Spawning target

Salmon production and spawning target

The spawning target is a stock-specific number of adult salmon that have to be present in the river at the time of spawning. This management target ensure that each river attain their production capacity. The target based management will ensure that the catch is done on a surplus of the salmon stock. The result is that it is possible to harvest on a surplus of the stock one salmon generation later. The result of overexploitation is in short; that the returning salmon stock will decline.

First-generation management (spawning) target have been established as spawning target for the River Tana system. Each main tributary has a separate management target.     

Spawning targets of River Tana. (Numbers from Tana river reseach and monitoring group/Hindar et al. 2007)

Target attainment

The target attainment describes the difference between the spawning stock and the spawning target. There are several possible ways to obtain an estimate of spawning stock size.

1)      Direct counting of spawners through diving.

2)      Combining fish counting and catch statistics. It is possible to count the ascending salmon through video (Lákšjohka and Utsjoki), or acoustics (Didson/simsonar in Kàràšjohka).

3)      Combine estimates of exploitation rate and catch statistic.

Stock evaluation in The Tana Watercourse

At the moment there are stock specific evaluations for only five of the Norwegian tributaries.

  • Máskejohka (in the lower part of the river system)
  • Lákšjohka (middel part)
  • Válljohka (middel part)
  • Iešjohka (upper part)
  • Kàràšjohka (upper part)

Target attainment in these five rivers function as an indicator of how the target attainment are in the lower, upper and middle part of the Tana watercourse.

The evaluation of target attainment in River Máskejohka, Válljohka, Iešjohka and Kàràšjohka is based on exploitation rate estimates. In river Kàràšjohka monitoring of ascending Atlantic salmon by radar sonar (DIDSON) gave a good estimate of exploitation rate in 2010. In River Lákšjohka video monitoring has given a precise number of ascending salmon since 2009. Combined with catch statistics gives the video monitoring gives good information about the size of the spawning stock.

The evaluation of target attainment in the five Norwegian tributaries incorporated in the evaluation reveals that all the stocks are far from reaching their spawning target. River Tana is at the moment overexploited.   

Value of target attainment; an example

The value of target attainment is enormous. The monitoring group of River Tana presented an example to illustrate this:

The Kàràšjohka has a spawning target of 11 501 kg females, and calculated smolt production capacity of 195 376 smolts. At 100 % target attainment combined with high sea survival, the pre-fisheries abundance would be enormous; almost 30 000 salmon of the Kàràšjohka stock would enter the coastal waters. This amount of salmon would stand a very high exploitation rate (tab 1). At a low target attainment (20 %) the smolt production would be much lower, and this would lead to smaller pre-fisheries abundance (6 000 at high sea survival). The Kàràšjohka stock would be able to sustain a much lower exploitation rate.

Tab 1: Value of spawning target attainment. Source: Research and monitoring group of River Tana.

I this example it is used high sea survival (15 %). With low sea survival (e.g. 5 %) fewer of the smolt would survive, and the pre-fishery abundance of adult salmon is a lot less. The sustainable exploitation rate would be lowered greatly.

This example shows that; 1) high target attainment in the different part of the watercourse all together would give enormous amounts of salmon one salmon-generation later, and 2) The importance of having a regulation that maximazes the probability  that enough samlmon survives to spawning.         

Mixed stock fishery

There are 20-30 different stocks of Atlantic salmon in the Tana River watercourse today. Fisheries in the main stem therefore are considered as mixed stock fisheries. This means that Atlantic salmon also from stocks that would have problems of reaching their spawning target are exploited. Different stocks have different run time. By learning when different stocks are present in the different part of the river it would to some extent be possible to have a sustainable mixed stock fishery in the main stem.

Sequential accumulated exploitation

In the Tana River watercourse there is a unique challenge of sequential accumulated exploitation. Each stock is exploited in several different stages. First, Tana salmons are exploited in the costal fisheries, then in the Tana Fjord, and in the estuary. Next, salmons are exploited in the main stem; first in the lower Norwegian part, and later on in the border area between Norway and Finland. The surviving salmons that manage to enter their natal river in e.g. one of the tributaries, are also exploited there.  

An example

The monitoring group of River Tana presents an example of the effect of the sequential accumulated exploitation in their latest report. The exploitation rate for Atlantic salmon of the Iešjohka stock is calculated for the costal fisheries (19 %), the lower main stem of the river (20 %), the border stretch (Finish-Norwegian) of the main stem (63 %),  and for the in river fisheries in River Iešjohka (50 %).

“If 10 000 Iešjohka salmon swims towards the coast in the spring 1 900 will be taken of the costal fisheries, and 8 100 will survive and arrive at the Tana estuary. Of these survivors, 1 620 (20 % of 8 100) will be taken in the lower (Norwegian) Tana main stem, and 6 480 salmon will be able to arrive the border area of the Tana main stem. In the border area, 4 082 salmon will be taken and 2 398 salmon will survive to enter the Iešjohka. In Iešjohka, 1 199 will be fished, and 1 199 will survive to spawn. This is 12 % of ther original 10 000.”   

This example is useful to demonstrate the effect of regulation in the different sequences of the total exploitation. If the management for some reason would close the fishery in the lower main stem (harvest 20 % of the total stock that arrive the Tana River), fisheries at the border stretch and in the tributary would be more successful (would take approximately the same percentage of the stock). Instead of the 12 % of the total run witch survived and spawned in the example, 15 % would survive.

If the fisheries were regulated in all sequences to 15 % exploitation at the cost and in the lower main stem, 50 % at the border stretch of the main stem, and 30 % in the tributary (River Iešjohka), the spawning stock would be more than doubled. Instead of 1 199 in the example, 2 528 salmon would have the possibility to spawn.

The main message of this example is that only with combined effort throughout the exploitation sequences it is possible to let the stocks in the upper part of the Tana River watercourse meet their spawning targets.   

Read more about the evaluation of the spawning target here:

Status of the river Tana salmon populations. Report 1-2012 of the working goup on salmon monitoring and research in the Tana river system