Strategic Marine Alliance for Research and · PDF file Strategic Marine Alliance for Research...

Click here to load reader

  • date post

  • Category


  • view

  • download


Embed Size (px)

Transcript of Strategic Marine Alliance for Research and · PDF file Strategic Marine Alliance for Research...

  • 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 ( and

    Dr Deirdre Brophy (

    Galway-Mayo Institute of Technology

  • Slide 2

    SMART Common Blended Learning Module Fisheries Surveys

    Design & Operation

    Fisheries and Oceans Canada

    Fisheries Science Operations at Sea

    Dr Rick Officer (

    Dr Deirdre Brophy (

    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

    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.

  • Slide 4

    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

    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

    1. Define objectives 2. Define the population to be sampled The 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 Steps DAB =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


  • Slide 6

    3. Decide what data will be collected 4. Decide on the degree of precision required

    Survey Planning Steps …

    Sorting by species before Catch Weight


    Extraction of otoliths (earstones) for Age


    Measurement of fish for Length Frequency


    Abundance / Biomass

    Size Structure

    Age Profile

    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.

  • 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.

  • Slide 7

    w w

    w .im

    ar es

    .w ur

    .n l

    N or

    th S

    ea h

    er rin

    g ac

    ou st

    ic s

    ur ve

    yht tp

    :// tin

    yu rl.

    co m

    /P la

    nk to

    n- ne


    5. Choose methods of measurement

    Survey Planning Steps …



    Egg and larval


    w w

    w .d

    fo -m

    po .g

    c. ca

    Fi sh

    er ie

    s an

    d O

    ce an

    s C

    an ad


    Direct counts

    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