Energy at any price? - Gwent Wildlife Trust

iStock PhotoThe global impactBirds and fish disperse the Estuary’s energy andproductivity to ecosystems across the globeEelsAfter maturing in fresh water, adultsswim across the Atlantic to spawn anddie in the Sargasso Sea. The larvae arethought to return on North Atlanticcurrents. Numbers of eels havedeclined hugely in recent years, forreasons which are not fully understood.The previous pages explain how the Estuarygenerates very large amounts of biomass (thesheer weight of living things), most of it fromthe intertidal zone.Because most of the Estuary’s predators arehighly mobile fish and birds, they effectivelytransport the energy and material they pick upin the mud to other places. This energy andmaterial is carried in these animals’ bodytissues and guts, and transmitted via theirparticipation in other ecosystems where theyfeed, are fed on, and eventually die anddecompose. Millions of fish and birds movingaround is impressive enough, but they keep upthese movements year after year, and thecumulative effect is astronomical. Without theSevern’s effect as a stopover and refuel facility,for example, the Arctic tundra couldn’tsupport so many migratory waders. Withoutits effect as a huge fish nursery, the Celtic Sea(and the Atlantic Ocean beyond) could notsupport so many salmon, sprats or mackerel.This extreme interconnectedness is whatmakes altering the Severn’s intertidal habitat sorisky. Does the Estuary’s energy prize justifysevering these global wildlife links, with theirbenefits to people which are not yet fullyunderstood?Dispersal happens across the UK tooLittle is known of the eel’s migration routesSalmonPrincipal feeding grounds are north ofthe Faeroes; adults are at sea for up tofour years. Most populations are indecline. The sea trout, another migrant,is thought to stay near the UK coast, asdo the allis and twaite shad (the Severnshad are genetically distinct species).Mike LanePREVIOUS PAGEThe Gwent LevelsHabitat for a huge diversityof birds. Many are relianton the Estuary’s mudflatsfor their source of foodThe Severn’s tributariesThe rivers are fine habitatsfor resident and migratoryfish which feed in (andpass through) the EstuaryThe Somerset LevelsThe Levels and the RiverParrett are excellenthabitats for resident andmigratory birds, many ofwhich feed in the EstuaryThe local effectAnimal movements disperse largequantities of energy and materialfrom the Severn Estuary toneighbouring systems in theregion. The key areas to benefitare the rivers Parrett in Somerset,Usk in Wales, the Wye either sideof the border, and the Gwent andSomerset LevelsReasons for salmon declines are complexBirdsWith several dozen species andsub-populations involved, the routesshown here are hugely simplified. Butessentially there are three sorts of birdmigration through the Estuary.The spring passage from Africa,branching west to breeding grounds inIceland and Greenland, or east tonorthern Europe and Russia.The return journey, from late June toOctober, by adults and young.The winter visitors, which don’t gofurther south than the Estuary, andreturn north in spring to breed.Bar-tailed godwits: 300 grams of pure energyLaurie CampbellColin VarndellNEXT PAGE18 The Wildlife Trusts’ Severn Estuary reportThe Wildlife Trusts’ Severn Estuary report 19