization, no single invention has contributed more to the effectiveness of purse seine net hauling than the power block. The Puretic power block line was introduced in the 1950s and was the key factor in the mechanization of purse seining. The combination of these blocks with advances in fluid hydraulics and the new large synthetic nets changed the character of purse seine fishing. The original Puretic power block was driven by an endless rope from the warping head of a winch. Nowadays, power blocks are usually driven by hydraulic pumps powered by the main or auxiliary engine. Their rpm, pull and direction can be controlled remotely. A minimum of three people are required for power block seining; the skipper, skiff operator, and corkline stacker. In many operations a fourth person stacks the leadline, and often a fifth person stacks the web. Drum In certain parts of the western United States as well as Canada, specifically on the coast of British Columbia, drum seining is a method of seine fishing which was adopted in the late 1950s and is used exclusively in that region. The drum seine uses a horizontally mounted drum to haul and store the net instead of a power block. The net is pulled in over a roller, which spans the stern, and then passes through a spooling gear with upright rollers. The spooling gear is moved from side to side across the stern which allows the net to be guided and wound tightly on the drum. There are several advantages to the drum seine over the power block. The net can