How many survive from eggs to spawning salmon?
In this assignment, you will read a text about wild salmon from egg to adult. The text contains a number of figures. You need to use these numbers to calculate how many eggs eventually become spawning salmon.
Competency goals for grade 7
- explain how organisms can be divided into main groups and give examples of the characteristics of different organisms.
- explain the importance of biodiversity and implement measures to preserve biodiversity in the local environment.
- explore and describe different food webs and use this to discuss interactions in nature.
Read the text and calculate how many survive
A female salmon spawns 5000 eggs. 250 of them die or are eaten by others before they have time to hatch. During the few weeks the yolk sac fry are down in the gravel devouring the food package, 470 more of them die. The first summer in the river is a tough time for the fry, who have to fight for food and good hiding places. By the time autumn arrives, 3,430 more fry have died. Those that remain grow a little bigger and stronger over time, and not as many die. However, by the time they become smolts, another 750 have died. Many of them have become food for birds and fish.
How many of the 5,000 eggs will become smolts?
Use the answer above. Let's see if the sea is a dangerous place for wild salmon. 35 of the smolts quickly become food for seals, birds and larger fish lurking in the waters when the small wild salmon swim out of the fjord in shoals. By the time they reach the good grazing areas, 20 more have dropped out. Time passes, and the wild salmon grow. Life in the open sea takes its toll, and before it's time to go home, 30 more are gone. Most have become food for other fish and animals. Before they get home to their river, 10 more disappear.
How many adult salmon did the 5,000 eggs eventually produce?
Female salmon produce 1,450 eggs for every kilogram of body weight. How big was the female salmon?
Naturally, many die
Usually there are 2 smolts out of 100 eggs.
Out of 100 smolts, about 5 return to the river as adult wild salmon.
For the teacher
This assignment is a good starting point for talking about how we humans affect nature. The film clip discusses several issues that affect wild salmon, and that lead to more wild salmon dying than normal.
The assignment is also a great opportunity to talk about food webs in both rivers and oceans.
The bottom line for the calculation:
- During their time in the river, 5,000 eggs become 100 smolts (5,000 - 250 - 470 - 3,430 - 750 = 100).
- Out of 100 smolts, 5 adult salmon return to the river (100 - 35 - 20 - 30 - 10 = 5).
- The female salmon weighed 3.44 kg (5000 eggs : 1450 eggs per kg = 3.44 kg).
The rule of thumb is no matter what: 100 eggs produce 2 smolts. The natural mortality rate in the sea is 95%.
Student booklet
The Nasjonalt villakssenter works for a stronger wild salmon and wants to promote the stream as an educational resource. Bring experiences from the classroom and put them in context with experiences outside. Many schools in Norway have a local stream. Even in our largest cities, there are streams that can be visited. Contact us for assistance, tips and professional support.
The student booklet "Who lives in your river?" can help with both preparation and follow-up work.