Potential Impacts of Climate Change on the Biota of the U.S. NES LME
There are many published, ongoing, and proposed studies conducted by NEFSC scientists that assess the potential impacts of climate variability and change on the biota of the U.S. NES LME. It is critical that these studies base their projections on historical and contemporary relationships between climate and the organism(s) in question.
A historical analysis of the NEFSC spring trawl survey from 1968 to 2007 revealed that the center of biomass of many fish stocks shifted north with many species concomitantly occupying greater depths (Table 1). However, stocks located in the southern region of the U.S. NES LME showed greater northward shifts and greater depth changes. Stocks in the northern regions, particularly those in the Gulf of Maine, showed little change in distribution. This is best exemplified in red hake (Figure 13). The most important factor associated with these observed shifts were warming temperature and circulation changes represented by the AMO.
An analysis of survey data from multiple regions in the U.S. and Canada revealed that marine species shifted latitude and depth by tracking local, as opposed to regional, changes in ocean temperature (Figure 14). Northern and deeper depth shifts were not observed for all regions (black arrows), particularly on the west coast and Gulf of Alaska where cooling has occurred, possibly due to natural climate variability. In the Gulf of Mexico where temperatures have warmed, marine species could not move north due to the land barrier and thus moved towards deeper water depths. However, a sub-regional analysis of the U.S. NES showed that the northern shift is no longer uniform among all regions (Figures 14.1 and 14.2). U.S. NES assemblages of species associated with shallower, warmer waters tended to shift west-southwest and to shallower waters over time, possibly towards cooler temperatures in the semi-enclosed Gulf of Maine, while species assemblages associated with relatively cooler and deeper waters shifted deeper, but with little latitudinal change. Conversely, species assemblages associated with warmer and shallower water on the broad, shallow continental shelf from the Mid-Atlantic Bight to Georges Bank shifted strongly northeast along latitudinal gradients with little change in depth. Shifts in depth among the southern species associated with deeper and cooler waters were more variable, although predominantly shifts were toward deeper waters.
Among four commercial and recreational species (black sea bass, scup, summer flounder, and winter flounder), all but winter flounder exhibited significant poleward shifts in distributions in at least one season (Bell et al. 2015). Changes in the centers of biomass for black sea bass and scup in spring were related to climate, while the change in the distribution of summer flounder was largely attributed to a decrease in fishing pressure and an expansion of the length at age structure. While the changes in ocean temperatures will have major impacts on the distribution of marine taxa, the effects of fishing can be of equivalent magnitude and on a more immediate time scale. It is important for management to take all factors into consideration when developing regulations for natural marine resources (Bell et al. 2015).
From the mid 1900s to 2002, periods of high catch rates of adult Atlantic croaker were associated with warm winter temperatures, a relationship driven by the overwintering survival of juveniles (Figure 15).
Variability in winter temperatures along the U.S. east coast corresponded to the NAO (Figure 15).
Landings of American lobster have substantially increased in the U.S. NES LME over the past two decades (Figure 16). However, landings in southern New England and in Long Island Sound have dramatically decreased whereas landings in the Gulf of Maine have more than tripled. The NEFSC spring trawl survey data also show increasing biomass in the north and decreases in the south (Movie 3). A variety of factors are thought to have contributed to this trend that include warming bottom temperature, increasing fishing effort, and decreasing predation rates. It is also suggested that the warming temperature lags landings suggesting a recruitment mechanism (Figure 16).
The record warm year of 2012 had a major impact on lobster in the Gulf of Maine. The warmer waters in spring of 2012 resulted in the lobsters moving inshore much earlier than usual (almost 1 month earlier; Figure 17a) and thus expanded the fishing season and caused record high landings (Figure 17b). This caused the market demand to drop as well as the price of lobster, which threatened the economic viability of both U.S. and Canadian lobster fisheries. The lobster experience in 2012 is an example of how quickly a change in the climate can impact a commercial marine species, cascading up through the economy.
Overexploitation is a large factor contributing to the decline but the increase in bottom temperature in these regions may also play a role (Figure 18).
Over the past decade, sea surface temperatures in the Gulf of Maine increased faster than 99% of the global ocean (Pershing et al. 2015). The warming, which was related to a northward shift in the Gulf Stream and to changes in the Atlantic Multidecadal Oscillation and Pacific Decadal Oscillation, may have led to reduced recruitment and increased mortality in the region’s Atlantic cod (Gadus morhua) stock (Figure 18.1). Palmer et al. (2016) argued that the “extra mortality” calculation driving this conclusion is an artifact. They argued that environmental factors affect all stocks, but attribution of additional mortality to temperature alone by Pershing et al. 2015 was unsupported by the data.
The spatial distribution of silver hake in the U.S. NES LME is highly correlated to the position of the Gulf Stream (Figure 19; Movie 5). This relationship is suggested to be caused by local bottom temperature changes that are responding to the same large scale forcing that causes changes in the circulation of the Gulf Stream.
From 1968 to 2008, the distribution of Northwest Atlantic mackerel shifted about 250 km to the northeast and from deeper off-shelf waters to shallower on-shelf waters (Figure 20). This shift was based on the NEFSC trawl surveys in the spring. These shifts were correlated to both interannual temperature variability and to warming ocean temperature in the U.S. NES LME.
The video pages linked below show distribution of the corresponding species. For more species distribution animations, please see our Spatial Analyses pages.
Black sea bass
Northern sand lance
Northern shortfin squid
Over the last four decades, fisheries in the U.S. NES LME have moved northward in correspondence with the northward shift of many commercially important species (Figure 21).
However, these fisheries shifted only about 10 to 30% as much as the respective target species, mostly due to economic and regulatory constraints (Figure 21).
In the U.S. NES, two large-scale ichthyoplankton programs sampled using similar methods and spatial domain each decade. Adult distributions from a long-term bottom trawl survey over the same time period and spatial area were also analyzed using the same analytical framework to compare changes in larval and adult distributions between the two decades. Changes in spatial distribution of larvae occurred for 43% of taxa, with shifts predominately northward (Figure 21.1). Timing of larval occurrence shifted for 49% of the larval taxa, with shifts evenly split between occurring earlier and later in the season. Where both larvae and adults of the same species were analyzed, 48% exhibited different shifts between larval and adult stages. Overall, these results demonstrate that larval fish distributions are changing in the U.S. NES ecosystem. The spatial changes are largely consistent with expectations from a changing climate. The temporal changes are more complex, indicating we need a better understanding of reproductive timing of fishes in the ecosystem. These changes may impact population productivity through changes in life history connectivity and recruitment, and add to the accumulating evidence for changes in the U.S. NES ecosystem with potential to impact fisheries and other ecosystem services.
Quantitative approaches have been developed to examine climate impacts on productivity, abundance, and distribution of various marine fish and invertebrate species. However, it is difficult to apply these approaches to large numbers of species owing to the lack of mechanistic understanding sufficient for quantitative analyses, as well as the lack of scientific infrastructure to support these more detailed studies. Vulnerability assessments provide a framework for evaluating climate impacts over a broad range of species with existing information. These methods combine the exposure of a species to a stressor (climate change and decadal variability) and the sensitivity of species to the stressor. These two components are then combined to estimate an overall vulnerability. Quantitative data are used when available, but qualitative information and expert opinion are used when quantitative data is lacking. A climate vulnerability assessment was conducted on 82 fish and invertebrate species in the U.S. NES including exploited, forage, and protected species. Climate vulnerability was defined as the extent to which abundance or productivity of a species in the region could be impacted by climate change and decadal variability. Overall climate vulnerability was high to very high for approximately half the species assessed (Figure 22; diadromous and benthic invertebrate species exhibit the greatest vulnerability). In addition, the majority of species included in the assessment had a high potential for a change in distribution in response to projected changes in climate (Figure 22.1). Negative effects of climate change are expected for approximately half of the species assessed, but some species are expected to be positively affected (Figure 22.2; e.g., increase in productivity or move into the region). These results will inform research and management activities related to understanding and adapting marine fisheries management and conservation to climate change and decadal variability.