Ecology and Vulnerability

Atlantic Sea Scallop

Marine bivalves are particularly vulnerable to ocean acidification due to their carbonate shells, with much of the literature focused on this aspect of climate change either alone or in combination with other stressors like elevated temperatures or dissolved oxygen. These stressors, both individually and in synergy, have documented physiological, morphological, and survival effects on both Bay Scallops and Atlantic Sea Scallops. The larval life stage appears to be the most vulnerable life stage for exposure to climate change stressors (Gobler and Talmage 2013, White et al., 2013; White et al., 2014; Clark and Gobler 2016; Gobler et al., 2017). 
 

Shifts in Range, Elevation, or Depth

Atlantic Sea Scallop are a sentinel species for climate change in marine systems (Stokesbury and Bethoney 2020). They range from Cape Hatteras, North Carolina, north to the Gulf of St. Lawrence, residing on the continental shelf in nearshore waters that typically range from 0℃ to 17℃ in temperature, with optimal growth between 10℃ and 15℃ and temperatures above 21℃ lethal. The benthic bivalve is typically found between 15 and 110 meters in depth but can be found in waters as shallow as 2 meters in the northern part of its range and are uncommon deeper than 60 meters (Cooley et al., 2015, Torre et al., 2018, Stokesbury and Bethoney 2020). Their preferred salinity is full-strength seawater of 35‰ with salinities lower than 16.5‰ lethal (Torres et al., 2018). Individuals reach ages of at least 18-20 years, with populations in the Mid-Atlantic exhibiting slightly faster growth rates than those in the Gulf of Maine (Cooley et al., 2015) and populations in the northern portion of the range showing smaller average shell sizes (Stokesbury and Bethoney 2020). Warming bottom ocean temperatures with climate change may exceed the thermal tolerance of Atlantic Sea Scallops in the southern portion of the current range or in inshore, shallower habitats (Cooley et al., 2015). Adult Atlantic Sea Scallops are generally sedentary, with their distribution dependent on larval recruitment (Cooley et al., 2015, Torre et al., 2018, Stokesbury and Bethoney 2020). 
The geologic record suggests that Atlantic Sea Scallops shifted their range on the continental shelf multiple times as sea level rose and fell and water temperatures changed (Stokesbury and Bethoney 2020). Between 1974-1977 to 2019-2022, the spring range of Atlantic Sea Scallop has shifted 1.52 degrees (168.83 km) north and contracted by 0.4 degrees (44.95 km). During the fall season, Atlantic Sea Scallop shifted 0.83 degrees (92.5 km) north and contracted its range by 0.31 degrees (34.69 km) between the periods of 1974-1976 to 2019-2022. Atlantic Sea Scallop shifted 12.8 meters and 9.9 meters deeper during spring and fall, respectively (NOAA Fisheries 2022). 

Model projections of future range shifts of Atlantic Sea Scallop in the Northeast estimate the population will move northward over the next 80 years in association with anticipated changes to bottom temperature and salinity. Sea scallop was projected to experience a significant loss of relative biomass during fall by 2050 across the Northeast continental shelf under mean model projections using the RCP 8.5 scenario (Allyn et al., 2020); however, small increases in relative biomass are projected in the Gulf of Maine during spring and fall, while losses are projected during fall in the Southern New England – Mid Atlantic region.

At the northern portion of the range, Atlantic Sea Scallop populations in the Bay of Fundy are positively correlated with temperature, which is thought to lead to rapid development of larva and improved survival of juveniles and adults (Dickie 1955 and Caddy 1979 as cited in Torre et al., 2018). Torre et al., (2018) developed a bioclimate envelope model for Atlantic Sea Scallop to evaluate habitat suitability with climate change in the Gulf of Maine from Massachusetts to Maine. Peak habitat suitability indices were a high annual bottom temperature of 15℃ for inshore habitats and 10℃ for offshore habitats, a lowest annual bottom salinity of 31‰ inshore and 33‰ offshore, a depth of 10 meters inshore and 37 meters offshore, and an average current speed of 0.05 meters per second inshore and 0.1 offshore. For inshore areas, bottom temperature, bottom salinity, and flow velocity were the most important environmental variables while bottom temperature, bottom salinity, and depth were most important in offshore areas. Changes in habitat suitability with changing climatic conditions from 1978-2013 showed increasing habitat suitability in inshore areas and decreasing habitat suitability in offshore areas, with the exception of offshore shoal areas which exhibited an increasing trend similar to inshore areas of the Gulf of Maine. Torre et al., (2018) suggest that changing climatic conditions have increased the habitat suitability of inshore areas in the Gulf of Maine while offshore habitat suitability remained relatively stable from 1978 to 2013. Asci et al., (2018) also found that offshore populations of Atlantic Sea Scallop in the central Gulf of Maine did not change significantly between 1986 and 2014, suggesting that the benthic communities of offshore Banks (shoals) experience high natural disturbance rates that could increase their resiliency to fishing activities and environmental drivers.

Ocean acidification over the past 150 years has increased by 26% with an additional acidification of two to three times that rate projected for the next century (Cooley et al., 2015). Increased ocean acidity affects the sea surface saturation state (or solubility) of aragonite, the form of calcium carbonate commonly found in larval bivalve shells. The calcium carbonate saturation state of the continental shelf decreases from south to north, offshore North Carolina to New Hampshire, in both summer and winter with winter saturation states lower than summer states. Ocean acidification increases the solubility of calcium carbonate (or decreases the saturation state), which can reduce growth and energy budgets of marine and estuarine bivalves (Cooley et al., 2015, Cameron et al., 2022). Warming ocean temperatures, on the other hand, can increase growth by increasing metabolism but only up to a limiting high temperature tolerance, above which growth can be impeded (Cooley et al., 2015).

Cameron et al., (2022) measured the effects of ocean acidification and water temperature on the calcification rate, carbonate chemistry of the extrapallial fluid, respiration, and survival of Atlantic Sea Scallops. Elevated ocean acidity inhibited calcification and respiration. The synergistic effects of higher ocean acidity with high water temperature resulted in mortality. They suggest that growth and survival declines were likely a result of dissolution of the external shell, thermal stress, and a reduced metabolism from ocean acidification. The negative physiological effects of ocean acidification on Atlantic Sea Scallop condition are not mitigated by current harvest regulations (Cameron et al., 2022).
Liu et al., (2021) modeled the inter-annual variation of scallop condition on Georges Bank with sea surface temperature and the concentration of chlorophyll-a from 1985 to 2019. Their results found a positive correlation between scallop condition in May with sea surface temperature variability in the preceding winter-spring months. The relationship between chlorophyll-a concentration and scallop condition was weak, likely due to vertical mixing of the water column that transports phytoplankton to its benthic habitat (Liu et al., 2021).
 

Changes to Morphology or Physiology
The energetic cost of calcification of shells is thought to increase with ocean acidification, with reduced energy budgets for reproduction or immunity (Cooley et al., 2015). A scope for growth model (a model for the level of energy available for growth) for Atlantic Sea Scallop developed by Zang et al., (2022) found that energy available for growth on the continental shelf of the Northeast US is spatially heterogeneous with higher levels in May-June and lower levels in January-February. Thermal stress in the Mid-Atlantic Bight resulted in negative energetics for growth from July to October. Food availability from particulate organic matter was higher in the cold seasons and lower in the warm seasons, which when combined with the thermal stress of the warm season for the Mid-Atlantic Bight led to a reduction in suitable habitat. No synergistic effects of warming and food deficiency were found on Georges Bank. The model also found sensitivities for increasing temperature and food availability with scallop size, indicating that larger scallops are more sensitive and that suitable habitats may decrease as scallops grow older or bigger (Zang et al., 2022).
 

Changes in Population

Stokesbury and Bethoney (2020) conducted a range-wide survey to quantify the abundance and distribution of Atlantic Sea Scallops from 2016 to 2018, estimating a total population of 34 billion individuals from North Carolina to Newfoundland. Georges Bank had the highest population density, providing 71% of the overall species abundance. The Mid-Atlantic contained 27% of the total population, while species abundance in the Gulf of Maine and northern range hosted the remaining 2% of the population. The naturally high climate variability in the range of the Atlantic Sea Scallop is anticipated to increase with future climate change. Large areas of the Georges Bank and Mid-Atlantic exhibited high productivity and may be self-sustaining, whereas the Northern grounds and Gulf of Maine may be reproductively isolated. Evidence suggests that recruitment patterns and extreme recruitment events may generate larval recruitment from highly productive Georges Bank and the Mid-Atlantic aggregations to the more isolated Northern grounds and Gulf of Maine (Stokesbury and Bethoney 2020). If larval recruitment and settlement of Atlantic Sea Scallops can be widespread during certain years, this suggests some adaptive capacity to respond to changing climatic conditions. Ocean acidification could affect larval recruitment of Atlantic Sea Scallop (Cooley et al., 2015), but populations do not appear to be limited by larval supply in Georges Bank currently (Cooley et al., 2015, Stokesbury and Bethoney 2020). Recruitment in Georges Bank could be limited by suitable settlement habitat, predation, or food availability. In the Mid-Atlantic, however, larval recruitment may be limited by larval supply, and a reduced larval supply due to ocean acidification in the future could reduce recruitment significantly in that region (Cooley et al., 2015). Periodic extreme recruitment events may not reach their full potential if warmer waters amplify the impact of fishing bycatch, such as what occurred in 2003 in the Mid-Atlantic when 10.4 billion juvenile Atlantic Sea Scallops (more than half of the year class) died as bycatch because they were raised to the surface through water temperatures above the lethal limit and then exposed to high air temperatures for several hours before being returned to the sea floor (Stokesbury et al., 2011).

Atlantic Sea Scallops are a commercial fishery in the US. Increasing bottom ocean temperatures and ocean acidification due to climate change are anticipated to increasingly threaten Atlantic Sea Scallop populations over the next several decades. Warming ocean temperatures could increase growth rates of the species, while ocean acidification could slow growth and decrease recruitment (Cooley et al., 2015). Cooley et al., (2015) developed an integrated assessment model to simulate Atlantic Sea Scallop populations in the near- and long-term under different CO2carbon dioxide emissions scenarios. Their model results indicate that Atlantic Sea Scallop populations could significantly decline by 2050 under the current emissions scenario (RCP 8.5), assuming current harvest regulations, decreased recruitment and slower growth due to ocean acidification, and increased growth due to warming oceans. An increased growth rate due to ocean warming appears to outweigh the negative growth impacts from ocean acidification until 2030, when the pattern reverses and the net impacts are negative growth rates for Atlantic Sea Scallop. With further increases in temperature and ocean acidification, the impacts of commercial harvest will be amplified, and current harvest regulations will become out of sync with the population size distribution (Cooley et al., 2015).

Model projections of the impacts of future climate change and harvest management of Atlantic Sea Scallop with a focus on ocean acidification estimated biomass reductions of 13% by the end of this century with lower emissions scenarios coupled with high harvest impact scenarios (Cooley et al., 2015; Rheuban et al., 2018). Under high emissions scenarios, Atlantic Sea Scallop biomass may be reduced by more than 50% by the end of this century. Commercial harvest of the species amplifies the impact of ocean acidification and vice versa. Harvest limits can mitigate some of the anticipated ocean acidification impacts, with a 10% increase in harvest closure areas increasing biomass by more than 25% under the highest impacts of ocean acidification scenario. A long-term decline in populations due to ocean acidification is projected regardless of the harvest management scenario (Rheuban et al., 2018).