Ecology and Vulnerability

American Lobster

Overview

American Lobster uses different habitats, each influenced by variable environmental
conditions, depending on its life stage. The larvae are pelagic and transition between three
planktonic instars (Phillips et al., 2013). During the post-larval instar phase, American Lobster
transition from being pelagic at the water’s surface, subject to transport by wind, to settling on
the benthic seafloor. Post-larvae can swim and move vertically and horizontally in the water
column, capable of diving to select a benthic habitat for settling that provides shelter. Typically,
this transition to benthic habitat occurs in shallower water above the summer thermocline. After
they settle, they transition to the juvenile life stage, where they exhibit cryptic behavior strongly
associated with rocky shelters. As very young juveniles, they are thought to be suspension
feeders. Their behavior and habitat use transitions again as the juveniles grow into adults, with
increasing use of habitats farther from shelters (Phillips et al., 2013). They also seasonally
migrate between inshore and offshore waters (Phillips et al., 2013; Goldstein and Watson, 2015).
Adult lobsters molt every one to two years. This shifting life history and habitat use exposes
American Lobster to variable climate impacts depending on the depth and temperature associated
with each life stage (Quinn et al., 2013; Goldstein and Watson, 2015; Quinn and Rochette, 2015;
Barret et al., 2017; Goode et al., 2019).
Juvenile and adult American Lobsters are positioned in the middle of the marine food
web, feeding primarily on benthic invertebrates but also plant material and potentially fish, while
being important prey for marine demersal and groundfish like Atlantic cod and other marine
fishes and crustaceans. With declining groundfish populations in the Gulf of Maine, predation
pressure on American Lobster has decreased and is thought to be a contributing factor alongside
warming temperatures for increasing lobster abundance in the northern portion of its range
(Phillips et al., 2013; Steneck and Wahle, 2013).

Shifts in Range, Elevation, or Depth

American Lobster ranges from North Carolina north to Labrador, Canada, in shallow
coastal waters in the northern portion of its range and deeper offshore waters in the southern
portion of its range (Phillips et al., 2013). In Atlantic Canada the species is primarily found in
waters less than 50 meters deep. American Lobsters are distributed widely from the coast to submarine canyons at the continental shelf break in the middle of its range. From Long Island Sound south through the Mid-Atlantic Bight, they are increasingly distributed in colder, deeper waters at the edge of the continental shelf (Phillips et al., 2013). As water temperatures increase with climate change, the southern end of the species range may likely become less metabolically favorable, while cooler, more northern waters will be more hospitable during summer months (Phillips et al., 2013). Casey et al. (2023) found that adult lobsters in southern New England have shifted offshore into deeper waters to release larvae in response to rising water temperatures in nearshore areas. // Over the past several decades, American Lobster populations have shifted northward, with increasing abundance in the Gulf of Maine and declining and disappearing populations in southern New England (Phillips et al., 2013; Wahle et al., 2015; Goode et al., 2019; Oppenheim et al., 2019; Mazur et al., 2020; NOAA Fisheries, 2022; Casey et al., 2023). In Narragansett Bay, Rhode Island, summer lobster nurseries contracted from the middle portions of the bay between 1990 and 2011-2012 to outer portions that are deeper, coincident with an increase in summer surface temperatures of about 2℃ (Wahle et al., 2015). Goode et al. (2019) suggested that the northward increase in lobsters results from complicated interactions between climate change, local oceanographic conditions, and the settlement behavior of lobster larvae. // Between 1974-1977 to 2019-2022, the spring range of American Lobster shifted 1.68 degrees (187.04 km) north and contracted by 2.58 degrees (285.34 km). During the fall season, American Lobster moved 1.54 degrees (171.08 km) north and contracted its range by 1.93 degrees (213.77 km) from 1974-1976 to 2019-2022. The spring distribution of American Lobster shifted 11.3 meters shallower during spring and 21.1 meters deeper during fall over the same periods (NOAA Fisheries, 2022). // Mazur et al. (2020) modeled the distribution of suitable habitats for American Lobsters in
the Northeast, which is strongly influenced by environmental conditions. They found that
abundance and distribution were driven by temperature, bathymetry characteristics, and
secondary productivity of zooplankton. Given these drivers, the availability of suitable habitat
has increased in the Gulf of Maine and decreased in southern New England. Evidence also
indicates a decline of inshore habitat suitability in the Gulf of Maine and an increase offshore in
deeper, cooler waters (Mazur et al., 2020). In Nova Scotia, Greenan et al. (2019) forecasted that
the distribution of American Lobsters is not likely to change or improve. // Tanaka et al. (2020) projected future range shifts of American Lobster and Atlantic Sea
Scallops in the Northeast with shifts in environmental conditions due to climate change. Their
results projected that American Lobsters will move further offshore over the next 80 years with
anticipated changes to bottom temperature and salinity. Tanaka and Chen (2016) similarly
projected increasing inshore habitat suitability for juvenile and adult lobsters during the spring (April to June) but no trend for the fall (September to November). // American Lobsters were projected to experience a significant loss of relative biomass
during spring and fall by 2050 across the Northeast continental shelf, with slightly larger
declines during fall in the Southern New England – Mid Atlantic compared to the Gulf of Maine
under mean model projections using the RCP 8.5 scenario (Allyn et al., 2020). Behan et al. (2021
& 2022) also modeled the distribution of American Lobster under the RCP 8.5 scenario, finding
that forecasted habitat suitability in the Gulf of Maine for 2028-2055 varies with the temperature
and salinity of bottom water, sediment grain size, the distance offshore, and latitude. Forecast
results depended on the spatial scale and assumptions about the models' environmental
parameters, generating uncertainty (Behan et al., 2021 & 2022; Hodgdon et al., 2021 & 2022;
Quinn et al., 2022).

Shifts in Phenology

Water temperature is a key driver of American Lobster post-larval settlement and population dynamics (Annis et al., 2013; Barret et al., 2017). Annis et al. (2013) found that settlement patterns of post-larval lobsters may be influenced by small variations in water temperature, with “either behavioral avoidance of colder settlement sites or elevated postsettlement mortality of post-larvae settling at colder sites”, resulting in a potential disconnect between the abundance and settlement of post-larvae in warmer inshore areas and cooler offshore areas. Haarr et al. (2020) documented an advancement in the timing of egg hatching in the southern Gulf of St. Lawrence by five weeks from 1989 to 2014 in response to warming temperatures. The rate of clutch development increased by 40% in the spring. Their results indicated that the earlier hatching time is a response to environmental conditions the females were exposed to 6 to 18 months earlier. The earlier hatch timing could result in a phenological mismatch with prey species for lobster larvae (Haarr et al., 2020). Monitoring studies by Staples et al. (2019) in three regions of the Gulf of Maine found different patterns in the timing and suddenness of initial molts between regions, stages of maturity, and sexes. Molting was variable with molts that were earlier associated with warmer temperatures, especially in inshore areas, but this response was not uniform and challenging to quantify as a molt time series connected with bottom temperatures (Staples et al., 2019). Some egg-bearing female lobsters make seasonal migrations between inshore and offshore areas, exposing their eggs to thermal conditions that differ from egg-bearing females who do not migrate (Goldstein and Watson, 2015). Goldstein and Watson (2015) found that the egg development rate did not change with exposure to different fall temperatures, but exposure to different spring temperatures led to faster development of eggs in warmer inshore areas. Eggs from inshore areas hatched approximately 30 days sooner than those in offshore areas and had significantly shorter development times from the onset of an eyespot to their hatch. Their results suggest that the seasonal movements of egg-bearing female lobsters influence the location and timing of egg hatching, with implications for larval transport and recruitment between inshore and offshore areas (Goldstein and Watson, 2015). Warming water temperatures at the southern end of the range of American Lobster are associated with an increase in seasonal outbreaks of epizootic shell disease (Groner et al., 2018). Groner et al. (2018) determined that a phenological mismatch is linked to disease outbreaks in Long Island Sound. Elevated water temperature in the spring correlates with earlier lobster molting in the spring, increasing the intermolt period during the summer when disease prevalence peaks in the fall. Warmer summer water temperatures increase the rate of new infections, with an 80% infection rate in warmer summers and a 30% infection rate in cooler summers. Lobsters that are infected had a mortality rate of greater than 50%. Groner et al. (2018) suggest that the phenological mismatch between molting and disease outbreaks due to rising temperatures will likely lead to future population-level impacts as waters continue to warm.

Changes to Morphology or Physiology

American Lobster larvae and post-larvae have been documented to have negative physiological responses to increasing water temperatures and acidification (Benestan et al., 2016; Hartington and Hamlin, 2019; Harrington et al., 2020a & 2020b; Lopez-Anido et al., 2021; Niemisto et al., 2021; Powell et al., 2023). McMahan et al., (2016) found that juvenile lobsters in Maine are growing faster in warmer years and molting more frequently. Waller et al. (2019) correlated a reduction in female size of maturity in Maine of up to 50% with increasing water temperature. Haarr et al. (2018) documented a reduction of up to 30% in the size of maturity of female lobsters in Canada over the past 10-80 years but did not find a correlation with increasing temperatures but rather with increasing fishing pressure. Goode et al. (2019) determined that the thermal tolerance of lobster post-larvae is between 12℃ and 20℃ and that surface waters that are thermally stratified in the summer in the southwestern Gulf of Maine have historically been within this thermal envelope. Surface waters in the Gulf of Maine are warming faster than bottom waters, however, creating a steep temperature gradient with depth. They conclude that the potential expansion of thermally suitable areas for settlement by lobster larvae is inhibited by the thermal stratification of bottom waters in the southwestern Gulf of Maine. In the northeastern Gulf of Maine, tidal mixing prevents thermal stratification, and the favorable area for larval settlement has expanded more (Goode et al., 2019). Casey et al. (2023) found that increasing water temperatures in the nearshore areas of southern New England has significantly increased thermal stress on recently settled post-larvae. Quinn (2017) investigated the thermal stress and tolerance levels of lobster larvae and documented sublethal thermal stress with exposures to water temperatures of 20-26℃ with potential lethal effects with longer exposures. With projected exposure to surface water temperatures over 30℃ with climate change, there could be negative population-level effects (Quinn 2017). Quinn et al. (2013) found that warmer temperatures caused faster larval development and shorter durations of larval stages, and colder temperatures resulted in 38% shorter development times in the northern Gulf of St. Lawrence. Larvae from this colder region had longer development times at warmer temperatures than previous studies have documented for larvae from warmer regions, indicating regional variability in the functional relationship between larval development time and temperature (Quinn et al., 2013). Harrington et al. (2019) found physiological trade-offs in post-larval lobsters with warming temperatures, with warming temperatures increasing growth rates but with physiological stress and loss of genetic diversity. These trade-offs could result in reduced adaptive capacity for climate change. Nielsen and McGaw (2016) determined that juvenile lobsters avoid temperatures lower than 8℃ and higher than 20℃, with a mean temperature preference of about 16.2℃. Their thermal preferences were not affected by size, origin (wild versus laboratory-raised), or prior acclimation (Nielsen and McGaw, 2016). When juvenile lobsters were offered a choice between food, shelter, or temperature, they consistently chose the environment with a shelter over those with food, even if the temperature was thermally unfavorable - suggesting juvenile lobsters make behavioral trade-offs to maintain optimal fitness and survival (Nielsen and McGaw, 2016). Wang et al. (2016) found that American Lobsters eat more food with increasing temperature and that they process their meals through their digestive system faster with increasing temperature. The results suggest that feeding cues are driven by stomach emptying regardless of temperature (Wang et al., 2016). Experiments conducted by Noisette et al. (2021) determined that lobster larvae have a higher tolerance to acidification, with no significant changes in development, morphology, mineralization, or survival time, but juvenile lobsters had the opposite pattern, with an increased time of development and reduced survival with acidification. They found that tolerance for acidification decreased until larvae metamorphosed into juveniles, after which their sensitivity increased (Noisette et al., 2021). Menu-Courey et al. (2019) found an increase in juvenile lobster mortality, an increase in aerobic capacity, and slower development with increasing acidification. McLean et al. (2018) found reduced growth and an increased vulnerability of juvenile lobsters to acidification. Acidification may negatively affect the calcification processes of lobsters during molting, with potential implications for mortality (Nagle et al., 2018). Waller et al. (2017) assessed the interactive effects of acidification with temperature on American Lobster, showing that projected warming by the end of the century will have a larger negative effect on larval survival than acidification but that acidification may affect larval behavior and metabolism in complex ways. Klymasz-Swartz et al. (2019) found that acidification negatively impacts every life stage of American Lobster and projected that by 2300, they likely would not be physiologically capable of acclimating to projected acidification levels.

Changes in Population

The size and age of maturity of American Lobster varies regionally across its range, with females in the southern and warmer portion of the range (i.e., southern New England) maturing three to four years faster than those in the northern portion of the range (i.e., Bay of Fundy; Phillips et al., 2013). Lobsters in warmer waters have been found to mature at smaller sizes (LeBris et al., 2017; Haarr et al., 2018). Khalsa et al. (2023) found that the size at maturity was an important driver of lobster productivity, suggesting that changes to lobster growth and maturity due to climate change may positively impact the population in the Gulf of Maine. American Lobster populations have skewed sex ratios, with different populations either male- or female-dominated (Jury et al., 2019). The skewed sex ratios are thought to be a result of both seasonal migrations inshore and offshore and selective harvest regulations and sex differences in catchability. Changing environmental conditions with climate change could impact lobster reproduction if local sex-specific populations shift as a result (Jury et al., 2019). Local populations of American Lobster post-larvae do not appear to be thermally adapted, with no significant differences in settlement behavior with the geographic origin of their female parents (Barret et al., 2017). Temperature was found to be a significant driver for larvae's survival and settlement behavior and the energetic conditions of post-larvae, suggesting that there are connections between development temperature and survival and settlement (Barret et al., 2017). Jaini et al. (2018) found that these connections with temperature and larval survival and settlement vary regionally, with positive associations for settlement with sea surface temperature in southern New England, Georges Bank, and southern Nova Scotia during the summer months. Settlement was only associated with sea surface temperature in the Gulf of Maine in the vicinity of the settlement site, and settlement in the Bay of Fundy had no correlation with sea surface temperature. Their results were consistent with residual oceanic flow driving larval distribution and settlement (Jaini et al., 2018). American Lobster populations are controlled by intense predation by large finfish, particularly Atlantic Cod. Steneck and Wahle (2013) argued that American Lobsters are evolutionarily adapted to predation pressures with an extended brood period, large larval size, and settlement habitat selection for shelters. Removing coastal predators from the Northwest Atlantic has allowed more population growth since 1980 in the US and Canada (Steneck and Wahle, 2013). Le Bris et al. (2018) modeled the synergistic effects of climate change, predation, and harvest on American Lobster population productivity. Their results found that climate change amplifies harvest impacts and vice versa, with the species’ reproductive potential driven by both (Le Bris et al., 2018). As population densities increase with warming waters, the vulnerability of American Lobsters to disease also increases (Steneck and Wahle, 2013; Groner et al., 2018). The prevalence of epizootic shell disease is increasing along the coast of Maine. Molting lessens the severity of the disease or eliminates it. Experiments conducted by Groner et al. (2018) found that diseased lobsters were more likely to molt and die and that those trends did not vary with temperature, and the temperature regime did appear to impact a physiological mechanism that could mitigate the effects of epizootic shell disease. Laufer et al. (2013) investigated a major die-off of the Long Island Sound population of American Lobster in 1999, finding that the causes of mortality were the cumulative effects of climate change stressors (temperature, acidification, and dissolved oxygen levels), shell disease, and pollution with endocrine-disrupting alkylphenol chemicals. The chemical pollutants negatively impacted the survival of larvae, molting and hardening of shells, and interfered with the metamorphosis between larvae and juvenile life stages (Laufer et al., 2013). A meta-analysis by Boudreau et al. (2015) found that, over the entire range of American Lobster, there is an interaction between predation and temperature, with predation the dominant population driver at the warm and cold extremes but not in the center of the range. Temperatures positively affected the recruitment of Lobsters at the warm range extremes. Lobster abundance did not follow the fishing harvest, but fishing effort followed Lobster abundance over time. Their results suggested that lobster populations will intensify at the thermal range boundaries of the species and weaken in the central core of the range (Boudreau et al., 2015). Oppenheim et al. (2019) enhanced population models for American Lobster to account for local bottom temperature and disease prevalence. Their results project that Gulf of Maine harvest populations will decline to near historical levels in the next decade and that the southern New England populations will not recover (Oppenheim et al., 2019). Tanaka et al. (2019) found that bottom temperature and salinity are drivers of lobster abundance during the spring and projected increasing populations in the Gulf of Maine. Tai et al. (2021) investigated the population effects of ocean acidification on American Lobster, finding that juveniles are the most vulnerable to negative impacts, but all life stages are impacted to some degree. Their population model indicated that the increasing magnitude of climate change effects would outweigh any population gains from harvest management (Tai et al., 2021).

Indirect Effects

The warming waters, thought to be the primary driver of the decline of juvenile American Lobsters from shallower inshore habitats in southern New England, may be amplified by the arrival and spread of invasive predatory and competitor species like the Asian Shore Crab (Hemigrapsus sanguineus) since the 1990s (Wahle et al., 2015). Overall, the American Lobster is a well-studied species, but many studies have focused on information needs with fisheries management or socioeconomic implications. Recent literature does not address how changing ocean circulation patterns and strengths may impact larval distribution and settlement, particularly any interactive effects with warming temperatures. Needed research to fill data gaps for American Lobster includes additional monitoring of larval settlement and juvenile nursery habitat in deep water in southern New England (Wahle et al., 2015), development of molt time series, and quantifying the connection to bottom temperature (Staples et al., 2017), quantification of the rate of larval development in nature and how it may vary over time and space (Quinn et al., 2022).