Exploitation and Predation
Consider the plight of a returning Wye or Usk salmon before 2000. Those that were fortunate enough to escape capture on their feeding grounds had to run a gauntlet of nets off the Irish coast before facing our own barrage of Putchers and nets in the Severn estuary. Once in the river, heavily manned rod fisheries using every known type of bait fly and spinner awaited them. That's just the legal fishing. The river, at various times, was haven to poachers of two types: the fast efficient nocturnal netter, operating on the lower rivers and the more 'traditional' lamp and gaff man taking fish from the spawning tributaries.
It hardly needs saying that leaving more fish to spawn should increase the number fish ultimately returning. However, this only holds true when fish numbers fall below a certain level. This threshold is termed the 'Spawning or Conservation Target'. If adult fish numbers are above this level, that number may, by and large, be safely taken without fear of reducing fish returning in future generations, since only a core of spawning adults is required.
Losses take place at all stages of the salmon’s life. Starting with the eggs laid in gravel: these can be lost through wash outs – high water events shifting gravels or drying out in extreme cold and frosty weather, poisoned by acid flushes or sheep dips, or even lost though excavation. When the alevins hatch, it is crucial that food is available when their yoke sac runs out and that suitable refuge from predators – mainly other fish – is available under suitable sized stones with ideal flows and depths of clean water. There are heavy losses at this stage (see habitat degradation) but this need not reduce final numbers as that is dependent on how the parr stage fares.
The survivors of the fry stage progress to parr where they ferociously complete with one another for the best and safest sites, their final number will depend on the quality of habitat and food availability and in any one year the capacity of a river system is finite in respect of this number. Our aim is to improve parr habitat and hence raise this ceiling. Losses at this stage are replaced in a healthy population of salmon, but not when numbers are low. Parr ultimately become silver in preparation for migration to sea where they become subject to more predation. Losses at this stage are not replaced as at fry or parr stages. And so off to sea where man and predator will reduce the number that finally returns either one, two or three years later (even four years for the monster Wye fish).
The upshot of this complex cycle is that there are stages of a salmon’s lifecycle where we can make a difference and some where we can’t. Please see salmon conservation for details of the actions we have taken.
Spawning and Management Targets
A Spawning Target is the amount (expressed as salmon egg numbers) needed to populate a river. This is computed via a complex assessment of juvenile survival and productivity rates together with an estimate of spawning area and quality. Sea survival rates are taken into account. At 100% there is no surplus for exploitation but the run could be said to be the bare minimum needed to keep the run sustainable.
A Management Target anticipates that more fish are needed to run an economic fishery.
The situation on the Wye is that the salmon spawning target has hovered between 19% and 60% (CEFAS: Annual assessment of salmon stocks and fisheries in England and Wales) The Usk, after failing '95 - '97, has just reached its target in subsequent years.
A significant criticism of the whole management process is that in England and Wales, fishery laws are too rigid to be altered for short terms and as a consequence often don’t get changed until stocks are far too low. Net Limitation orders occur only once a decade. Rod fishing byelaws are similarly too slow to take into account prevailing stock levels.
Predation
Thirty years ago, there were no goosanders on the Wye. According to the 1999 MAFF survey up to 98% of the salmon parr produced on the Upper Wye are eaten by goosanders. Additionally, Cormorants are seen in groups of up to 55, eating 2 - 3kg of fish each per day. More recently, the increase in otter numbers has resulted in quite significant mortalities of unspawned fish especially where they struggle to get over falls. The effects of these natural predators would not be so marked on a salmon stock at former levels but are likely to impact significantly when stocks are low and trying to recover, as on the Wye today. Predation has a more significant effect on low density populations.
