Assumptions regarding development of fisheries and key biological parameters

Table 4.3.1.1 provides a summary of assumptions on which the stock projections are based. The fishing scenarios differ from the ones in previous assessments, but the overall structure is similar to, for example, the previous full assessment (ICES, 2019). Furthermore, the reference year for assessing the effects of different fishing options in the advice year on smolt production has been shifted one year earlier, as explained below. This was done due to the recent successive change in fishing pattern towards harvesting mature (instead of immature) salmon to higher extent than in the past.

Fishing scenarios

Scenario 1 illustrates stock development in case all fishing (both at sea and in rivers) is closed, whereas scenario 2 is similar with the exception that only sea fisheries (both recreational and

commercial) are closed but fishing is allowed in all rivers except those where it is currently banned (Kågeälven, Ljungan, Testeboån and Emån). Scenarios 3–6 illustrate fishing scenarios with the current fishing pattern and a differing degree of total removal at sea (both recreational and commercial). Note that there is no longer a scenario corresponding to a “base case”, i.e. the same future removal as advised by ICES for the current year (total commercial sea catch of 116 000 salmon in 2021), as in earlier assessments. Scenarios 7–10 introduce a new fishing pattern in which offshore fisheries (both recreational and commercial) are closed, and coastal fisheries in SD29-31 would be allowed with a differing amount of total removal. In scenarios 3–10 river fisheries are taking place similarly as in scenario 2.

As in previous years, fisheries in the interim year (2021) follow the scenarios, except for longline fishing during the first months of the year, which is estimated based on the effort observed dur-ing the corresponddur-ing months of 2020.

Scenarios were modified to account for annually varying harvest rates in recreational trolling and catchabilities in commercial longlining. We assumed that the longline catchability in the future will remain the same as in the last observed year (2020). Similarly, the harvest rate esti-mated for trolling in 2020 was used also for future years. To obtain the desired total removal for each fishing scenario, the effort values from 2020 were given for the future years, and optimiza-tion was performed to find an effort multiplier that resulted in a total sea catch corresponding to the desired (scenario-specific) total removal in the advice year (2022). Total sea catch was ob-tained as the sum of catches from coastal trapnet, offshore longline and recreational trolling fish-eries. The same multiplier was used for coastal trapnet, offshore longline and recreational trolling in scenarios 3–6. In scenarios 7–10 an effort multiplier was applied only for coastal trap-netting, whereas harvest rates for offshore fisheries were set to zero. It should be noted that the current methodology keeps the fishing pattern the same between the scenarios in terms of rela-tive differences in harvest rates, not in catches. Thus, in scenarios with high total removal, a greater share of the catch will be taken in the offshore areas compared to ones with lower re-moval, because offshore fisheries are first in order in the fishing pattern.

The recreational trolling fishery is now handled in a slightly different way to earlier years, since it has been added as a separate fishery to the scenarios. Earlier (e.g. in 2019), it was included as a part of the offshore longline fishery and it was assumed that the recreational sea effort would stay the same over all scenarios, while the number of salmon available to the fishery varied ac-cording to the commercial removal.

Because the scenarios are technically defined in terms of future fishing effort, the predicted catches have probability distributions according to the estimated population abundance, age-specific catchabilities and assumed fishing effort. Scenarios 3–6 assume the same fishing pattern in commercial fisheries (division of effort between fishing grounds) as realized in 2020. Figure 4.3.2.8a–c shows the harvest rates prevailing in scenarios 4, 6, 7 and 10.

Survival parameters

In both the M74 and the post-smolt mortality (Mps) projections, an autoregressive model with one year lag (AR(1)) is fitted at the logit-scale with the historical estimates of the survival param-eters. Mean values of the mean of the post-smolt survival over years 2016–2019 (16%), variance over the same time-series and the autocorrelation coefficient are taken from the historical analy-sis into the future projections. The method for M74 is similar, but the stable mean for the future is taken as the mean over the whole historical time-series. In addition, the forward projection for Mps is started from 2019 to replace the highly uncertain model estimate of the last year of the historical model and the future uncertainty is adjusted to accommodate the range of historical variation in M74. The starting point of M74 projections is 2021. Time-series for Mps and M74 survival are illustrated in Figure 4.3.2.1.

Adult natural mortality (M) is assumed to stay constant in future, equalling the values estimated from the historical assessment. Different fisheries occur at different points in time and space, and many catch only maturing salmon, which have been subject to several months’ natural mortality within a year. Thus, to increase comparability of abundances and catches, the abundances at sea have been calculated by letting M first decrease the PFA (stock size at the beginning of year) of multi-sea-winter salmon for six months. Moreover, the stock size of grilse has been presented as the abundance after the period of post-smolt mortality and four months of adult natural mortal-ity. This period is considered because the post-smolt mortality period ends in April, after which eight months of that calendar year remain during which grilse are large enough to be fished.

Half of that period, i.e. four months, is considered to best represent the natural mortality that takes place before the fishing.

Maturation

Annual sea-age group-specific maturation rates are given as the average level computed over the historical period, separately for wild and reared salmon. This projection starts from 2022, as the maturation rates of 2021 can be predicted based on sea surface temperature (SST) information from early 2021 (ICES, 2014, Annex 4). The time series of maturation rates are presented in Figure 4.3.2.2.

Releases of reared salmon

The number of released reared salmon per assessment unit is assumed to remain at the same level in the future as in 2020 (Table 3.3.1).

Evaluation of stock status under various catch options for 2022

For other fish stocks assessed by ICES, biological reference points often apply to spawning stock (typically expressed in terms of biomass, SSB) at the end of the advice year. For Baltic salmon, however, there is a half-century-long tradition of using smolt production as the main metric of abundance (ICES 2020b). Accordingly, reference points and stock status for Baltic salmon are expressed and evaluated in terms of smolts (i.e. recruits produced by a certain spawning stock) rather than the spawning stock itself. Because of the time lag between spawning and smoltifica-tion, fishing in any specific year will not affect smolt production until some years later.

The schematic and approximate figure below illustrates how sea fishing for Baltic salmon in a particular calendar year affects future smolt production and status (e.g. evaluated using R^lim). As shown by blue arrows, fishing in 2022 will mainly affect smolt production in 2026 (or 2025, de-pending on the AU), whereas current stock status – i.e. smolt production in 2020 (last year with data) – reflects past fishing and spawning stocks (mainly 2016).

Based on results for the 10 fishing scenarios presented earlier, stock status corresponding to smolt production in 2026 (AU 1-3) or 2025 (AU 4) is evaluated below (Section 4.3.2). The one-year difference between AU’s reflects latitudinal differences in average smolt age. Note that the time lag of 3 or 4 years from the advice year until smoltification is one year shorter than what has been used for corresponding evaluations in previous years. The reason for this change is the recent shift in fishing pattern with an increased share of coastal catches from the Gulf of Bothnia, targeting only maturing salmon during their spawning migration. In the past, when fishing on the Main Basin feeding grounds was taking a much larger part of the total sea catch than at present, using smolt runs 4-5 years ahead from the advice year as reference years was considered more accurate, as fishing was then targeting a larger share of immature salmon. Also, note that the new scenarios 7-10 (added this year) are entirely based on Gulf of Bothnian coastal fishing on spawning migrating salmon.