Absolute Recruitment: The number of fish which grow into the
catchable size range in a unit of time (usually a year).
Abundance Index: Information obtained from
samples or observations and used as a measure of the weight or number
of fish which make up a stock.
Age: 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+).
Annual (or seasonal) Total Mortality Rate: The number of fish which
die during a year (or season), divided by the initial number. Also
called: actual mortality rate, *coefficient of mortality .
Availability: 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.
Assessment Level: 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).
Biomass: 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.
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.
Catchability : 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
Catch Curve: 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 .
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.
Critical Size: 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.
Equlibrium Catch: 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.
Equlibrium Yield: 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.
Exploitation pattern: 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.).
Exploitation rate: 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.
Fishing Effort: 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.
Fishing Intensity: 1. Effective fishing effort. 2. Fishing effort per
unit area 3. Effectiveness of fishing.
Fishing Mortality: Deaths in a fish stock caused by fishing.
Fishing Power: 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.
Fmax: 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.
F0.1: 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).
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 Fishing Mortality: When fishing and natural
mortality act concurrently, F is equal to the instantaneous total
mortality rate, multiplied by the ratio of fishing deaths to all
deaths. Also called: rate of fishing; instantaneous rate of fishing;
*force of fishing mortality .
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 Natural Mortality: When natural and fishing
mortality operate concurrently it is equal to the instantaneous total
mortality rate, multiplied by the ratio of natural deaths to all
deaths. Also called: *force of natural mortality .
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.
Instantaneous Rate Of Surplus Production: Equal to rate of growth plus
rate of recruitment less rate of natural mortality--all in terms of
weight and on an instantaneous basis. In a "balanced" or equilibrium
fishery, this increment replaces what is removed by fishing, and rate
of surplus production is numerically equal to rate of fishing. Also
called: *instantaneous rate of natural increase.
Length Frequency: 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
Length Occurrence Interval Occurrence
10 2
11 0 10-12 6
12 4 13-15 3
13 0 or 16-18 0
14 1 19-21
15 2
16 0
17 0
18 0
19
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.
Maintainable Yield: 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: 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.
Mortality occurs at all life stages of the population.
Depending on the species, mortalities suffered from the egg to
larval stages are usually very high, less so from the larval to
juvenile stage. In young fish, death can occur from several
causes: starvation, predation, or disease. If fish survive their
first year, these natural causes of death usually decline
dramatically, and in many cases, fishing becomes the dominant
source of mortality. Pollution may also add to the death rate of
the population. Generally, young fish are more vulnerable to
pollution mortalities than are older fish.
Knowing the sources and levels of mortalities affecting fish
populations is a critical ingredient of forecasting both landings
and spawning stock sizes, and in evaluating the changes in
populations that may be induced by regulations such as minimum mesh
sizes, minimum fish lengths, quotas, effort limits, and area
closures. The rate at which the stock is harvested is usually
estimated by calculating the abundance of a cohort or
year class over successive years to determine how rapidly it is declining.
The total mortality of the population is the sum of deaths due to
both natural and fishing-related causes.
The fishing mortality rate (F) on each age group of a stock is
determined by two factors: (1) the proportion of that age group
that is big enough to be captured by the gear (usually termed the
partial recruitment of each age), and (2) the overall amount of
fishing effort on the stock. At intermediate stock abundance
levels, the relationship between effort and F is direct. A doubling
of effort translates into a doubling of the fishing mortality rate.
At very low or very high stock sizes (when the stock is either hard
to locate or unavoidable), the relation between effort and F may
change.
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.
Nominal Catch: 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.
As an illustration, suppose regulations state that fishermen can
only land 5,000 lbs. of cod per trip. A fisherman makes 10 trips with
the intent to not exceed the 5,000 lbs. limit. Before landing each
trip, he estimates that his total catch for each was approximately
5,000 lbs. However, after landing each trip, weigh-in's at a dealer
showed that every catch was just about 5,500 lbs. The fisherman's
estimates of his catch were, therefore, precise but not accurate.
Two fictional series of research vessel tows were made in a single
stratum. The first series resulted in catches of 61, 55, 60, 64, 63 and
59 pounds. The second resulted in 10, 20, 45, 60, 110 and 115 pounds.
Both resulted in mean catches per tow of 65 lbs. The first series of
tows is a very precise estimate of abundance while the estimate of the
second series is very imprecise. The range of values about the average
in the first series (55-64) is much narrower than that of the second
(19-115); therefore, confidence in the first average as an estimate of
relative abundance is much greater than confidence in the second
average. Nothing can be stated about the accuracy of either of the
series of tows in providing estimates of true abundance. The degree of
accuracy is affected by fish behavior, gear performance, and a possible
mismatch between the timing and area of surveys in relation to fish
movements and distribution.
Parameter: A "constant" or numerical description of some property of a
population (which may be real or imaginary). Cf. statistic.
Pelagic: Fish that spend most of their life swimming in the water column as
opposed to resting on the bottom are known as pelagic species.
Pieces: Individual items, as in the expression "two dollars a piece" .
Individual fish.
Production: 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.
Quota: 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.
Recruitment: 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.
Recruitment Curve, Reproduction Curve: A graph of the
progeny of a spawning at the time they reach a specified age (for
example, the age at which half of the brood has become vulnerable to
fishing), plotted against the abundance of the stock that produced
them.
Recruitment overfishing: 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.
Relative Abundance: 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.
Sample: 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.)
Standardization: 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.
Stock: 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.
Stratified Mean (Average) Catch Per Tow: From
research vessel surveys, for separate
species of fish, each average catch per tow --determined from a series
of tows--in each geographic stratum of a region is multiplied by that
area (square nautical miles) of the stratum in which the tows were
made. All of the individual products are added together and the total
is divided by the sum of the entire area of the region. The final
result is the stratified mean catch per tow; this is used as an index
of relative abundance. For example, a scientist wishes to calculate the
stratified mean catch per tow of cod in a region (perhaps Georges Bank)
that measures 100 square nautical miles. The region has been divided
into 5 strata on the basis of depth. In each stratm, 5 tows are made
and the average catch of cod is calculated.
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.
Spawning Stock Biomass pe Recruit (SSB/R): The expected lifetime
contribution to the spawning stock biomass for a recruit 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 SSB/R can be calculated for each level of F.
A useful reference point is the level of SSB/R that would be
realized if there were no fishing. This is a maximum value for
SSB/R, and can be compared to levels of SSB/R generated under
different rates of fishing. For example, the maximum SSB/R for
Georges Bank haddock is approximately 9 kg for a recruit at age 1.
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.
Success (of fishing): Catch per unit of effort.
Surplus Production: 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 .
Survival Rate: Number of fish alive after a
specified time interval, divided by the initial number. Usually on a
yearly basis.
Sustainable Yield: 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.
TAC: Total allowable catch is the total regulated catch from a
stock in a given time period, usually a year.
Usable Stock: 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.
Vessel Class: 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 Population: Utilized stock.
Virtual Population Analysis (or Cohort Analysis) (VPA): An analysis of
the catches from a given year class over its life in the fishery.
If 10 fish from the 1968 year class were caught each year for 10
successive years from 1970 to 1979 (age 2 to age 11), then 100 fish
would have been caught from the 1968 year class during its life in
the fishery. Since 10 fish were caught during 1979, then 10 fish
must have been alive at the beginning of that year. At the
beginning of 1978, there must have been at least 20 fish alive
because 10 were caught in 1978 and 10 more were caught in 1979. By
working backward year by year, one can be virtually certain that at
least 100 fish were alive at the beginning of 1970. A virtual
population analysis goes a step further and calculates the number
of fish that must have been alive if some fish also died from
causes other than fishing.
For example, if in addition to the 10 fish caught per year in the
fishery, the instantaneous natural mortality rate was also known then a
virtual population analysis calculates the number that must have been
alive each year to produce a catch of 10 fish each year plus those that
died from natural causes. If one knows the fishing mortality rate
during the last year for which catch data are available (in this case,
1979), then the exact abundance of the year class can be determined in
each and every year. Even when an approximate fishing mortality rate
is used in the last year (1979), a precise estimate of the abundance
can usually be determined for the stock in years prior to the most
recent one or two (e.g., 1970-1977 in the example).
Accuracy depends on the rate of population decline and the correctness
of the starting value of the fishing mortality rate (in the most recent
year). This technique is used extensively in fishery assessments since
the conditions for its use are so common; many fisheries are heavily
exploited, the annual catches for a year class can generally be easily
determined, and the natural mortality rate is known within a fairly
small range and is low compared with the fishing mortality rate.
Year 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.
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