97 increases because sea ice reflects solar radiation back into space. One of the primary changing water variables that threaten Alaskan fisheries is ocean acidification, a result of increased levels of dissolved carbon dioxide in seawater. Like other water chemistry values, acidification is expected to accelerate. Predictions are that within the next 15–30 years pH levels will drop low enough to endanger several Alaskan marine species (Johnson, 2016). Changes in Fishery Stock Levels Depending on their catch, particular Alaskan fisheries are expected to feel more financial impact from climate change than others. The combined catch of salmon, pollock, and Pacific cod makes up the large majority of both value and volume of seafood landed in the state, comprising 70% of the seafood ex-vessel value and 83% of the total volume. Here, ex-vessel refers to the value of the catch directly paid to fishermen prior to processing. Moreover, highvalue catch, including crab, halibut, and sablefish (black cod), are large drivers of value for the sector. They constitute 20% of the value produced by the industry but only 2% of volume. Flatfish and rockfish are also key Alaskan products, constituting 9% of total value and 14% of volume (McDowell Group, 2020). With oceanographic variables changing, though, such species have responded by changing distribution and productivity as well as by adapting their physiological behavior. As oceans warm, range dispersion and extension have become increasingly common in the northern hemisphere for many species. Typically, the center of a species’ biomass shifts to colder waters, either north or to deeper water, to reestablish their environment equilibrium. For example, among native Alaskan species, snow crab biomass has shifted 54 miles north, and pollock are inhabiting ever deeper waters. Invasive species not native to the region continue to shift north too. Globally, scientists estimate a 30% to 70% increase in total biomass in colder subarctic waters like the Bering Sea; at the same time, a 40% to 60% decrease in biomass is predicted in the tropics (Holmyard, 2014). Such a shift constitutes an increasing threat of invasive species inducing increased predation, competition, and disease for native Alaskan species. Currently, only behavioral adaptations have been seen in species, such as changes in distribution and species targeting alternative prey. However, physiological change has yet to be well proven among Alaskan species; physiological changes typically occur over longer periods of time than behavioral ones. Of the types of Alaskan seafood, the combined catch of the five species of salmon constitutes the largest value for the industry, annually averaging $744M in ex-vessel value and approximately $1.73B in (post-processing) first wholesale value in 2017 and 2018 (McDowell Group, 2020). Because salmon go through two freshwater phases and extensive ocean migrations annually, the species is susceptible to many changing variables. The impact of increased northern Pacific temperatures over the last 40 years on salmon stock abundance and productivity in Alaskan waters differs by species. However, the relative sizes and lengths of all five Alaskan species have trended downward. By 2010, compared to pre-1990 values, the average length of Chinook salmon decreased by 8%, sockeye by 2.1%, and the other three species somewhere in-between, signifying the continual increased stress placed on the species due to changing waters. Decrease in size, specifically for Chinook, resulted in a 16% decrease in average egg production, 28% decrease in nutrient transport, and 21% decrease in commercial value per fish (Oke et al., 2020). Periods of warm temperatures tend to increase the return of all five species to Alaskan seas thanks to increased zooplankton availability for juvenile salmon post-spawn. The early ocean stage is critical for survival post-freshwater juvenile development, where migration has evolved to synchronize with peaks in plankton availability. That migration to Alaska, however, reduces return and abundance throughout British Columbia and the US Pacific Northwest. Pink, chum, and sockeye salmon all have long-term trends of increasing abundance in Alaskan seas. But the three species spend a lesser time during juvenile development in freshwater streams, thus exhibiting increased biomass due to greater food availability in the warmer waters. By contrast, coho and Chinook salmon tend to spend a year or more of juvenile development in freshwater streams, making them more susceptible to increasing stream temperature and low flow, which stress these
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