A salmon that swims back out is not necessarily a saved salmon

Returning to the water and using warm water: survival does not guarantee unaltered reproductive success

In Atlantic salmon, the 97% figure only tells part of the story.

A salmon swims off after being unhooked. For the fisherman, it's a relief. But this single departure doesn't, in itself, guarantee that the fish will fully recover, continue its migration, and contribute to the next generation.

The distinction is essential: surviving a release and maintaining intact reproductive success are two different things.

Science helps us better understand this difference. It also shows why capture conditions, particularly water temperature, must play a central role in our fishing decisions.

What the 97% really means

In a study conducted on the Alta River in Norway, Thorstad and colleagues reported that 97% of the 30 radio-tagged salmon survived release and were present in known spawning areas during the reproductive period . The captures took place in water temperatures of 10–14.5 °C . [1]

This is a favorable result for releasing the fish back into the water. But it is not a measure of the offspring produced.

The presence of a fish in a spawning area does not tell us how many offspring it will be attributed, nor whether its contribution will be comparable to that of an uncaught fish.

The study also noted adverse effects associated with factors such as fight duration, injuries, bleeding, handling, and exposure to air and temperature. These effects affected the fish's condition, stress levels, and behavior. [1]

The Alta results therefore demonstrate high survival rates under the studied conditions. However, they do not demonstrate a 97% reproductive success rate.

Releasing the animal back into the water has conservation value.

Recognizing these limitations does not mean condemning the practice of releasing the fish back into the water.

A salmon kept before spawning will not participate in the next spawning season. A released salmon can still contribute. Releasing them thus helps preserve breeding stock that would otherwise be harvested.

But its value as a management tool does not mean that every catch is without consequence. Rather, it compels us to examine the conditions under which we choose to fish.

The question then becomes: how to reduce the effects of capture on a fish that still has to complete its migration and reproduce?

When the water warms up, the risk increases.

In 2020, Van Leeuwen and his colleagues combined data from several studies to model post-release mortality as a function of temperature. [2]

Their analysis indicates generally low mortality in fresh water, less than about 5% below 12°C. For the 18-20°C range, the model provides an average estimate of about 16% , with a reported range of 7-33% taking uncertainty into account . [2]

These figures do not constitute a universal rate. The authors note significant variability between studies at high temperatures and recommend caution in interpreting the predictions. [2]

It would therefore be excessive to claim that, in each river, exactly 16 out of 100 salmon will die after being released back into the water between 18 and 20 °C.

But if such a rate were applied to a group of 100 fish caught before spawning, it would represent 16 fish that would not participate in that reproduction. Survival thus remains a prerequisite for any future reproductive contribution.

The temperature at the time of capture matters

An analysis published in 2024 by Van Leeuwen and colleagues sheds further light on the issue. It re-examines data collected in Newfoundland between 2018 and 2020 and focuses on mortality within five days of release . [3]

Among salmon caught at 20°C or above , 4 out of 18 died , or about 22%. In a subgroup of fish caught at 20°C or below, but subsequently exposed to an average temperature of at least 20°C, 1 out of 13 died . [3]

These small sample sizes do not allow the percentages to be transformed into general rules. The authors nevertheless conclude that, under the conditions examined, the temperature at the time of capture remains the best indicator of survival after release. [3]

This does not make subsequent temperatures unimportant. It underscores the significance of the conditions present when the fish is subjected to the fight and handling.

Reproduction requires a different approach.

To go beyond simply observing survival, researchers have used genetics to link fry to their parents.

Richard, Dionne, Wang, and Bernatchez did so in a study published in 2013. Their results suggest a size-dependent effect of release, with larger individuals being more affected in their reproductive contribution. [4]

They also detected an interaction between temperature and air exposure. However, the observed relationships were complex: they should not be reduced to a universal formula for calculating reproductive loss for each additional degree or each second out of the water. [4]

This study concerned one population and one year. It measured the contribution to the sampled fry, and not the number of offspring that would one day return to reproduce. [4]

However, it shows why it is necessary to examine the possible consequences of capture beyond immediate survival .

What the Rimouski study shows

Bouchard and his colleagues published a study in 2022 on the reproductive success of salmon released back into the Rimouski River. [5]

At least 83% of the fish released back into the water that reached the upstream side of the dam reproduced , including some released at temperatures above 20°C. [5]

However, the relative reproductive success of released females reached approximately 73% of that of uncaptured females . This is a difference in reproductive contribution, not a survival rate or a proportion of females that became unable to spawn. [5]

This 27% difference should also not be attributed directly to heat. In trapped fish, researchers did not detect a decrease in reproductive success with increased temperature at the time of release. [5]

In contrast, fish caught in warmer water were less likely to become trapped. Therefore, the reproductive outcomes observed in fish that migrated upstream do not necessarily predict the fate of all released fish. [5]

Fish not found in the trap: an uncertainty to be respected

We need to be rigorous in both directions.

A salmon absent from the trap cannot automatically be considered dead or as having failed to reproduce. Another publication from the same team specifies that spawning also occurs downstream of the dam , in an area excluded from their upstream reproductive monitoring. Fish that reach upstream are, in fact, trapped and then transported over the obstacle. [6]

The question of what will happen to the missing fish therefore remains open. This limitation prevents us from stating that they have all recovered fully, or from concluding that they have all been lost to reproduction.

18°C is not an absolute limit

The studies cited do not allow us to conclude that at exactly 18°C , each salmon would experience a specific reduction in its reproductive success. [2, 4, 5]

Nor do they allow us to consider any lower temperature as a guarantee of no effect. The risk described by the analyses varies with the conditions; it is not simply a matter of switching on and off. [1, 2, 4]

For me, 18°C is a benchmark for caution, not a proven biological boundary. As the water warms, particularly around 20°C and above, mortality data provide further reasons to limit avoidable stresses. [2, 3]

This position stems from a precautionary choice informed by research. It does not claim to replace local regulations or establish a threshold valid for all rivers.

Adapting our decisions to the state of the population

The abundance of a population must also be taken into account. But a river that hosts many salmon does not, in itself, allow us to conclude that the losses would be negligible.

The challenge is to decide what risk we are willing to accept, given the state of the stock, environmental conditions, and uncertainties.

We don't have to wait for a statistically significant effect to be detected in every river before choosing to reduce avoidable pressure. The absence of a significant result is not a guarantee of the absence of consequences.

The salmon before the first cast

Releasing a boat back into the water deserves more than just a number used as a guarantee. It deserves careful practice and an honest assessment of the available knowledge.

Studies show that many released salmon survive and reproduce. They also show that results vary, that warm water increases survival concerns, and that reproductive contribution may be affected. [1–5]

Seeing a fish swim away is always an important moment. But what we want to protect goes further: its ability to reach a breeding habitat and contribute to the next generation.

That's why my decision to fish shouldn't begin with the question: "Can I still successfully catch and release?"

She should start with this question: "Under the current conditions, is it reasonable to subject this fish to this fight?"

Giving up a few casts when conditions become difficult is also a fisherman's gesture.

Salmon first and foremost. And especially before the first cast.

Sources

[1] Thorstad, EB, Næsje, TF, Fiske, P. and Finstad, B. (2003). Effects of hook and release on Atlantic salmon in the River Alta, northern Norway. Fisheries Research, 60(2–3), 293–307. Consult the study

[2] Van Leeuwen, TE et al. (2020). Mortality of Atlantic salmon after catch and release angle: assessment of a recreational Atlantic salmon fishery in a changing climate. Canadian Journal of Fisheries and Aquatic Sciences, 77(9), 1518–1528. Consult the study

[3] Van Leeuwen, T.E., Keefe, D., Young, M. and Adams, B. (2024). Water temperature at the time of the catch-and-release event is a better predictor of survival in Atlantic salmon (Salmo salar) than acute water temperature changes before and after. Journal of Fish Biology, 104(5), 1623–1627. Consult the study

[4] Richard, A., Dionne, M., Wang, J. and Bernatchez, L. (2013). Does catch and release affect the mating system and individual reproductive success of wild Atlantic salmon (Salmo salar L.)? Molecular Ecology, 22(1), 187–200. Consult the study

[5] Bouchard, R., Wellband, K., Lecomte, L., Bernatchez, L. and April, J. (2022). Effect of catch-and-release and temperature at release on reproductive success of Atlantic salmon (Salmo salar L.) in the Rimouski River, Quebec, Canada. Fisheries Management and Ecology, 29(6), 888–896. Consult the study

[6] Bouchard, R., Wellband, K., Lecomte, L., Bernatchez, L. and April, J. (2022). Effects of stocking at the parr stage on the reproductive fitness and genetic diversity of a wild population of Atlantic salmon (Salmo salar L.). Evolutionary Applications, 15(5), 838–852. Consult the study