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The Population of Pollock Under Climate Change as Determined by Age, Distribution, and Prey Energy Content
Pollock, like many other species, respond to the threats of climate change within their home in the Bering Sea. Living in an ecosystem hugely affected by its seasonal ice sheet, pollock are dependent on the timing and extent of its annual movement. The connection examined in this paper is the relationship between algal blooms, cold water stratification, juvenile pollock predation, and adult pollock fishery recruitment. As the reach and lifespan of the ice sheet fluctuate, so does the amount of cold water habitat and ice algae that juvenile pollock depend on to survive to adulthood. During a year with an earlier retreat of/or less ice a smaller cold water area is established, leaving juvenile pollock open to their cannibalistic adult counterparts. Also, during such a year, ice algae production does not provide the high energy lipids needed to fuel the juvenile pollock population through their growth. This chain effect, while not threatening for the survival of the entire population, does have significant implications for fishery recruitment.
Climate change as a global phenomenon acts uniquely in different environments to a wide range of possible effects on almost every species. In the Arctic, many of these individual systems draw back to the infamous retreating ice sheet, upon which Arctic species live, hunt, reproduce, and die. One Arctic species with major implications to humans may be experiencing difficulties due to climate change as retreating sea ice alters its habitat in the Bering Sea. Walleye pollock, (Gadus chalcogrammus), is a billion dollar industry in the US. This industry depends on the natural seasonal variability of the Bering Sea ice sheet as it annually descends and retreats over the Bering Sea. This is the environmental clock that marks the algal blooms pollock depend on. In this way, as climate change alters the ice landscape the energy content of the lower food chain is also affected, leading to a possible decrease in survival for adult pollock.
Physical Oceanography of the Bering Sea
There are three hydrographic areas within the southeastern Bering Sea shelf: the coastal shelf, with a depth of less than 50 meters; the middle shelf, with a depth of 50-100 meters; and the outer shelf, with a depth of 100-200 meters (Bering Sea, 2014). Pollock can be found over most of the Bering Sea, but much of the population and studies occur in the Eastern Bering Sea (EBS), where the research is centered. Pollock spend much of their time over the 500 kilometer wide sea shelf, which is generally less than 180 meters deep (Hunt, et. al., 2011; Bering Sea, 2014). The processes that occur within the central shelf are most critical to pollock. (Stabeno, et. al., 2012)
A comparison between the -2 degree water in the cold pool during a warm year (2003) and a cold year (Blue) (2007) with depth contours of the EBS marked. Note that the warm year highlight has been moved down 2 degrees of latitude to show comparison.
The middle part of the southeastern Bering Sea shelf is the region within the Bering Sea most affected by climate change. In this area, a well-mixed water column appears in winter due to the strong winds; however, in summer two clearly separated layers appear. The surface layer of the summer water column is mixed by the wind while the bottom layer is mixed by the tide. The nutrient-rich bottom layer is insulated from warming by the surface layer once the water column stratifies. This insulation during the summer months causes the bottom layer to warm only slightly. Because the temperature of the bottom layer, the cold pool, depends on the water column’s temperature during the time of stratification, the time of ice retreat affects it greatly (Stabeno, et. al., 2012). The cold pool’s temperature stays below two degrees Celsius for the summer in cold years when extensive spring ice remains through April, while, during warm years with early ice retreat, the cold pool’s temperature remains above two degrees Celsius during summer.
Seasonal Ice Sheet Data
According to historical records, the continuous decline of the Arctic sea ice extent began in the late 1800s and has rapidly increased over the last three decades. The rate of ice loss in this period is unequaled by any other sea ice recession in the last thousand years (Polyak et. al., 2010). Additionally, the annual mean temperature in the Arctic is now measured at being more than 1.5 degrees Celsius higher than it was in the period of time between 1971 and 2000. (Overland, et. al. 2013).
Compiled historical records relating to Arctic ice margins have shown that a general retreat of seasonal Arctic ice has been occurring since early in the twentieth century. This retreat has particularly accelerated in the last five decades in regards to both seasonal and perennial ice. Though reliable satellite records of ice margins have only been available since 1979, in the three decades of their existence, the recorded data has exhibited generally negative trends in sea-ice extent; the month of September is particularly significant with a decline of 11% per decade. (Polyak, et. al., 2010).
Since the 1980s, Arctic sea ice volume has declined by 75% (Overland, et. al., 2013); between 1982 and 2007, perennial sea ice over five years of age decreased by 56%. The general coverage of perennial ice decreased by 88%, and any ice exceeding nine years of age all but disappeared. (Stroeve, et. al., 2008). A seasonally nearly ice free
Arctic, an Arctic devoid of almost all perennial ice, should appear within the next fifty years. (Overland, et. al., 2013; Polyak, et. al., 2010; Stroeve, et. al., 2008). This eventuality will increase Arctic warming and may also affect weather systems that range beyond the Arctic. (Polyak, et. al. 2010).
Pollock, (Gadus chalcogramma) was our main species of consideration. These groundfish are a relative of cod that commonly populate the Eastern Bering Sea. During their growth an individual can be expected to reach 30-91cm. Their range of habitat extends from roughly 100 meters below the surface to 300 meters, but they have been spotted at depths as low as 1000 meters. Pollock, with a twelve year life span, go through several life phases based on age that dictate behavior and position on the food chain. These life phases will be referred to as adult; over two years, or juvenile; less than two years. Juvenile can also be broken into age 0, which hatched that year, and age 1.
Distribution of pollock is dependant mainly on age and temperature (by season), and predator locations (Benoit-Bird et. al. 2013). Younger fish generally subsist on zooplankton such as copepods, while adults eat euphausiids (krill), tunicates, copepods, shrimp, and other fish as well as sometimes resorting to cannibalism of juvenile Pollock. Juvenile pollock success is dependent on timing and location overlap with their prey copepods, and they enjoy a much greater overlap during cold years than in warm years (Siddon et. al. 2013). Pollock success is also directly linked to the lipid content of copepod prey sources (Heintz et. al. 2013).
For age-0 pollock distribution the factors of original spawning ground and subsequent survival, as well as the regular stresses that produce schooling behavior also determine success (Benoit-Bird et. al. 2013). Overlap of adult and age-0 pollock that allows for cannibalism happens primarily during autumn and winter while cannibalism of age-1 pollock occurs farther Northwest during the summer months (Mueter et, al, 2011).
Implications of Climate Change
The warm year vs. cold year effect is a key factor in the distribution of pollock based on their age and prey. Earlier sea ice retreat leads to an earlier plankton bloom, juvenile pollock’s main prey and so those pollock move to and feed in those areas where copepods live off that bloom. For juvenile pollock, this creates a spike of surviving juvenile pollock fueled by the temporarily expanded prey source, but later on in the year pollock cannot get enough energy from their food to survive through the winter, and so later age class populations are reduced. In contrast, algal blooms on the ice sheet in cold years create a higher lipid content copepod source, so the population of pollock can be more abundant (Heintz et. al. 2013). There is a 33% increase (Heintz et. al. 2013) in energy of pollock when a cold year produces high-lipid copepods in overlap with juvenile pollock. In this way the success of juvenile pollock determines the success of the species.
The success of juvenile pollock during cold vs. warm years also is affected by distribution. Age 1 pollock can take refuge in the cold pool due to their greater temperature tolerance, while the older fish are pushed to outer shelf outside the cold pool. This keeps adult pollock from cannibalizing their juvenile counterparts in excess. The decrease in cold pool size during warm years reduces the availability of this safe habitat, which causes a cannibalism increase as pollock are the best food for other pollock when copepods and other prey have a low energy content (Siddon, personal communication). With more warm years in the Bering Sea due to climate change, the cold pool will be warmer and lipid content of copepods will decrease. In this way the population recruitment of pollock will suffer. (Stabeno, et. al., 2012).
The pollock catch has annually averaged 1.3 million tons ever since the late 1980s when United States vessels first began fishing for pollock. Today, the pollock fishery is the largest in the United States by volume. Since 1998, pollock prices have hovered at approximately one dollar per pound.
A table of age two fish caught shows a correlation between year temperature, or previous year temperature and the amount of two-year-old (new adult) fish caught.
The pollock fishery is currently the second largest in the world and made up 61.9% of the total Alaskan groundfish catch in 2012 (Walleye Pollock Research, 2012). The U.S. fishery landed roughly 1.26 million tons between 2012 and 2014. In 2012 the products derived from the catch were worth over 1 billion dollars, and the catch itself valued $343 million. This massive resource fuels the imitation crab industry and is the fillet component in fried fillet sandwiches. This use is in part due to the natural oil content which is both higher than the content in similar species and considered more flavorful. (NOAA, 2014) To a much lesser extent, money from the pollock fishery goes back into native villages on the west coast of Alaska. This happens through jobs, subsidies and money given back to the tribal government (Pollock Provides, 2008).
As the amount of pollock recruited to adulthood will greatly deteriorate with the increase of warm years in the southeastern Bering Sea shelf, it is to be recommended that fisheries begin to consider the recruitment of other species to serve as a buffer for certain pollock products. Arrowtooth flounder (Atheresthes stomias), could be a possible alternative to pollock for surimi, which is more commonly known as imitation crab. Though the arrowtooth flounder has not been commercially fished in the past because of an enzyme that quickly breaks down the fish when heated, additives have been developed that can stop the flesh from degrading.
These additives will open up opportunities for the arrowtooth flounder’s commercial fishery; its marketability will be greatly benefited as well (Arrowtooth Flounder Overview, 2014; Arrowtooth Flounder Research, 2014). This makes a surimi product that originates from arrowtooth flounder a viable alternative to the current pollock surimi; instituting arrowtooth flounder based surimi products will reduce the human-related strain on the pollock population while also reducing human dependence on the continually deteriorating pollock fishery.
Pollock is a vital component to the Bering Sea ecosystem, both for the food chain and the humans who fish from it. As the Arctic’s mean temperature has risen by approximately 1.5 degrees Celsius in the last four decades and the ice sheet volume has
decreased by 75% (Overland et. al. 2013), it is reasonable to conclude that the temperature will only rise higher and higher as the Bering ice sheet retreats earlier and earlier. This would greatly affect the southeastern Bering Sea shelf by raising the temperature of the summer cold pool perpetually above 2 degrees Celsius, therefore instituting a repeating cycle of continuous warm years that would be detrimental to pollock population recruitment, as the plankton prey that juvenile pollock depend on would bloom earlier, leaving pollock with less energy during the later months. (Stabeno et. al. 2012; Heintz et. al. 2014).
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