Figure 4 |
Figure 5 |
As an illustration, (Figure
5) shows the relationship between fishing
mortality and biomass, relative to status determination criteria.
The vertical lines representing ½ BMSY and BMSY
and the horizontal line representing FMSY form a
grid indicating resource status. If the status determination
criteria for a stock were ½ BMSY for the MSST
and FMSY for the MFMT, the stock in Sector A, with
biomass below ½ BMSY and F above FMSY
would be classified as "overfished" (B is below BTHRESHOLD)
, and "overfishing is occurring" (F is above FTHRESHOLD).
In this case the FMC would be required to develop a rebuilding
plan and an F reduction plan within one year. A stock falling
in Sector B (biomass is below ½ BMSY and F is
below FMSY) would again be classified as "overfished"
although here "overfishing is not occurring" (F is
below FTHRESHOLD). In this case only a rebuilding
plan would be mandated. A stock falling in Sector C, between
½ BMSY and BMSY and above FMSY,
is not "overfished" (B is above BTHRESHOLD),
although "overfishing is occurring" (F is above FTHRESHOLD),
requiring an F reduction. Finally, for a stock for which B is
between ½ BMSY and BMSY, and F is
below FMSY (Sector D) no action is required; however,
since stocks are intended to be managed to produce MSY, the
FMCs must pursue management policies to rebuild the stocks to
and maintain them at BMSY in such cases.
PATHWAYS
OF ASSESSMENT ADVICE
Stock
assessments and related analyses and documentation are sometimes
provided directly to the Councils through Scientific and Statistical
Committee meetings or to ASMFC via section meetings. Increasingly,
however, managers are depending upon the Northeast Regional
Stock Assessment Workshop (SAW) process for assessment advice.
The
SAW originated in 1985 as a vehicle for in house or local peer
review of stock assessments and related research. As the condition
of fishery resources in the Northeast deteriorated and pressure
for assessment and management advice intensified, the SAW evolved
into an intensive biannual review process involving four components:
a Steering Committee, to oversee the process and determine priorities;
working groups, responsible for completion of stock assessments
and working papers; a Stock Assessment Review Committee (SARC)
that reviews assessments and prepares management advice; and
a Public Review Workshop that presents SARC reports and advice
at meetings of the New England and Mid Atlantic Councils. SARC
membership was structured to include experts from the NEFSC
and other NMFS Centers, the Councils and ASMFC, state agencies
and academic institutions, and Canada; and all SAW related meetings
and workshops are open to participation by industry representatives
and other interested parties. Since 2004, the SARC has consisted
of a small panel of independent experts who convene to review
the scientific merit of the assessment analyses and determine
whether the assessments are sufficiently rigorous to form the
basis of management advice. The SAW has been very effective
in generating high quality assessment advice while enhancing
the credibility of this advice through intensive peer review
and participation by fisheries scientists, industry and the
general public.
Since
2002, assessments of the 19-20 large mesh groundfish stocks
managed by the New England Fishery Management Council under
the Northeast Multi-Species Fishery Management Plan have been
assessed on a three year cycle at Groundfish Assessment Review
Meetings (GARM).
One
additional pathway has recently become available, the
Transboundary Resource Assessment Committee (TRAC) process
for completion and peer-review of assessments for transboundary
resources shared by Canada and the United States. The
two countries have cooperated closely for many years
in collecting and sharing data, preparation of joint
assessments and peer review activities; and in recent
years it has become obvious that a formal cooperative
arrangement would promote efficiency and consistency
in reporting. As a result, a unified process was started
in 1998. This consists of both periodic assessment benchmark
meetings and annual assessment update meetings. The
benchmarks take place off the management cycle and perform
in-depth model formulation and model comparison reviews.
The annual updates apply the benchmark standards in
the updated assessments and forward consensus-based
reports for use in resource management by both countries.
To date, the TRAC process has been used only for Georges
Bank stocks of cod, haddock and yellowtail, as well
as the Georges Bank/Gulf of Maine Atlantic herring complex
and it provides a logical option for other transboundary
resources.
DEFINITIONS OF TECHNICAL
TERMS
[ Return to Scup ]
Assessment
terms used throughout this document may not be familiar to all.
A brief explanation of some of these terms follows, organized
alphabetically.
Assessment
level: Categories of the level of complexity of each
assessment included in this document are as given above (Index,
Surplus production, Yield per recruit, and Age/size structured).
The latter may include projections of future catch and stock
sizes or modeling of relationships between recruitment and spawning
stock size.
BMSY:
The long-term average stock biomass level required to achieve
Maximum Sustainable Yield or MSY, when the stock is fished at
FMSY. Biomass is usually measured in terms of metric
tons (mt).
BTHRESHOLD
(Minimum stock size threshold or MSST): One of two
Status Determination Criteria, specified in the national standard
guidelines as the greater of (a) ½ BMSY, or
(b) the minimum stock size at which rebuilding to BMSY
will occur within 10 years when fishing at the Maximum Fishing
Mortality Threshold or MFMT. At stock sizes below BTHRESHOLD,
the stock is considered to be overfished.
Biological reference
points: Specific values for variables that describe
the state of a fishery, used to evaluate its status. These may
include "target" reference points, corresponding to
a desired goal or level and "limit" reference points
or "thresholds" carrying an unacceptably high risk
to the stock if exceeded. Examples are F0.1, FMSY,
FTHRESHOLD, and Fmax.
Biomass-weighted F: An estimate of fishing
mortality in which F for each age group in the stock, as determined
from virtual population analysis or VPA, is weighted by corresponding
stock biomass-at-age values. This calculation is needed to make
F from age structured assessments comparable to FMSY
estimates obtained from surplus-production (e.g. ASPIC) modeling
of all components in the stock.
Catch per unit
effort (CPUE): A measure of relative success of fishing
operations, often used as a proxy for relative abundance under
the assumption of a linear relation to stock size. To be valid,
should be standardized to account for differences in fishing
power or temporal/spatial changes in catchability.
Control rule (MSY Control Rule): A protocol
for specifying harvest rates in relation to stock status and
limit and target reference points. Technically, a harvest strategy
which, if implemented, would be expected to result in a long-term
average catch approximating MSY.
Exploitation pattern: The distribution of fishing
mortality over the age composition of the fish population, determined
by the type of fishing gear, areal 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 (or annual fishing mortality rate) would be 0.72. Note
that this rate cannot exceed unity; obviously, more fish cannot
die than were originally present.
Fishing
mortality rate (F): That part of the total mortality
rate applying to a fish population that is caused by fishing.
Fishing mortality is usually expressed as an instantaneous rate,
as discussed under Mortality rates, and can range to values
exceeding unity, such as 2.0 or higher.
Fmax: The fishing mortality rate
that results in the maximum level of yield per recruit. This
is the point that defines growth overfishing.
Fmed:
The fishing mortality rate at which recruitment balances removals
over time, as estimated from stock recruitment data.
FMSY:
The fishing mortality rate that produces MSY by taking a constant
fraction of a stock that is fluctuating around BMSY.
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 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 one tenth
the slope of the curve at its origin).
F20%:
The fishing mortality rate at which spawning per recruit (usually
using spawning biomass per recruit as a proxy) is reduced to
20% of the unfished level. Other levels may be used depending
on biological characteristics of the target species and/or management
objectives.
FREBUILD:
The fishing mortality rate(s) that, when applied over a specified
time frame, will result in stock biomass increasing to BMSY
with some specified probability level).
FTARGET:
The fishing mortality rate which (with some specified probability
level) will prevent FTHRESHOLD from being exceeded.
FTHRESHOLD
(Maximum fishing mortality threshold or MFMT): One
of two Status Determination Criteria specified in the national
standard guidelines as the fishing mortality rate associated
with the MSY Control Rule. Usually, FTHRESHOLD is
FMSY if stock biomass is moderate to high, and a
lower value if it is low. Exceeding FTHRESHOLD for
one year or more constitutes overfishing.
Fully-recruited
F: An estimate of fishing mortality for all age groups
fully vulnerable to fishing. It may or may not be weighted by
population size in number.
Growth overfishing: The rate of fishing, as
indicated by a yield-per-recruit curve, greater than that at
which the loss in weight from total mortality equals the gain
in weight due to growth. This point is defined as Fmax.
Maturation:
Reported in this document wherever possible as median length
or age at maturity (L50 or A50) as determined
from length and age-specific maturation ogives.
Maximum sustainable yield (MSY): The largest
long-term average catch or yield that can be taken from a stock
or stock complex under prevailing ecological and environmental
conditions. In order to achieve the maximum average yield over
the long term it will usually be necessary to vary annual yields
in response to natural fluctuations in stock size (e.g. by applying
a constant fishing mortality rate of FMSY).
Maximum
spawning potential reference points: Reference points
based on some fraction of maximum spawning potential (MSP) as
determined from spawning stock biomass per recruit models, used
to define overfishing. MSP is the spawning stock biomass per
recruit in the absence of fishing; it is then reduced to a percentage
of the maximum as F increases. If the %MSP level is reduced
below the overfishing definition level, then the stock is considered
to be overfished. This level is typically determined by stock-recruitment
modeling, to determine the fishing mortality rate beyond which
the stock will be unable to replace itself.
Mean
biomass: The product of mean abundance (numbers) and
the average weight of individual fish. Mean abundance is calculated
from abundance at the beginning of the year and the annual mortality
rate, while average weights are derived from population size
and weight at age data. Mean abundance and biomass are usually
calculated for each age group separately and then summed to
estimate the mean biomass of the population.
Mortality rates:
The rates at which fish die from fishing and/or natural causes.
Mortality rates can be described in several ways.
One
conceptually simple approach is to express mortality on an annual
basis, i.e., A, the annual mortality rate, expressed
as a proportion (5% or 0.05 per year). This is the fraction
of the population alive at the beginning of the year which dies
during the year. The survivors may be represented by (1-A) =
S, the annual survival rate.
In
exploited populations, however, it is important to account for
both fishing and natural mortality. This can pose complex problems,
because population changes tend to be exponential; and different
components tend to be multiplicative (that is, in any given
period of time, individuals that die from natural causes would
otherwise be killed by fishing and vice versa).
For
these reasons, biologists tend to work with instantaneous
rates, in which time intervals are sufficiently short so as
to allow separation of the primary components as instantaneous
fishing mortality (F) and instantaneous natural mortality (M).
Together the two are equivalent to instantaneous total mortality
(Z), i.e. Z = F+M.
The
necessary mathematics are based on a logarithmic scale which
relates well to biological processes (since they tend to be
exponential); and effects which are multiplicative in nature
become additive on a logarithmic scale.
The concept of instantaneous
rates can be illustrated by a simple example. Imagine a year
of a fish’s life to be divided into a large number (n)
of equal time intervals, and Z/n is the number dying within
that interval. If n = 1,000 and Z =1.0, then during the first
time interval 1/1000 = 0.1% of the population dies. For a population
of 1,000,000 fish, 1000 would die, leaving 999,000 survivors.
In the next time interval 0.1% of 999,000 fish, or 999 fish
die, leaving 998,001 survivors, and so on. Repeated 1,000 times,
we would have:
| 1,000,000 (1-
0.0010)1000 = 367,695 survivors |
Or, we may use the relation:
| S = e -Z
= 0.3679 (1,000,000) = 367,879 survivors |
where
e is the base of natural logarithms (2.71828).
The
calculation provides the same approximate result. Note that
the annual mortality rate A = 1-e -Z, hence, 1-0.3679
or 0.6321 or 63% in our example. Again, A can never exceed
unity, although F and Z can considerably exceed unity for
heavily exploited stocks.
The utility of instantaneous rates for dealing with different
sources of mortality over time can be illustrated as follows.
Assume a population at the beginning of a given year consists
of 1,000 fish, and that during the year it is subjected to
an instantaneous fishing mortality rate of F = 0.5, while
instantaneous natural mortality (M) = 0.2. The instantaneous
total mortality rate (Z) is equal to (F+M) = 0.7. Removals
by fishing are calculated by applying the annual exploitation
rate.
| |
F(1-e -Z)
Z |
| = |
0.5 (1-e-0.7
)
0.7 |
| = |
0.3596 |
During
the year, 0.3596(1000) = 360 fish are caught, and S = e -0.7
= 0.4966(1000) = 497 fish survive. The difference from the
original number of 1,000 fish (1,000-360-497), or 143 fish,
is the number dying from natural causes. The additive property
of instantaneous rates allows us to obtain approximately the
same result for natural mortality, i.e.,
| |
M(1-e-Z
)
Z |
| =
|
0.2
(1-e-0.7 )
0.7 |
| =
|
0.1438, or,
144 fish. |
In
the absence of fishing this number would be A = (1-e -0.2)1000
= 0.1813 x 1000 = 181 fish, with 819 fish surviving to the
beginning of the following year. If the process is continued
for another year, the catch in the exploited population would
be 179 fish, 71 fish would die from natural causes, and 247
fish would survive, while in the unfished population 149 fish
would die, leaving 670 survivors. Continued for 10 years the
exploited population would be essentially eliminated (1 surviving
fish) whereas 14% of the unfished population (135 fish) would
survive.
This
example uses an annual exploitation rate (36%) for the exploited
population that is somewhat high but was sustained historically
by some Northeast stocks. For some heavily fished stocks exploitation
rates have in some years exceeded 50-60 percent. The number
of fish alive after 5 years from a year class of 1,000,000
fish exploited at F=1.0 and M=0.2 (58% exploitation rate)
would be:
1,000,000
[e-1.2 x 5] = 2,478 fish! |
Natural
mortality rate (M): That part of total mortality
applying to a fish population that is caused by factors other
than fishing. It is common practice to consider all sources
together since they usually account for much less than fishing
mortality. It is usually expressed as an instantaneous rate
as discussed above.
Nominal
catch: The sum of the catches that are landed (expressed
as live weight or equivalent). Nominal catches do not include
discards.
Optimum yield (OY): The amount of fish that
will provide the greatest overall benefit to the nation, particularly
with respect to food production and recreational opportunities
and taking into account the protection of marine ecosystems.
Under the Magnuson-Stevens Fishery Conservation and Management
Act or MFCMA, OY cannot exceed MSY.
Overfishing/overfished:
According to the National Standard Guidelines, "overfishing
occurs whenever a stock or stock complex is subjected to a
rate or level of fishing mortality that jeopardizes the capacity
of a stock or stock complex to produce MSY on a continuing
basis." Overfishing is occurring if FTHRESHOLD
is exceeded for a year or more. An "overfished"
stock has been reduced below BTHRESHOLD requiring
management actions to rebuild to the MSY level within an acceptable
time frame.
Overfishing
definition: An objective and measurable guideline
or guidelines for a given stock defining a fishing mortality
rate that constitutes overfishing, and/or the point at which
the stock reaches an overfished condition; formerly required
for each fishery management plan or FMP under revised guidelines
(50 CFR Part 602) to National Standards 1 and 2 of the Magnuson
Fishery Conservation and Management Act or MFCMA. Reauthorization
of the Act (Magnuson-Stevens Fishery Conservation and Management
Act or MSFCMA) under the Sustainable Fisheries Act or SFA
resulted in a requirement for status determination criteria
to describe both overfishing and the condition of being overfished.
Quota:
A portion of a total allowable catch (TAC) allocated to an
operating unit, such as a vessel size class or a country.
Recruitment:
The amount of fish added to the exploitable stock each year
due to growth and/or migration into the fishing area. The
number of fish that grow to become vulnerable to the fishing
gear in a given year would be the recruitment to the fishable
population in that year. The term is also used in referring
to the number of fish reaching a certain age or size.
Recruitment
overfishing: The rate of fishing above which 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.
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 per 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.
Statistical
Catch at Age Model: An approach to estimating population
abundance and fishing mortality rates from catch at age data
and indices of abundance. These models start at the earliest
point of available data and project forward through the time
series. Fishing mortality is separated into age and year components.
The models change parameters such as recruitment, fishing
mortality by year, selectivity at age, and catchability coefficients
until the predicted catch and indices most closely match the
observed values. In essence, statistical catch at age models
create a simulated population using a set of parameters that
generates predicted values. The parameters are changed until
the predicted values most closely match the observed values.
There is a great deal of flexibility in the approach because
missing data are easily handled and many types of data can
be matched by the simulated population.
Status
Determination Criteria: Objective and measurable
criteria used to determine if overfishing is occurring or
the stock is in an overfished state according to National
Standard Guidelines.
TAC:
Total allowable catch is the total regulated catch from a
stock in a given time period, usually a year.
Vessel class:
Commercial fishing vessels are classified according to their
gross registered tons (grt) of displacement. Vessels displacing
less than 5 tons were not routinely monitored prior to the
new mandatory reporting system implemented in the Northeast
in 1994, and were referred to as undertonnage. The current
classification scheme is as follows:
| Vessel Class |
GRT |
| 1 |
<5 |
| 2 |
5 - 50 |
| 3 |
51 - 150 |
| 4 |
151+ |
Virtual
population analysis (or cohort analysis): An analysis
of the catches from a given year class over its life in the
fishery. If 10 fish from the 1988 year class were caught each
year for 10 successive years from 1990 to 1999 (age 2 to age
11), then 100 fish would have been caught from the 1988 year
class during its life in the fishery. Since 10 fish were caught
during 1999, then 10 fish must have been alive at the beginning
of that year. At the beginning of 1998, there must have been
at least 20 fish alive because 10 were caught in 1998 and
10 more were caught in 1999. By working backward year by year,
one can be virtually certain that at least 100 fish were alive
at the beginning of 1990.
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, 1999), 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 (1999), 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., for 1990-1996 or
1997 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 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 2005 year class of cod includes all
cod born in 2005, which would be age 1 in 2006. 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 for a 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.
[ Return to Scup ]
For further
information
Anon. 1998a. Improving Fish Stock Assessments. Committee on
Fish Stock Assessment Methods. Ocean Studies Board. National
Research Council. National Academy Press, Washington, D.C.,
177 p.
Anon.
1998b. Review of Northeast Fishery Stock Assessments. Committee
to Review Northeast Fishery Stock Assessments. Ocean Studies
Board. National Research Council. National Academy Press,
Washington, D.C., 128 p.
Applegate,
A., S. Cadrin, J. Hoenig, C. Moore, S. Murawski and E. Pikitch.
1998. Evaluation of existing overfishing definitions and recommendations
for new overfishing definitions to comply with the Sustainable
Fisheries Act. Final Report of the Overfishing Definition
Review Panel to the New England Fishery Management Council.
179 p.
Gabriel,
W.L. and P.M. Mace. 1999. A review of biological reference
points in the context of the precautionary approach, p. 34-45.
In: Restrepo, V.R. ed. Proceedings of the Fifth National NMFS
Stock Assessment Workshop. NOAA [National Oceanic and Atmospheric
Administration] Tech. Memo. NMFS-F/SPO-40. 161 p.
Gabriel,
W.L., M.P. Sissenwine, and W.J. Overholtz. 1989. Analysis
of spawning stock biomass per recruit: an example for Georges
Bank haddock. N. Am. J. Fish. Mgmt. 9: 383 391.
Gulland,
J.A., ed. 1988. Fish population dynamics. John Wiley &
Sons, Inc., New York. 422 p.
Hillborn,
R. and C.J. Walters. 1992. Quantitative fisheries stock Assessment:
choice, dynamics and uncertainty. London: Chapman and Hall.
570 p.
O’Brien,
L., J. Burnett, and R.K. Mayo. 1993. Maturation of 19 species
of finfish off the northeast coast of the United States, 1985-
1990. NOAA [National Oceanic and Atmospheric Administration]
Tech. Rept. NMFS 113. 66 p.
Quinn,
T. J. and R.B. Deriso. 1999. Quantitative fish dynamics. Oxford
University Press, Inc., New York. 542 p.
Restrepo,
V.R., G.G. Thompson, P.M. Mace, W.L. Gabriel, L.L.Low, A.D.
MacCall, R.D.Methot, J.E. Powers, B.L. Taylor, P.R. Wade,
and J.F. Witzig. 1998. Technical guidance on the use of precautionary
approaches to implementing National Standard 1 of the Magnuson-Stevens
Fishery Conservation and Management Act. NOAA [National Oceanic
and Atmospheric Administration] Tech. Memo. NMFS-F/SPO-31.
54 p.
Restrepo,
V. R., P. M. Mace, and F. Serchuk. 1999. The precautionary
approach: a new paradigm or business as usual? P 61-70. In:
NMFS. 1999. Our living oceans. Report on the status of U.S.
living marine resources, 1999. U.S. Dep. Commer., NOAA Tech.
Memo. NMFS-F/SPO-41, 301 p.
Rosenberg, A., convener.
1994. Scientific review of definitions of overfishing in U.S.
fishery management plans. NOAA [National Oceanic and Atmospheric
Administration] Tech. Memo. NMFS F/SPO 17. 205 p.
