Chapter 20: Sampling Commercial Fisheries
Debra J. Murie, Daryl C. Parkyn, and Sandra L. Diamond
Commercial fisheries involve the harvest and sale of a variety of wild fishes, crustaceans, mollusks, echinoderms, and other aquatic species (e.g., kelps) for a variety of uses, including food for human consumption, pet food, bait, and fish meal products. These fisheries play a vital role in the economies of many countries and provide essential nutrition for billions of people worldwide (FAO 2010). Commercial fisheries are sampled to monitor the quantity of fish caught, seasonal closures, minimum sizes, the presence of toxins, and catches of protected species. However, the most important reasons for sampling commercial fisheries are to determine if fished stocks are at sustainable abundances and if they are being harvested at sustainable levels. Basic data collected from commercial fisheries to monitor stock status include the number or weight of individuals caught, fishing effort, and biological characteristics of the catch. For some species, commercial catch data are the only data available for managing fisheries and are therefore used as indices of abundance or in simple models to monitor trends over time. In many instances, catch data are supplemented by data on the age or length of individuals so that more complex models can be used to assess stock status. For other stocks, multiple commercial fisheries using different gears, recreational or recreational-for-hire fisheries, or fisheries-independent data may exist; in such cases, commercial fisheries data are used in conjunction with these other data sources in a formal and complex process called a stock assessment (section 20.7). Because sampling of commercial fisheries is an integral part of stock assessment, it is critical that the data are collected in a scientifically valid manner.
In commercial fisheries, catch (or total catch) refers to all fishes, invertebrates, or other aquatic organisms that are captured by the gear and brought on deck, whether they are targeted (intentionally caught) or not (FAO 1996). Landings or landed catch refers to that part of the catch that is physically retained and brought to a point of sale (e.g., factory vessel, port, or processing plant) and may include both target and incidental species that have commercial value. The part of the catch that is brought aboard but was not specifically targeted is known as bycatch. Bycatch may be retained for sale, kept for personal use, or used as bait, but in many cases it must be discarded (i.e., thrown overboard or released at sea). Bycatch includes individuals that are controlled by regulations (e.g., smaller than the minimum size limit or caught during a closed season), prohibited species or sexes (common in crab fisheries), unmarketable species or sizes, and protected species (e.g., sea turtles caught in shrimp trawls).