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Lifting barriers to range expansion: the European green crab Carcinus maenas (Linnaeus, 1758) enters the Salish Sea.

ABSTRACT Although the European green crab Carcinus maenas has persisted in Oregon and Washington coastal estuaries, and thrived in the inlets of the west coast of British Columbia since 1998, populations of this species had not established themselves in the inner Salish Sea, between southern Vancouver Island, the mainland, and Puget Sound. It has been hypothesized that the Strait of Juan de Fuca acts as a semipermeable barrier preventing C. maenas larvae from entering this inland sea. Most years, the water is too cold (<10°C) for larvae to develop and the predominately estuarine surface outflow flushes larvae out to sea. In late August and September of 2016. a total of five live Carcinus maenas were discovered in Westcott Bay and Padilla Bay, Washington, suggesting that this species recently entered the inland sea as larvae. Unusually warm surface water in the northeast Pacific (>2.5°C above average) from the fall of 2013 to 2015 would have allowed larvae to survive alongshore transport off the coasts of Oregon, Washington, and southern British Columbia. Reversals of the estuarine current in the Strait of Juan de Fuca forced by strong southerly winds associated with major storms along the outer coast in late October 2014 and 2015 could have transported the warm ocean water and larvae inward through the strait and, with the aid of local winds and tidal currents, into the inner Salish Sea. Preferential inward transport would have been with the Olympic Peninsula Countercurrent that forms along the U.S. side of the channel.

KEY WORDS: Carcinus maenas. El Nino, temperature limitation, range expansion, invasive species, wind reversed estuarine circulation, Olympic Peninsula Countercurrent


The European green crab Carcinus maenas has invaded temperate marine communities around the world (Behrens Yamada 2001, Carlton & Cohen 2003). When abundant, C. maenas are voracious predators on native species, including clams, oysters, and crabs (Cohen et al. 1995, DFO 2011). They are also ecological engineers in that they transform valuable eelgrass beds into bare mudflats by ripping up the plants in search of food (DFO 2011, Neckles 2015). On the west coast of North America, C. maenas first arrived in San Francisco Bay in the 1980s as transplants from the east coast (Cohen et al. 1995, Grosholz & Ruiz 1995). Most likely, they hitchhiked on the algal packing material used to keep Atlantic lobsters or baitworms from Maine moist during transit. Northward range expansions into the coastal Oregon and Washington estuaries, and to the west coast of Vancouver Island, can be traced to the northward transport of larvae by prevailing ocean currents, especially during the winter of the major 1997/1998 El Nino event (Behrens Yamada et al. 2005).

Since this initial colonizing event, Carcinus maenas has persisted at low densities in the coastal Oregon and Washington estuaries, and thrived in the inlets of the west coast of Vancouver Island. In Oregon and Washington, new year classes are detected only after warm winters, characterized by high positive values of the Pacific Decadal Oscillation and multivariate El Nino Southern Oscillation, weak southward shelf currents and late biological spring transitions (Behrens Yamada et al. 2015). On the west coast of Vancouver Island, C. maenas has not only thrived, but also spread to the central British Columbia coast around Bella Bella (detected 2011), Sooke Inlet on the Strait of Juan de Fuca portion of the Salish Sea (detected 2012), and northern Queen Charlotte Strait near Port Hardy (detected 2015) (Gillespie et al. 2015, G. E. Gillespie & T. C. Norgard, unpublished data) (Fig. 1A, B). The northward expansion was predicted from larval transport models (Therriault et al. 2008), but the Sooke population may be due to shellfish transplants from C. maenas-infected sites (Curtis et al. 2015).

Despite extensive sampling by Fisheries and Oceans Canada (DFO), the Washington Department of Fish and Wildlife, Washington Sea Grant, the Padilla Bay National Estuarine Research Reserve, the United States Fish and Wildlife Service and others, Carcinus maenas was not detected in the inner Salish Sea, between Southern Vancouver Island, the mainland, and Puget Sound for 19 y (Gillespie et al. 2007, 2015, Eissinger 2010, Grason 2016). Trapping effort in Canadian and Washington waters was 1,550 and 6,000 traps, respectively. Then on August 30, 2016, a live male C. maenas was trapped in a salt marsh channel in Westcott Bay, San Juan Island, by volunteers as part of the Washington Sea Grant Crab Team early detection program (Fig. IB, Table 1; Yamada & Staude 2016, Grason 2016). Subsequently, biologists from Washington Sea Grant and the Washington Department of Fish and Wildlife set 194 traps around this, and nearby sites, but no more live C. maenas were found. Only one molt was found. In September, an additional live crab was discovered in Padilla Bay, and follow-up trapping by the previous sampling crew and biologists from the Padilla Bay National Estuarine Research Reserve captured three additional live crabs (Fig. 1B, Table 1; Grason 2016). Prior to the discovery of these live crabs, another potential sighting of C. maenas on the Canadian side of Boundary Bay was reported in June 2015 to DFO by a credible source. Subsequent sampling (365 traps) in the southern Gulf Islands, Vancouver Island, and Boundary Bay produced no C. maenas.

Why were no Carcinus maenas detected in the inner Salish Sea for 19 y, when there have been source populations in Oregon and Washington coastal estuaries, and very abundant populations in the coastal inlets of Vancouver Island, including Sooke Inlet, just west of Victoria? Why now? To answer these questions, one needs to

1. Estimate the time the crabs would have entered the inner Salish Sea as larvae.

2. Examine oceanographic and meteorological conditions in the Strait of Juan de Fuca before and after this colonization event.


Settlement Window of Carcinus maenas Larvae

The ages of the crabs captured in Washington were estimated based on their size (Grason, personal communication) and size criteria developed from following the growth of the strong 1998 El Nino year class of Carcinus maenas in Oregon and from mark-recapture studies in Oregon and in Pipestem Inlet, British Columbia (Behrens Yamada et al. 2005, Behrens Yamada & Gillespie 2008). Crabs with less than 60 mm CW at the end of the summer would be classified as 0-age crabs, which most likely settled from the plankton during the previous winter. Crabs between 55 and 80 mm CW would be classified as 1-age crabs.

Oceanographic and Meteorological Conditions

Sea surface temperature (SST) is recorded by U.S. and Canadian government agencies at numerous sites along the west coast of Oregon, Washington, and British Columbia, as well as in the Strait of Juan de Fuca and Salish Sea (Fig. IB). Time series measurements of SST, wind velocity, significant wave height, and other environmental parameters are also available from meteorological buoys (MBs) moored through out the northeast Pacific and inner coastal waterways. Although records from several coastal sites, such as the Race Rocks light station off Victoria and Tatoosh Island off northwest Washington, date back over 30-50 y, near-continuous records from moored MBs are typically shorter than about 15 y.

For this study, most of the available records were examined in detail and time series from NOAA MB C46087 and C46088 were selected (Fig. 1B) to represent wind and SST conditions at the extreme ends of the Strait of Juan de Fuca, the channel connecting the northeast Pacific to inner coastal waterways. These two sites provide near continuous hourly data for the period 2013 to 2016, the period most relevant to recent movement of green crabs into the Salish Sea. (Time series for MBs off the coasts of Oregon, Washington, and southern Vancouver Island closely resemble those for MB C46087, at the western end of the strait.) Similarly, SST records for Port Angeles (Water Level observation site PTAW1) and Cherry Point (site CHYW1) were used to represent the temperature of surface waters connecting the eastern Strait to the Salish Sea. Port Angeles is especially representative of this process because, based on several years of current meter observations across the central strait (Thomson et al. 2007), maximum wind-driven inflow occurs along the U.S. side of the channel. Sea surface temperature records for Tatoosh Island and Neah Bay were omitted because both have large segments of missing data for the period of interest. (Note that the data that are available for these two locations are fully consistent with data from the sites used in this study.)

To determine seasonal to interannual variability in the study region over the past decade, hourly to daily SST time series were used to calculate the monthly mean water temperatures for all sites. The corresponding monthly anomalies for each site were then calculated by first averaging the monthly SST records for each month over the period 2005 to 2010; a longer common averaging period was not possible owing to the brevity of the MB records. Monthly anomalies were obtained by subtracting the mean SST for each month from the monthly series.

Principal component analysis was used to define the alongshore and cross-shore components of the wind stress for each MB. Wind stress rather than wind velocity was used because wind stress, combined with the Coriolis force, appears as the primary forcing mechanism in the equations of motion. Directions are expressed in terms of degrees true compass bearing. Wind stress is positive in the direction of the principal component.


Settlement Window for Carcinus maenas Larvae

The crabs discovered in Westcott and Padilla Bays represent two year classes: 2015 in Westcott (74 mm CW) and 2016 in Padilla Bay (33-58 mm CW) (Table 1). The crab that left the 69 mm molt would have grown to around 80 mm, using a 20% molt increment (Behrens Yamada et al. 2005), and thus could also be attributed to the 2015 year class. These two year classes probably settled from the plankton during the winters of 2014/ 2015 and 2015/2016, respectively.

Oceanographie and Meteorological Conditions

Figure 2 shows monthly mean SST and corresponding temperature anomalies over the past decade for the four selected sites within the Strait of Juan de Fuca (Fig. IB). The moored buoys measure SST at 0.6 m below the waterline (Fig. 2A, B), whereas near-shore platforms measure SST at several meters below Mean Lower Low Water (Fig. 2C, D). Temperature records for the Race Rocks light station on the Canadian side of the strait resemble those for Port Angeles but with slightly lower anomalies; time series for meteorological stations along the outer coast of Washington, Oregon, and southern Vancouver Island resemble those for MB C46087 at the entrance to the Strait of Juan de Fuca.

As a result of anomalously warm surface water in the northeast Pacific from late 2013 to 2015 (McCabe et al. 2016, Di Lorenzo & Mantua 2016), all sites in the Strait of Juan de Fuca recorded anomalously high SST from the fall of 2014 to the spring of 2016. Particularly noteworthy are the marked positive anomalies recorded in October 2014 and 2015 at MB C46087 (strait entrance) and Port Angeles (eastern strait). With the exception of January and February, monthly mean SST at the mouth of the strait and at Cherry Point in the Washington Salish Sea remained above 10°C from the fall of 2014 to end of the available record in early 2016. Surface temperatures at Port Angeles remained above 10°C until late fall and fell to only about 9°C in winter to early spring (Fig. 2C). As a result, surface water temperatures from the oceanic entrance and along the U.S. side of the strait and into the southern Salish Sea were well above normal from the fall of 2014 to the spring of 2016 and typically exceeded the 10°C threshold for larval survival.

As illustrated by Figure 3A, periods of anomalously warm water in the strait in the fall of 2014 and 2015 were accompanied by periods of strong southerly (downwelling favorable) winds off the west coast of Washington and southern Vancouver Island. These storm-generated winds, combined with the earth's rotation (Coriolis effect), led to a cross-shore pressure gradient that reversed the estuarine flow in the strait, leading to warm surface waters and crab larvae being transported from the outer coast to the eastern Strait (Fig. 3). It is not inconceivable that larvae from sites on the Canadian side of the strait could also have been transported eastward by the reversed estuarine flow. Once larvae reached the eastern strait, they would have been redistributed through the Gulf and San Juan Islands to sites within the Salish Sea by tidal currents and regional winds. Strong estuarine-reversing winds also occurred in the fall and winter of 2013 to 2014, but water temperatures were well below the 10°C threshold for larval survival.


Two physical barriers appear to have prevented the movement of Carcinus maenas larvae into the Salish Sea: the dynamical barrier provided by estuarine circulation in the Strait of Juan de Fuca and the thermal barrier presented by cold water temperatures. The surface component of the estuarine circulation in the Strait of Juan de Fuca, driven by the discharge of fresh water into the Salish Sea from the Fraser, Skagit, and other rivers, typically flows seaward out of the strait at mean speeds of around 0.25 m/sec (Thomson et al. 2007). This flow (Fig. 4A) makes it impossible for larvae from coastal oceanic regions or from sites along the Canadian side of the channel to reach the eastern strait under normal flow conditions. Similarly, because of strong tidal mixing through the Gulf and San Juan Islands and in Admiralty Inlet, temperatures of surface waters in the Strait are typically less than 10°C, too cold for green crab larvae to develop (deRivera et al. 2007). However, storm-driven reversals in the estuarine circulation of the strait, lasting for several days to a week, are not uncommon in fall and winter (Thomson et al. 2007). The strongest inflow, sometimes in excess of 1.0 m/sec, is generally found along the U.S. side of the Strait and has been named the Olympic Peninsula Countercurrent (Fig. 4B). When flow reversals occur in the fall during periods of anomalously warm surface ocean conditions, such as during major El Nino events, the dynamical and thermal barriers can be lifted and larvae transported to the eastern Strait of Juan de Fuca and into the adjoining Salish Sea.

Although Carcinus maenas has extended its distribution north of Vancouver Island into appropriate microhabitats in the central coast of British Columbia and were detected in northern Queen Charlotte Strait near Port Hardy in 2015, oceanographic conditions in Johnstone Strait and Discovery Channel are strong barriers to larval development and dispersal southward into the Salish Sea. Vigorous tidal mixing limits stratification and keeps surface temperatures below 10°C year-round and near 7°C in the winter and spring (Thomson 1981). The central portion of Discovery Channel is typically tidally mixed from top to bottom, resulting in water temperatures less than 10°C throughout the water column at all times. Similarly, surface temperatures in Queen Charlotte and Broughton Strait range between 7°C and 10°C from late fall to early spring, only rising above 10°C in the summer. Cold temperatures in this region have been an effective barrier to northward dispersal of Manila clams (Venerupis philippinarum) and Pacific oysters (Crassostrea gigas) (Heritage et al. 1998, Gillespie et al. 2004. Gillespie & Bourne 2005a, 2005b, Gillespie 2007, Gillespie et al. 2012).

Temperature is not the only barrier to southward larval transport to the Salish Sea. The prevailing estuarine circulation in Johnstone Strait is driven by the same processes (primarily, river discharge and tidal mixing) that drive the estuarine circulation in the Strait of Juan de Fuca. Moreover, this circulation, consisting of seaward outflow in the upper layer and compensating inflow in the lower layer, is equally strong in both straits (Thomson 1976, 1977). Thus, water in the upper 75100 m of Johnstone Strait is carried seaward, acting as a barrier to inward larval transport. There are also additional barriers that make it unlikely that the crab larvae used the northern passages to enter the Salish Sea: (1) Unlike the Strait of Juan de Fuca, Johnstone Strait is narrow relative to the internal radius of deformation, r = NH/f ? 10 km, rendering the flow in Johnstone Strait much more strongly two-dimensional than in the Strait of Juan de Fuca (here N is the Brunt-Vaisala frequency determined by the vertical density structure, H is the water depth, and/is the Coriolis parameter associated with Earth's rotation; cf. Thomson & Emery 2014). As a consequence, wind-induced coastally intensified current reversals, such as the Olympic Peninsula Countercurrent observed in Juan de Fuca Strait (Fig. 4B), are not observed in Johnstone Strait. (2) The rare wind-forced reversals of the general estuarine flow in Johnstone Strait only prevail for day or so and are not able to transport surface oceanic water southward from Queen Charlotte Sound to the Salish Sea.

Researchers long feared that Carcinus maenas would eventually enter the inner Salish Sea, either through human-mediated factors, such as shellfish transplants, or through natural larval dispersal (Behrens Yamada & Gillespie 2008). Once a satellite population is established in these prey-rich, protected waters, crabs could spread quickly through larval dispersal, as has happened for other invasives, including Pacific oysters, Manila clams, and varnish clams (Nuttallia obscurata) (Quayle 1964, Gillespie et al. 2001). European green crab C. maenas has also established and spread in other areas once overcoming oceanographic barriers through anthropogenic means [e.g., Australia (Thresher et al. 2003) and Newfoundland (Blakeslee et al. 2010, DFO 2011)]. An earlier C. maenas sighting in 1999 in Price Bay, near Victoria, did not result in a self-maintaining population. It appeared to have been a lone individual brought in as a larva during the prior major El Nino of 1997 to 1998 (Table 1). The size of the Westcott Bay crab, 74 mm in carapace width, suggests that it probably settled from the plankton late in 2014 or early in 2015. The smaller crabs in Padilla Bay would have settled in late 2015 or early 2016. The unique coincidence of anomalously warm water during the stormy season of 2014/2015 and 2015/2016 could have provided a window of favorability for transport of these two year classes to the inner Salish Sea.

It is not known how extensively Carcinus maenas is distributed throughout the inner Salish Sea. It is likely that more sightings will be reported as the two year classes of crabs grow and become easier to detect by professional and volunteer samplers. Recent trapping results suggest that C. maenas abundance in the inner Salish Sea is still low. DFO found no C. maenas in 365 traps set in the southern Gulf Islands, Vancouver Island, and Boundary Bay. The abundance at Westcott Bay of 1/200 traps and in Padilla Bay of less than 1/100 traps suggests that green crabs are rare and may not yet constitute a breeding population. As the recent El Nino has now dissipated, a temporary reprieve from further green crab exogenous larval dispersal into the inner Salish Sea can be expected.

Both temperature and currents play a role in the establishment of expatriate populations of Carcinus maenas around the world (Behrens Yamada et al. 2015). Larvae require sea water temperatures greater than or equal to 10°C to develop and females need temperatures less than or equal to 18°C to successfully incubate their eggs (Crothers 1967, deRivera et al. 2007). If these critical temperatures are not reached, for at least part of the year, C. maenas cannot establish self-perpetuating populations, as was the case for all tropical sightings (Carlton & Cohen 2003). In addition to favorable temperatures, currents during the critical larval stages can determine whether a cohort or a population can become established (Behrens Yamada et al. 2015). For example, green crab populations have persisted in Cape Town, South Africa, since 1980, but have not expanded their range in over 35 y, despite the presence of wave-sheltered lagoons with abundant prey just 90 km to the north (C. L. Griffiths, personal communication). The north-flowing Benguela current off the South African coast is characterized by strong upwelling (Boyer et al. 2000). Seaward Ekman transport in the surface layer carries larvae off-shore, thus creating a barrier to dispersal to the favorable habitats to the north. European green crab in Australia and Tasmania showed a pattern of long periods of relative stasis punctuated by rare episodes of longer-distance and widespread expansion, possibly facilitated by unusual oceanographic conditions or anthropogenic transfers (Thresher et al. 2003). These examples, together with our observations in Oregon, Washington, and British Columbia, confirm the important roles temperature and currents play in limiting the successful range expansion of C. maenas.


We thank Allen Pleus, Emily Grason, and Lorenz Sollmann for providing Washington trapping data and Lyanne Curtis for providing recent DFO trapping data. Roy Hourston of DFO assisted with the oceanographic figures. The reviews and comments by James Carlton, Michael Kosro, and Emily Grason greatly improved a former version of this manuscript. GEG and TCN were employed by DFO while collecting trapping data, no other funding sources were used to prepare this manuscript. SBY thanks the Oregon State University Valley Library for use of a research room during preparation of this manuscript.


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(1) Integrative Biology, Oregon State University, CorvaUis, OR 97331; (2) Institute of Ocean Sciences, Fisheries and Oceans Canada, Sidney, BC V8L 4B2, Canada; (3) Pacific Biological Station, Fisheries and Oceans Canada, Nanaimo, BC V9R 6N7, Canada

* Corresponding author. E-mail:

DOI: 10.2983/035.036.0121

Caption: Figure 1. (A) Locations of sampling sites for Carcinus maenas by Fisheries and Oceans Canada, 2006 to 2016. (B) Locations of MB moorings and water level-SST sites in the Strait of Juan de Fuca and in the Salish Sea and sites where C. maenas were discovered. Legend: 1. MB C46087--strait entrance; 2. PTAW1--Port Angeles; 3. MB C46088--eastern strait; 4. CHYW1--Cherry Point; 5. Acoustic Doppler current profiler array. PaB, Padilla Bay; PrB, Price Bay; SB, Sooke Basin; SH, Sooke Harbour; WB, Westcott Bay.

Caption: Figure 2. (A-D) Monthly mean SST (solid line) and the mean monthly SST (broken line) at four sites in the study region. Sea surface temperature anomalies are the difference between the two curves; red denotes higher than average SST values, blue lower than average values. Data presented in (A) and (B) are from MB moored in the middle of the entrance to the Strait of Juan de Fuca and in the middle of the strait at the eastern end, respectively. Data in (C) and (D) are from coastal sites at Port Angeles and Cherry Point, respectively. See Figure 1B for location.

Caption: Figure 3. (A) Daily alongshore component of wind stress for MB C46087 at the entrance to the Strait of Juan de Fuca. Positive wind stress is toward 280 degrees true; (B) the same but for MB C46088 at the eastern end of the strait. Note the warm daily temperatures and relatively weak alongshore (northward) wind stress at this site.

Caption: Figure 4. Gridded cross sections of the observed daily mean along-strait component of residual current in the Strait of Juan de Fuca. (A) Normal estuarine circulation (observations of June 2003) and (B) wind-forced reversed flow (observations of 16 December 2002). Velocity scale ranges from 80 to 65 cm/sec with positive ("in") values denoting landward flow and negative ("out") values denoting seaward flow. A no-slip boundary condition has been assumed in the mapping with the no-slip boundary positioned slightly landward of the actual bathymetry. Velocity data are from upward looking acoustic Doppler current profilers. Speeds are listed for selected depths. For reference purposes, a sustained speed of 50 cm/sec equates to a speed of 43 km/day and to a distance of roughly 300 km over a period of a week (from Thomson et al. 2007).

First sightings of Carcinus maenas in the Salish Sea and subsequent
trapping results.

Location Date (first discovered Sex Carapace
 or sampled) width (mm)

Price Bay, Esquimalt, August 1999 M 65
 British Columbia
 Summer to Fall 1999

Sooke, British 2012 3 M >79
 Columbia 2 F >69

Wescott Bay, August 30, 2016 M 74
Roche Harbor

Wescott Bay August 30, 2016
Sea Farm
Wescott Bay, September 12-14, 2016 ? 69
 Roche Harbor,
 and Henry Island
Padilla Bay September 19, 2016 F 34

Padilla Bay September 27, 2016 F 39

Padilla Bay September 27, 2016 F 40

Padilla Bay September 28, 2016 M 58

Location Condition Discovered or
 subsequently sampled

Price Bay, Esquimalt, Barron Carswell, B.C.
 British Columbia Ministry of Fisheries

Sooke, British Public report/DFO

Wescott Bay, Molted Craig Staude, Jack Bell,
Roche Harbor recently Bruce Robinson,
 WSG volunteers,
 and SBY
Wescott Bay SBY and WSG
Sea Farm volunteers
Wescott Bay, Molt, covered WSG, WDFW
 Roche Harbor, with algae
 and Henry Island
Padilla Bay Glen Alexander,
Padilla Bay WSG, PBNERR,
Padilla Bay WSG, PBNERR.
Padilla Bay WSG, PBNERR.


Location (no. of Estimated
 crabs/no. of traps) year class

Price Bay, Esquimalt, 1998
 British Columbia

Sooke, British 415/102 2009 or 2010

Wescott Bay, 1/11 2015
Roche Harbor

Wescott Bay 0/15
Sea Farm
Wescott Bay, 0/194 2015
 Roche Harbor,
 and Henry Island
Padilla Bay 2016

Padilla Bay 2/182 2016

Padilla Bay 2016

Padilla Bay 1/186 2015 or 2016

Location Estimated time Comments
 of settlement

Price Bay, Esquimalt, Fall 1997 to El Nino
 British Columbia Spring 1998
 No more green
 crabs found
 in area
Sooke, British 2010 or earlier Possibly from
 Columbia shellfish

Wescott Bay, Fall 2014 to El Nino
Roche Harbor Spring 2015

Wescott Bay
Sea Farm
Wescott Bay, Fall 2014 to El Nino
 Roche Harbor, Spring 2015
 and Henry Island
Padilla Bay Fall 2015 to El Nino
 Spring 2016
Padilla Bay Fall 2015 to El Nino
 Spring 2016
Padilla Bay Fall 2015 to El Nino
 Spring 2016
Padilla Bay Fall 2014 to El Nino
 Spring 2016

SBY, Sylvia Behrens Yamada; PBNERR, Padilla Bay National Estuarine
Research Reserve; WDFW, Washington Department of Fish and Wildlife;
WSG, Washington Sea Grant Crab Team.
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Article Details
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Author:Yamada, Sylvia Behrens; Thomson, Richard E.; Gillespie, Graham E.; Norgard, Tammy C.
Publication:Journal of Shellfish Research
Article Type:Report
Geographic Code:1USA
Date:Apr 1, 2017
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