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A brief review on the Ricker Curve to help you study for the final

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Pacific salmon life cycle

Adults

(RECRUITS arriving back at river)

Adults

(ocean)

Spawners

(fresh water)

Juveniles

(estuary, early ocean)

Juveniles

(fresh water)

2 – 7 years later

1. Relate the Ricker curve to the life cycle diagram

Adults

(RECRUITS arriving back at river)

Spawners

(fresh water)

The Ricker model predicts how many recruits (adult offspring) are produced by the number of spawners (parents) that swim upstream in a particular year.

Those recruits come back to the river 2 – 7 years after the spawners, depending on the species of Pacific salmon (for a particular species, all recruits typically arrive back during the same year.)

2 – 7 years later

2. Draw the axes of the graph, label them.

Recruits

(these are the adult offspring who are now back at the mouth of the river where they were born)

Spawners

(these are the parents)

2. Draw the line where the number of recruits = number of spawners (this is the 1:1 line).

At any point along this line, the number of recruits returning is identical to the number of parents who swam up the river some number of years earlier.

The parents would have had laid many, many more eggs, but for many reasons that we discussed in class, not all those eggs became adult offspring (check out your notes). It is also possible that not all spawners had the opportunity to lay eggs.

Recruits

(these are the adult offspring who are now back at the mouth of the river where they were born)

Spawners

(these are the parents)

3. Draw the Ricker Curve. (Note that there are other fisheries models that relate recruits to spawners. We explored this one, because it is the traditional model and one that was used by fisheries managers for a long time.)

Note that the Ricker can take different shapes (the peak and slopes can change) depending on the parameter values. (Also, I’ve drawn this one in powerpoint, so it’s not very representative of the actual shape. Check out the lecture slides.

Recruits

(these are the adult offspring who are now back at the mouth of the river where they were born)

Spawners

(these are the parents)

4. Think a more about what the curve means.

Natural equilibrium point, where number of recruits = number of spawners. That happens where these two lines cross. You can convince yourself why that is.

Below the natural equilibrium point, we get more recruits than spawners. (Think about population models below their carrying capacity. Plenty of resources.)

Recruits

(these are the adult offspring who are now back at the mouth of the river where they were born)

In this region, there are fewer recruits than spawners. The model predicts this, because it relies on a carrying capacity. Remember what happens when a population goes beyond its carrying capacity? Lots of competition for resources. In this case the limiting resources include space for laying eggs (too many salmon means they can dig up each others nests), or not enough food for their offspring (there are potentially others).

Spawners

(these are the parents)

5. Where is the MSY?

I’ve shown the solution for MSY graphically. It is calculated from a mathematical equation that uses several parameters that control the shape of the curve.

MSY for this curve is here (although this curve is not exactly a Ricker due to drawing it quickly in powerpoint)

Recruits

(these are the adult offspring who are now back at the mouth of the river where they were born)

Red arrow indicates potential catch at the MSY. The MSY maximizes the distance between the Ricker curve and the 1:1 line.

Spawners

(these are the parents)

6. How many fish can you catch at the MSY?

MSY is here

Number of recruits at MSY

Recruits

(these are the adult offspring who are now back at the mouth of the river where they were born)

Number of recruits to keep population at same size as number of spawners.

Red arrow indicates potential catch at the MSY. The MSY maximizes the distance between the Ricker curve and the 1:1 line.

Spawners

(these are the parents)

If you want to understand this better, you will need to do a little thinking/drawing notes on the curve to convince yourself how this works. It’s hard to explain briefly. You can use the lecture notes to help, too. Think about if we let the rest of the recruits go, they will become spawners. How many recruits will come back in the next generation?

7. Compare number of spawners at MSY versus number of spawners at equlibrium

MSY for this curve is here (although this curve is not exactly a Ricker due to drawing it quickly in powerpoint)

If this doesn’t make sense to you, extend those orange lines up to hit the Ricker curve.

Recruits

(these are the adult offspring who are now back at the mouth of the river where they were born)

Number of spawners at equilibrium

Number of spawners at MSY

Spawners

(these are the parents)

Brief review of the Ricker Model and what you need to know (repeat from lecture slide from March 28)

- Number of recruits is a function of number of spawners (their parents)
- Carrying capacity places a limit number of recruits as density of spawners increases
- MSY reduces population size to a lower equilibrium
- Model limitations

I’ve gone over all these things in this review except the last. Check out your lecture notes & discuss among yourselves what the limits of the model are.

Example of survival through the salmon life cycle: Fraser River sockeye salmon

Eggs per female

Survivors to fry stage

Survivors to smolt stage

Survivors to adult

Fishery harvest rate

Fish remaining for spawning

From the Cohen commission report (2012), work by M. LaPointe

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