Ecology and Vulnerability

Atlantic Cod

A growing number of studies provide strong evidence that marine fish and invertebrate species are responding to climate change through geographical range (Nye et al., 2009) and phenology shifts (Staudinger et al., 2019; Langan et al., 2020). Studies of range shifts generally use long-term federal datasets from spring and fall bottom trawl (NOAA Fisheries 2022) and ichthyoplankton Ecosystem Monitoring (EcoMon) (Walsh et al., 2015) surveys of the continental shelf, or by state and private surveys of regional bays and estuaries (Morson et al., 2019, Snyder et al., 2019; Langan et al., 2020). These extensive monitoring programs have shown that centers of population biomass for the majority of marine fish species are shifting poleward towards higher latitudes and to greater depths at rates that exceed terrestrial species (Lenoir et al., 2020). These general trends are projected to continue under future climate scenarios of modeled habitat abundance (Kleisner et al., 2017), suitable thermal habitat (Morley et al., 2018), and relative population biomass (Allyn et al., 2020). 
An analysis of 42 species of marine fish and invertebrates (RSGCN and non-RSGCN species) found in continental shelf waters of the Northeast region between 1974 and 2022, showed the community has shifted on average 0.75 degrees (83.83 km) north during spring, and 0.61 degrees (68.27 km) north during fall. The community also experienced a change in average depth of 22.4 meters and 8.6 meters deeper in spring and fall, respectively (NOAA Fisheries 2022). Similar to patterns observed at the community scale and for non-RSGCN species, the direction, rate, and magnitude of range shifts varies widely among RSGCN marine fishes and elasmobranchs sampled during this period; responses in range changes span -0.51 km southward in Winter Skate, to 2.25 km northward in Black Sea Bass during spring, and -0.21 km northward in Atlantic Herring to 2.05 km northward in Black Sea Bass during fall. Depth range shifts spanned from -61.08 meters deeper in Atlantic Herring to 12.41 km shallower in Atlantic Cod in spring, and -50.88 meters deeper in Atlantic Herring to 31.20 meters shallower in Atlantic Halibut during fall (NOAA Fisheries 2022). 
Shifts in phenology are more difficult to track in comparison to range shifts in aquatic species because observations need to have repeated observations within a narrow and set area over multiple decades (Staudinger et al., 2019). Several multi-species analyses have shown phenological shifts in the occurrence of larval phases of demersal groundfishes (Walsh et al., 2015), growth and productivity of population biomass (Henderson et al., 2017), as well as seasonal migration patterns (both ingress and egress) and residence times in estuaries systems such as Narragansett Bay, Rhode Island (Langan et al., 2021).
As species make distributional changes in range and phenology, primarily in response to warming sea surface and bottom temperatures, entire assemblages of marine species are reshuffling and reorganizing across Northeast coastal ecosystems (Weiskopf et al., 2020; Staudinger et al., 2021). Examples of changes in community structure through altered species diversity, population size, and density have been documented across the Northeast continental shelf (Friedland et al., 2020) and in estuaries including Little Egg Inlet, New Jersey (Morson et al., 2019), Narragansett Bay, Rhode Island (Langan et al., 2021), and Long Island Sound off of New York and Connecticut (Snyder et al., 2019). Ensembles of warm-water associated species that are moving north into temperate habitats are generally replacing cold-water associated species, which are moving to deeper depths and higher latitudes in search of cold water refugia (Friedland et al., 2020; Pershing et al., 2021).
On a global scale, warming has led to a decrease in body size (Sheridan and Bickford 2011) but this varies widely by species and local conditions. The indirect effects of morphological and behavioral changes combined with shifting distributions is changing predator-prey and competitive relationships among associated marine fish species and resulting in novel trophic interactions (Staudinger et al., 2021). Most models that project range shifts currently do not account for species interactions (Pearson and Dawson 2003, MacPherson et al., 2021); this is an area of active research as changes in trophic relationships are hypothesized to be more important for species survival under future conditions than environmental conditions (Louthan et al., 2015, Staniczenko et al., 2017). In addition, the combined stress from warming ocean temperatures, decreased dissolved oxygen, and ocean acidification is increasing the prevalence and vulnerability to disease and predation.

Shifts in Range, Elevation, or Depth

Between the periods of 1974-1977 and 2019-2022, the spring range of juvenile and adult Atlantic Cod shifted 0.32 degrees (36.56 km) north and contracted by 1.13 degrees (125.08 km) across the Northeast continental shelf; conversely, during the fall season, Atlantic Cod shifted 0.15 degrees (16.74 km) south and contracted its range by 1.12 degrees (124.7 km) between the 65 periods of 1974-1976 to 2019-2022. Atlantic Cod have also shifted 0.5 meters into shallower waters during spring and 12.4 meters shallower during fall over the two periods (NOAA Fisheries, 2022). Larval Atlantic Cod shifted their distribution northward and to deeper waters between the periods of 1977-1987 and 1999-2008 in the Southern New England to Gulf of Maine regions (Walsh et al., 2015). Over the same time period, adult and larval life phases of Atlantic Cod exhibited synchronous distributional shifts to deeper waters during spring across the Northeast continental shelf region (Walsh et al., 2015). While Atlantic Cod increased in number off of Rhode Island in Southern New England waters between 2000 and 2018 (Langan et al. 2020), they are projected to experience significant losses of relative biomass during spring and fall by 2050 across the Northeast continental shelf with larger declines in the Southern New England – Mid-Atlantic regions compared to the Gulf of Maine under mean model projections using the RCP 8.5 scenario (Allyn et al., 2020). This loss of biomass (and range) along the Scotian Shelf to the southernmost parts of the range is likely due to increasing water temperatures and declining oxygen levels (Deutsch et al., 2015), though modeling suggests that warming temperature might be more limiting than oxygen levels except in the deepest parts of the Atlantic Cod’s range (Brennan et al. 2016). There is low certainty that Atlantic Cod is projected to shift the centroid of their range along the coast of Eastern Canada by 111.6 km under RCP 2.6 and medium uncertainty they will shift 428.2 km under RCP 8.5 by the end of the century (2081-2100), with projected losses of available thermal habitat of -1.7% under a RCP 2.6 and -34.0% under RCP 8.5 when compared to the period of 2006-2020 (Morely et al., 2018). Mechanistic distribution models based on climate change projections and various habitat suitability for Atlantic Cod at different phases of their life history suggest a general northern shift; however, habitat shifts for different life stages vary in projections (Cote et al., 2021). Specifically, the best habitat for egg survival may expand by 153% by 2010, but the habitat for post-hatching growth may decrease by 74% (Cote et al., 2021). Another model that takes into account projected changes in temperature and hypoxic conditions in the Gulf of St. Lawerence projects that Atlantic Cod will lose ~5% of biomass by 2065 due to losing habitat in deeper areas of the Gulf as they become more hypoxic and will remain in shallower, more oxygenated waters (Stortini et al. 2017).

Shifts in Phenology

In an experimental study, larval Atlantic Cod were exposed to ocean acidification levels projected in the year 2100 (1179 µatm CO2), and the larval mortality rates were doubled compared to those exposed to control levels of acidification (503 µatm CO2; Mittermayer et al., 2019). Similarly, another experimental study exposing Atlantic Cod larvae to the same projected ocean acidification levels (1179 µatm CO2) and varying levels of food limitations found that under high acidification levels (but limited in food), larvae were larger at 36 days and showed organ defects and smaller gills, suggesting trade-offs between growth and organ development that can have serious ramifications for the larvae as they continue to develop (Stiasny et al., 2019). In another study, larvae hatched from parents acclimated to high acidification levels (1100 µatm CO2) still had much higher levels of mortality than those raised in a controlled environment, but some of the mortality was mitigated by high food availability, demonstrating that acclimation may help survival in high acidification levels, but only in ideal circumstances (Stiasny et al., 2018). Eggs exposed to a control and elevated acidification levels (400 and 1100 µatm CO2, respectively) and temperature at the upper end of their thermal tolerance (12℃) were significantly less likely to hatch, and those that did were much more likely to be deformed (Dahlke et al., 2017).

Changes in Population

Between the years 1982 and 2013, the Gulf of Maine warmed faster than 99.1% of the rest of the world’s oceans (Pershing et al. 2015), which is correlated with an increase of mortality of age 4 Atlantic Cod (Pershing et al. 2015), which is the year they start preparing for spawning and switching between benthic to pelagic prey (Sherwood et al., 2007), which makes them less efficient predators (Pershing et al. 2015), something that would be further worsened by the increased metabolic costs of rising water temperatures (Deutsch et al., 2015). In the Gulf of St. Lawrence, models suggest that warming waters make it likely that the Atlantic Cod population may only slightly recover their population by 2040 with severe limitations on fishing and predation by Harp Seals (Pagophilus groenlandicus; Bousquet et al., 2014).

Indirect Effects

Warming temperatures will likely lead to changing predator-prey dynamics, as range overlap of Atlantic Cod and their major prey (e.g., Sandlace [Ammodytes dubius] and Atlantic Herring) off of the Northeast U.S. decreased from 1968 to 2014 and is projected to further decrease by as much as 50% in the next ~80 years by based on the RCP 8.5 emissions scenario, severely impacting the food web and top-down control of Cod in the region (Seldon et al., 2018)

References

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