Stock assessment
Encyclopedia
Stock assessments provide fisheries managers
with the information that is used in the regulation of a fish stock
. Biological and fisheries data are collected in a stock assessment.
A wide array of biological data may be collected for an assessment. These include details on the age structure
of the stock, age at first spawning
, fecundity, ratio of males to females in the stock, natural mortality (M), fishing mortality (F), growth rate of the fish, spawning behavior, critical habitats, migratory
habits, food preferences, and an estimate of either the total population or total biomass
of the stock.
The following data regarding fisheries activities is collected: the kinds of fisherman
in the fishery, commercial
versus recreational
, and the gear
that is used (longline, rod
and reel
, nets
, etc), pounds of fish caught by each type of fisherman, the fishing effort each kind of fisherman expends, the age structure of the fish harvested by each group of fisherman, the ratio of males to females that are captured, how the fish are marketed
, the value of the fish to the different fisherman groups, and the time and geographic location of the best catches. Also in the assessment, geographical boundaries of different stocks or populations are defined. From the combined biological and fisheries data, the current status and condition of the stock is defined and managers use this assessment to predict how in the future, stocks will respond to varying levels of fishing pressure. Ultimately managers want to reduce the levels of overfishing
that occurs and restore stocks that have been overfished.
, stock refers to a harvested or managed unit of a fish. Typically stocks are divided based on geographical location and not based on individual population. Spanish mackerel
are distributed from Maine
to the Yucatan Peninsula
in Mexico
. They are divided into two stocks, based on whether they migrate northward along the eastern United States coast or if they migrate into the Gulf of Mexico. Each stock of Spanish mackerel does not represent discrete populations. Stocks are not always composed of a single species. Stocks can be composed of multiple species due to their being harvested together or as a form of convenience for managers. An example of a multispecies stock is river herring
. Alewives and blueback herring are labeled as river herring for management purposes due to their similar physical appearances and being harvested together. Individuals within a stock are subdivided into cohorts. A cohort is a group of fish born in the same year within a population or stock.
, self-reporting, telephone surveys, and vessel-monitoring
surveys.
Fishery-independent data is obtained in the absence of any fishing activity. The majority of this data is collected by state and federal agencies. A wide variety of methods and gear types are used to acquire fishery-independent data. Sampling equipment can include trawls, seines, acoustic
and/or video surveys. The study may focus on a single species, multiple species, or a specific age range or cohort. Regardless of the method or approach, these surveys provide managers with an estimate of abundance. Mark and recapture studies are commonly used to estimate movement, migration, growth rate, natural mortality, and discard
mortality. Stock assessments are often completed using both fishery-dependent and fishery-independent data.
of a stock is lower than what is required to support the maximum sustainable yield
. Overfishing
describes the rate of removal from a stock and can be categorized as two different types: recruitment and economic. Recruitment overfishing is when fishing pressure is too heavy to allow a fish population to replace itself. Economic overfishing occurs when the level of fish harvesting that is higher than that of economic efficiency and more fish are harvested than necessary to have maximum profits for the fishery.
Some of these benchmarks are combined in the form of ratios in order to better understand the status of the stock. A Fcurr/Fmsy ratio greater than one indicates that overfishing is occurring. When SSBcurr/SSBmsy is greater than one, the MSY will be produced, but if it is less than one it means that the stock is overfished. MFMT and MSST are used as limit reference points. When MFMT is exceeded or the spawning stock size dips below MSST the fishery is shut down. A benchmark that is currently gaining in popularity is SPR, spawning potential ratio. The SPR is the average fecundity of a recruit over its lifetime when the stock is fished divided by the average fecundity of a recruit over its lifetime when the stock is unfished. The SPR is based on the principle that certain levels of fish have to survive in order to spawn and replenish the stock at a sustainable level.
models. Of these models, surplus production models are the least complex and require the least amount of data. This model describes the stock solely in regards of biomass and the only used total catch and effort data. These are most commonly used in situations limited data is available on a stock. Statistical catch at age models are based on the age structure of a fished population. These models use the proportional catch-at-age to predict the relative abundance of each age class. These calculated relative abundances are then used to estimate future abundances of the stock and harvest regulations are set based on the predicted future abundances. In virtual population analysis models, catch-at-age data is used to estimate historical stock abundance. From this analysis, the manager then determines if overfishing is occurring. The type of model used depends on the data that is available. Modern stock assessment methods use statistical approaches to "integrate" multiple sources of information to estimate management quantities and their associated uncertainty.
Fisheries management
Fisheries management draws on fisheries science in order to find ways to protect fishery resources so sustainable exploitation is possible. Modern fisheries management is often referred to as a governmental system of appropriate management rules based on defined objectives and a mix of management...
with the information that is used in the regulation of a fish stock
Fish stock
Fish stocks are subpopulations of a particular species of fish, for which intrinsic parameters are the only significant factors in determining population dynamics, while extrinsic factors are considered to be insignificant.-The stock concept:All species have geographic limits to their...
. Biological and fisheries data are collected in a stock assessment.
A wide array of biological data may be collected for an assessment. These include details on the age structure
Age class structure
Age class structure in fisheries and wildlife management is a part of population assessment. Age can be determined by counting growth rings in fish scales, otoliths, cross-sections of fin spines for species with thick spines such as triggerfish, or teeth for a few species. Each method has its...
of the stock, age at first spawning
Spawn (biology)
Spawn refers to the eggs and sperm released or deposited, usually into water, by aquatic animals. As a verb, spawn refers to the process of releasing the eggs and sperm, also called spawning...
, fecundity, ratio of males to females in the stock, natural mortality (M), fishing mortality (F), growth rate of the fish, spawning behavior, critical habitats, migratory
Fish migration
Many types of fish migrate on a regular basis, on time scales ranging from daily to annually or longer, and over distances ranging from a few metres to thousands of kilometres...
habits, food preferences, and an estimate of either the total population or total biomass
Biomass (ecology)
Biomass, in ecology, is the mass of living biological organisms in a given area or ecosystem at a given time. Biomass can refer to species biomass, which is the mass of one or more species, or to community biomass, which is the mass of all species in the community. It can include microorganisms,...
of the stock.
The following data regarding fisheries activities is collected: the kinds of fisherman
Fisherman
A fisherman or fisher is someone who captures fish and other animals from a body of water, or gathers shellfish. Worldwide, there are about 38 million commercial and subsistence fishermen and fish farmers. The term can also be applied to recreational fishermen and may be used to describe both men...
in the fishery, commercial
Commercial fishing
Commercial fishing is the activity of catching fish and other seafood for commercial profit, mostly from wild fisheries. It provides a large quantity of food to many countries around the world, but those who practice it as an industry must often pursue fish far into the ocean under adverse conditions...
versus recreational
Recreational fishing
Recreational fishing, also called sport fishing, is fishing for pleasure or competition. It can be contrasted with commercial fishing, which is fishing for profit, or subsistence fishing, which is fishing for survival....
, and the gear
Fishing tackle
Fishing tackle, is a general term that refers to the equipment used by fishermen when fishing. Almost any equipment or gear used for fishing can be called fishing tackle. Some examples are hooks, lines, sinkers, floats, rods, reels, baits, lures, spears, nets, gaffs, traps, waders and tackle...
that is used (longline, rod
Fishing rod
A fishing rod or a fishing pole is a tool used to catch fish, usually in conjunction with the pastime of angling, and can also be used in competition casting. . A length of fishing line is attached to a long, flexible rod or pole: one end terminates in a hook for catching the fish...
and reel
Fishing reel
A fishing reel is a "cylindrical device attached to a fishing rod used in winding the line". Modern fishing reels usually have fittings which make it easier to retrieve the line and deploy it for better accuracy or distance. Fishing reels are traditionally used in the recreational sport of angling...
, nets
Fishing net
A fishing net or fishnet is a net that is used for fishing. Fishing nets are meshes usually formed by knotting a relatively thin thread. Modern nets are usually made of artificial polyamides like nylon, although nets of organic polyamides such as wool or silk thread were common until recently and...
, etc), pounds of fish caught by each type of fisherman, the fishing effort each kind of fisherman expends, the age structure of the fish harvested by each group of fisherman, the ratio of males to females that are captured, how the fish are marketed
Fish marketing
Fish marketing, is the marketing and sale of fish products.-Live fish trade:The live fish trade is a global system that links fishing communities with markets, primarily in Hong Kong and mainland China...
, the value of the fish to the different fisherman groups, and the time and geographic location of the best catches. Also in the assessment, geographical boundaries of different stocks or populations are defined. From the combined biological and fisheries data, the current status and condition of the stock is defined and managers use this assessment to predict how in the future, stocks will respond to varying levels of fishing pressure. Ultimately managers want to reduce the levels of overfishing
Overfishing
Overfishing occurs when fishing activities reduce fish stocks below an acceptable level. This can occur in any body of water from a pond to the oceans....
that occurs and restore stocks that have been overfished.
Defining stock
In fisheries managementFisheries management
Fisheries management draws on fisheries science in order to find ways to protect fishery resources so sustainable exploitation is possible. Modern fisheries management is often referred to as a governmental system of appropriate management rules based on defined objectives and a mix of management...
, stock refers to a harvested or managed unit of a fish. Typically stocks are divided based on geographical location and not based on individual population. Spanish mackerel
Spanish mackerel
Spanish mackerel may refer to:* Chloroscombrus chrysurus* Elagatis bipinnulata* Schedophilus maculatus* Scomber japonicus* Scomberomorus brasiliensis* Scomberomorus cavalla* Scomberomorus commerson* Scomberomorus guttatus...
are distributed from Maine
Maine
Maine is a state in the New England region of the northeastern United States, bordered by the Atlantic Ocean to the east and south, New Hampshire to the west, and the Canadian provinces of Quebec to the northwest and New Brunswick to the northeast. Maine is both the northernmost and easternmost...
to the Yucatan Peninsula
Yucatán Peninsula
The Yucatán Peninsula, in southeastern Mexico, separates the Caribbean Sea from the Gulf of Mexico, with the northern coastline on the Yucatán Channel...
in Mexico
Mexico
The United Mexican States , commonly known as Mexico , is a federal constitutional republic in North America. It is bordered on the north by the United States; on the south and west by the Pacific Ocean; on the southeast by Guatemala, Belize, and the Caribbean Sea; and on the east by the Gulf of...
. They are divided into two stocks, based on whether they migrate northward along the eastern United States coast or if they migrate into the Gulf of Mexico. Each stock of Spanish mackerel does not represent discrete populations. Stocks are not always composed of a single species. Stocks can be composed of multiple species due to their being harvested together or as a form of convenience for managers. An example of a multispecies stock is river herring
Herring
Herring is an oily fish of the genus Clupea, found in the shallow, temperate waters of the North Pacific and the North Atlantic oceans, including the Baltic Sea. Three species of Clupea are recognized. The main taxa, the Atlantic herring and the Pacific herring may each be divided into subspecies...
. Alewives and blueback herring are labeled as river herring for management purposes due to their similar physical appearances and being harvested together. Individuals within a stock are subdivided into cohorts. A cohort is a group of fish born in the same year within a population or stock.
Gathering data
Data used in stock assessments can be classified as fishery-dependent data or fishery-independent data. Fishery-dependent data is collected from the fishery itself, using both commercial and recreational sources. There are a variety of methods for obtaining fishery-dependent data. The most common approach is to use recorded landings. Landings are a record of the amount of fish sold and the numbers are typically reported in total weight. Another common mode for acquiring fishery-dependent data is through portside sampling of the catch of both recreational and commercial fisherman to obtain age and length information on the stock. Other less common methods for obtaining data is through the use of onboard observersFisheries observer
A fisheries observer is an independent specialist who serves on board commercial fishing vessels, or in fish processing plants and other platforms, and is employed by a fisheries observer program, either directly by a government agency or by a third party contractor. Observers spend anywhere from...
, self-reporting, telephone surveys, and vessel-monitoring
Vessel monitoring system
Vessel monitoring systems are used in commercial fishing to allow environmental and fisheries regulatory organizations to monitor, minimally, the position, time at a position, and course and speed of fishing vessels. They are a key part of monitoring control and surveillance programs at the...
surveys.
Fishery-independent data is obtained in the absence of any fishing activity. The majority of this data is collected by state and federal agencies. A wide variety of methods and gear types are used to acquire fishery-independent data. Sampling equipment can include trawls, seines, acoustic
Fisheries acoustics
Fisheries acoustics includes a range of research and practical application topics using acoustical devices as sensors in aquatic environments. Acoustical techniques can be applied to sensing aquatic animals, zooplankton, and physical and biological habitat characteristics.-Basic Theory:Biomass...
and/or video surveys. The study may focus on a single species, multiple species, or a specific age range or cohort. Regardless of the method or approach, these surveys provide managers with an estimate of abundance. Mark and recapture studies are commonly used to estimate movement, migration, growth rate, natural mortality, and discard
Discards
Discards are the portion of a catch of fish which is not retained on board during commercial fishing operations and is returned, often dead or dying, to the sea...
mortality. Stock assessments are often completed using both fishery-dependent and fishery-independent data.
Overfished versus overfishing
Overfished refers to the number of fish in the stock. Typically a stock is described as being overfished when current biomassBiomass (ecology)
Biomass, in ecology, is the mass of living biological organisms in a given area or ecosystem at a given time. Biomass can refer to species biomass, which is the mass of one or more species, or to community biomass, which is the mass of all species in the community. It can include microorganisms,...
of a stock is lower than what is required to support the maximum sustainable yield
Maximum sustainable yield
In population ecology and economics, maximum sustainable yield or MSY is, theoretically, the largest yield that can be taken from a species' stock over an indefinite period...
. Overfishing
Overfishing
Overfishing occurs when fishing activities reduce fish stocks below an acceptable level. This can occur in any body of water from a pond to the oceans....
describes the rate of removal from a stock and can be categorized as two different types: recruitment and economic. Recruitment overfishing is when fishing pressure is too heavy to allow a fish population to replace itself. Economic overfishing occurs when the level of fish harvesting that is higher than that of economic efficiency and more fish are harvested than necessary to have maximum profits for the fishery.
Biological reference points
Biological reference points are the primary output of stock assessments and fishing regulations are set to meet these biological benchmarks. There are several types of benchmarks are used depending on the preference of the regulating agency. The benchmarks are used to control different aspects of the fishery or population. Benchmarks that regulate fishing mortality include Fcurr, Fmsy, Emsy, and MFMT. Fcurr is the current level of fishing mortality, Fmsy is the fishing mortality that produces the maximum sustainable yield, and Emsy is the expected level of fishing that will produce the maximum sustainable yield. SSBcurr, SSBmsy, and MSST are used in the regulation of biomass levels. SSBcurr stands for the current spawning stock biomass, SSBmsy is the amount of spawning stock biomass needed to produce the maximum sustainable yield, and MSST is the minimum standing stock threshold. The amount that is available to be harvested is controlled by the benchmarks MSY and OY. MSY is the maximum sustainable yield and OY is the optimum yield of a stock. The main difference between MSY and OY is that MSY considers only the biology of the fish while the OY considers the economic aspect of the harvest.Some of these benchmarks are combined in the form of ratios in order to better understand the status of the stock. A Fcurr/Fmsy ratio greater than one indicates that overfishing is occurring. When SSBcurr/SSBmsy is greater than one, the MSY will be produced, but if it is less than one it means that the stock is overfished. MFMT and MSST are used as limit reference points. When MFMT is exceeded or the spawning stock size dips below MSST the fishery is shut down. A benchmark that is currently gaining in popularity is SPR, spawning potential ratio. The SPR is the average fecundity of a recruit over its lifetime when the stock is fished divided by the average fecundity of a recruit over its lifetime when the stock is unfished. The SPR is based on the principle that certain levels of fish have to survive in order to spawn and replenish the stock at a sustainable level.
Catch curve
A catch curve is a descriptive figure that describes catch by age and length. A catch curve only reflects fish that have recruited to a fishery and does not reflect the full age structure of a stock. A catch curve illustrates the proportions that different age and size classes are harvested by a fishery.Assessment models
The mathematical and statistical techniques used to complete a stock assessment are referred to as assessment models. Three commonly used models are surplus production models, statistical catch at age models, and virtual population analysisVirtual Population Analysis
Virtual population analysis is a modeling technique commonly used in fisheries science for reconstructing historical fish numbers at age using information on death of individuals each year. This death is usually partitioned into catch by fisheries and natural mortality.VPA is the most commonly...
models. Of these models, surplus production models are the least complex and require the least amount of data. This model describes the stock solely in regards of biomass and the only used total catch and effort data. These are most commonly used in situations limited data is available on a stock. Statistical catch at age models are based on the age structure of a fished population. These models use the proportional catch-at-age to predict the relative abundance of each age class. These calculated relative abundances are then used to estimate future abundances of the stock and harvest regulations are set based on the predicted future abundances. In virtual population analysis models, catch-at-age data is used to estimate historical stock abundance. From this analysis, the manager then determines if overfishing is occurring. The type of model used depends on the data that is available. Modern stock assessment methods use statistical approaches to "integrate" multiple sources of information to estimate management quantities and their associated uncertainty.