Lake Erie watersnake COSEWIC assessment and status report: chapter 7

Population Sizes and Trends

Search effort and abundance

Records show that Lake Erie Watersnakes were found in abundance in the early 1800s and up to the early 1960s around the Lake Erie islands in the U.S. and Canada near the west end of Lake Erie (Ballou 1878; McDermott 1947; Wright and Wright 1957; Langlois 1964). Searches of Middle Island were conducted from 2001-2005 in 25 visits (D. Jacobs pers. comm. July 2005). Between 2001 and 2005, OMNR personnel visited East Sister Island 17 times to search for Lake Erie Watersnakes (D. Jacobs pers. comm. July 2005). King focused most of his work in the U.S. Islands, but also sampled in some years on Canadian Islands (King 1986, 1987a, 1987b, 1989, 2002).

Pelee Island

Surveys in the mid-1900s found Lake Erie Watersnakes to be numerous on Pelee Island (Fetherston 1949; Watts 1951). Fetherston (1949) collected 27 watersnakes in one afternoon on the east shore of Fish Point. However, by the 1970s, the numbers of specimens collected had declined. Campbell (1977) (cited in Campbell et al. 1991) reconsidered his opinion that the Lake Erie Watersnake was not endangered in the early 1970s and decided it was declining and probably endangered. King (1986) estimated the population of Lake Erie Watersnakes along 4.8 km of shoreline on Pelee Island to be 489 adults (95% confidence intervals between 205 and 1547 adults) and capture rate and density to be 0.87 adult snakes per hour or 102 per km respectively. Mark-recapture surveys along 4.65 km of shoreline on Pelee Island between 1988-1992 estimated that the population had decreased to 391 adults from 467 adults estimated between 1980-1985 (King 2002). The 4.6-4.8 km of shoreline sampled were in protected areas and likely had higher densities than most of Pelee’s shoreline. Sampling has become more difficult in many parts of Pelee Island because permission is needed to access many of the shoreline areas (D. Jacobs pers. comm. July 2005).

Middle Island

Thomas (1949) believed that a large number of mature individuals observed on Middle Island in 1945 represented the population size attained when the snakes were left undisturbed, because humans had not inhabited the island since 1933. Seven members of the Ohio Herpetological Society collected several hundred specimens, 254 of which were adults, in April 1949 (Camin et al. 1954). In 1957, Camin and Ehrlich (1958) found only 21 additional new adults. A search in early June 1958 yielded 84 adults captured by four people in two days, but killing of snakes began again once humans started returning to Middle Island during the summer months (Ehrlich and Camin 1960). Between 2001-2005, during a search for suitable watersnakes to be fitted with radio transmitters, a total of 54 adult Lake Erie Watersnakes were found on Middle Island (D. Jacobs unpubl. data; D. Jacobs pers. comm. September 2005). In 2002, the minimum number of known adults on Middle Island was 43 (assuming no immigration) (D. Jacobs pers. comm. September 2005).

East Sister Island

During 1980 and 1983, East Sister Island was surveyed four times. King (1986) found 0.21 adult snakes per hour over 1 km of shoreline and a mean population estimate of 25 adults for the entire island. From 1988-1992, the population density estimate was 52 adults per km, and King (2002) estimated 109 adults as the total population. Searches for snakes for a telemetry study during 2001-2005 on East Sister Island found only 30 adult Lake Erie Watersnakes (D. Jacobs unpubl. data; D. Jacobs pers. comm. November 2004). In 2002, the minimum number of known adults on East Sister Island was 19 (assuming no immigration) (D. Jacobs pers. comm. September 2005).

Hen Island

There have been no new published reports on numbers of Lake Erie Watersnakes on Hen Island since the last COSEWIC report (Campbell et al. 1991). The 1991 report notes that in 35 minutes in May 1990, 10 to 15 watersnakes were seen along the shoreline. OMNR researchers have attempted to gain access to this island, but permission has been refused by the landowner, though Lake Erie Watersnakes are still believed to occur on Hen Island (D. Jacobs pers. comm. July 2005).

Fluctuations and trends

The restricted range of the Lake Erie Watersnake on islands of western Lake Erie creates a population that is vulnerable to stochastic events and increasing human activities. Historically, as the number of people inhabiting islands in the watersnake’s range increased, persecution by humans likely became a major cause of the watersnake's decline. The local abundance of snakes, their moderately large size, and their propensity to bite made them a target for extermination. In the past, Lake Erie Watersnakes occurred on 22 offshore islands and rock outcrops between mainland Ohio and Ontario within the western portion of Lake Erie. There are now only 11 U.S. and four Canadian islands where the Lake Erie Watersnake is believed to be extant. Persecution of watersnakes by humans is still apparent today and may still be the number one cause of the watersnake’s decline (USFWS 2003). However, populations are also declining due to increasing residential and commercial development, resulting in loss of suitable habitat and increased road mortality (Willson et al. 2002). Other events, such as changing water levels (and subsequent mortality due to flooding of communal hibernacula), prey availability (should there be an event that affects current prey availability), predation, and possible inbreeding depression (Frankham 1998; Madsen et al. 1996) are concerns given the fragmented nature of the Lake Erie Watersnake’s geographic range and limited population size.

King (2002) summarized his research and surveys covering 36 sites on nine U.S. and three Canadian (East Sister, Middle, and Pelee) islands for one or more periods between 1980 and 2002. For U.S. populations from 1980-1985 to 1988-1992, King (2002) estimated that the watersnake populations declined by 33 adults per km of shoreline at three sites. From 1988-1992 to 1996-1998, they declined a further 13 adults per km of shoreline at four sites (see Tables 3 and 4 in King 2002). However, from 1988-1992 to 2000-2002, numbers increased from 23-88 adults per km of shoreline, depending on site and time period. These increases coincide with efforts of the Ohio Division of Wildlife and the USFWS to halt persecution of snakes and with increases in densities of round gobies (King 2002).

From 1980-1985 to 1988-1992, the average adult density on the three Canadian islands declined by 48.6 snakes per km of shoreline (from a total of 8.7 km sampled). However, this decline largely reflects a decline on one site (Fish Point south) on Pelee; the other sites remained more or less unchanged (Table 4 in King 2002). The decline at Fish Point may have resulted from the inadvertent destruction of a known communal hibernaculum that was located immediately adjacent to Fish Point (D. Jacobs pers. comm. June 2005).

Rescue effect

There is evidence that watersnakes may occasionally move among islands and also from the mainland to island. Therefore, immigration is possible, but long-distance movement is rare (King 2001; Bishop and Rouse unpubl. data 1999). In 2002, a PIT tagged Lake Erie Watersnake was located during surveys on Middle Island. It was later confirmed that this animal was originally captured and marked on the south shore of Kelleys Island in Ohio. The watersnake had travelled a straight-line distance of 11 km (Jacobs unpubl. data. 2002/2003). However, King (1987b) noted that movement among islands occurs at very low rates. Thus, immigration of individuals between U.S. and Canadian islands is possible, though it is unknown whether this would have any positive effect on maintaining the viable population threshold for continued survival in Canada.

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