ORIGINAL PAPER

The potential for mitten crab Eriocheir sinensis H. MilneEdwards, 1853 (Crustacea: Brachyura) invasion of PacificNorthwest and Alaskan Estuaries

Erik Hanson Æ Mark Sytsma

Received: 7 August 2006 / Accepted: 10 August 2007

� Springer Science+Business Media B.V. 2007

Abstract Eriocheir sinensis H. Milne Edwards,

1853 is on the list of top 100 invaders compiled by

the International Union for Conservation of Nature

and Natural Resources. The recent establishment of a

large Chinese mitten crab population in San Fran-

cisco Bay and the potential for introductions from

California, Asia and Europe pose a significant

invasion potential for estuaries and rivers from

California to Alaska. This alien species would place

at risk the catchment areas of the Pacific Northwest

including the economic and social activities that

depend upon intact aquatic systems. An analysis of

ecological conditions that define the mitten crab’s

native and introduced range suggests that large stable

estuaries with long flushing times are necessary to

sustain significant populations. Most Pacific North-

west estuaries have limited salinity intrusion,

estuarine habitat and short flushing times and face a

reduced risk of population establishment. Large,

stable estuaries, such as the Puget Sound, may

support significant populations. River-dominated

estuaries, such as the Columbia River, have flushing

times less than the duration of larval development

and wouldn’t support populations. An application of

a temperature based larval development rate to near-

shore and estuary sea surface temperatures suggests

that estuaries in Oregon and Washington have

sufficient thermal regimes to support larval develop-

ment. Most estuary systems in Alaska have limited

periods where water temperatures are above the

mortality threshold for the larval stages and are at a

low risk for the establishment of populations. A

potential sea temperature rise of two degrees Celsius

would permit larval development in Alaskan estuar-

ies, where sufficient estuarine and freshwater habitats

exist.

Keywords Eriocheir sinensis � Mitten crab �West coast estuaries � Invasive species � Invasion

prediction � Zoeal development

Abbreviations

PNW Pacific Northwest

NOAA National Oceanic and Atmospheric

Administration

CMCWG Chinese Mitten Crab Working Group

Introduction

The Chinese mitten crab, Eriocheir sinensis, is a

catadromous species that invades estuary-river cou-

pled systems and impacts their ecological functions

and economic and human uses (CMCWG 2003). The

E. Hanson (&) � M. Sytsma

Center for Lakes and Reservoirs, Portland State

University, P.O. Box 751, Portland, OR 97201, USA

e-mail: ehanson@pdx.edu

M. Sytsma

e-mail: sytsmam@pdx.edu

123

Biol Invasions

DOI 10.1007/s10530-007-9156-3

recent establishment of a large population of mitten

crabs in the San Francisco Bay (Hieb 1997) and

capture of individual crabs in the Saint Lawrence

Seaway (de Lafontaine 2005), Chesapeake Bay (Ruiz

et al. 2006), Iran (Robbins et al. 2006) and Iraq

(Clark et al. 2006) demonstrates that the mitten crab

is being introduced to new habitats. The potential for

unintentional and intentional introductions from Cal-

ifornia, Asia, and Europe pose a significant invasion

risk to estuaries and rivers in the Pacific Northwest

(PNW) and Alaska that meet the mitten crab’s habitat

and environmental requirements (CMCWG 2003).

The mitten crab is native to the Yellow Sea region

in a latitude range of 24–42� North, from Hong Kong,

China to the Yalu River in South Korea (Hymanson

et al. 1999; Panning 1939). Introduced into Germany

in the 1912, they spread in Europe to a latitude range

of 36–55� North (Herborg et al. 2003). This latitude

range in North America covers a region from Mon-

terey Bay, California to Northern British Columbia.

The mitten crab’s life history consists of two larval

phases (zoea and megalopa) and adult. The planktonic

zoea occur primarily in lower estuary and near-ocean

habitats. The magalopa are a transitional phase

between the planktonic zoea and the benthic adult

that undergo a metamorphosis into a juvenile crab in

brackish and fresh water (Rudnick et al. 2003). The

early juvenile crab then resides in tidally influenced

low salinity areas through the winter with a migration

upstream to brackish and fresh water rearing areas the

next year (Panning 1939). The adults occupy a wide

range of brackish and freshwater habitat with a

migration to higher salinity estuarine waters for

reproduction (Veldhuizen and Hieb 1998; Rudnick

et al. 2005).

In Europe, main mitten crab population centers

occur in the Elbe, Weser, Ems, Rhine, Humber, and

Thames river systems, with crabs found in the

coastal areas of Denmark, Germany, the Netherlands,

Belgium, and France, (Panning 1939; Ingle 1986;

Gollasch 1999). Asian watersheds that support

significant populations are the Liao, Yangtze, Ouji-

ang, and Hai Rivers (Jin and Li 1998). No

populations are established in estuary-river systems

that open into the Mediterranean (Petit and Mizoule

1974) or Baltic Sea (Haahtela 1963). Mitten crabs

have been collected in Norway, Sweden and Finland

but no established populations have been documented

(Christiansen 1988).

The highest densities of mitten crabs usually occur

within estuaries and near river mouths but high

densities have been reported as far as 90 km upstream

of the mouth of the Thames River (Attrill and

Thomas 1996) and 450 km upstream of the mouth of

the Elbe River (Panning 1939). The reported maxi-

mum migratory distance traveled by mitten crabs is

1,000 km in China (Panning 1939).

The larvae comprises five zoeal (occasionally six

under certain conditions) and one megalopal stage,

over a one to four month period (Anger 1991; Kim

et al. 1995). The zoea last 4–15 days per stage and

the megalopa between 20 and 30 days dependant

upon temperature (Anger 1991). Larvae are present in

estuaries from winter through summer (Anger 1991;

Rudnick and Resh 2002; Panning 1939).

Larval development and survival is temperature

and salinity dependant, with survival in a range of

salinities from 15 to 32 ppt and temperatures from 12

to 25�C (Anger 1991). Optimal survival occurs in

salinities of 20–25 ppt and temperatures from 15 to

25�C (Anger 1991; Kim et al. 1995; Huang et al.

2001). Complete mortality in the first zoea stage

occurs at 9�C (Anger 1991).

The evaluation of invasion potential is based on

the premise that mitten crab populations will become

established in estuary-river systems that match eco-

logical conditions that define the native and

introduced range. This approach is an established

method used to predict expansions and distributions

of invasive species. Numerous studies have demon-

strated that a species ecological niche represents a

long-term stable constraint on distributional potential

(Kolar and Lodge 2001). This approach is combined

with an analysis of thermal regimes in select estuaries

in comparison to the larval temperature requirements

to determine if a sufficient period exists for larval

development. Temperature dictates survival limits

and the length of the larval period.

Materials and methods

Habitat comparison

Data for variables that define the available estuarine

and freshwater habitat of systems with established

mitten crab populations was collected from published

literature. Limited information exists on the habitat

E. Hanson, M. Sytsma

123

use of adult mitten crabs. The characteristics that are

known are broad scale. For example, the adult crab is

not limited to wetlands, tidal areas, or lowland

streams. All brackish and freshwater areas are

potential habitat, so measures of total available

habitat were used.

Detailed information does exist on the salinity and

temperature requirements and development times of

the larval stage. The interaction between salinity,

estuarine circulation, and temperature that would

explicitly define larval habitat in an estuary is

complex and can only be resolved on an individual

estuary basis. Broad scale variables that define the

interaction between estuarine circulation and salinity

were utilized in this analysis. Other researchers have

utilized these variables to define environmental

processes, estuary classifications and self-recruitment

to estuarine populations (Sponaugle et al. 2002).

Variables that were used include:

• Watershed area: the land area that drains to the

estuary. An indicator of freshwater habitat.

• Estuary area: the total surface area of the estuary,

defined variously. In general, the area between

the river mouth and the last extent of land

before the ocean. This indicates the area avail-

able for reproduction, larval, and early juvenile

development.

• Tidal influence: the distance upstream from the

mouth of the estuary that water level is affected

by the tides. The tidal length is a secondary

indicator of estuarine habitat.

• Salinity intrusion: the distance into an estuary that

salinity penetrates. Salinity intrusion is correlated

with the area of a salinity mixing in an estuary

and indicates the area that is available for larval

development.

• Flushing time: the time it takes to replace the

freshwater volume of an estuary at the rate of net

flow through the estuary. The flushing time defines

the rate at which water masses and larvae are

exchanged between the river, estuary and ocean.

Flushing time is an indicator of larval retention.

Pacific Northwest and Alaskan systems with

habitat and environmental values within the range

of values calculated from the introduced and native

range are expected to provide the necessary condi-

tions for the development of substantial populations.

Larval development in PNW and Alaskan

estuaries

The rate of larval development is a function of

temperature. Laboratory rearing times at 12, 15, and

18�C were used by Anger (1991) to develop temper-

ature-based stage-specific models of development.

We utilized these stage-specific development models

to derive total larval (zoeal and megalopal) develop-

ment times at 12, 15, and 18�C; 90, 62, and 43 days,

respectively. These development times were then

used to derive a temperature-based model of total

development time for the larval period. The equation

for larval development is

D ¼ 393.99e�0:123x

where X = water temperature (�C), and D = devel-

opment duration in days.

This equation was utilized to derive estuary

specific larval development times for select PNW

and Alaskan estuaries. The larval development equa-

tion in conjunction with temperature limits was used

to determine a potential larval period when temper-

atures are sufficient for the completion of larval

development for each estuary. The development

times at 12 and 9�C were compared to the average

number of contiguous days above 12 and 9�C to

determine the thermal suitability of each estuary.

Analysis proceeded on the premise that all

temperatures above 9�C permitted development.

Mean daily water temperature was obtained from

National Oceanic and Atmospheric Administration’s

(NOAA 1985) website of observed United States

water levels and meteorological and oceanographic

information (http://www.co-ops.nos.noaa.gov/data_

res.html) for PNW and Alaskan sites within estuar-

ies and near-shore (Table 1). Several sites, San

Francisco Bay, Puget Sound, and Prince William

Sound are composed of data from several stations

(Table 1). Temperature data was collected within one

meter of the surface and was recorded for up to a

20-year period. Daily averages were calculated from

temperatures that were recorded at up to a 10-min

interval. A daily increment (1/D) in development was

calculated based upon the average daily temperature

(X). The (1/D) values were summed until the frac-

tions equaled one and indicated the completion of a

larval or zoeal period.

The potential for mitten crab Eriocheir sinensis

123

Ovigerous females are found in estuaries beginning

in October (Rudnick et al. 2003) and provided the

start date (October 1) for the period tested for larval

development. An end date of August 31 was chosen

because it provides a minimal period for juvenile

crabs to accumulate enough resources before low

temperatures limit feeding activity and growth and

roughly corresponds to when megalopae are typically

last seen in Germany (Panning 1939). The start date

was delayed when temperatures dropped below 9�C.

The potential larval period was the total number of

days from the first start date until the final date that

would permit complete larval development by August

31. The development time for each estuary was

calculated as an average of bi-monthly incremented

development times over the potential larval period.

As an additional step, larval development times

were calculated for sites after inclusion of a 2�C

water temperature rise. A 2�C was chosen because it

is a midpoint of the predicted range of ocean

warming over the next 100 years (IPCC 2001).

Results

Habitat comparison

Systems with established mitten crab populations

exhibited a wide habitat range, with watershed from

12,700 to 1,808,000 km2 and estuary area from 200

to 1,328 km2 and a relatively narrow mean flushing

time from 23 to 65 days (Table 2). The habitat data

(watershed and estuary area and tidal intrusion)

indicates that the Columbia River, Puget Sound,

and Cook Inlet are within the range of systems with

significant mitten crab populations (Table 3). The

salinity intrusion and flushing time indicates that

Puget Sound, Taku, Chikat, and Stikine River estu-

aries and Cook Inlet can support larval development

(Table 3). Puget Sound and the Alaskan systems

match all of the conditions found in systems with

mitten crab populations.

Larval development

In San Francisco and Coos Bay, the larval period

begins on October 1 and runs continuously through

the summer. All other estuaries have an extended

period of temperatures below 9�C that would pre-

clude larval development until Spring (Fig. 1). San

Francisco Bay has the longest potential larval period

of 293 days, from October 1 until July 20. In contrast,

the potential larval period in Willapa Bay, Puget

Sound, and Alaskan estuaries is 126, 63, and 25,

respectively (Table 4). For Alaskan waters, the

average period for larval development begins on

May 25 and ends on June 19. For sites north of the

Table 1 List of estuaries and corresponding NOAA stations used in larval development analysis

Area NOAA station Lattitude Longitude

San Francisco Bay 9414523 Redwood City, CA 37�30.40 N 122�12.60 W

9414863 Richmond, CA 37�55.70 N 122�24.00 W

9414290 San Francisco, CA 37�48.40 N 122�27.90 W

Yaquina Bay 9435380 South Beach, OR 44�37.50 N 124�2.60 W

Columbia River 9439040 Astoria, OR 46�12.50 N 123�46.00 W

Willapa Bay 9440910 Toke Point, WA 46�42.50 N 123�57.90 W

Puget Sound 9447130 Seattle, WA 47�36.30 N 122�20.30 W

9446484 Tacoma, WA 47�16.00 N 122�24.80 W

Ketchikan 9450460 Kethikan, AK 55�20.00 N 131� 37.50 W

Juneau 9452210 Juneau, AK 58�17.90 N 134� 24.70 W

Prince William Sound 46060 West Orca Island, AK 60�34.50 N 146� 50.00 W

9454240 Valdez, AK 61�7.50 N 146� 21.70 W

Cook Inlet 9455920 Anchorage, AK 61�14.30 N 149� 53.40 W

Kodiak 9457292 Kodiak Island, AK 57�43.90 N 152� 30.70 W

E. Hanson, M. Sytsma

123

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The potential for mitten crab Eriocheir sinensis

123

Columbia River an increase of 2�C doubles the length

of the available period for larval development

(Fig. 1) and has a limited effect on the average

length of larval development (Table 4).

The number of contiguous days above 9�C com-

pared to larval development at that temperature

(131 days) indicates that most systems south of

Alaska are thermally suitable (Fig. 2). Coos Bay,

Table 3 Habitat values for Pacific Northwest and Alaskan estuaries

Watershed

area (1,000 km2)

Estuary area (km2) Tidal intrusion (km) Maximum salinity

intrusion (km)

Mean flushing

time (days)

Min–Max Min–Max Min–Max Min–Max Min–Max

12.7–1,808 200–1,328 100–326 67–107 23–65

Rogue River 13.2a 2.6a 6d 1d xp

Coos Bay 2.7a 33.7a 54d 30a 35h

Umpqua River 1.6a 25.9a 45d 27d xp

Siuslaw River 11.8a 10.4a 40d 27d xp

Alsea River 1.2a 5.2a 26d 22d gi

Yaquina River 0.6a 12.9a 41d 32d 7j

Siletz River 0.9a 5.2a 38d 21d xp

Netarts Bay 0.04a 5.2a 8d 8d 2j

Tillamook Bay 1.4a 31.1a 21k 21g 10k

Nehalem Bay 2.2a 5.2a 25d 23d xp

Columbia river 670a 735.6a 234e 43e 3e

Willapa Bay 1.9a 238.3a 45a 40a xl

Grays Harbor 6.3a 150.2a 50f 45a xm

Puget Sound 237a 2,632b [300a [300a 152b

Taku estuary 13c 17c – – –

Chikat estuary 13c 18c – – –

Stikine estuary 19c 88c – – –

Cook Inlet 101n –o –o –o –o

The range of values for estuaries with mitten crab populations is indicated by minimum and maximum values listed under each

column

Bold indicates within range, – indicates estimated within range, · indicates estimated below rangea NOAA (1985)b Friebertshauser and Duxbury (1972)c Charstansen (2004)d Percy et al. (1974)e Hamilton (1984)f Uncles et al. (2002)g Komar (1997)h Arnerson (1976)i Matson (1972)j Zimmermann (1972)k Colbert and McManus (2003)l Hickey et al. (2002)m Duxbury (1979)n Brabets et al. (1999)o Okkonen and Howell (2003)p —Insufficient data

E. Hanson, M. Sytsma

123

Yaquina Bay, and Puget Sound do not have a

sufficient number of days greater than 12�C (90 days)

(Fig. 3). Alaskan estuaries are below the minimal

threshold for both 9 and 12�C (Figs. 2, 3). All sites

from Ketchikan south would meet the 9 and 12�C

threshold with the inclusion of a 2�C temperature rise

and two northern Alaskan sites, Valdez and Anchor-

age would meet the 12�C threshold (Figs. 2, 3).

Discussion

Most estuaries in the Pacific Northwest are much

smaller than the average estuary that supports a

significant mitten crab population. The average

system with a mitten crab population has an estuary

area of 602 km2 with a salinity intrusion of 85 km

and a flushing time of 45 days. Large, stable estuaries

provide salinity regimes for optimal larval survival

and flushing times of sufficient duration for develop-

ment within the estuary. A flushing time of 45 days is

greater than the zoea development time at tempera-

tures equal to or greater than 14�C.

The estuaries with habitat to support mitten crab

populations are Puget Sound and the Alaskan Estu-

aries. Most European and Asian estuaries with mitten

crab populations are large. This contrasts with most

PNW estuaries that tend to be small and quickly

transition to open ocean waters. While the Columbia

River is a large estuary, most surface salinities are

below 10 ppt with limited areas with salinities high

enough to permit larval development (NOAA 1985).

The Columbia River’s short flushing time suggests a

low probability of zoea being retained within the

estuary.

Coos Bay is smaller than most systems with mitten

crabs, yet its long flushing time suggests that larval

development could occur within the bay. If mitten

crab populations are limited by the retention of larvae

near adult habitat, then Coos Bay may support mitten

crab populations. The lack of significant estuary

habitat in Coos Bay that is necessary for the

development of young adult crabs, may limit popu-

lations in Coos Bay. The combination of these two

factors will probably determine the potential for

mitten crab populations in Coos Bay.

The lack of a significant population in the Seine

River supports the use of flushing time as a predictive

measure. Between 1943 and 1996, only 50 crabs were

caught in the Seine River and estuary (Vincent 1996).

Its watershed and estuary area are within the range

expected to support mitten crab populations, but its

flushing time (10 days) and salinity intrusion (46 km)

(Gilles and Fitch 2000) likely limit larval recruitment.

A main characteristic of estuaries with mitten crab

populations not captured by this analysis is a

connection to the ocean through large shallow marine

areas and seas that are protected from ocean circu-

lation patterns. These areas act as secondary estuaries

with extensive salinity mixing zones and extended

retention times. The Wadden Sea, into which the

Ems, Weser, and Elbe River flow, is a shallow area of

Fig. 1 Bar graph indicating

potential larval period in

each estuary. The Black bar

indicates mean daily water

temperature and the white

bar indicates mean daily

temperature with a 2�C

increase

The potential for mitten crab Eriocheir sinensis

123

roughly 13,000 km2 sheltered by barrier islands that

extends from the Netherlands to Denmark (deJonge

2003). The flushing time of this coastal area is around

73 days (OSPAR 2000). The Bohai and Yellow Seas

into which the Yangtze, Haihe, and Oujiang Rivers

flow also exhibit shallow depths, extended retention

times and extensive mixing zones (Li and Qin 2003;

Dingman 2003). Larvae flushed out of estuaries into

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e2

2n

dA

ug

5th

Jul

26

thJu

n3

0th

Jul

4th

May

26

thJu

l1

0th

Jul

6th

Jun

4th

To

tal

len

gth

(day

s)o

fla

rval

dev

elo

pm

ent

31

02

68

26

51

48

29

92

73

76

14

60

48

15

Fig. 2 Mean number of contiguous days greater than or equal

to 9�C for Pacific Northwest and Alaskan estuaries. The

number of days to complete larval development at 9�C

(131 days) is indicated by the black bar

Fig. 3 Mean number of contiguous days greater than or equal

to 12�C for Pacific Northwest and Alaskan estuaries. The

number of days to complete larval development at 12�C

(90 days) is indicated by the black bar

E. Hanson, M. Sytsma

123

The West Coast has few areas that serve as

secondary retention zones. The continental shelf

along the PNW is narrow; the 200 m depth contour

is from eight to 32 km offshore (Hickey and Banas

2003). Circulation consists of alongshore currents

dominated by wind-driven coastal upwelling and

river plumes, primarily the Columbia River plume

that extends 300–400 km seaward. The strong cur-

rents and lack of shallows would disperse larvae

flushed into the ocean and prevent return to natal or

nearby estuaries.

Alaskan estuaries contain habitat that is suitable

for mitten crab populations. Temperature is more

likely to limit mitten crab populations in Alaskan

systems. Mitten crabs feed at temperatures down to

7�C, molt at 10�C and survive short periods of lower

temperatures. Temperatures in rivers that flow into

the Taku, Stikine, and the Prince William Sound

have extended periods of temperatures below 5�C

and are normally below 10�C in the summer (USGS

2004). The temperature in these systems typically

rises to 7�C in May with approximately 30 days

above 9�C. In contrast, the mean annual tempera-

tures of large rivers in Europe range from 11 to 14�C

(OSPAR 2000). The lack of established populations

in Sweden and Norway suggests that mitten crabs

either have a limited tolerance of long cold periods

or require a longer period of high temperatures for

growth.

Larval development

This present study considers that estuaries South of

Puget Sound have a sufficient period of temperatures

suitable for larval development (Chart 1). A conser-

vative estimate is used for the minimum temperature

for larval development in our analysis—the temper-

ature at which complete mortality occurred (9�C),

because the survival rate at temperatures between 9

and 12�C is unknown. If temperatures closer to 12�C

cause high levels of mortality, the number of

estuaries capable of supporting larval development

would be lower. Puget Sound, Yaquina Bay, and

Coos Bay have significant periods when the water

temperature is between 10 and 12�C. A more precise

estimate of lethal temperature limits would greatly

enhance the ability to predict the potential for larval

development in these estuaries.

The limited period available for larval develop-

ment in Alaskan estuaries is probably not sufficient to

maintain populations (Fig. 1). Given all the other

factors involved in reproduction (reproductive matu-

ration, migration, mating, and egg development),

there is a low probability that a window of 25 days

would permit larval development. In years with

below average temperatures, the potential develop-

ment period would not be sufficient for larval

development. Larval development is only a portion

of the reproductive cycle that requires moderate to

high temperatures. It is unknown what effect low

temperatures would have on reproduction, egg devel-

opment, and juvenile growth.

A 2�C increase in water temperature greatly

increases the probability of larval development in

all estuaries. This increase would not only effect

larval development but would also increase the

potential for juvenile crab development prior to the

onset of low wintertime temperatures and probably

adult growth rates (Rudnick et al. 2005). This

temperature increase would place Alaskan estuaries

at risk, especially as they contain freshwater and

estuarine habitat that is suitable for the development

of large mitten crab populations.

A similar approach was used to examine the

potential green crab, Carcinus maenas, invasion of

North American (deRivera et al.2006). The study

found a temperature range for larval survival similar

to mitten crabs, with 10�C as the lethal threshold

(deRivera et al. 2006). Using a larval development

regression model, and some of the same temperature

data, they concluded that some Alaskan waters were

at risk because the number of days above 12.5�C was

greater than the 59 days required for larval develop-

ment (deRivera et al. 2006). Our mitten crab range

expansion predictions differ from their predictions for

green crabs, primarily due to a difference in devel-

opment time. Green crab larvae (4 zoeal stages, 1

megalopal) develop into juvenile crabs in about

59 days at 12.5�C versus 85 days for mitten crabs.

The flushing time is important to mitten crabs

because a major portion of their larval development

period is spent as a planktonic zoea. At 14�C, the

zoeal period lasts 44 days. This suggests that reten-

tion of larvae within estuaries is an important factor

for the development and maintenance of mitten crab

populations. Many PNW estuaries have a flushing

time that is a fraction of the period necessary for

The potential for mitten crab Eriocheir sinensis

123

larval retention. Puget Sound and the Alaskan

estuaries have flushing times that would permit larval

development within the estuary. In estuaries such as

Coos Bay and Willapa Bay the larvae would probably

spend an extended period in near-ocean waters where

they would quickly be moved off and along-shore.

Larval transport out of estuaries probably even

greater in the Columbia River where the larvae

would spend almost all of the larval period in the

ocean with a very low probability of return. In

general, the probability larval return to natal estuaries

is inversely related to the rate at which larvae flux

away from the parent population (Sponaugle et al.

2002).

The direct comparison of flushing time to larval

development time simplifies variation in flow and

ignores geographic, climatic, and local conditions

such as headlands that create local retention zones,

river plumes that increase and concentrate dispersion,

fronts, upwelling effects, and wind driven surface

currents (Sponaugle et al. 2002). These conditions

can lengthen or shorten the potential dispersion and

create local retention zones within or near estuaries.

On average, larvae that spend a significant amount of

time outside of an estuary are likely to be transported

beyond the ability of the megalopae to return to the

estuary. Retention and dispersal can also be affected

by larval behavior in the form of vertical migration

and selective tidal stream transport by zoea and

megalopa. It is unknown if mitten crab larvae exhibit

these behaviors.

Flushing times are flow dependant; during periods

with high flow rates, flushing times will be greatly

reduced. The rivers and streams in the indigenous

range of the mitten crab experience high flows in the

summer and low flows in the spring when larvae are

likely to be present in the estuary. This increases the

likelihood of estuarine retention in the native range.

The PNW experiences high flows when mitten crab

larvae are likely to be in estuaries. This increases the

likelihood that mitten crabs will be flushed from

PNW estuaries.

Conclusion

The examination of the key factors necessary for

larval development of mitten crabs, habitat suitabil-

ity, and environmental condition suggests that the

majority of PNW and Alaskan estuaries are not at risk

of establishment of significant mitten crab popula-

tions. Puget Sound is the only estuary with the proper

combination of habitat, salinity, flushing time, and

temperature for larval development and maintenance

of mitten crab populations. Alaskan waters were

deemed at a low risk due to insufficient temperatures

to support larval development. The potential for a

period of increased temperatures due to global

warming or natural warming trends may place

Alaskan waters at a higher risk.

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