European Green Crab (Carcinus maenas) Dispersal: The Pacific ...

Crabs in Cold Water Regions: Biology, Management, and Economics Alaska Sea Grant College Program • AK-SG-02-01, 2002

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European Green Crab (Carcinus maenas) Dispersal: The Pacific Experience G.S. Jamieson Fisheries and Oceans Canada, Pacific Biological Station, Nanaimo, British Columbia, Canada

Mike Foreman and Josef Cherniawsky Fisheries and Oceans Canada, Institute of Ocean Sciences, Sidney, British Columbia, Canada

Colin Levings Fisheries and Oceans Canada, West Vancouver Lab, West Vancouver, British Columbia, Canada

Abstract The European green crab, Carcinus maenas (Decapoda: Portunidae), is extending its range much more rapidly on the Pacific coast of North America than it is on the Atlantic coast. This crab was first observed on the west coast of North America in San Francisco Bay in 1989, and by 2001, green crabs had been found in Esperanza Inlet on the west coast of Vancouver Island, about 1,500 km northward. This dramatic rate of range expansion on the Pacific coast to British Columbia may have been facilitated by 1997/ 1998 El Niño currents, as the crabs found in Canada were all of a size expected of the 1997/1998 year class. Ocean current analyses derived from sea surface elevation anomalies do not indicate likely larval transport via sea level–induced currents after February 25, 1998, but from November 26, 1997, to the former date, north-flowing currents were stronger than normal. Currents resulting from short-term storm event(s) might have also transported larvae, but these were not resolvable from the seasonal satellite altimetry data we considered. However, the wide spatial distribution of green crab occurrences in Canada suggests ocean currents in late 1997/early 1998 were the most likely vector for the first transport of green crab larvae to British Columbia.

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Jamieson et al. — European Green Crab Dispersal

Introduction Green crabs (Carcinus maenas L.) are indigenous to temperate waters in the Mediterranean Sea and the eastern Atlantic from Mauritania to Norway. This species has been introduced, presumably accidentally, into a number of locations worldwide, including the eastern Pacific in the late 1980s. A number of papers describe this latter introduction (Cohen et al. 1995, Grosholz and Ruiz 1995, Grosholz 1996, Dumbauld and Kauffman 1998, Hunt et al. 1998, Jamieson et al. 1998) in a general sense, but few have investigated the oceanographic features which have influenced the subsequent dispersal and range extension of this species (Jamieson 2000). This information is relevant for a number of reasons: 1.

Since green crabs are new to the northeastern Pacific, the species’ dispersal is relatively easy to document and correlate with oceanographic events. This allows improved evaluation of the role of oceanography in influencing the dispersal of meroplanktonic larvae from indigenous species, information that would otherwise be difficult to obtain. The larvae of indigenous species from different “source” sites (Pulliam 1988, Roberts 1998) are generally mixed, confusing the dispersal patterns from specific sources.

2.

From a metapopulation conservation perspective, it may be particularly important to identify significant source populations of indigenous species and to protect them. Having a nonindigenous species (NIS) model to help identify potentially important sites may present a unique opportunity in this regard. Knowing the dispersal patterns of a NIS allows research on its impacts on the indigenous ecosystem to be evaluated. Ideally, preinvasion monitoring can be conducted in areas where it is not yet known to occur, but where it will be likely to occur as its distribution range expands. Then, when it is known to be present, the consequences of its presence can also be investigated.

3.

Alternate sites for the approved dumping of ballast water are presently being investigated by computer models that predict the dispersion of particles from a point source (M. Foreman, pers. comm.). Having a known dispersion pattern from a nonindigenous “model” species helps validate predictions. Ideally, potential discharge locations should be “sink” dispersal sites (Pulliam 1988, Roberts 1998), such as frontal convergences with an offshore transport. Unfortunately, many such “sinks” have seasonal, if not short-term, characteristics. Since oceanic variability is not negligible, there can be no guarantee that some ballast water will not come ashore in an unpredicted event. Considering the up-to-80-day survival of green crab larvae, there may be enough time for them to wash ashore and settle during short-term wind reversal events even in the summer, when the prevailing surface transport is offshore.

Crabs in Cold Water Regions: Biology, Management, and Economics

4.

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Where the impacts of a NIS may have ecological or economic consequences, forewarning of incipient occurrence of the NIS may encourage mitigative action (e.g., altered bivalve culture procedures to prevent predation by green crabs) to be developed and/or taken in anticipation of possible negative consequences.

We therefore describe here the general patterns of green crab range extensions that have occurred to date in the northeastern Pacific Ocean, present an analysis of relevant seasonal satellite altimetry-derived ocean currents in the El Niño year of 1998 (the year green crab larvae were likely transported to British Columbia), and evaluate the possibility of human transport vectors that might have taken green crab larvae from their known source locations in 1998 to British Columbia.

Green Crab Biology Few biological data are available for North American waters, but green crabs, and their planktonic dispersal, have been studied to some extent off Europe by Quieroga (1996), and his observations are presented here. While larvae are typically hatched in estuaries, their behavior with the tidal cycle (Zeng and Naylor 1996) ensures that most larvae are exported to the sea. Larval development includes four zoeal stages and one megalopa stage (Rice and Ingle 1975), and megalopae settle as first crab instars 4-9 weeks after hatching, depending on water temperature during development (Dawirs 1985, Mohamedeen and Hartnoll 1989, Nagaraj 1993). At 13.5ºC and 35 ppt, development takes about 56 days (Quieroga 1996). Megalopae are the stage that reenters estuaries and the other sheltered habitats required for adult survival. In Maine, U.S.A., ovigerous females occur in the warmer months (Berril 1982). There is no published data on the occurrence of ovigerous females in Pacific waters, but it is expected to be in the warmer months too. In Europe, larvae can be found in coastal waters during most of the year, but abundance peaks between April and July (Rees 1952, Lindley 1987). Two spawnings occur in Portuguese estuaries: between February and April, and between June and July (Gonçalves 1991, Paula 1993, Quieroga 1995). Queiroga (1996) described the spatial distribution of C. maenas larvae off Portugal. Carcinus maenas larvae were restricted to the inner and middle shelf, with the later zoeae occurring furthest offshore, mostly about 15-20 km from the coast. All larvae found occurred within 45 km of the coast. Megalopae showed evidence of moving onshore, which appeared to occur at depths less than 30 m. Combining all larval stages, larvae were at depths of 20-25 m during the day and about 30-45 m during twilight, with greater depth variability in the later larval stages. More than one green crab spawning may occur each year in at least some of the estuaries in the northeastern Pacific (S. Behrens Yamada, Oregon State University, Corvallis, Oregon, pers. comm.), leaving a longer

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time period of settlement than occurs with larger native crab species, which typically seem to have only one spawning per year. While currents could theoretically transport green crab larvae considerable distances, there are no data on the planktonic occurrence and spatial distribution of green crab larvae off western North America.

Green Crab Dispersal in the Northeastern Pacific in the Context of Regional Oceanography Observations of green crabs on the West Coast of North America are summarized in Fig. 1. They were first observed in San Francisco Bay from 1989 to 1990, where they remained confined until 1993, when they were found about 100 km north in Bodega Bay (Cohen et al. 1995, Grosholz and Ruiz 1995, Jamieson et al. 1998). They gradually extended their range northward in California at a rate of about 55 km per year to Humbolt Bay, inhabiting the small estuaries found along the outer coast (Miller 1996). However, in 1997, adult crabs were found in Coos Bay, Oregon, 300 km north of Humbolt Bay (Richmond 1998, Behrens Yamada et al. 2000); in 1998 in Grays Harbor, Washington, 425 km further northward (Figlar-Barnes et al. 2001); and in 1999, in Barkley Sound, British Columbia, another 225 km northward (Table 1). A single crab was also found in Esquimalt Harbour outside Victoria in 1999. This represents a movement of 950 km in just 2 years. In 2000, green crabs were found in both Clayoquot and Nootka sounds, an additional 120 km northward, and in 2001, in Esperanza Inlet, another 40 km northward (Table 1). The general oceanography off western North America north of San Francisco is characterized by a seasonal change in current flow direction along the continental shelves, with a northward flowing Davidson Current in the winter months and a southward flowing current on the outer shelf and/or California Current in the summer (Fig. 3 in Thomson et al. 1989). These currents are predominantly wind-driven and the time periods over which directional change occurs are called the spring and fall transitions. These transition periods are not instantaneous, and in 1998, for example, the spring transition lasted from February 25 to about May 6 (R. Thomson, Fisheries and Oceans Canada, Sidney, B.C., pers. comm.). During these transition periods, currents are irregular and no clear flow direction dominates. Off Vancouver Island, a surface estuarine outflow from Juan de Fuca Strait is driven by Fraser River discharge and flows northward throughout the year in a narrow coastal current, called the Vancouver Island Coastal Current. After the spring transition, this current appears to be a barrier to onshore movement of surface-dwelling planktonic larvae that are found farther offshore, possibly arriving from more southerly and continental shelf locations. Onshore movement of such organisms from offshore locations seems to occur only in the spring/summer when storm events cause the temporary suspension of the Vancouver Island Coastal Current


Figure 1.

Dates of first observations of adult European green crab on the West Coast of the United States and Canada from 1989 to 2000. Also shown are altimeter tracks from TOPEX/Poseidon and ERS-2 satellites and coastal sea-level stations (triangles), which provide sea level data for calculations of seasonal ocean current anomalies.

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Table 1.


Green crab recoveries in British Columbia to 2002.

Location

Date Jun 1999 Jun 1999 Jul 1999 Jul 1999 Jul 1999

Sex

Size (mm)

Female Male Male Female Unknown

58.4 74.5 Unknown Unknown Unknown

Barkley Sound, Useless Inlet

17 29 13 13 13

Esquimalt Harbour

8 Aug 1999

Male

65

Clayoquot Sound, Lemmens Inlet

11 May 2000 11 May 2000

Male Male

67.8 55.2

Nootka Sound, Bligh Island

20 Jul 2000

Male

61

Little Espinosa Inlet, Esperanza Inlet

15 Aug 2001 15 Aug 2001

Male Male

72.8 79.3

15 Aug 2001

Male

80.3

(Jamieson et al. 1989). On the other hand, if larvae are hatched in the current and become entrained in it, they are carried north by it and could be deposited in some suitable inshore habitat by the variable action of winds, tides (riding a flood tide), and eddies. There is a noticeable current pattern change between northern California and British Columbia between major El Niño (e.g., 1997/1998) and non-El Niño years. In El Niño years, the spring transition may be delayed and may not maintain itself as rigorously as normal; i.e., resulting in more northward transport, less upwelling and less offshore movement in coastal areas, and water temperatures up to 5ºC warmer off British Columbia (Fisheries and Oceans Canada 2002). In 1997, the south-flowing shelf break current was virtually nonexistent after the spring transition (Cherniawsky et al., in press; R. Thomson, Fisheries and Oceans Canada, Sidney, B.C., pers. comm.) and it is possible that larvae were dispersed to British Columbia in quantity that year. There is also the possibility of secondary spawnings in the fall of 1997 and larval transport to British Columbia by ocean currents then. A lack of reported crabs in the summer of 1998 might be because transported crabs were still small and relatively few in number, and there was insufficient searching for them. Short-term storm activity and rapid northward transport via currents is typical of the winter months. The hypothesis that we investigate here was: Could the anomalous currents during the 1997/1998 El Niño have favored the northward transport of green crab larvae from southern Washington to British Columbia in the spring of 1998? Nearshore sea surface temperatures from northern


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California to British Columbia in the winter (e.g., December 1999) are usually 9-11ºC, while in the summer (e.g., July 1999) they are typically low, particularly off northern California because of upwelling, and are usually about 12-14ºC. In the El Niño year of 1997/1998, December 1997 and July 1998 sea surface temperatures from Oregon to Vancouver Island were 1013ºC and 14-17ºC, respectively (Fisheries and Oceans Canada 2002). However, water temperatures in shallow estuaries, and particularly in larger systems such as Willapa Bay and Grays Harbor, Washington, may be much higher in the summer, and stay warmer over a longer time period compared to estuaries in Atlantic Canada. This is important because these warmer temperatures may allow multiple spawning, i.e., larvae may be present in these areas during both the summer and fall. Green crab growth ceases below 10ºC (Berrill 1982). Since green crabs are likely to spawn in the spring, their larvae are probably present in coastal waters in this season. A C. maenas range extension during periods of northward currents would likely be mostly downstream, as shown by the range of this species extending relatively rapidly northward from California. The rate of the observed movement, hundreds of kilometers a year, suggests that natural dispersal of green crab larvae by currents, hypothesized to be significant for Dungeness crab (Cancer magister) larvae (McConnaughey et al. 1992), is likely a significant factor. Both Hunt et al. (1998) and Dumbauld and Kauffman (1998) suggested that larval transport was the probable main vector of introduction of green crabs to Oregon and Washington, respectively.

Methods Due to the scarcity of current meter moorings in our study area off northern Washington and southern British Columbia, our 1998 surface currents were estimated from sea surface elevation fields that combined TOPEX/ Poseidon (TP) and ERS-2 satellite altimetry with coastal tide gauge data. A similar approach was used by Polovina et al. (1998) to simulate the transport of spiny lobster larvae (Panulirus marginatus) off the northwestern Hawaiian Islands. Although that study used only TP altimetry, ERS-2 altimetry was also included here to permit better spatial resolution along the continental shelves, while the coastal tide data were included to permit the calculation of elevation fields right up to the coast. Actual larvae transport simulations were not conducted because, as is shown below, the average surface current anomalies off the Washington coast were generally southward between February 26 and May 25, 1998. A joint American and French venture, the TOPEX/Poseidon (TP) satellite was launched in September 1992 and despite its expected 5-year lifetime, is still operational in 2002. Radar altimeters on the satellite measure sea surface elevation to an accuracy of about 2 cm. The satellite repeat orbit is close to 10 days and ground track separation is about 150 km near the Washington and British Columbia coasts (Fig. 1). The ERS-2 satellite data

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became available in May 1995 from the European Space Agency. While its orbit is repeated every 35 days, its ground track separation is correspondingly finer, namely about 45 km off Washington and British Columbia. The altimeter measurements from both satellites require numerous corrections before sea surface elevations, relative to a fixed geoid, are produced. The most important of these corrections is the removal of tides. Although several global models have been developed for removing tide effects, these models are generally inaccurate on continental shelves (Foreman et al. 1998). Therefore, for this study, tides were removed from TP sea levels by using the harmonics from a detailed analysis of close to 7 years of TP data (Cherniawsky et al. 2001). Tides in ERS-2 sea levels were subtracted using a recent model of the northeastern Pacific Ocean that accurately resolves the complicated coastal features (Foreman et al. 2000). For both satellites, data are lost when the ground track gets to within about 20 km of the coast. As this may be a critical region for the transport of crab larvae, we augmented our satellite coverage with sea surface elevations from coastal tide gauges. These data were acquired from the Canadian Hydrographic Service and the Center for Operational Oceanographic Products and Services of the National Oceanic and Atmospheric Administration. Twenty-two coastal stations were used for a large region that extended from central Oregon to southeastern Alaska, with data from stations between 44ºN and 50ºN (see Fig. 1) having direct relevance to the smaller region of this study. In order to remove sea level changes not impacting the ocean currents and to maintain consistency with processing of the altimeter data, the same inverse barometer correction (Benada 1993) was applied to each tide gauge time series. For this we used atmospheric pressure data measured at nearby Environment Canada and U.S. weather buoys and several coastal stations, spatially interpolated to locations of the coastal sea level stations. In order to avoid the presence of short-wavelength geoid errors in satellite orbit calculations, a mean sea level was taken as our reference elevation. This level was calculated over a 4-year period of June 1995 to May 1999 (common to both satellites) after removing the tide at each of the along-track satellite sites and coastal tide gauge locations. Seasonal sea surface anomalies were then computed relative to this mean value for winter (November 26 to February 25) of 1997/1998 and for spring (February 26 to May 25) of 1998 and interpolated to a triangular grid using spatial objective analysis known as kriging (Krige 1966). A finite element method, similar to that described in Foreman et al. (1998), was then used to compute surface current anomalies associated with these elevation anomalies.

Results Green Crab Occurrences in British Columbia To date, green crabs have been found in British Columbia each year from 1999 to 2001. In 1999, five adult specimens were found at the head of


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Barkley Sound, and one crab was found in Price Bay at the head of Esquimalt Harbour near Victoria. In 2000, three more adult crabs in total were found in sheltered waters in Clayoquot and Nootka sounds, the next consecutive large embayments to the north, respectively. In 2001, three adult male crabs were found in Esperanza Inlet, again the next bay northward. The crabs found are described in Table 1, and although crabs cannot be aged and green crab growth information is limited, the size of these crabs caught in each year could likely be achieved if they had settled as larvae in late 1997 or early 1998. S. Behrens Yamada (Oregon State University, Corvallis, pers. comm.) reported that Oregon 2-year-old green crabs were 45-84 mm carapace width (CW), while the ones found in British Columbia in 1999 were 58-75 mm CW. If green crabs had been transported to British Columbia earlier, we believe that green crabs would likely have been discovered earlier, as there was considerable publicity about their anticipated arrival and knowledgeable people were on the lookout for them. No formal surveys for green crabs were conducted prior to 2001, but the west coast of Vancouver Island has many knowledgeable marine biologists (Bamfield Marine Station academics, Pacific Rim National Park staff, shellfish aquaculturists, etc.) who had been informed through brochures, posters, and a Web site about what green crabs look like and that any found should be retained and immediately sent to Fisheries and Oceans staff. Since there were no prior observations, it is presumed that green crab settlement likely first occurred in British Columbia in late 1997 or early 1998.

Oceanographic Analyses The sea level anomalies resulting from our processing are shown in Figs. 2a and 2b. The El Niño collapse along the coasts of British Columbia and Washington occurred dramatically on February 25, 1998, and so these anomalies were primarily representative of the 1997/1998 winter and the 1998 spring transition period. The computed surface currents (Fig. 2) are essentially geostrophic, with high sea level (pressure) on the right. Because these are anomalies, we are missing any long-term mean alongshore currents, albeit these are thought to be much smaller than the seasonal currents (cf. Foreman et al. 1998). We also note that certain small-scale features in Fig. 2 may be affected by local outliers in ERS-2 seasonal mean sea level data because of inadequate temporal sampling (every 35 days) of this satellite. Figure 2b shows that, unlike in the previous winter (Fig. 2a), the current anomalies in the spring of 1998 are generally southward. Consequently, apart from episodic storm events not resolved by satellite altimetry, it is unlikely that ocean currents could have transported any passive drifter (including crab larvae) from Willapa Bay to either Barkley Sound or Juan de Fuca Strait after February 25, 1998. However, northward winter currents in both El Niño and non-El Niño years are often higher than 20 cm per second (Thomson et al. 1989) for periods of several days. Thus it is conceivable that near-surface larvae could

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Figure 2.


Seasonal sea level (in meters) and ocean current anomalies near Washington State and southern British Columbia during (a) winter and (b) spring 1998.

have traveled from Willapa Bay to the Vancouver Island shelf if this journey was completed prior to the spring transition. In fact, calculation of the elevation anomalies and associated surface currents for the winter period of November 26, 1997 to February 25, 1998 (Fig. 2a) indicates that these north-flowing currents were even stronger during this El Niño winter. So, if larvae were present in coastal waters prior to February 25, it is possible that they might have been transported by currents to the Vancouver Island shelf.

Humans as Potential Vectors: Regional Ballast Water Movement When green crabs were first found in Barkley Sound in 1999, the possibility that they had been transported to British Columbia via vessels seemed to be a distinct possibility because there was known to be self-dumping barge traffic in 1998 between Willapa Bay, Washington, where green crabs were known to be present, to the head of Barkley Sound, British Columbia. About 100 barge loads of rock were transported from the head of Barkley


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Sound to Willapa Bay between July and September 1998. The barges returned empty of rock but each contained an estimated 500 m3 of scoopedup water (G.S. Jamieson, unpubl. data). The likelihood of barge transport seemed initially to be enhanced by the fact that no green crabs were found near the mouth of Barkley Sound, where we had expected green crabs to also be present if many green crab larvae had been swept northward by currents. With biological researchers from both the Bamfield Marine Station and Pacific Rim National Park active year-round at the mouth of Barkley Sound, we felt that if green crabs were present there, they would surely have been discovered. However, two factors seem to argue against human transport of green crabs. First, the subsequent reports of additional green crabs in three sounds northward over the next 2 years gives more credence to natural transport via currents, although simultaneous transport via vessels into Barkley Sound cannot be ruled out. Using the logic of Occam’s razor, an initial widespread diffuse settlement by ocean currents seems more likely than solely a human-mediated introduction into the head of Barkley Sound, and subsequent natural transport of larvae from there northward in subsequent years. Second, available green crab data (S. Behrens Yamada, Oregon State University, Corvallis, pers. comm.) suggest that green crab larvae could not have grown from settlement size to 75 mm CW in 1 year in Barkley Sound. Since all green crabs found to date in British Columbia were of sizes consistent with a 1997/1998 year class, natural settlement in 1997/1998 seems most likely. Since all green crabs found to date have been increasingly large, it would appear that there is only one year class of settled green crabs in British Columbia; i.e., 1 year of significant transport. To date, there is no evidence that green crabs are successfully reproducing along the west coast of Vancouver Island. However, because a number of adult crabs have been found at most of the aforementioned B.C. sites, we suggest that this may be likely. The transport mechanism to explain the single green crab found in the Victoria/Esquimalt area in 1999 remains an enigma. Currents are known to transport crab larvae from the outer coast to the eastern end of Juan de Fuca Strait (Dinnel et al. 1993), but when this has occurred, most crab settlement has occurred along the Washington side of the strait, where inflows mostly occur. Typically, there is only an outflow along the northern, Canadian side of the strait (Thomson 1981). No green crabs have been found along the Washington side of Juan de Fuca Strait to date. The Esquimalt Harbour area is a major regional dry dock area and there is considerable ship traffic between Victoria and California, but again, while vessel transport is always a possibility, there is no specific evidence to suggest a vessel was the vector.

Discussion The scale of movements to Oregon and Washington suggests natural dispersal, as green crabs were found in a number of estuaries almost simul-

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taneously and in locations in the estuaries which argue against human transport (Hunt et al. 1998, Dumbauld and Kauffman 1998). Over the past few years, the green crabs found in British Columbia were spread over a relatively large geographic area. It therefore seems likely that green crab larvae were transported via ocean currents to British Columbia from either Oregon or Washington green crab populations. However, our satellite data analysis suggests that this did not occur after February 25, 1998, even though that is the time when green crab larvae are most likely present if their seasonal timing of egg hatching was similar to that of the indigenous Dungeness crabs. By doing back-calculations of size based on temperature and growth rates observed in Europe, S. Behrens Yamada (Oregon State University, Corvallis, pers. comm.) estimated that most green crabs settled in Oregon in early 1998, but it was possible that some settled earlier. They stated that “a [shellfish] grower in Coos Bay saw ‘thousands’ of little green crab less than 10 mm [in carapace width] in April of 1998.” Since he observed high growth rates when some juvenile crabs were put in a tank, S. Behrens Yamada (pers. comm.) believed that those small crabs were of the same cohort that they monitored during the summer of 1998. They found the El Niño cohort in Oregon was fairly well defined by size, but with a rather wide size distribution that they felt might result from an extended settlement period from the plankton, differential growth rates, or both. Size ranges they observed in the fall of each recent growing season were 32-60, 45-84, 64-92, and 70-99 mm CW, in 1998, 1999, 2000, and 2001, respectively. It thus seems likely that green crabs so far found in Vancouver Island locations originated from natural current dispersal from U.S. waters in either late 1997 or early 1998. While crab larvae that dispersed out of Barkley Sound would likely be transported northward in the Vancouver Island Coastal Current along coastal Vancouver Island (Thomson et al. 1989), given the few adult green crabs found in Barkley Sound to date, we suggest larval production there would have been minimal and would have been unlikely to have resulted in substantial green crab recruitment, i.e., enough to allow the recent findings of green crab adults at the northern locations to date. S. Behrens Yamada (Oregon State University, Corvallis, pers. comm.) observed that green crabs in Oregon in fall 2001 ranged from 70 to 99 mm. Since our August 2001 specimens from Esperanza Inlet are within this size range, they are also likely of the same 1997/1998 year class. Given the time frames involved, this seems to suggest that there was a natural dispersal event that transported green crab larvae to British Columbia from U.S. waters to the south sometime in 1997 or 1998. Given the data we present here, the event probably occurred prior to February 25, 1998, or it was the result of one or a number of short-term storm events that could not be resolved by seasonal satellite altimetry data. Whether green crabs presently in British Columbia constitute a viable population or not is unknown at this time. If this is not yet the case, it would seem likely that substantial new introductions will continue to oc-


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cur by natural dispersal from established green crab populations to the south, particularly in El Niño years, until ultimately an adequate reproducing green crab population results in Canada. There is also always the possibility of introductions of green crab larvae by ballast water. Finally, given the extensive range expansion that has occurred with this species on the northeast Pacific coast to date, further introductions of green crab larvae to Canadian waters are likely to occur sooner rather than later.

Acknowledgments We would like to acknowledge Cheryl Dunsmore, Norm and Marleen Truesdell, Barron Carswell, Barry Sealey, and Fred Lazuk, who through their keen observation of the marine environment found the first green crabs in British Columbia. Jim Morrison has helped extensively in educating fishers and the public about the need to report green crab findings. “Tidalist” versions of TOPEX/Poseidon and ERS-2 altimeter data were kindly provided by Brian Beckley of the Goddard Space Flight Center, NASA. Coastal sea level data were supplied by Bodo de Lange Boom and Tony Ma of the Canadian Hydrographic Service and by Melvin McCray and Stephen Lyles of the NOAA CO-OPS Program. Eugene (Zhenia) Kulikov kindly provided assistance in the conversion of Fig. 2 to an appropriate format.

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