Question 3: Describe Achieved

Practises Navigation over long distances

Masu salmon (Oncorhynchus masou) are native to north-east Asia. Like all salmon they home very well, coming back to breed in the stream they hatched in, apparently because as young fish they imprinted on chemicals in that stream. In the river the females release eggs and the males swim over them releasing sperm, which is spawning; many adults die afterwards, and neither the fertilised eggs nor the young get any parental care. The paper’s photos show the fish just hatched, very young, as young fish heading downstream to the sea, and as adults heading back upstream.

Describe a navigation method the salmon uses to get from the sea to its stream, the taxic response that takes it upstream, an advantage of homing, and the salmon’s strategy of having a lot of young.

The 2023 paper, page 8 ↗ Marking schedule, page 4 ↗

Work it out

Try it on paper first. Stuck? Take one hint at a time.

1 What is homing?
Returning to a particular place, here the stream where the salmon hatched, across territory it does not know.
2 Name a navigation method that could bring a salmon from the sea back to its stream.
The Earth’s magnetic field, which salmon can sense; or the smell of the home stream, whose chemicals they imprinted on as young fish. Landmarks and polarised light have also been suggested.
3 What is a taxic response, and which one takes it up the stream?
A taxis is movement directed by a stimulus, towards or away from it. Swimming upstream into the current is positive rheotaxis; swimming towards the home stream’s chemicals is positive chemotaxis.
4 What is an advantage of homing to the stream it hatched in?
That stream is known to have places to spawn and conditions young salmon survive in, and other salmon from it will be there to mate with.
5 What kind of reproductive strategy is having a lot of young, and what does it cost and gain?
An r-strategy: very many eggs and sperm, but no parental care, so the eggs and young are unprotected and most die. Producing so many means some still survive.

Answer Homing is returning to a particular place, here the stream where the salmon hatched. A salmon may navigate from the sea by the Earth’s magnetic field, and to its stream by the smell of chemicals it imprinted on when young. A taxis is movement directed by a stimulus, towards or away from it; the salmon swims upstream by positive rheotaxis (into the current) and towards its stream’s smell by positive chemotaxis. Homing takes it to a stream with proven spawning sites and conditions, where other salmon are spawning too. Having many young is an r-strategy: lots of eggs and sperm with no parental care, so most young die unprotected, but enough survive.

What the marker wanted

Achieved Describes, one point each: what homing is; one thing homing gains; one navigation method; what a taxis is; which taxis (positive chemotaxis or positive rheotaxis); what the lack of parental care costs; what the huge output of eggs and sperm gains; why spawning in the home river helps; and that this is an r-strategy. Four make A4.

The schedule marks the question as one answer from N0 to E8. Achieved points count first: one or two give N1 or N2, three A3, four A4. Above that, two Merit points give M5 and three M6, then one Excellence point E7 and both E8.

In our words, from NZQA’s 2023 marking schedule.

Common mistake

Calling the many young an advantage without its cost. With no parental care most of them die; NZQA’s 2023 report says Not Achieved answers often could not give the advantages and disadvantages of this strategy.

How did it go?

Got it moves it further out. Review brings it back tomorrow.