Whether we are aware of it or not, we are surrounded everyday by alien species. Our favourite houseplant or pet is likely not native to its current home and has been introduced there by human means. Some species can also spread unintentionally through human actions, hitchhiking in some shells collected on holiday, or in the soil of an exotic plant bought overseas. Many of these species will stay put and don’t pose any threat to the natural environment of their new area. However, some alien species can become invasive and have the ability to spread in the wild, causing significant ecological and economic disruption.
As our world has become increasingly connected, both for leisure and commerce, the frequency of inadvertent introductions of alien species has similarly increased. As a result, invasive alien species have become a common part of many natural environments. Once established, invasive species can often be hard to fully eradicate. This is often due to the very traits which may make them invasive such as reproducing in large numbers, spreading quickly or lacking any natural predators or parasites. For this reason, preventing introductions through education and biosecurity is often the most cost effective way to manage the threat of invasive species.
One invasive alien species which has serious ecological and economic impacts in the habitats it invades is the zebra mussel. Zebra mussels are an invasive aquatic species which have spread across Europe from their native range in the Black and Caspian seas. They have been present on the island of Ireland since the 1990s with the first record occurring in Lough Derg in 1997. These mussels live in lakes, canals, slow moving rivers and reservoirs and can be recognised by their striped, D-shaped shells (Figure 1). They differ from our native freshwater mussels because they attach to hard surfaces using byssal threads. This means they can build up thick encrustations which cause disruption to both natural environments and man-made structures. This is referred to as biofouling. They can also reproduce in high quantities, with a single female producing >1 million eggs in a year. Adults generally spawn in summer in Ireland and microscopic larvae (called veligers) are produced which can travel up to 300km through the water before settling on a hard surface and developing into adults.
Spread
Zebra mussels have already spread to lots of water bodies across Northern Ireland and Ireland including the Shannon, Boyle, Corrib and Erne navigations. Given they can only travel passively through water, this spread has been inadvertently facilitated by human actions. Zebra mussels can be spread through a range of means both in their adult and juvenile forms, a key one being transportation on boat hulls. Importantly this can include overland transport between water bodies as zebra mussels can close up and survive outside of water for up to three weeks in damp conditions. Hitchhiking mussels can then become dislodged or spawn when they have reached a new water body. Boating activity also overlaps with the main season of zebra mussel spawning and larvae can be transferred in ballast water.
Boats which are slipped more often tend not build up as large encrustations and any attached mussels can be noticed and removed earlier. As a result, hire boats often have lower numbers of zebra mussels than private boats. Angling boats in particular represent a spreading risk due to the fact that they are often transferred overland, between water bodies, to follow fishing seasons and competitions. Competitions of any kind which bring together vessels from a range of sites can pose a high risk of introductions. Angling activities can also contribute to the spread of zebra mussels on gear and in bait buckets if equipment isn’t cleaned properly in between usages or bait bucket water is transferred between water bodies.
Economic impacts
Due to their ability to form dense encrustations on solid surfaces, zebra mussels pose a threat to industries which use intake pipes from infested water bodies as these can become biofouled and blocked. This applies to fisheries, power stations and water treatment plants and can lead to high costs in management and modifications. On a more individual level, zebra mussel encrustations can cause drag on boats and lead to greater fuel and maintenance costs.
Environmental impacts
The ecological impacts of zebra mussels can also be severe as they significantly alter the nutrient cycles of lakes they invade. Zebra mussels are filter feeders and can filter up to a litre of water a day to obtain food and oxygen. The result of this is that zebra mussels reduce the amount of phytoplankton and zooplankton in the water, increasing visibility. Whilst filtering the water, zebra mussels also secrete anything that isn’t food, this concentrates energy and nutrients to the bottom of the lake. The knock-on effects of these processes can be changes in fish and phytoplankton communities and abundance. It is important to point out that, though the water may appear clearer due to zebra mussel filtering it does not mean it is cleaner, and in fact zebra mussels can have difficult to predict impacts on input levels in lakes.
A direct impact of zebra mussels in Irish lakes has been a reduction in native unionid freshwater mussels, including complete eradication in some loughs. This happens because zebra mussels colonise the shells of these mussels, preventing them from opening and closing properly. Zebra mussels have also been observed to impact the occurrence of algal blooms in water bodies through selective feeding on different phytoplankton species and affecting nutrient ratios. Because of this, where increased nutrient inputs would normally cause increased algal blooms, zebra mussels can complicate this relationship and lead to algal blooms in the absence of increased nutrient inputs.
Management
Whilst control measures do exist for zebra mussels, it is often not possible for established infestations to be completely removed as the mussels spawn in such significant quantities and large-scale application of chemicals would damage other species in the lake. Therefore, most management is focussed on reducing population numbers and mitigating impact. However, if an infestation is new and concentrated in a small area, zebra mussels can be removed by hand, potentially to the point of eradication. Similarly, aggregations can be covered with tarpaulins to restrict oxygen to the mussels. However, this approach needs to be maintained for over a month as zebra mussels can survive under low oxygen up to this time.
If zebra mussels sense toxins in the water they can close up and survive up to a month of chemical treatments. Chlorine is a popular treatment in industrial contexts where zebra mussels can be an issue. However, it isn’t suitable for usage in open water due to its negative effects on other species. Potassium chloride can also kill zebra mussel infestations and has limited toxicity to other species. Maintaining an effective concentration of this chemical in open water, however, can require repeat applications in large quantities.
Due to the damage zebra mussels cause, new control measures are being developed which are lower effort than manual removal but less damaging to other species than many chemical treatments. One of these measures is Zequanox©, a product made from dead bacteria which is toxic to zebra mussels but still recognised by them as food. This means a shorter treatment time than chemical applications in order to be effective. As of 2019, Zequanox has only been licenced in the US but has been trialled in Irish waters. BioBullets encapsulate an ingredient toxic to zebra mussels, like potassium chloride, in a coating palatable to the mussels so they remain open and feeding. This can greatly reduce the quantity of product needed for treatment.
Prevention and regulations
As zebra mussels are unable to travel between unconnected waterbodies on their own, human actions make a huge difference to preventing their spread through awareness and good biosecurity practice. The Check, Clean, Dry campaign aims to educate water users in best biosecurity practice:
- Checking for the presence of plants, animals or mud on equipment and water vessels;
- Cleaning equipment and vessels properly using high pressure sprays or hot water (minimum 45°C); and
- Allowing equipment and vessels to properly dry out for at least 48 hours between usage in different water bodies as many species can survive for a long time in damp environments.
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‘Invasive Alien Species in Ireland’ is an all-Ireland approach to invasive species which provides identification and educational materials. Including Codes of practice for marina operators and water users and pathway action plans for Angling and recreational boating and watercraft. Members of the public can report sightings using CEDAR. There are also legal requirements impacting introduction and movement of invasive species. The EU regulation No 1143/2014 prevents the introduction of invasive species but also their sale, trade, breeding and is enforced in Northern Ireland. The Wildlife (Northern Ireland) Order 1985, as amended, makes it an offence to introduce into the wild any animal which is not ordinarily resident in, or a regular visitor to, Northern Ireland or is listed in Schedule 9, which includes zebra mussels. Invasive Species Northern Ireland also provides a short field guide for identifying zebra mussels.