The Invasive Island Apple Snail (Pomacea maculata) in Langan Park and Three Mile Creek
Background. The island apple snail (Pomacea maculata) is the iconic invasive animal species affecting the Three Mile Creek Watershed and threatening the downstream fishery nursery habitats in the Mobile-Tensaw Delta. This exotic invader is native to South America, but it has been introduced around the world, where it often causes severe damage to native habitats and agricultural crops. The island apple snail was first anecdotally reported in the Three Mile Creek (TMC) Watershed in 2004, but its presence was not confirmed until 2008, when densities escalated around the lakes of Langan/Municipal Park (LMP). It is likely that the population of apple snails there originated from aquarium pet releases.
Biology. Like other introduced, exotic species, apple snails in Alabama lack natural predators which could limit their spread. They are tolerant to and active across a range of temperatures between 59° and 97°, but they have been observed to be active and able to survive colder temperatures. When introduced into suitable habitats, the snails have quickly established themselves due to fast maturation and high rates of fecundity. Female island apple snails can lay more than 2,000 eggs in each garish pink egg mass laid above water level at night on emergent vegetation or infrastructure. Hatching rates are generally low, but each snail can lay a new clutch of eggs nearly every two weeks. After emerging from their eggs after a one-to-two-week incubation period, juvenile snails can reach reproductive maturity within 60 to 80 days.
The Threats. The introduction of apple snails in TMC caused resource managers great concern. The Creek drains directly into the Mobile River Ship Channel and the Alabama State Port. While salinities at the mouth of TMC, in the Mobile River, and in Mobile Bay, could limit snail movement, their tolerance is approximately 6.8 parts per thousand. With water in those three locations quite fresh at certain times, depending upon rainfall levels, tides and currents could move snails, potentially allowing them to invade more than 20,000 acres of the Mobile Tensaw Delta. If apple snails become established in the Mobile Tensaw Delta, there could be detrimental effects through habitat degradation and direct competition with native species. Apple snails are voracious herbivores, documented to consume substantial amounts of important native wetlands plants, with demonstrated consumption rates between 55% and 9% of spider lilies, widgeon frass, arrowhead, and coontail. Apple snails can convert productive submerged aquatic vegetation habitat into a non-productive “desert” of algae. Furthermore, once apple snails reach the mouth of TMC, they could attach to and lay eggs on ships and barges, potentially making the Port a source of apple snails to the Tombigbee River, Alabama River, and ports outside of Alabama.
Eradication Efforts. The Alabama Department of Conservation and Natural Resources Wildlife and Freshwaters Fisheries Division (AWFF), in cooperation with the U. S. Fish and Wildlife Service and other partners, began efforts in fall 2009 to control snail population in the LMP lakes and within TMC. Control measures included conducting initial labor-intensive manual efforts to remove egg masses and adults snails on October 3, 2009, with the help of 40-50 volunteers. Another volunteer effort was undertaken by 30-40 volunteers on October 10, 2009. An initial copper sulfate molluscicide treatment was conducted on the LMP lakes on October 6, 2009 and on TMC October 7 and 8, 2009. A second round of molluscicide was applied on the LMP lakes over a week later on October 15 and 16. Snail trapping to monitor populations post-chemical treatment began October 20, 2009, with the deployment of 20 traps and five traps within the lower and upper LMP lakes, respectively. Catch rates, initially 74 snails per day pre-treatment, fell to 4.6 snails per day post treatment, reflecting a 16-fold decrease in catch rates.
The AWFF continued herbicide treatment of emergent vegetation to eliminate egg-laying substrates and application of copper sulfate molluscicide in 2010, 2011, and 2012 with funding from the USFWS and in 2013, 2014, and 2015 with funding from the Coastal Impacts Assistance Program with some successes, but the apple snail population in the LMP lakes has proven to be resilient. With City of Mobile and the MBNEP addressing upstream sediment inputs from 12 Mile Creek and the City anticipating restoration of the Lakes beginning in late 2021, the importance of eradicating the snails before excavated material is relocated took on additional urgency.
In April 2020, with the City of Mobile having secured RESTORE Act funding to dredge and restore the LMP lakes, the MBNEP staff developed an effective strategy to eradicate and count snail egg masses from the shoreline, vegetation, and infrastructure of the lower LMP lake while opportunistically collecting, counting, and destroying adult snails. With snail incubation periods between one and two weeks, control efforts were undertaken twice weekly every Tuesday and Friday, and beginning in April and extending into October 2020. Data were recorded through this period; see the graph below to view weekly snail and egg removal data (updated bi-weekly). Initially, the MBNEP recruited volunteers to help, with walking volunteers or staff using stout, 15-foot bamboo poles to knock the egg masses from emergent vegetation as they walked from the dam near Ziegler along the north and south lower lake shorelines. At least three staffers in kayaks worked the island shorelines, bridge pilings and infrastructure, and wetland edges, including along the base of the dam at the lower lakes western extent. The City of Mobile Parks and Recreation Department aggressively applied herbicide to shoreline and wetlands vegetation to eliminate egg-laying opportunities for the snails. In August, MBNEP contracted Osprey Initiative to take over manual efforts through October with RESTORE Act implementation funding.
The MBNEP will release a Request for Proposals in fall 2020 for RESTORE Act-funded implementation of a comprehensive and aggressive molluscicide application program to eradicate island apple snails from the LMP lakes prior to the dredging and restoration. USA students are currently monitoring manual snail removal efforts using trapping protocols established by AWFF. With the snail source in the LMP lakes addressed, the MBNEP will use a TMC Invasive Species Control Plan-prescribed molluscicide drip treatment to eradicate snails from downstream in TMC.