Question 3: Explain Merit

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.

Explain how the salmon navigates home and moves up its stream, an advantage of homing, and why having a lot of young is a successful reproductive strategy.

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 The describe answer named the magnetic field. How does it help, and why is it a good method at sea?
Young salmon going to sea are thought to imprint on the magnetic field where their river meets the coast, and as adults to swim until they find it again: the leading explanation, with growing evidence, though not yet settled. It works in open ocean, where there are no landmarks and the water looks the same in every direction.
2 How does homing work once the salmon reaches the river?
As young fish they imprinted on the chemical signature of their home stream. At each fork the adult follows the water that carries that smell, swimming up its gradient, while positive rheotaxis keeps it heading into the current, upstream.
3 Why is returning to the home stream an advantage?
A random river may lack spawning sites or have conditions that do not suit masu salmon. The home stream raised the parent, so it is likely to suit the eggs and young.
4 Why does spawning where the others spawn matter?
Fertilisation is external: eggs and sperm are released into the water. With many salmon from the same stream spawning at the same time and place, eggs are far more likely to be fertilised.
5 Why is having a lot of young an advantage?
So many eggs and young are produced that predators cannot eat them all, so enough survive to reach the sea and come back to breed, replacing the parents.

Answer Young salmon are thought to imprint on the magnetic field where their river meets the sea, and as adults to use it to find that coast again, a method that works in open ocean with no landmarks. In the river they follow the chemical signature of their home stream, imprinted when young, choosing at each fork the water that carries it, and positive rheotaxis keeps them swimming into the current, upstream. Returning home avoids rivers that may lack spawning sites or suitable conditions: the home stream already raised the parent. Spawning where many others from the same stream spawn at the same time matters because fertilisation is external, so crowded spawning makes it far more likely. Producing huge numbers of young means predators cannot eat them all, so enough survive to return and breed, replacing the parents.

What the marker wanted

Achieved The describe points: homing and its advantage, a navigation method, taxic behaviour and its type, the cost of no parental care, the gain from many eggs, spawning in the same river, and r-strategy.
Merit Explains: homing and its advantage; how homing works; a navigation method and its advantage; an advantage of having a lot of young; the advantages of spawning in the same river. Three make M6.

One answer, N0 to E8. Merit points sit above the Achieved ones: two give M5, three M6.

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

Common mistake

Naming and defining homing, a navigation method and the taxis without explaining how any of them work. NZQA’s 2023 report says many answers stopped there.

How did it go?

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