Slide 1
Strategic Marine Alliance for Research and
Training
Digital Resources for Common Module in Offshore
Multidisciplinary Operations in Marine Science
Fisheries Science Operations at Sea
Dr Rick Officer ([email protected])
and
Dr Deirdre Brophy ([email protected])
Galway-Mayo Institute of Technology
Slide 2
SMART Common Blended Learning Module
Fisheries Surveys
Fisheries and Oceans Canada
www.dfo-mpo.gc.ca
Design &
Operation
Fisheries Science Operations at Sea
Dr Rick Officer
([email protected])
Dr Deirdre Brophy
([email protected])
Galway-Mayo Institute of Technology, Galway, Ireland
This online lecture will introduce you to the Design and Operation of Fisheries
Surveys. The lecture aims to give you a practical understanding of the:
Purpose of fisheries surveys,
Survey Design for fisheries assessment,
Survey Gears and Methods, and,
Data Processing and Analysis.
The design and operation of fisheries surveys are huge fields of scientific endeavour,
and a vast amount of information and literature exists on these subjects. This lecture
gives a basic introduction to the common themes and issues encountered when
designing and conducting fisheries surveys. A deeper understanding of the subject
will require independent investigation of the topics and links provided in the lecture.
Slide 3
Fisheries Surveys are required
for Fisheries Management
Surveys provide estimates of:
• Abundance,
• Mortality, and,
• Recruitment
These estimates are used to:
• Assess the status of fish stocks
• Evaluate species & ecosystem interactions
• Develop management measures
Purpose of fisheries surveys
Estimates of the abundance and dynamics of fish populations are basic requirements
for the management of fisheries resources. The demand for this information has
increased markedly in recent years as resource managers have broadened their
attention beyond targeted stocks and species. Increasing needs to consider
interacting species and the ecosystems in which fish stocks live have both increased
the demand for fisheries surveys, and the complexity of their design and operation.
Fisheries surveys have also become increasingly important due to difficulties with the
use of commercial fisheries catch and effort data as a basis for fisheries stock
assessment. Until the 1960s many stock assessments relied upon an assumption
that a simple linear relationship existed between catch per unit effort (CPUE) and
stock biomass (B):
where q is a coefficient of catchability (Gunderson, 1993). Continuing development
of the skill of commercial fishers increases the effectiveness of their fishing effort,
and invalidates this basic assumption. Fisheries surveys attempt to more reliably
estimate fluctuations in fisheries resources by maintaining constant catchability, and
by controlling and standardising their fishing effort.
Slide 4
Survey Planning Steps
1. Define objectives
2. Define the population to be sampled
3. Decide what data will be collected
4. Decide on the degree of precision required
5. Choose methods of measurement
6. Map the complete “frame” of sampling units
7. Select units to be sampled
8. Plan, organise and conduct the field work
9. Analyse the data
10. Revise the survey plan, conduct & analysis
Survey Planning Steps
Cochran (1977) and ICES (2004) describe in detail the steps involved in planning
and executing a survey. These steps provide a useful framework for introducing
fisheries surveys to those new to the topic, and describe the major aspects that
require our consideration. We’ll consider each step and focus our attention on those
of particular interest to seagoing and shore-going fisheries survey scientists.
Objectives: Clearly defining the objectives of fisheries surveys is of fundamental
importance. The assessment of fish stocks generally requires estimation of historic,
current and future abundance of fish so that appropriate management measures may
be implemented (ICES, 2004). Surveys therefore generally aim to estimate the
abundance of animals within a surveyed area.
Actual survey objectives are often expressed with greater detail and specificity than
this general aim, e.g: To detect changes in stock size over time, To detect changes
in the abundance of year classes or cohorts, To detect changes in spatial distribution
over time, To detect the abundance of the incoming year class (i.e. recruitment).
The high cost of ship-time often demands that ancillary objectives are also pursued.
Surveys therefore generally have ancillary biological objectives (such as sampling of
maturity, sex-ratio, weight, gut contents) or physical, chemical or geological
sampling objectives (e.g. temperature, nutrient distribution, or sediment type).
Slide 5
Survey Planning Steps
1. Define objectives
Bˆ = AD
2. Define the population to be sampled
The surveyed population must be:
Available
and
Vulnerable
Survey area
;
;
;
; ;
;
;
;
;
; ;
:
:
:
;
:
Sampling gear must have Known catchability
The surveyed sites must be Representative
These assumptions are difficult to satisfy
Defining the population: The general aim of fisheries surveys involves
extrapolating the number (or weight) of animals per unit area, observed using
particular sampling gear, to a larger, entire survey area (Gunderson, 1993). This
conceptual framework is essentially identical to that of quadrat sampling:
where
= biomass estimate, A = total survey area, and
= the
mean density (or mean weight) observed using the sampling gear.
This implicitly assumes that:
1. The entire population of surveyed species remain within the survey area, and are
available to be surveyed.
2. The surveyed species cannot avoid being surveyed.
3. The unit of area or volume sampled by the gear is known exactly, and,
4. The sampled sites are representative of the entire survey area.
These assumptions are very difficult to satisfy. Much of the work of survey design,
operation and analysis seeks to evaluate and/or overcome violations of these
assumptions.
Slide 6
Survey Planning Steps …
3. Decide what data will be collected
4. Decide on the degree of precision required
Abundance /
Biomass
Sorting by species
before Catch Weight
Estimation
Size
Structure
Measurement of fish for
Length Frequency
Estimation
Age
Profile
Growth
rings
Extraction of otoliths
(earstones) for Age
Estimation
Data to be collected: The data required are usually defined by the objectives of the
survey. As a minimum fish are usually sorted by species, then counted and/or
weighed within each sampling unit to determine abundance per unit area or time
sampled. The location of the sampling unit within the survey area is a critical piece of
information to be recorded and related to the sample data.
Usually the catch at each sampling station are sub-sampled for ancillary data such as
individual fish length, age (via sampling of structures such as otoliths) weight, sex,
reproductive status, and gut contents.
We’ll look at some of the data collected when we consider particular types of surveys
in more detail.
Degree of Precision required: The precision of surveys is determined by the
quality and quantity of sampling. Sample quality can be impaired by poor species
identification, inaccurate weight and abundance recording, inaccurate or biased
length estimation, and poor record keeping. Fisheries survey scientists employ
standardised sampling operation and data checking procedures to minimise the
chances of sampling errors occurring.
Precision is generally improved by increasing the number of samples taken. Methods
for evaluating the precision of survey data have undergone rapid development in
recent years. Statistical treatment of the data to minimise sampling variance in now
commonplace in post-survey data processing and analysis.
www.dfo-mpo.gc.ca
Fisheries and Oceans Canada
Slide 7
Survey Planning Steps …
Direct
counts
Acoustics
www.imares.wur.nl
North Sea herring acoustic survey
Egg and
larval
sampling
Trawl
http://tinyurl.com/Plankton-net
5. Choose methods
of measurement
Methods of measurement: Comprehensive overviews of the sampling equipment
and methods used in fisheries surveys are provided by Gunderson (1993). Survey
sampling equipment can be divided into four broad categories:
1. Trawl, including otter board and beam trawling methods that target bottom
dwelling (demersal) species (e.g. cod, haddock, whiting, plaice and sole) and midwater trawls that target pelagic species (e.g. mackerel and herring).
2. Acoustic, utilising echo sounders to identify fish shoals (particularly useful for
pelagic species).
3. Egg and Larval gear, utilising plankton nets and sampling equipment.
4. Direct counts, usually visual methods such as diver transect surveys or deployed
video systems.
We’ll explore the gear and methods used by each category in more detail later.
Common to each method is the use of standardised sampling devices (e.g. consistent
trawl size and type, defined sampling areas, consistent mesh size), and their
deployment using standardised procedures (consistent tow duration, fixed acoustic
sounding frequency and periods, fixed recording periods).
Slide 8
Survey Planning Steps …
6. Map the sampling “frame”
Marine Institute
Groundfish
Survey:
Trawl station
locations
Marine Institute
Herring
Acoustic
Surveys:
North West
Celtic Sea
Map the sampling frame: Mapping the potential distribution of sampling units
divides the area to be surveyed into units that will ideally cover the whole population
of interest without overlap (ICES, 2004).
For surveys deploying towed gears (e.g. trawl and eggs and larval surveys) the
sampling frame usually describes the locations of all possible non-overlapping tows,
each defined by a proscribed tow length, or, more commonly, a fixed tow duration
(ICES, 2004). Station locations on the Marine Institute’s groundfish survey are
confined to depths inhabited by the main species of interest and to areas of trawlable
bottom type.
Sampling frames for visual surveys similarly standardise the suite of locations
(usually referred to as “stations”) at which sampling may be undertaken for defined
periods.
Acoustic survey attempt to survey large areas in short time periods to obtain data on
spatial distribution as well as abundance (Jennings et al, 2001). Their sampling
frame therefore follows a defined scouting track, and may also include more
intensively surveyed tracks in regions of particular interest. The Marine Institute’s
herring acoustic surveys follow this design.
Slide 9
Survey Planning Steps …
7. Select units to be sampled
Systematic sampling:
Good for mapping
spatial distribution
But:
May give biased
estimates of the mean,
Ignores biogeographic
influences on species
distribution.
Random sampling:
Gives unbiased
estimates of the mean
But:
May be imprecise,
Ignores biogeographic
influences on species
distribution.
20m
Stratified random
sampling:
Utilises ancillary
information on
50m
biogeographic
influences on species
distribution (e.g.
100m
depth, habitat type).
Strata with high fish
density are sampled
more intensively.
Stations may be allocated randomly within
strata (e.g. : 20-50m strata), or
systematically (e.g. : 50-100m strata).
Stratified random sampling
increases precision for the
same level sampling effort
Select the units to be sampled: This is a critical element of the survey design.
Well informed and clever allocation of the sampling effort can improve the precision
of survey data and reduce biases.
Simple allocation of sampling locations in a fixed grid pattern or randomly within the
entire sampling frame may ignore patterns in the distribution of species and their
habitats, and therefore result in biased, imprecise survey data.
In many surveys prior knowledge of the habitat and depth preferences (and other
biogeographic factors) of species are used to identify areas of expected high and low
abundance (strata). Areas of high abundance are usually associated with high
variability. Sampling is allocated more intensively to strata with high abundance and
variability in an effort to improve the precision of the survey and reduce biases
(ICES, 2004). Stations may be allocated within strata randomly to reduce bias, or
systematically to identify distribution patterns within strata.
Resources spent on surveys are expensive so stratified random designs are a
sensible way to improve data quality for the same or less sampling effort. They are
routinely employed on towed gear surveys, and on visual surveys that employ
discrete stations.
Acoustic surveys usually follow systematic tracks but may operate with different
spatial intensity in particular strata (e.g. the MI Celtic Sea herring acoustic survey
samples more intensively in shallower, inshore waters).
Slide 10
Survey Planning Steps …
8. Plan, organise and conduct the survey
The challenges of running a survey are:
Logistical, Technical, Mechanical,
Physical, Analytical & Inter-personal
Clearly defined roles and responsibilities at
sea help ensure that data quality is
maintained and that survey costs are
minimised
Plan, organise and conduct survey:
The organisation and operation of fisheries survey are considerable logistical
challenges. A huge amount of effort must be put into the planning of survey
activities to avoid unforeseen difficulties at sea, or in the field. On large ship-board
trawl surveys the roles and responsibilities of the fisheries scientists involved are
clearly defined:
The Chief Scientist has overall responsibility for all scientific operations at sea,
including the operation of the primary survey tasks as well as ancillary and multidisciplinary data collection. The Chief Scientist plans the survey route in conjunction
with the ship’s crew in a manner that minimises costs (fuel, victuals, gear and time)
whilst remaining flexible to weather conditions. The Chief Scientist is the primary
interface between the scientific and operational crews of the vessel, and may also
communicate at sea with other research vessels on co-ordinated international
surveys.
A Fishing Skipper may be employed to supervise the deployment of the fishing
gear, adapt the schedule of sampling and station selection, and to ensure repairs to
damaged gear are made in accordance with the proscribed gear design.
A Deck Scientist directs the processing of the catch including its sorting, weighing
and sub-sampling for biological data, and records the data. They are crucial to the
maintenance of high quality data.
Other Fish scientists are involved in sorting the catch and sub-sampling it for
ancillary biological data. They may bring specialist skills in species identification or
other ancillary disciplines.
Slide 11
Survey Planning Steps …
9. Analyse the data
Source: ICES (2004). Report of the Workshop on Survey Design and Data Analysis
Analyse the data:
Abundance estimation from surveys can be broken down into three related but quite
distinct components (ICES, 2004). These are conducted independently, often by
different people, in very different places:
1. Estimation of fish density at a sampling station, carried out at sea:
Abundance per unit area or time may be derived from a trawl (catch per unit effort),
an echosounder (area of acoustic scattering per unit area sampled) or a plankton net
(number of eggs or larvae per unit area sampled). These data are often collected by
scientists with specialised sampling capabilities.
2. Interpolation of fish density to a whole survey estimate, carried out in the
laboratory: This considers the entire sampling frame of sampled stations and can be
as simple as deriving an arithmetic mean CPUE index, or involve complex
geostatistical analyses to estimate the abundance and uncertainty in total biomass.
These analyses are usually carried out by senior scientists with statistical and
analytical skills.
3. The incorporation of survey estimates into stock assessment, often
carried out at international stock assessment meetings: These analyses
consider time series (i.e. collected over several years) of survey estimates of
abundance, and other biological parameters (e.g., maturity, fecundity and weights at
age). Stock assessment scientists use survey data to “tune” the information from
commercial catches to estimate historic, current and future stock abundance, fishing
mortality and recruitment.
Slide 12
Survey Planning Steps …
10. Revise the survey
Past surveys provide useful experience to
inform the re-design of future surveys
Lets now look in some more detail at the
particular types of surveys we introduced
earlier: Egg and Larval surveys
Trawl Surveys
Acoustic Surveys
Direct counts
Revise the survey:
The results from previous surveys may be used to improve subsequent survey design
and operation. Care must be taken, however, to avoid adaptations that alter the
fundamental premise of constant catchability and standardised fishing effort.
Analytical tools are emerging that allow survey scientists to post-stratify their survey
abundance estimates (re-allocating station data to alternative strata) in an attempt
to improve the precision and reduce the bias of overall survey abundance estimates.
Such processes are extremely worthwhile in that they improve the value for money
obtained from surveys, and may substantially improve the conduct of subsequent
surveys. Exploration of the analytical methods employed is, however, beyond the
scope of this course.
From our review of the 10 steps to fisheries surveys it is clear that survey design and
operation involves all steps consecutively, but often concurrently.
We’ll consider each of the four survey types we introduced earlier to see how the
steps to survey design and operation are integrated in practice.
Let’s start with egg and larval surveys.
Slide 13
Why study the early life stages?
Pelagic fish eggs and recently hatched larvae have a relatively restricted distribution
and given their positive buoyancy are usually concentrated in upper layers of water
column. Eggs are entirely passive while early larvae have weak swimming abilities
making them very vulnerable to capture with appropriate sampling gears.
Egg and larval surveys provide a means of sampling the reproductive output of the
spawning population that is independent of commercial fisheries and so not affected
by sources of bias such as changes sampling gear and fishing fleet behaviour. Gear
avoidance is much less of an issue than it is with adult fish and multiple species can
be sampled at the same time with no additional costs. Egg and larval surveys are
used in to provide fishery independent estimates of spawning stock size which are
used in the assessment of stocks in species such as mackerel, herring sardine and
anchovy.
As well as providing estimates of the current size of the spawning population,
surveys of the early life stages can be used to predict the size of a year class that
will recruit to the fishery at some in the future. Late larval and juvenile fish surveys
are often used to derive recruitment indices. Management strategies can be
adjusted in advance of a particularly strong or weak year class entering the fishery.
Slide 14
Why study the early life stages?
Surveys of the early life stages can also be used to collect important information on
the distribution and ecology of fish during this critical period of their life cycle.
Information on horizontal and vertical distribution of eggs and larvae can be used to
define boundaries between different stocks of the same species, to describe how
fish are dispersed during the larval phase and to estimate the extent to which
populations are connected by mixing of their early life stages.
Eggs and larvae suffer extremely high rates of mortality (over 99% of the eggs
released can be lost during the larval period) so small variations in growth and
survival can have dramatic effects on subsequent abundance. A huge amount of
variability in the abundance of fish populations arises during the first year of life. By
examining how distribution, abundance and growth of larvae and juveniles varies
with environmental conditions such as temperature and food availability, fisheries
scientists can use information from surveys to improve understanding of the factors
governing variability in recruitment. The ultimate goal of such studies is often to
identify environmental parameters that can be used as an indicator of recruitment
strength.
Slide 15
Relative abundance
Surveys of planktonic eggs and larvae can be
used to derive a relative index of stock
abundance
This index can be used to compare relative stock
size between years
When the spawning stock is large, the index is high
When the spawning stock is small, the index is low
Mortalities of early life stages are very high so
abundance index must focus on a specific stage
or size range
Slile 16
Absolute abundance
Egg and larval surveys can also be used to
derive absolute estimates of stock size
Many potential sources of error need to be
considered
Careful survey design and planning is
required
Statistical treatment of survey data
necessary to incorporate sources of error
Slide 17
Estimating stock size using egg
production methods
P = BRF
so
B=
P
RF
Estimated from egg survey
From http://www.cefas.defra.gov.uk
Determined from samples of fish
collected before the spawning
season
P = Total egg production of the stock,
B = Spawning stock biomass
R = Proportion of the stock that are egg
producing females
F= Fecundity (number of eggs produced
per unit weight of female)
Egg production methods are used to provide a fishery-independent estimate of stock
size for fish that spawn in clearly defined areas. These methods have been used to
estimate stock size in a range of species including mackerel (Lockwood et al 1981),
sardine (Lo et al 1996), cod, plaice and sole (Armstrong et al 2001).
The total number of eggs produced by the stock is a function of the total size of the
stock, the proportion of the stock that are egg producing females and the number of
eggs produced by each female per unit weight (fecundity). The total egg production
(P) is estimated from plankton surveys while the proportion of spawning females in
the stock (R) and their individual fecundity (F) are estimated by collecting samples
of fish just before spawning commences. These parameters are used to estimate the
size of the spawning stock (B).
Slide 18
Egg production methods
There are two approaches to estimating stock size from egg abundance. The annual
egg production method (AEPM) is suitable for determinate spawners for which total
fecundity can be estimated by examining the gonad prior to the onset of spawning.
The entire spawning period is sampled, usually using multiple surveys. Females are
sampled at random immediately prior to the spawning season and total potential
fecundity is estimated taking small samples of ovary tissue and counting the number
of yolked eggs (oocytes) either macroscopically or from histological sections of
gonads. Counts are raised to the total number of eggs in the ovary. Stock size is
estimated by dividing total egg production by fecundity and the proportion of
spawning females in the stock.
The daily egg production method (DEPM) is more appropriate for indeterminate
spawners whose ovaries continue to develop after the onset of spawning, preventing
estimation of total fecundity. Daily egg production is estimated from a single survey
during the period of maximum spawning activity and combined with daily fecundity
rates (estimated using the same methods as above) to estimate stock size.
Slide 19
Egg production methods: some
considerations
Fecundity estimates must be
corrected to allow for atresia
(when fully developed eggs
are resorbed rather than
spawned)
Proportion of atretic oocytes
estimated from histological
sections of gonads
Histological cross section through
a mature gonad of a female herring
from Bucholtz et al 2008
A substantial proportion of developed oocytes in the gonads are never released
during spawning but are resorbed by the fish through the process of atresia. If not
accounted for atresia can lead to overestimation of fecundity and variation in rates of
atresia can introduce bias to estimates of stock size. Atresia can occur both before
and during the spawning season, so regular sampling is required to estimate the
numbers of oocytes lost this way.
Slide 20
Egg production methods: some
considerations
Egg production estimates must be
corrected for mortality
Eggs assigned to a series of
developmental stages to estimate age
and likely rates of post-spawning
mortality
Images of cod eggs at various
stages of development (from
Geffen and Nash 2012)
In order to relate the abundance of eggs in the plankton samples to the original
number of eggs spawned, corrections must be applied to account for mortality of
eggs between time of release and time of capture by the survey. In order to do this,
the ages of eggs in the sample are estimated by assigning them to developmental
stages based on their visual appearance, determining egg development rates based
on temperatures in the survey area (relationship between temperature and
incubation can be determined experimentally) and back-calculating spawning dates.
Estimated daily mortality rates are used to scale up current egg numbers to the
number originally released.
Slide 21
International mackerel and horse mackerel
egg survey
Calculation of spawning stock biomass for NE Atlantic
mackerel in 2010 using AEPM (from ICES WGWIDE
report 2011)
Total annual egg production
2.12*1015
Realised fecundity
(oocytes/g female)
1070
Proportion of females in
stock
0.50
Raising factor (used to
convert spawning fish to
total fish)
1.08
SSB
4.289 million tonnes*
2010 International Mackerel &
Horse Mackerel Egg Survey - Area
Covered (from www.marine.ie
*
2.12 X 1015
X 1.08 = 4.289 million tonnes
1070 X 0.5
The International Mackerel and Horse Mackerel Egg Survey has been running since
1977 under the coordination of ICES and takes place every three years. The survey
covers the spawning distribution of the two species from Gibraltar to the north coast
of Scotland between January and July. Multiple surveys are conducted using research
vessels from several countries. In 2010, ten research institutes from nine countries,
Scotland, Norway, Germany, the Netherlands, Spain, Portugal, Ireland, Iceland and
the Faeroes, took part in the programme. Sixteen surveys were carried out over 334
days of ship time. The surveys started in early February off the coast of Portugal,
and the Marine Institute finished the programme at the end of July.
The aim of the survey program is to estimate the spawning stock biomass of the
North-east Atlantic mackerel and horse mackerel stocks using the annual egg
production method. It provides the only fishery independent indices and direct
biomass estimates of mackerel and horse mackerel and is an essential component of
the annual assessment of the stocks.
The Irish Marine Institute leg of the survey covers the area off the northwest of
Ireland. The survey employs a Gulf VII plankton sampler (see later) which is
deployed at a series of stations to a maximum depth of 5m, towed at a speed of 4
knots through a V-shaped/oblique profile. A series of trawls are also taken in order to
sample the gonads of the fish for fecundity analysis.
More information:
http://www.marine.ie/home/services/surveys/fisheries/Mackerel+and+horse+macke
rel+egg+surveys.htm
Slide 22
Estimating stock size from larval
abundance
For species that spawn demersal eggs (eg. Atlantic herring Clupea harengus)
effectively sampling the egg stage is very difficult. In such cases the larval stages
may be used to estimate the size of the spawning population. After hatching and
during larval development mortality rates are extremely high and can also vary in
space and time due to fluctuations in the environment. The correlation between
larval abundance and spawning stock biomass will become weaker as larvae get
older. Therefore, larval surveys aimed at estimating stock size focus on estimating
the abundance of larvae as soon after hatching as possible. Corrections for both
larval and egg mortality need to be applied.
Egg and larval production methods are based on the assumption that rates of
mortality, growth and development do not vary in space or time. This assumption is
often violated, due to variation in factors such as temperature, food availability,
predator density etc, thus introducing a source of error into estimates of stock size.
Variability in rates of development can be incorporated into estimates of stock size
by using the temperatures recorded at individual stations to calculate station specific
rates of development.
Slide 23
Irish Sea larval herring survey
Estimates of larval herring abundance in the
Northern Irish Sea in 2010. Crosses indicate
sampling stations. Areas of shading indicate
proportional larval abundance. (from ICES HAWG
report 2011)
Herring larval surveys have been carried out in the Irish Sea during the autumn
spawning period since 1974. These surveys are used to produce larval production
indices which are combined with estimates of stock size from acoustic surveys of the
adult stock and input to the assessment of the stock. The larval surveys also provide
information on annual variation in the timing and location of spawning (Dickey-Collas
et al 2001).
Larval densities by length class are used to estimate larval production rates and birth
date distribution rates, assuming constant rates of growth and mortality (based on
estimates made in 1993-1997). It is recognised that variation in growth and
mortality rates are a potential source of bias in the larval production index and the
influence of annually varying mortality rates on the estimate has been investigated
by the ICES herring working group. the During the survey, a systematic grid of
stations covering the spawning grounds and surrounding areas is sampled using a
Gulf-VII high-speed plankton sampler. Double-oblique tows are made to within 2m of
the seabed at each station.
Slide 24
Recruitment
indices
r2 = 0.7
p<0.0001
Relative abundance of early juvenile cod
Plot showing the relationship between
the relative abundance of early juvenile
Arcto-Norwegian cod (Gadus morhua
L.) (approx. 3 months old) and the
abundance of the same year class at
age 3 as indicated by acoustic surveys.
The juvenile abundance estimates were
obtained by mid-water trawl surveys
conducted by the Norwegian Institute of
Marine Research from 1978-1991. From
Helle at al (2000)
Early indictors of year class strength, before a cohort is available to the fishery are
extremely valuable for providing advice on the status of exploited stocks. There is a
lot of debate and conflicting evidence in the literature as to when in the life cycle
year class strength becomes established (see Helle et al 2000; van der Weer 2000)
and what factors determine variability in recruitment (see review in Houde 2008).
Generally the egg and larval phases are subject to extreme variability and for most
species more reliable indicators of year class strength are provided by surveys of late
larvae and early juveniles. Trawl surveys on large research vessels (e.g. Marine
Institute annual groundfish survey) are used to derive recruitment indices for
commercial species such as cod, haddock and whiting. However, for nursery grounds
of many species are located in shallow coastal areas where shore based or small boat
surveys are more appropriate.
Some of the sampling methods used to survey post-larval and juvenile fish are
presented in the following slides. As for adult fish, issues of vulnerability and
availability to gear must be considered. Generally, these surveys provide a relative
index of abundance and so long as standardised methods are used and capture
efficiency does not vary annually, a reliable index of abundance can be obtained.
Slide 25
Gear considerations: Egg and
larval sampling
Usually sampled using plankton nets
Mesh size and method of deployment selected
based on size and stage of development of
target organisms
Simple plankton net design has been modified to
reduce problems of gear avoidance and
extrusion from net
Fish undergo dramatic changes in their size, swimming and sensory abilities during
early larval development. As a result their vulnerability to sampling gear changes
quite rapidly.
The type of plankton sampling gear and the method of deployment (e.g. towing
speed and depth) should be selected to maximise capture efficiency.
A larva’s ability to avoid a plankton net will depend on its maximum swimming speed
and the distance at which it can detect it. Directly estimating capture efficiency
based on motor skills and sensory capabilities is quite complex . A more common
approach is to estimate capture efficiency from the ratio of night-to-day catches.
When light levels are low and larvae are relatively inactive, rates of capture should
be at a maximum.
Net avoidance can be reduced by increasing the speed at which the net is towed or
increasing the radius of the net.
Although a fish egg is unlikely to pass through a net if the mesh size is smaller than
its dimensions, small elongate larvae may be extruded through the mesh, thus
reducing capture efficiency.
In late larvae and early juveniles capture efficiency can be difficult to determine and
are often quite low. Surveys of these later stages usually provide indices of relative
abundance and are not used to estimate absolute abundance.
Slide 26
Survey gears: egg and larval sampling
Eggs and early yolk sac larvae are concentrated at
the spawning area at a relatively high abundance.
These stages can be effectively sampled using a
vertical tow of a small plankton net from a stationery
vessel (large or small) e.g. a PairoVET net (right)
http://swfsc.noaa.gov
As larvae disperse and become capable of avoiding nets a
greater volume of water must be sampled at higher speeds.
Double bongo nets (right) can be towed from large or small
vessels depending on their size. The double net design
Plankton net – simple design
Gulf designs
increases sampling efficiency
while the positioning of the
Mokness
Automatic sampllers
bridle between the nets rather
than in the mouth reduces gear
Visual techniques
avoidance. Depressors stabilize the net which is towed in a vshaped or oblique profile at a towing speed of 1.5-2 knots. At
faster speeds filtration is less effective and net may be
damaged
http://www.spartel.u-net.com
Slide 27
Survey gears: egg and larval sampling
The Gulf VII plankton net is designed to be
towed from large vessels at high speeds (6-7
knots). The net is encased in a metal frame for
protection and stability. A cone at mouth of net
reduces displacement and facilitates the higher
towing speeds. There is no bridle at mouth of
net, reducing gear avoidance.
Sensory attachments can be attached to collect
additional data (e.g. temperature, salinity,
depth, chlorophyll concentration) and facilitate
continuous monitoring of net during deployment
This design is commonly used in egg and larval
surveys for the assessment of commercial fish
stocks.
Slide 28
Survey gears: egg and larval sampling
MOCNESS designs : multiple opening and
closing net and environmental sensing system.
Allows sampling at multiple selected depths
Slide 29
Survey gears: egg and larval sampling
Continuous underway fish egg and
sampler (CUFES)
Consists of a machine that pumps a
sample of water from the top 3 meters
of the water continuously while the ship
is moving and a filtration system which
traps all of the floating fish eggs which
are collected and identified on board the
ship (http://cufes.ucsd.edu/ ). The
identification and staging of the eggs
can be done manually, or the system
can be combined with a camera and
image analysis/recognition system
which automates the inditifcation,
staging and counting of the eggs
(REFLICS, Iwamoto et al 2001)
A number of technological developments have automated the collection and
identification of fish eggs and larvae.
Slide 30
Survey gears: juvenile sampling
Reilly pushnet – designed to capture small
juvenile fish in shallow areas, inaccessible by
boat. Widely used to sample juvenile flatfish.
Trawled area can be estimated but capture
efficiencies are low.
Beach seines can also be used to sample
juvenile fish from shallow nursery areas.
Standardisation of tows is difficult and conditions
will vary depending on substrate, slope of beach
etc.
Small beam trawls can be deployed from small
vessels in shallow water. Capture efficiencies
are better than with the pushnet. Semiquantitative.
Slide 31
Trawl surveys
Standardisation is critical:
This is achieved by
agreement (often between
international partners) on
trawl geometry and survey
design
International
co-ordination &
cooperation on
European
demersal trawl
surveys
Standard
net design
The three remaining types of trawl surveys to be considered in subsequent
presentations include:
Trawl surveys:
www.dfo-mpo.gc.ca
Fisheries and Oceans Canada
Slide 32
Direct
counts
Marine Institute
tinyurl.com/MI-UWTV-survey
Abalone
dive surveys
Towed camera survey
of prawn burrows
Aerial surveys
of herring milt
www.dfo-mpo.gc.ca
Fisheries and Oceans Canada
Baited underwater
video surveys
Stony Brook University
tinyurl.com/BRUV-survey
A vast range of approaches, …
but a similar fundamental design
Direct counts:
The range of gears and methods deployed for direct count surveys is too vast to list
here. Direct count surveys are conducted underwater by divers, by camera systems,
or by sonar detection systems. They can also be conducted from water surface or
from the air.
Despite the vast range of gears employed the underlying principle of relating
observed abundance to a standard area or period of sampling remains. Similar
design issues therefore must be considered when conducting direct count surveys.
The logistical challenges involved in conducting these surveys may, however, be
particularly difficult.
Slide 33
Acoustic surveys
• An echo-sounder is used to
identify and enumerate schools
of fish in the water column.
• Trawls are taken through
selected sounded schools to
confirm that the acoustic
targets are the particular fish
species identifiable by distinct
echo patterns
• Mid-water trawl nets
suspended have:
Echogram of mid-water net haul from the 2011 MI
Northwest Acoustic Survey (tinyurl.com/MI-NWHS).
Floats on the top edge
Weights at the bottom, and,
The pull of the trawl ‘doors’ which
are attached to the ships’ warps.
Acoustic Surveys:
Acoustic methods allow fish to be identified within the water column. They are
therefore particularly useful for estimating the distribution and abundance of pelagic
fish species (such as mackerel, herring and horse mackerel) (Jennings et al. 2001).
Slide 34
References:
Cochran WG (1977). Sampling Techniques
(3rd ed.). Wiley, New York, 428pp.
Gunderson DR (1993). Surveys of fisheries
resources. Wiley, New York, 248pp.
ICES (2004). Report of the Workshop on
Survey Design and Data Analysis. ICES
CM 2004/B:07, 65pp.
Jennings S, Kaiser M & Reynolds JD (2001).
Marine Fisheries Ecology. Blackwell
Science, Oxford, 417pp.