| dc.description.abstract | Anon. 2022. Tana/Teno stock recovery and sustainable fisheries. Report from the Tana/Teno Monitoring and Research Group nr 1/2022.
Since the early 2000s, the entire Tana/Teno salmon stock complex has had a negative development, recently culminating in a situation with no exploitable surplus left and a complete closure of the salmon fishery in 2021 and 2022. Concurrently, neighboring rivers have had the opposite trend, recovering from a precarious stock situation in the 1990s to reaching their respective management targets in combination with high catches.
An attempt was made to rebuild the Tana/Teno salmon stocks with the new agreement in 2017. But while this agreement successfully reduced the overall exploitation rate of Tana/Teno salmon, it also coincided with a prolonged period of poor sea survival from 2019 and onwards. Results from the Utsjoki video monitoring indicates that the return rate of grilse (1SW salmon) 2019-2021 was 18-40 % of the previous average. Consequently, the reduction in exploitation rate from the 2017 agreement was not large enough to result in any increased spawning stocks. With the 2021 fishing closure, a clear effect was found in spawning stock sizes across the Tana/Teno river system, an increase that is also expected to be found in 2022.
All evaluated stocks in Tana/Teno are currently found to be at a level that indicates the need for a stock recovery plan. In order to have a successful recovery, several key elements have to be in place. Firstly, an unambiguous set of criteria needs to be established concerning the relation between exploitation and the status assessment. A threshold for formal stock recovery should be established (corresponding to going from the yellow to the orange status category) and associated with lowered exploitation to counter what is a seriously depleted salmon status. The four largest salmon stocks of the Tana/Teno system (the main stem, Anárjohka, Kárášjohka, Iešjohka) are however in an even more precarious situation, having fallen from orange to the red status category. This latter category is reserved for a situation with no exploitable surplus over at least two of the last four years. This is a critical situation and no fishing should be permitted.
The negative stock development in Tana/Teno is the result of having removed too many spawners over a too long period. The pressure of a chronic high accumulated exploitation rate has consistently pushed the salmon stocks towards a situation with low fish density. In recent years, fisheries biologists have become increasingly aware of the dangers of pushing fish stocks towards very low densities as the stocks then become increasingly vulnerable to natural mortality factors. This phenomenon is called Allee effects or depensation. With Allee effects, the stock growth rate will decline at low stock densities, even to the point of becoming negative (meaning that more individuals die than is replaced by recruitment, effectively leading to a collapse). Because of Allee effects, getting a recovery process to effectively start might be problematic and to counter the existence of Allee effects, care should be taken to reduce exploitation as much as possible during the first generation of a recovery process.
In order to help finding a robust recovery process and establish criteria for a reopening of the salmon fisheries following two years of no fishing, the Tana/Teno Monitoring and Research Group (MRG) was tasked with answering a number of questions summarized below.
The first question concerned criteria for safely reopening the salmon fisheries, including how large the exploitable surplus should be. Most importantly, stocks placed in the red status category are in a critical situation and should not be exposed to any exploitation for any reason. This is particularly true for exploitation from a mixed-stock fishery. A sea survival returning to more normal levels would be followed by increased return rates of adult salmon and would enable the most critical stocks to return to a situation with at least some exploitable surplus. This might allow for a limited fishery, but care should be taken to 1) use forecasting to establish a quota that can be safely exploited within the terms of the recovery process, and 2) establish a regulation that allows for strict enforcement in order to avoid overexploiting the quota. We outline a concrete system for achieving this in the report.
The second question concerned the precision of monitoring used to establish estimates of exploitable surplus, and what safety margins would be needed to counter uncertainty in the estimates. The existing monitoring is likely to continue to be an effective tool at tracking stock status and developments in pre-fishery abundance. However, we need a probability framework to enhance our assessment of the recovery process with probability projections towards a successful recovery. These needs can be met with a Bayesian population model based on the Baltic salmon population model tailored to the needs of the Tana/Teno river system.
The third question concerned recovery time span. Here it is important to realize that a salmon stock recovery will have a stair-shaped form, increasing in steps towards the overall recovery goal. The lengths of these steps follow the salmon generation length, meaning that an effect of a decrease in exploitation in year 1 will not be observed in the number of returning adults until 7-8 years later (7-8 years being the time the salmon needs in order to hatch from eggs, live as juveniles, smoltify and then grow at sea before returning). The shortest possible recovery time would then be one generation, followed by two generations at approximately 15 years.
The fourth question concerned different strategies for combining recovery and a sustainable fishery and we provide a detailed discussion of three different approaches. The first strategy involves using the main stem sonar counts to dynamically adjust the fishery during the fishing season. The counting could either be used to provide a reopening criteria (closed fishing at the start, then a limited reopening later in the summer if the count exceed a preagreed criteria) or a criteria for closing (if the counting show a weaker than expected run of salmon). There are difficulties to these approaches. The second option is likely not a viable tool during recovery, but could possibly be used as a tool to increase the likelihood of keeping a stock at or around its spawning target. The first option might work during a recovery, but with the caveat that it might fail to protect late-run stocks such as the main stem and Anárjohka/Inarijoki salmon. For this reason, using the main stem sonar might work best as an additional criteria for the quota/forecasting-based approaches.
The second strategy concerned using forecasting to establish a total quota for a fishing season. There is a strong correlation between the number of 1SW one year, the number of 2SW the following year and the number of 3SW two years after. We demonstrate that this correlation were able to provide a fairly accurate forecast of the pre-fishery abundance in years with a normal sea survival. The forecasting underestimated the pre-fishery abundance in years with higher than normal sea survival, which is a consequence that have no adverse effects on the recovery. However, the forecasting severely overestimated the pre-fishery abundance in years with low sea survival, and additional criteria should therefore be used to avoid this. One mitigating option could be the use of the main stem sonar count.
The third strategy concerned the use of quotas in different forms. Our evaluation here finds that general and non-dynamic daily and seasonal quotas are unlikely to protect against overexploiting during the recovery process. These measures are expected to perform a bit better for keeping the stocks at or around their spawning target.
The last question concerned the potential effects of sea survival, predation, climate change and pink salmon on the recovery. Overall, sea survival emerges as a key factor. With the current low survival, fewer than expected salmon return and the scope for exploitation in combination with recovery is exceedingly restricted. An improved sea survival will increase the potential for reopening the fishery. Predation is significantly affecting salmon stock recovery when stocks are depleted down to the most serious status categories. Attempting predator control in this situation is likely difficult with uncertain effects, some of which might even be negative for salmon. Predator control is therefore not advisable, and restricting mortality through exploitation is likely to be a significantly more efficient approach. Climate change will affect the Tana/Teno salmon, both negative and positive. Very little is still known about the possible effects of pink salmon. What is certain, however, is that significant attempts to eliminate the pink salmon will have adverse effects on salmon and the prospect of successfully recovering the salmon stocks, further limiting the scope for fishing in odd years during the stock recovery process. | en_US |
