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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

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Slide 2

SMART Common Blended Learning ModuleFisheries Surveys

Design & Operation

Fisheries and Oceans Canadawww.dfo-mpo.gc.ca

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.

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Slide 3

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

Fisheries Surveys are required for Fisheries Management

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.

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Slide 4

1. Define objectives2. Define the population to be sampled3. Decide what data will be collected4. Decide on the degree of precision required5. Choose methods of measurement6. Map the complete “frame” of sampling units7. Select units to be sampled8. Plan, organise and conduct the field work9. Analyse the data10. Revise the survey plan, conduct & analysis

Survey Planning Steps

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

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maturity, sex-ratio, weight, gut contents) or physical, chemical or geological

sampling objectives (e.g. temperature, nutrient distribution, or sediment type).

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Slide 5

1. Define objectives2. Define the population to be sampledThe surveyed population must be:

Available and Vulnerable

Sampling gear must have Known catchability

The surveyed sites must be Representative

These assumptions are difficult to satisfy

Survey Planning StepsDAB =ˆ

Survey area

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.

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Slide 6

3. Decide what data will be collected4. Decide on the degree of precision required

Survey Planning Steps …

Sorting by species before Catch Weight

Estimation

Extraction of otoliths(earstones) for Age

Estimation

Measurement of fish for Length Frequency

Estimation

Abundance /Biomass

SizeStructure

AgeProfile

Growth rings

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.

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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.

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Slide 7

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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 mid-

water 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).

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Slide 8

6. Map the sampling “frame”Survey Planning Steps …

Marine InstituteHerringAcousticSurveys:

North WestCeltic Sea

Marine InstituteGroundfish

Survey:Trawl station

locations

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.

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Slide 9

7. Select units to be sampledSurvey Planning Steps …

20m

50m

100m

Systematic sampling:Good for mapping spatial distribution But:May give biased estimates of the mean,Ignores biogeographicinfluences on species distribution.

Random sampling:Gives unbiased estimates of the mean But:May be imprecise,Ignores biogeographicinfluences on species distribution.

Stratified random sampling:Utilises ancillary information on biogeographicinfluences on species distribution (e.g. 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.

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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).

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Slide 10

8. Plan, organise and conduct the surveySurvey Planning Steps …

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 multi-

disciplinary 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.

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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.

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Slide 11

9. Analyse the dataSurvey Planning Steps …

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

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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.

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Slide 12

10. Revise the surveySurvey Planning Steps …

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 SurveysAcoustic SurveysDirect 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.

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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.

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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.

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Slide 15

Relative abundanceSurveys 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 highWhen 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 abundanceEgg and larval surveys can also be used to

derive absolute estimates of stock sizeMany potential sources of error need to be

considered Careful survey design and planning is

requiredStatistical treatment of survey data

necessary to incorporate sources of error

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Slide 17

Estimating stock size using egg production methods

P = Total egg production of the stock,B = Spawning stock biomassR = Proportion of the stock that are egg producing femalesF= Fecundity (number of eggs produced per unit weight of female)

RFPB

BRFP

=

=so

Estimated from egg survey

Determined from samples of fish collected before the spawning season

From http://www.cefas.defra.gov.uk

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).

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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.

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Slide 19

Egg production methods: some considerationsFecundity estimates must be

corrected to allow for atresia(when fully developed eggs are resorbed rather than spawned)

Proportion of atretic oocytesestimated 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.

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Slide 20

Egg production methods: some considerationsEgg production estimates must be

corrected for mortalityEggs 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.

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Slide 21

International mackerel and horse mackerel egg survey

2010 International Mackerel & Horse Mackerel Egg Survey - Area Covered (from www.marine.ie

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* nnesmillion to 289.408.15.01070

1012.2 15*

=XXX

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

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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

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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.

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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.

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Slide 24

Recruitment indices

Plot showing the relationship between the relative abundance of early juvenile Arcto-Norwegian cod (Gadus morhuaL.) (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)

Relative abundance of early juvenile cod

r2 = 0.7p<0.0001

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.

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Slide 25

Gear considerations: Egg and larval samplingUsually sampled using plankton netsMesh 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.

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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.

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Slide 26

Survey gears: egg and larval sampling

Plankton net – simple designGulf designsMoknessAutomatic sampllersVisual techniques

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)

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 increases sampling efficiency while the positioning of the bridle between the nets rather than in the mouth reduces gear avoidance. Depressors stabilize the net which is towed in a v-shaped 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://swfsc.noaa.gov

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.

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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

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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.

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Slide 30

Survey gears: juvenile samplingReilly 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. Semi-quantitative.

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Slide 31

Trawl surveysStandardisation 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:

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Slide 32

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Aerial surveys of herring milt

Directcounts

A vast range of approaches, …but a similar fundamental design

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Abalonedive surveys

Towed camera survey of prawn burrows

Baited underwater video surveys

Marine Institutetinyurl.com/MI-UWTV-survey

Stony Brook Universitytinyurl.com/BRUV-survey

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.

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Slide 33

Acoustic surveys

Echogram of mid-water net haul from the 2011 MI Northwest Acoustic Survey (tinyurl.com/MI-NWHS).

• 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:

Floats on the top edgeWeights 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).

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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.