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Blue Whiting: A Guide to Handling and Quality

19 October 2015

An Bord Iascaigh Mhara

The aim of this guide from the Irish Sea Fisheries Board (BIM) is to provide practical and contemporary guidelines on the handling and quality of Blue Whiting, at all stages, from capture to the first point of sale.

Introduction

Blue whiting (Micromesistius poutassou) is a pelagic species closely related to cod, haddock and hake. Although widely distributed in the eastern and western North Atlantic and the Mediterranean, Irish vessels target this species in a small area of its distribution off the Porcupine Bank (VIa, VIIb,VIIc) and the Rockall Bank (VIb), landing catches for fishmeal and more recently, for human consumption.

Species distribution map for blue whiting (Micromesistius poutassou). Source: FAO

The body colour of fresh fish progresses from a dark blue-grey on the back, through silver on the flanks to a creamy white on the belly, over a general background of pale blue.

Although normally between 25cm and 35cm in total length, the species may reach 50cm. Growth is rapid in the first two years of life and slows down once the fish reaches maturity (2 to 4 years).

Once mature, blue whiting undertake an annual migration in the spring from feeding grounds in the Norwegian sea to major spawning areas off the west of Ireland and Scotland, and minor locations along the Norwegian coast. Spawning occurs from February in the south of the range, to May in the north, at depths from 180m to 360m. A return migration to the feeding grounds takes place after spawning occurs.

Blue whiting form mid-water shoals at depths from 160m to 1500m. Adult fish are most common between 200m and 600m and tend to form characteristic, horizontal layers in the water column as the shoals follow their major food item (meso-pelagic crustaceans) from deep water during the day, to the surface waters at night.

Quality Assessment

The objective assessment of quality is vital to enable industry agree and implement common trading specifications so that fish can be traded in a fair manner. Defining quality is not easy, as it can include a range of factors, which depend on market preferences such as: species, size, capture method, seasonal condition and freshness.

Of major importance to all consumers is freshness, a characteristic, which relates to the degree of spoilage a fish has undergone. Very importantly and unlike many other quality attributes, this is something that the fishing industry has certain control over.

As a result of good manufacturing practices, spoilage at all stages in production and processing can be assessed and minimized. A test used to regularly assess the freshness of blue whiting is the determination of total volatile base nitrogen (TVBN).

TVBN measures the key products of bacterial spoilage (ammonia, dimethylamine and trimethylamine) from a sample of fish and is carried out using specialised laboratory equipment.

Sensory assessment remains the most popular method of assessing freshness. This type of assessment uses smell, texture and visual appearance to determine the quality of fish. It is a particularly useful technique as it is low cost and requires nothing other than careful and exact training. It is a widespread and reliable assessment method and provides the foundation for the design and application of this guide.

Eye

Very High Eye Quality High Eye Quality Medium Eye Quality Low Eye Quality Very Low Eye Quality

Gill Colour

Very High Gill Colour High Gill Colour Medium Gill Colour Low Gill Colour Very Low Eye Quality
 

Stomach Contents

Stomach contents - Very High Stomach contents - High Stomach contents - Very Low

Rigor

Rigor - Very High Rigor - High Rigor - Very Low
 

Sexual Maturity Stages

MALE (TESTES)
STAGE SIZE COLOUR OTHER
FEATURE
I
(Immature virgin)
Very Small Translucent Thin, narrow ribbon
II
(Developing virgin)
Length less than 1/2 body cavity Becoming opaque, white Slightly lobed and coiled
III
(Early mature)
Length 3/4 body cavity Whitish-grey Strongly coiled, blood vessels visible
IV
(Late mature)
Length 3/4 body cavity, swelling Opaque, white Strongly coiled
V
(Ripe)
Filling body cavity Opaque, creamy-white Tightly convoluted lobes, milt does not flow
VI
(Running)
Filling body cavity Creamy-white Milt easily extruded
VII
(Spent)
Length less than 1/2 body cavity White, bloodshot Crinkled and shrunken
FEMALE (OVARY)
STAGE SIZE COLOUR OTHER
FEATURE
I
(Immature virgin)
Length less than 1/4 body cavity Translucent, white Small,
oval sac
II
(Developing virgin)
Length 1/3 body cavity Reddish-orange or translucent Small,
oval sac
III
(Early mature)
Length 1/2 body cavity Pinkish-white Opaque eggs clearly visible
IV
(Late mature)
Length 2/3 body cavity Whitish-yellow Opaque eggs clearly visible
V
(Ripe)
Very swollen Pale yellow Some eggs transparent, not easily extruded
VI
(Running)
Very swollen Transparent Eggs easily extruded
VII
(Spent)
Length 1/2 body cavity Bloodshot Flaccid and shrunken



Examples of Sexual Maturity Stages

Male (Testes)

Female (Ovary)

Instructions

  1. Photocopy the assessment sheet to enable scores to be recorded.
  2. Take a random sample of ten fish and score each one separately.
  3. Take one fish and assess each quality category i.e. Eye, Skin, Rigor etc. separately.
  4. Look at the first category, Eye, and decide which description matches the fish you are examining i.e. Very High, High, Medium, Low or Very Low.
  5. When one of the five options has been chosen, place a tick in the shaded box directly below your choice.
  6. Now move to the next quality category, Skin and repeat steps 4 and 5, for this category and all following categories for the fish.
  7. You now should have one tick for each quality category.
  8. Repeat steps 3-6 for nine more fish, ignoring any previous ticks from other fish examined.
  9. After examining all ten fish, you should have a total of ten ticks for each quality category.
  10. Now look at your columns i.e. Very High, High, in turn.
  11. Add all cells in the column and put the resulting figure into the space at the bottom of the column.
  12. Multiply this number by the appropriate weighting for the column, which is 5, for example, in the case of the 'Very High' column.
  13. Repeat steps 11 to 13 for all columns.
  14. Add the multiplied column totals and divide this number by 10 (the number of fish used) to achieve the average numerical quality score for the fish, examined.
  15. This numerical score can then be assigned a quality grade.
  16. Repeat the same procedure described above for the market specifications.
  17. Using the sexual maturity sheet, identify the sex and maturity stage.
  18. Note the approximate number of parasites (nematode worms) and their location (fillet, gonad, liver).



Freshness attributes Worked example
 

October 2015

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