| Resources |
| For
this activity you will need to get hold of some fish scales
from a fish shop or processing factory. Snapper scales
are best - but not from very large fish. Blue cod, kahawai
and tarakihi are also possible. If you get a whole fish,
measure it so the students can calculate the length of
the fish in previous winters. |
 |
| Aim |
| To
assess your ability to work out the age of a fish. |
|
| Task |
| The
age of fish can be worked out by counting the rings on
scales, the same as the age of trees can be assessed by
counting the rings in the tree trunk. MFish usually uses
otoliths, part of the inner ear, to age fish, but the
principle is the same. Each ring indicates annual growth. |
|
|
In
the winter, fish stop growing and this shows up as a
dark ring. Fish grow in summer and autumn and on their
scales this appears as a lighter growth zone (juveniles
grow in spring, but adults put growth into gonads rather
than body and scale development in spring). Sometimes
double rings occur if the fish has been under stress,
for example, if there was a cold autumn storm.
|
|
| Use
the snapper scale diagram as a guide so you know what
you're looking for. |
|
Click for larger image
|
|
| 1 |
Preparation.
If you have a whole fish, take a few scales from the midbody
area, below the lateral line. Soak them for about one
minute in water. Rub them between your fingers to remove
any traces of skin and dry them. Leave them for a few
minutes so they are thoroughly dry. |
| 2 |
Mount
them between two microscope slides, taped around the ends.
Show your slide to the teacher. View it at a magnification
of 10x to 40x. A normal school binocular microscope should
do. (Alternatively, you could use a projector; using 35mm
slide mounts with thin glass windows. Or you could use
a microfilm reader if it has appropriate lenses and a
stage to place the scales on.) |
| 3 |
Look
at the slide under the microscope and count the rings
on at least three scales. Record this data and work out
the age of the fish. |
| 4 |
Have
these scales got any double rings? Can you suggest a reason
for this? |
| 5 |
If
you know the size of the fish, measure the distances between
the scale rings (e.g. with an eyepiece micrometer) and
calculate the length of the fish at each previous winter,
by assuming the scale grows proportionately to fish length. |
top
|
| Assessment
A: criteria and marking |
| Assessment
criteria |
 |
Level
1: Carries out some activities in an investigation. |
|
Level
2: Carries out an investigation. |
|
Level
3: Carries out an investigation skilfully. |
|
Level
4: Carries out an investigation skilfully and thoroughly. |
|
Level
5: Carries out an investigation skilfully, thoroughly
and with initiative. |
|
| Marking
schedule |
|
Level
1: Prepares a few scales. |
|
Level 2: Mounts a few scales between two microscope slides
(or uses an alternative method). |
|
Level
3: Good microscope work. Is able to clearly identify the
rings and growth zones (or alternative method). |
|
Level 4: Collects data from at least three scales, but
is unable to work out the fish's age. |
|
Level
5: Accurately assesses the fish's age. If the fish has
a double ring, can identify this and give a reason (eg,
autumn storm). If they know the size of the fish, they
can accurately calculate the length of the fish at previous
winters. |
