Canal Boat - - This Month -

The wa­ter­ways are un­der threat from a host of alien in­vaders

From galloping weeds to can­ni­bal cray­fish, be­ware of the nat­u­ral preda­tors

Pur­ple-pink is not the usual colour as­so­ci­ated with a Po­lice­man’s Hel­met. At the height of sum­mer the dome-shaped flow­ers of Bri­tain’s tallest grow­ing an­nual plant, Hi­malayan Bal­sam, can dom­i­nate the canal bank.

A sweet scent wafts from the nec­tar in a short spur on the lower sepal at­tract­ing bees that are tem­po­rar­ily trapped in the flower, en­sur­ing ef­fi­cient pol­li­na­tion, be­fore back­ing out. With just their back end vis­i­ble, the charm­ing lo­cal name of ‘bee-bum’ has arisen.

The suc­cu­lent stems and sharply-toothed green leaves are char­ac­ter­is­tic of the genus Im­pa­tiens, named for the fast method of seed dis­per­sal.

Ex­plo­sive seed cap­sules hurl seeds more than ten me­tres and the plant has be­come very com­mon. Wel­comed in some waste ar­eas, it is con­sid­ered a pest where its ro­bust growth sup­presses na­tive flora. This Asian beauty is an alien in­vader, first in­tro­duced to botan­i­cal gar­dens in c.1839 from In­dia, it es­caped to form wa­ter­way colonies by 1855.

Other in­va­sive non-na­tive wildlife are also caus­ing alarm along our canals. Ze­bra mus­sels spread to West­ern Europe from Rus­sia in the early 19th cen­tury as the canal sys­tems were built, hitch­ing a lift on the hulls of ships.

They are smaller than marine mus­sels, with a dis­tinc­tive D shape, and or­na­mented with blue or brown alternating with yel­low-white wavy bands.

Na­tive fresh­wa­ter mus­sels live partly buried in the sed­i­ment at the bot­tom of lakes and rivers, but Ze­bra mus­sels are able to at­tach them­selves to hard sur­faces by ad­he­sive fil­a­ments (the byssus), the only fresh­wa­ter mus­sels able to form colonies in this way.

They clog up water­works and sys­tems across the UK, im­ped­ing the work­ing of lock gates and sluices.

Ze­bra mus­sels pro­duce mil­lions of mi­cro­scopic lar­vae when wa­ter tem­per­a­tures are above 12C (roughly May to Septem­ber). The lar­vae drift in cur­rents un­til set­tling on hard sur­faces such as ma­sonry and wooden posts where they de­velop into adults that live two to three years.

They are eaten by fish, cray­fish and wa­ter­fowl, but in in­suf­fi­cient num­bers to stop them from smoth­er­ing na­tive species and al­ter­ing ecosys­tems by re­mov­ing phy­to­plank­ton and es­sen­tial nu­tri­ents.

Float­ing Pen­ny­wort has sim­i­lar eco­log­i­cal con­se­quences. In­tro­duced to Bri­tain in the 1980s by the aquatic nurs­ery trade, its sale be­came il­le­gal in 2014.

Dense float­ing mats of lush fo­liage are swamp­ing wa­ter­ways in the south-east of Eng­land and Nor­folk and spread­ing into the Mid­lands, crowd­ing out na­tive plants and starv­ing fish of oxy­gen.

The shiny, kid­ney-shaped leaves have crin­kled edges and fleshy stalks and are in­ter­wo­ven by fine roots. The tiny white flow­ers are rare but, if present, can be seen be­tween July and Au­gust. The com­bi­na­tion of rapid growth rate – up to 20cm a day in late sum­mer – and the abil­ity of the plant to spread by frag­men­ta­tion – a sin­gle piece can float away and re­gen­er­ate into an en­tire plant – make con­trol of Float­ing Pen­ny­wort es­pe­cially dif­fi­cult.

The restau­rant trade in­tro­duced Amer­i­can Sig­nal Cray­fish in the 1970s as an al­ter­na­tive to lob­sters. On wet nights the am­phibi­ous cray­fish clam­bered out of farm ponds and walked over­land to pop­u­late our wa­ter­ways where they have now spread out of con­trol, erad­i­cat­ing the smaller na­tive White-Clawed Cray­fish by tak­ing over their ter­ri­tory, eat­ing them, and spread­ing a deadly fun­gal dis­ease.

You can see these un­wel­come in­vaders along our canals where they bur­row into the bank us­ing their tail as a scoop. The dam­age, ex­ac­er­bated by boat wash, can cause the banks to col­lapse. Ac­tive dur­ing the day, they are red­dish-brown with large ro­bust claws that are bright red on the un­der­sides and have a white ‘sig­nal’ patch near to the hinge.

They eat prac­ti­cally any­thing, be­ing both scav­engers and preda­tors. They can grab plants, in­ver­te­brates, snails, and small fish with their claws and tear them apart.

Clip­ping off weed near the bed of the wa­ter­way is es­pe­cially dev­as­tat­ing to the en­vi­ron­ment.

The nat­u­ral balance is in­creas­ingly frag­ile.

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