Non-native species

Non-native species

In 2012, the Scottish Parliament approved the document ‘Non-native species: code of practice’. The code defines NNS as animals and plants that have been moved to a location out-with their native range by human action, whether intentionally or not. In other countries or contexts they may also be referred to as ‘alien’, ‘exotic’, or ‘non-indigenous’ species.

As well as posing a threat to biodiversity, invasive NNS (INNS) can disrupt a number of marine economic activities by fouling or smothering of shellfish and fish farms, marinas and mooring pontoons, or in extreme cases, fouling of waterway. It has proven to be costly to manage these impacts and to date, there has been no wholly successful eradication of a marine NNS (Sambrook et al., 2014).

The globally agreed response in the marine environment is to focus on preventing introduction and spread of NNS. This is reflected in various international conventions, agreements, EU directives and national strategies. Action has been progressed through

• global measures, such as the International Maritime Organisation’s International Convention for the Control and Management of Ships’ Ballast Water and Sediments (known as the ‘Ballast Water Management Convention’) (IMO, 2019) has set the standards of ‘no living biota’ to be discharged from water or ballast sediments to prevent spread of NNS between countries;
• GB Invasive Non-Native Species Strategy
• national measures such the UK Pathways Project (NNSS, 2019); and
• regional biosecurity plans to deal with specific outbreaks of an NNS.

Pathway management

Collectively the various strategies and action plans that currently exist look at activities that can spread NNS, known as pathways, and seek to manage them through a series of actions to improve ‘biosecurity’. Known marine pathways include ballast water transfer, aquaculture equipment and stock movements, vessel fouling and even deliberate (and illegal) release of some species. Actions to manage such pathways are identified by considering which points in an activity NNS may be transferred or released, and which mitigation (i.e. biosecurity) measure can be introduced to minimise the risk. In some cases legal powers can be used for action that can be taken to help manage pathways (Payne et al., 2014).

NNS impacts

NNS are identified as a pressure on biodiversity. Whilst there is evidence from around the world that invasive NNS can colonise, displace, smother, compete with and disrupt native species and habitats, there is scant understanding of their levels of establishment or impacts within UK waters.

Within the UK a list of NNS is regularly revised as knowledge of their impacts is developed. Only NNS that are assessed (GBNNSS, 2019) as having a ‘high-impact’ are considered a pressure on native marine biota and habitats. For the purposes of this Scottish Marine Assessment 2020 the ‘high impact’ species list for 2018 water body classification was used, as identified in the UKTAG Working Paper 7.6 (2015a).

High impact species

The high-impact marine species within Scotland are:

Common cord-grass, Townsend’s grass or rice grass (Spartina anglica).

Carpet sea squirt, colonial tunicate, (non-native Didemnum vexillum).

Leathery sea squirt (Styela clava).

Japanese kelp, Wakame (Undaria pinnatifida) (subject to final confirmation as high-impact species).

Common cord-grass, Townsend’s grass or rice grass (Spartina anglica)

This species is a terrestrial, coastal species, native to eastern USA that has an impact on coastal habitats, such as saltmarsh. However, it has down-stream impacts on intertidal mudflats and shallow subtidal habitats, and has recognisable impacts in seagrass beds (Davison & Hughes, 1998). Common cord-grass is an invasive pioneer species, a hybrid of introduced and native cord-grass species, which colonises the upper parts of mudflats. Its rapid growth consolidates sediment, raises mudflats and reduces sediment availability elsewhere. Studies in England have shown that this species has been linked to the reduction of the success of seagrass species and caused a reduction in seagrass density in Lindisfarne, Northumberland. This was caused by common cord-grass encroachment and consequential changes in sediment dynamics (Davison et al., 1991).

Before the impacts of this species were understood it was deliberately planted in coastal areas to stabilise soft sediments during land reclamation (Hubbard & Stebbings, 1967). More recently, it has been subject of eradication exercises, with some success in the Cromarty Firth (SNH, 2011).

Carpet sea squirt; colonial tunicate; (non-native Didemnum vexillum)

This is a colonial sea squirt, from the North-West Pacific, and was first reported in Scotland in 2009 (Beveridge et al., 2011). The species was initially found in the Firth of Clyde and has now been found in Argyll. This highly invasive species can rapidly colonise artificial substrates and smother sessile habitat and is thought to be easily spread via a number of vectors, including larval release in temperate climates, and attachments of fragments to substrates and biota (Carman et al., 2014). Despite several attempts, there have been no recorded successful eradication attempts for this species in marine waters (Holt & Cordingley, 2011; Ordóñez, 2015).

Leathery sea squirt (Styela clava)

Figure a: Leathery sea squirt (Styela clava) © J. Khan

This sea squirt, from the North-West Pacific, was first found in Scotland in 1981 (Lutzen, 1999) and has spread up the west coast as far north as the Isle of Skye. Leathery sea squirt is capable of colonising a variety of intertidal and shallow subtidal habitats, taking advantage of gaps in substrata and artificial substrates where it displaces native sea squirts (Ciona intestinalis) (Lutzen, 1999). The species is established in a variety of natural habitats, including low or variable salinity habitats, and soft sediment habitats (subtidal muds and sands). It has also colonised hard substrates and can foul sedentary species, such as oyster, blue mussels and native sea squirts. It can be an aggressive invader and in Canada without any natural predators it has produced dense local populations that disrupt native biota (Clark & Therriault, 2007). There have been no successful eradication attempts of this species in the UK.

Other species

Japanese kelp (Undaria pinnatifida)

Figure b: Japanese kelp (Undaria pinnatifida) © J. Khan

Japanese kelp, from the North-West Pacific, was first recorded in Scotland in 2016, on an artificial substrate where it has colonised man-made structures. This species has not yet been classified as a high-impact species by the GB non-native species secretariat, and is currently recorded as being of unknown impact. However, due to its impacts in other countries, there has been sufficient concern that a risk assessment of the impacts is being undertaken. In other countries, it has colonised vulnerable habitats such as kelp habitats and subtidal seaweed habitats. Japanese kelp is currently unknown on any natural substrates in Scottish seas.

There have been no studies of impacts of established population of Japanese kelp in the UK. However in Argentina, where this species has been established for a number of years, they have seen a reduction of native species (Casas et al., 2004). In areas of New Zealand with similar climate and conditions to Scotland, studies show that rather than directly compete with native species, this species appears to rapidly colonise where native species have been damaged or not yet colonised (Scheil & Thompson, 2012). In Europe Japanese kelp has been observed to be a rapid coloniser of new habitats such as marina pontoons (Munchen & Nunn, 2014). There have been no recorded successful eradication attempt for this species.

Wireweed (Sargassum muticum)

Wireweed is from Japan and not classified as a high-impact species by the GB non-native species secretariat (UKTAG, 2015a), but it is included in this summary as it was the only species that was included in the 2011 Scottish assessment (Baxter et al., 2011). It is recognised as a significant threat to habitats where it can displace other alga and disrupt habitats. Wireweed is widespread and established in Scotland, and as far as it is known has not yet disrupted any habitats (Harries et al, 2007). Habitats at risk include subtidal and intertidal seaweed communities and kelp habitats. So far wireweed tends to occur in isolated clumps in the intertidal and has not yet been recorded as having displaced other species.

Figure c: Wireweed © NatureScot.

What is known about this species has been derived from studies in England; the brown seaweed (Halidrys siliquosa) has been reported to be displaced as the dominant species in rock pools by wireweed and, where wireweed was dominant in the subtidal, a decrease in other seaweed species have been noted (Eno et al., 1997). On the south coast of England, wireweed competes for space in the lower shore lagoons of the Solent (Strong et al., 2006) and there are records in England that wireweed has been found attached to shells and pebbles, and has successfully colonised areas of seagrass beds left open by disturbance (Davison & Hughes, 1998). However, as wireweed is not categorised as a high-impact species, and there is no evidence of this species causing ecological harm in Scottish waters, then this species has not been included in this assessment.

There are a number of high-impact species established in UK waters however, they have not yet been recorded, or verified in Scotland. These are:

Slipper limpet (Crepidula fornicata).

American oyster drill (Urosalpinx cinerea).

Marine tubeworm (Ficopomatus enigmaticus).

Chinese mitten crab (Eriocheir sinensis).

Slipper limpet (Crepidula fornicata)

Figure d: Slipper limpet (Crepidula fornicata) © J. Khan

The slipper limpet, from the East coast of USA, has not yet been verified in Scotland, however is found in Strangford Lough, Northern Ireland. This species was accidentally introduced to the UK in the 1880s and has now been recorded as having a serious impact on natural habitats. The slipper limpet competes for space, while the build-up of its faeces, pseudo-faeces and siltation smothers native species and renders the substratum unsuitable for re-settlement by many native species. Where successful, this invasive species can change the entire biotope, to produce a slipper limpet dominated biotope. This can result in the loss of native populations of vulnerable species and habitats, such as blue mussels in Germany (Thielteges, 2005), native oyster beds in the South of England (Blanchard, 1997; Eno et al., 1997). In northern France, severe damage to maerl beds has been recorded where coralline algae and many of its associated species have been smothered (Grall & Hall-Spenser, 2003).

Within Scotland vulnerable habitats include oyster beds, mussel, maerl, kelp and seaweed communities. All are identified as being at risk should slipper limpets become established, changing the character of the substratum and thus preventing the recruitment of native species. There has only been one record of a partially-successful eradication attempt of this species in the UK (Bohn, 2014).

American Oyster drill (Urosalpinx cinerea)

The American oyster drill is not currently recorded in Scotland, but is established in south east and south west of the UK. Native oyster beds are a highly sensitive to impacts from this species as it is a major predator of oyster spat and is also considered to be a major pest on commercially cultured oyster beds (Korringa, 1952; Yonge, 1960). This species is thought to be a contributor to the more recent declines in native oyster populations and their associated living reefs and species in the south of the UK and France.

Australian tubeworm (Ficopomatus enigmaticus)

This species is confined to brackish waters and thrives in brackish lagoons and currently is only known within controlled, ‘warmed’ waters of ports and harbours in England and Wales. Its known northern limits within the UK are Cumbria on the west coast. It is thought that the water temperature has so-far limited its distribution to warmer southern sites in the UK (Zibrowius & Thorp, 1989). Where established in large populations, this filter-feeder competes with native filter-feeders and its pseudo-faeces can impact on water quality (Davies et al., 1989).

Chinese mitten crab (Eriocheir sinensis)

Figure e: Chinese mitten crab. © Crown 2009 The Food and Environment Research Agency.

There was a report of a Chinese mitten crab found in the River Clyde in 2016 (Yeomans & Clark, 2016). However, this was only the empty shell thought to have been discarded from a food retailer. Subsequent surveillance exercises have not recorded any live specimens, nor established populations of this species in the Clyde, nor in Scotland. The nearest verified population of Chinese mitten crab has been found in North Wales. This is a highly invasive species that can impact on freshwater, brackish and marine systems. It spends much of its life in brackish and freshwaters, but returns to the sea to breed (GSID, 2019). It is an opportunistic omnivore and predates on any sessile species including algae and invertebrates, but can have devastating effects on fish egg populations (Gollasch & Nehring, 2006). Its burrowing activities can undermine natural and artificial coastal and river embankments and has been recognised in California as a threat to infrastructure, such as flood protection (Rudnick et al., 2000).

General information about NNS can be found on the GB Non-native species secretariat web site.

Biosecurity at a regional level

Understanding activities within an SMR that can spread marine NNS (i.e. pathways), can help assess the likelihood of NNS being introduced or spread within that region. Knowledge of specific biosecurity measures recommended in an area, and whether they are being implemented can help assess whether the risk is adequately managed. Information on pathways can also direct surveillance for NNS in general or high-impact species for WFD classification Current understanding of activities and biosecurity measures that are in place are summarised in Table a:

The existing biosecurity plans for Scotland range from the simple “Check-Clean-Dry” advice to all water users (GBNNSS campaigns 2020 website), to bespoke community-led plans such as the Loch Creran Community Biosecurity Action Plan (Brown, et al., 2017). Biosecurity plans can be drawn up to reflect unique situations and activities following advice for marine biosecurity planning (Payne et al., 2014). This is a risk-assessment based planning that highlights vectors of spread and contamination that can be addressed by all water-users within the area of risk.