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NOAA - Fisheries index
Category: Agriculture and Industry > Fisheries
Date & country: 09/09/2007, USA
The number of fish which grow into the catchable size range in a unit of time (usually a year).
Information obtained from samples or observations and used as a measure of the weight or number of fish which make up a stock.
The number of years of life completed, here indicated by an arabic numeral, followed by a plus sign if there is any possibility of ambiguity (age 5, age 5+).
Categories of the level of complexity of, and data available for each assessment: index of abundance (INDEX), yield-per-recruit analysis (YIELD), analysis of the age structure of the catch (AGE STRUCTURE), analysis including the relationship between recruitment and spawning stock size (SPAWNING STOCK) and assessment that allows prediction of future (one or two years ahead) stock sizes and catches (predictive).
1. : The fraction of a fish population which lives in regions where it is susceptible to fishing during a given fishing season. This fraction receives recruits from or becomes mingled with the non-available part of the stock at other seasons, or in other years. (Any more or less completely isolated segment of the population is best treated as a separate stock.) 2. : Catch per unit of effort.
Biological Reference Points
Fishing mortality rates that may provide acceptable protection against growth overfishing and/or recruitment overfishing for a particular stock. They are usually calculated from equilibrium yield-per-recruit curves, spawning stock biomass-per-recruit curves and stock recruitment data. Examples are F0.1, Fmax and Fmed.
Measure of the quantity, usually by weight in pounds or metric tons (2,205 pounds = 1 metric ton), of a stock at a given time.
A graph of the logarithm of number of fish taken at successive ages or sizes.
Catch Per Unit Of Effort
The catch of fish, in numbers or in weight, taken by a defined unit of fishing effort. Also called: catch per effort, fishing success, availability .
: The fraction of a fish stock which is caught by a defined unit of the fishing effort. When the unit is small enough that it catches only a small part of the stock -- 0.01 or less -- it can be used as an instantaneous rate in computing population change. (For fractions taken of various portions of the stock, see 'vulnerability.') Also called: catchability coefficient, *force of fishing mortality
Conditional Fishing Mortality Rate
The fraction of an initial stock which would be caught during the year (or season) if no other causes of mortality operated. (Also called fishing mortality rate).
Conditional Natural Mortality Rate
The fraction of an initial stock that would die from causes other than fishing during a year (or season), if there were no fishing mortality. Also called: annual natural mortality rate, seasonal natural mortality rate.
The average size of the fish in a year class at the time when the instantaneous rate of natural mortality equals the instantaneous rate of growth in weight for the year-class as a whole. Also called: *optimum size.
Effectiveness Of Fishing
A general term referring to the percentage removal of fish from a stock (but not as specifically defined) as either rate of exploitation or instantaneous rate of fishing.
The catch (in numbers) taken from a fish stock when it is in equilibrium with fishing of a given intensity, and (apart from the effects of environmental variation) its abundance is not changing from one year to the next.
The yield in weight taken from a fish stock when it is in equilibrium with fishing of a given intensity, and (apart from effects of environmental variation) its biomass is not changing from one year to the next. Also called: sustainable yield, equivalent sustainable yield.
The distribution of fishing mortality over the age composition of the fish population, determined by the type of fishing gear, area and seasonal distribution of fishing, and the growth and migration of the fish. The pattern can be changed by modifications to fishing gear, for example, increasing mesh or hook size, or by changing the ratio of harvest by gears exploiting the fish (e.g., gill net, trawl, hook and line, etc.).
The proportion of a population at the beginning of a given time period that is caught during that time period (usually expressed on a yearly basis). For example, if 720,000 fish were caught during the year from a population of 1 million fish alive at the beginning of the year, the annual exploitation rate would be 0.72.
The fishing mortality rate at which the increase in yield per recruit in weight for an increase in a unit of effort is only 10 percent of the yield per recruit produced by the first unit of effort on the unexploited stock (i.e., the slope of the yield-per-recruit curve for the F0.1 rate is only one-tenth the slope of the curve at its origin).
1. The total fishing gear in use for a specified period of time. When two or more kinds of gear are used, they must be adjusted to some standard type. 2. Effective fishing effort.
1. Effective fishing effort. 2. Fishing effort per unit area 3. Effectiveness of fishing.
Deaths in a fish stock caused by fishing.
The catch which a particular gear or vessel takes from a given density of fish during a certain time interval. For example, larger vessels (horsepower) have a greater ability to catch more fish, thus the greater their fishing power. Also, improvements in a vessel or gear, such as fish finders, Loran, etc., can increase fishing power.
The rate of fishing mortality for a given exploitation pattern rate of growth and natural mortality, that results in the maximum level of yield per recruit. This is the point that defines growth overfishing.
The rate of fishing, as indicated by an equilibrium yield-per-recruit curve, greater than which the losses in weight from total mortality exceed the gain in weight due to growth. This point is defined as Fmax.
Instantaneous Rate Of Growth
The natural logarithm of the ratio of final weigl1t to initial weight of a fish in a unit of time, usually a year. When applied collectively to all fish of a given age in a stock, the possibility of selective mortality must be considered .
Instantaneous Rate Of Mortality
The natural logarithm (with sign changed) of the survival rate. The ratio of number of deaths per unit of time to population abundance during that time, if all deceased fish were to be immediately replaced so that population does not change. Also called: *coefficient of decrease.
Instantaneous Rate Of Recruitment
Number of fish that grow to catchable size per short interval of time, divided by the number of catchable fish already present at that time. Usually given on a yearly basis: that is, the figure just described is divided by the fraction of a year represented by the 'short interval' in question. This concept is used principally when the size of the vulnerable stock is not changing or is changing only slowly, since among fishes recruitment is not usually associated with stock size in the direct way in which mortality and growth are.
An arrangement of recorded lengths which indicates the number of times each length or length interval occurs. For example, 10 measurements of lengths are taken in the following order: 10, 12, 12, 14, 12, 15, 15, 19, 12, and 10. A length frequency would be:
Length Occurrence Interval Occurrence
11 0 10-12 6
12 4 13-15 3
13 0 or 16-18 0
14 1 19-21
Long-term Potential Catch
The largest annual harvest, in weight, that can be removed from a fish stock year after year, under existing environmental conditions. This can be estimated in various ways, from maximum values from production models to average observed catches over a suitable period of years.
The largest catch that can be maintained from the population, at whatever level of stock size, over an indefinite period. It will be identical to the sustainable yield for populations below the level giving the MSY, and equal to the MSY for populations at or above this level.
Maximum Sustainable Yield (MSY)
The largest average catch or yield that can continuously be taken from a stock under existing environmental conditions. (For species with fluctuating recruitment, the maximum might be obtained by taking fewer fish in some years than in others.) Also called: maximum equilibrium catch ; maximum sustained yield; sustainable catch.
Mortality rates are critical for determining the abundance of fish populations and the effects of harvesting strategies on yield and spawning potential from a stock. Fish abundance is a balance between the factors that act to increase the stock (births) and factors that decrease population numbers (deaths). When births exceed deaths, the stock increases, and vice-versa. The stock is brought into stability when the number of recruits entering the fishery balances the number of deaths. Fishery managers can control deaths caused by fishing by manipulating the sizes of fish vulnerable to the gear. Fishing mortality can be changed through indirect methods, such as regulating mesh size to make fish of certain ages less vulnerable to the gear. Direct control measures, such as catch quotas or effort limits, determine the rate of fishing mortality on the vulnerable sizes. The total number of births is determined by the abundance of breeders in the population the spawning stock which can also be manipulated by managers.
Natural Mortality (M)
Deaths in a fish stock caused by predation, pollution, senility, etc., but not fishing.
Net Increase (or Decrease)
New body substance elaborated in a stock, less the loss from all forms of mortality.
The sum of the catches that are landed (expressed as live weight or equivalents). Nominal catches do not include unreported discards.
Optimum Yield (OY)
The yield from a fishery which provides the greatest overall benefit to the nation with particular reference to food production and recreational opportunities; it is based on MSY as modified by economic, social or ecological factors. Precision and Accuracy Precision is the closeness to each other of repeated measurements of the same quantity or object, while accuracy is closeness of a measured or computed value to its true value.
A 'constant' or numerical description of some property of a population (which may be real or imaginary). Cf. statistic.
Fish that spend most of their life swimming in the water column as opposed to resting on the bottom are known as pelagic species.
Individual items, as in the expression 'two dollars a piece' . Individual fish.
1. The total elaboration of new body substance in a stock in a unit of time, irrespective of whether or not it survives to the end of that time. Also called: *net production ; *total production. 2. *Yield.
A portion of a total allowable catch (TAC) allocated to an operating unit, such as a vessel class or size or a country.
Rate Of Exploitation
The fraction, by number, of the fish in a population at a given time, which is caught and killed by man during the year immediately following . The term may also be applied to separate parts of the stock distinguished by size, sex, etc. Also called: *fishing coefficient .
Rate Of Removal
An inexactly-defined term that can mean either rate of exploitation or rate of fishing--depending on the context .
Rate Of Utilization
Similar to rate of exploitation, except that only the fish landed are considered. The distinction between catch and landings is important when considerable quantities of fish are discarded at sea.
The amount of fish added to the exploitable stock each year due to growth and/or migration into the fishing area. For example, the number of fish that grow to become vulnerable to the fishing gear in one year would be the recruitment to the fishable population that year. This term is also used in referring to the number of fish from a year class reaching a certain age. For example, all fish reaching their second year would be age 2 recruits.
The rate of fishing above which the recruitment to the exploitable stock becomes significantly reduced. This is characterized by a greatly reduced spawning stock, a decreasing proportion of older fish in the catch, and generally very low recruitment year after year.
An estimate of actual or absolute abundance; usually stated as some kind of index; for example, as bottom trawl survey stratified mean catch per tow.
A proportion or a segment of a fish stock which is removed for study, and is assumed to be representative of the whole. The greater the effort, in terms of both numbers and magnitude of the samples, the greater the confidence that the information obtained is a true reflection of the status of a stock (level of abundance in terms of numbers or weight, age composition, etc.)
Spawning Stock Biomass (SSB)
The total weight of all sexually mature fish in the population. This quantity depends on year class abundance, the exploitation pattern, the rate of growth, fishing and natural mortality rates, the onset of sexual maturity and environmental conditions.
The procedure of maintaining methods and equipment as constant as possible. Without standardization one cannot determine whether measurements of yearly differences in relative abundance are caused by actual fluctuations in stock abundance or by differences in the measurement procedure used. The lack of standardization is one reason why surveys using different commercial fishing vessels in different years do not produce comparable information. For example, if two vessels of different horsepower are used in separate years, the results can't be compared, unless vessel mensuration experiments are performed. This would involve a comparison of the two vessels' catches to determine the influence of their fishing power on the size of the catch, and a determination of a correction factor.
Status of Exploitation
An appraisal of exploitation is given for each stock discussed in the Species Synopsis section using the terms unknown, protected, not exploited, underexploited, moderately exploited, fully exploited, and over-exploited. These terms describe the effect of current fishing effort on each stock, and is based on current data and the knowledge of the stocks over time.
A part of a fish population usually with a particular migration pattern, specific spawning grounds, and subject to a distinct fishery. A fish stock may be treated as a total or a spawning stock. Total stock refers to both juveniles and adults, either in numbers or by weight, while spawning stock refers to the numbers or weight of individuals which are old enough to reproduce.
Success (of fishing)
Catch per unit of effort.
Production of new weight by a fishable stock, plus recruits added to it, less what is removed by natural mortality. This is usually estimated as the catch in a given year plus the increase in stock size (or less the decrease). Also called: natural increase, sustainable yield, equilibrium catch .
Number of fish alive after a specified time interval, divided by the initial number. Usually on a yearly basis.
The number or weight of fish in a stock that can be taken by fishing without reducing the stock biomass from year to year, assuming that environmental conditions remain the same.
Total allowable catch is the total regulated catch from a stock in a given time period, usually a year.
The number or weight of all fish in a stock that lie within the range of sizes customarily considered usable (or designated so by law). Also called: *standing crop.
Utilized Stock, Utilized Population
The part, by number, of the fish alive at a given time, which will be caught in future.
Commercial fishing vessels are classified according to their gross registered tons (grt) of displacement. Vessels displacing less than 5 tons are not routinely monitored, and are referred to as undertonnage. Larger vessels are classified as follows:
Vessel Class GRT
2 5 - 50
3 51 - 150
4 151 - 500
Virtual PopulationYear Class (or Cohort)
Fish in a stock born in the same year. For example, the 1987 year class of cod includes all cod born in 1987, which would be age 1 in 1988. Occasionally, a stock produces a very small or very large year class which can be pivotal in determining stock abundance in later years.
Yield per Recruit
The expected lifetime yield per fish of a specific age (e.g., per age 2 individual). For a given exploitation pattern, rate of growth, and natural mortality, an expected equilibrium value of Y/R can be calculated for each level of F.