Noteworthy Topics (pdf)
Here we present items that are new or noteworthy and of potential interest to fisheries managers.
Factors Affecting 2022 Western Alaska Chinook Salmon Runs & Subsistence Harvest
Western Alaska Chinook salmon runs have concurrently declined to low abundance levels for over a decade (ADFG, 2013; Schindler et al., 2013; KRITFC, 2022; Liller, 2021)4. Salmon are integral to the Western Alaska ecosystem, bridging marine and freshwater habitats, filling both prey and predator niches, and supporting vital subsistence harvests (Courtney et al., 2019; KRITFC, 2022). Figure 3 highlights the factors that contributed to the 2022 run sizes of Chinook salmon across Western Alaska as evidenced by Western science, Indigenous Knowledge, and community observations from the Kuskokwim and Yukon rivers.
Cumulative ecosystem factors since 2016 impacted the spawning adults, to the marine-stage juveniles, and ultimately the returning adults in 2022. For the parent spawners in 2016 and 2017, marine heatwave conditions (p. 31), smaller and younger size at maturity (Lewis et al., 2015; Oke et al., 2020), and warm river temperatures during the adult spawning migration likely contributed to reduced reproductive success (von Biela et al. (2020), Howard & von Biela (in review)). Low summer water levels and warm river conditions had the potential to impact eggs (2016-2017), and freshwater conditions could continue to influence fry and smolt growth and survival (2017-2018), but those relationships vary in different places depending on the absolute values of temperature, flow, and water level and are not fully understood across different tributaries.
Marine juveniles experienced heatwave conditions again in the eastern Bering Sea in 2019 when low zooplank- ton productivity (p. 79) contributed to empty stomachs and decreased fish condition (Murphy et al., 2021). In 2019 and 2020 combined, approximately 28,300 immature Chinook salmon from Western Alaska (Yukon and Coastal Western Alaska regions) were caught as bycatch 5). The estimated impact rate of bycatch to combined Western Alaska Chinook salmon stocks averaged 1.9% for the 2011-2021 runs (see Table 9 here 6) or annual estimates of 6,331-10,614 fewer spawners to Western Alaska (see Table 7 here 6). The impact rate for the 2022 run is not yet available, but is expected to be higher based on low run sizes in 2022 (i.e., impact rate is inversely related to run size).
Marine temperatures largely relaxed to average conditions over the past year (Figure 23), which may have a positive effect on 2022 spawning success. However, amounts necessary for subsistence use of Chinook salmon in Kuskokwim and Yukon communities have not been met since 2010 and were not met again in 2022. Food security impacts associated with Chinook salmon declines in Western Alaska have been compounded by declines regionally in other salmon species, such as coho and chum salmon (KRITFC, 2022).
Figure 3: Factors affecting 2022 Western Alaska Chinook salmon runs and subsistence harvest.
Contributed by Kevin Whitworth and Terese Schomogyi - Kuskokwim River Inter-Tribal Fish Commission
High Resolution Climate Change Projections for the Eastern Bering Sea
"Carbon mitigation" includes national and global policies and technologies to reduce greenhouse gas emissions and increase atmospheric carbon recapture in order to reduce global warming and climate change. In the absence of immediate implementation of widespread carbon mitigation measures, significant warming of sea surface and bottom water temperatures (SST and BT, respectively) are projected to occur across the Bering Sea over the next century, driving average water temperatures at the end of the century to be as warm or warmer than those observed during recent marine heatwaves. Specifically, under a low carbon mitigation scenario ('ssp585') modeled bottom temperatures consistently exceed average historical (1980-2013) ranges by 2040-2060. In contrast, in scenarios with immediate implementation of high carbon mitigation actions, warming is projected to be much more gradual over the next century and by 2080-2100 only moderately warmer than present day. In essence, scenarios that include immediate and large-scale implementation of carbon mitigation measures predict a future Bering Sea that is slightly warmer but relatively similar to contemporary conditions, while scenarios with delayed or minimal implementation project warming that drives the modeled Bering Sea system to conditions well beyond those observed to date (Figures 4 and 5).
Figure 4: Bias-corrected summer sea surface temperature (top row) and bottom temperature (bottom row) for the southern Bering Sea (SEBS) from the hindcast (dark blue line) and projections under high (ssp126, left column; cool colors) and low (ssp585, right column, warm colors) mitigation scenarios. A ten year running mean is shown in the dark line and shading indicates the standard error of mean values; individual Earth System Models are shown as individual lines. Average modeled temperatures from the reference period (1980-2013) of the hindcast are shown as the horizontal blue line; dashed lines represent ±1 standard deviation of the mean. Note different scales between rows.
Projected warming differs slightly across seasons as well as mitigation scenarios. Warming is generally larger across all regions and seasons under low carbon mitigation scenarios. However, in the northeastern Bering Sea (NEBS) there are large differences in winter bottom water warming between low and high carbon mitigation scenarios. We examined three contrasting earth systems models to evaluate the spread and characterize the agreement in projections (see methods in Hermann et al. (2021) for more detail). Under high mitigation scenarios, two of the three models projected continuation of cold winter conditions, indicating the potential for sea ice and cold bottom water temperatures to be preserved to some extent over the next century in these scenarios.
Global Warming Levels (GWLs) are an index used internationally by policy makers to standardize discussions around future climate change impacts. GWL indices represent average warming across the entire globe (all seasons and regions) in degrees Celsius relative to pre-industrial average global temperatures from the years 1850-1900. Present day GWL is around +1.1°C, meaning that on average the earth's atmosphere near the surface is 1.1°C warmer than it was during the pre-industrial era at the end of the last century. This warming is unprecedented in the last 2000 years, and temperatures in the most recent decade (2011-2020) are warmer than any period in the last 125,000 years (IPCC, 2021). Based on multiple lines of evidence, the IPCC and other experts have identified critical GWLs of +1.5 and +2°C, beyond which climate change impacts and risks across sectors and nations rapidly increase, and the feasibility and effectiveness of adaptation actions become highly uncertain (IPCC, 2022). Of note, GWLs of +1.5 and +2°C represent the target and limit respectively of the Paris Agreement, a legally binding international treaty on climate change (i.e., UNFCCC Paris Agreement and Nationally Determined Contributions (NDCs)7).
While the earth as a whole has warmed approximately 1.1°C, warming to date has not been even across regions and the Arctic has warmed roughly +2 to +3°C to date. To understand what GWLs mean for the Bering Sea marine ecosystem, we used high resolution model projections to translate GWL indices into regional changes in SST and BT. A GWL of +1.5°C (over the pre-industrial average, or roughly +0.4°C global warming relative to present day) is projected to result in eastern Bering Sea SSTs and BTs that are similar to present day conditions (Figure 6). However, at GWL of +3 and +4°C (or +1.9 to +2.9°C global warming relative to present day), significant warming is projected to push water temperatures well beyond those observed to date, even during recent marine heatwaves. Ongoing work as part of the Alaska Climate Integrated Modeling (ACLIM) project 8 and numerous climate change studies find evidence of increasing risk for Bering sea ecosystems, fisheries, subsistence resources, and coastal communities associated with higher warming rates (IPCC, 2022). Differences in trends between low and high carbon mitigation scenarios demon- strate the scope for warming of the Bering Sea to be ameliorated through carbon mitigation. Importantly, there is high potential to limit summer bottom temperature warming to less than ∼3°C (over 1980-2013 averages), provided sufficient global cooperation results in necessary reductions in carbon emissions.
For more details on these high resolution climate change projections for the eastern Bering Sea, please see p. 209.
Kirstin K. Holsman - NOAA Fisheries, Alaska Fisheries Science Center
Figure 5: Bias-corrected summer sea surface temperature (top row) and bottom temperature (bottom row) for the northern Bering Sea (NEBS) from the hindcast (dark blue line) and projections under high (ssp126, left column; cool colors) and low (ssp585, right column, warm colors) mitigation scenarios. A ten year running mean is shown in the dark line and shading indicates the standard error of mean values; individual Earth System Models are shown as individual lines. Average modeled temperatures from the reference period (1980-2013) of the hindcast are show as the horizontal blue line; dashed lines represent ±1 standard deviation of the mean. Note different scales between rows.
Figure 6: Southern and Northern Bering Sea ('SEBS' and 'NEBS', respectively) modeled summer bottom and sea surface temperatures ('BT' and 'SST', respectively) as a function of CMIP6 Global Warming Levels (mean global increase in temperature relative to pre-industrial temperatures (1850-1900)). Recent hindcast ranges are reported ('2010-2021') as well as bias corrected projections from the Bering10K model for each GWL (+1 to +4°C GWL). Boxplots represent the 25thand 75thpercentile (i.e, the interquartile range) with the horizontal line representing the median temperature, and the error bars representing the min or max (IQR ± IQR*1.5). Outliers are represented by points (e.g., marine heatwave years if above the boxplot). For more information on interpretation of boxplots see https://r-graph-gallery.com/boxplot.html.