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
Gill Colour
Stomach Contents
Rigor
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
- Photocopy the assessment sheet to enable scores to be recorded.
- Take a random sample of ten fish and score each one separately.
- Take one fish and assess each quality category i.e. Eye, Skin, Rigor etc. separately.
- 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.
- When one of the five options has been chosen, place a tick in the shaded box directly below your choice.
- Now move to the next quality category, Skin and repeat steps 4 and 5, for this category and all following categories for the fish.
- You now should have one tick for each quality category.
- Repeat steps 3-6 for nine more fish, ignoring any previous ticks from other fish examined.
- After examining all ten fish, you should have a total of ten ticks for each quality category.
- Now look at your columns i.e. Very High, High, in turn.
- Add all cells in the column and put the resulting figure into the space at the bottom of the column.
- Multiply this number by the appropriate weighting for the column, which is 5, for example, in the case of the 'Very High' column.
- Repeat steps 11 to 13 for all columns.
- 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.
- This numerical score can then be assigned a quality grade.
- Repeat the same procedure described above for the market specifications.
- Using the sexual maturity sheet, identify the sex and maturity stage.
- Note the approximate number of parasites (nematode worms) and their location (fillet, gonad, liver).
October 2015
