Under the Microscope:The salmon, an aristocrat among fishes, is frequently in the news because of its economic and cultural importance to Ireland, most recently of course in light of the ban on drift-net salmon fishing.
And one of the most thrilling images on nature TV programmes is the bear standing in the shallows of a river where salmon are returning upriver to spawn, casually scooping the big fish out of the water. Obviously the salmon provides a delicious meal for the bear, but until I read this amazing story by SM Gende and TP Quinn in Scientific American, August 2006, I didn't know that it also richly fertilises the entire local ecosystem.
Gende and Quinn have intensively studied this phenomenon in the forest national parks of Alaska. They discovered that bears fertilise the forest by liberally discarding partially eaten salmon carcasses. The flow of nutrients in riverside ecosystems has traditionally been thought to flow unidirectionally - from forest to stream to sea. This newly discovered flow of nutrients runs from ocean to river to woodland. Pacific salmon emerge from gravel in lakes and streams in spring and, after a while, migrate to the ocean. They remain at sea for up to four years and then return to their native streams to spawn and die. Leaving freshwater, the young salmon weigh between 1kg and 20g each but when they return from sea they weigh between 2kg and 10kg. This represents a large net flow of nutrients from sea to river and lake ecosystems.
An adult male chum salmon contains about 130g of nitrogen, 20g of phosphorous and over 20,000 kilojoules of energy as protein and fat. The authors estimated that one 250m stretch of stream received over 80kg nitrogen and 11kg phosphorous in salmon tissue in one month.
The bears over-winter in dens, where they neither eat nor drink. They do not truly hibernate because their body temperature remains significantly above ambient levels and they must maintain this body heat. Also, females give birth and lactate in this period. The survival and reproductive success of bears depends on the stores of fat they have deposited in late summer and autumn, mainly from eating salmon.
Bears very often bring captured salmon to the river bank or into the adjacent forest to eat them. This is to avoid violent confrontations with other bears. Bears are mostly solitary animals as adults except when courting for a few weeks in spring and summer. They are aggressive when they congregate in shallows to feed on salmon and some confrontations may result in serious injury or death. Dominant bears capture more salmon than subordinate bears and eat less of each fish. In some cases subordinate bears fare so poorly on salmon they prefer to feed on berries.
Also, bears often eat only the most nutritious part (highest in fat) of the salmon and leave the rest lying on the ground. Bears particularly like female salmon full of eggs. Bears deliver huge amounts of nutrients to the riverside and lakeside ecosystem that otherwise would not benefit from this resource.
A host of other animals feed on the salmon carcasses abandoned by the bears - many birds and small mammals feed on the carcasses and flies, beetles and slugs colonise them and lay eggs. The insects that colonise the carcasses are in turn eaten by wasps and other insectivores. Densities of the insect-eating songbirds are higher along salmon streams than along waterways that do not support salmon.
Eventually the salmon carcasses are broken down, releasing their nitrogen, phosphorous and other elements into the soil where it is available to plants. The authors found that nitrogen and phosphorous made available to plants in this way by salmon in Alaska equals the recommended concentrations of fertiliser for similar plants in northern forests. Growth of Sitka spruce was found to be three times greater along salmon than along non-salmon rivers.
Bears and salmon are key components in these ecosystems. Both have been severely depleted and sometimes eliminated in many areas where they were once plentiful. In some areas where bears and salmon populations have been decimated, authorities are now dispersing salmon carcasses by helicopter in an attempt to mimic natural processes.
Here in Ireland a complete ban on drift-netting of salmon, as well as angling curbs on more than 30 rivers, is about to begin. A recent report from an independent salmon group warned salmon stocks would fall catastrophically if drift-net fishing was not banned immediately.
Clearly we blunder into natural ecosystems at our peril - as illustrated by my final story. The old Native American chief sat in his home on the reservation, smoking his ceremonial pipe, eyeing the two US government officials sent to interview him. "Chief Two Eagles," one official began, "you have observed the white man for many years. You have seen all his progress and all his problems." The chief nodded. The official continued, "What do you think of all the white man has done?"
The chief stared at the officials for more than a minute, and then calmly replied. "When white man found the land, Indians were running it. No taxes. No Debt. Plenty buffalo, plenty beaver. Women did most of the work. Medicine man free. Indian men hunted and fished all the time." The chief paused, then added, "Only white man dumb enough to think he could improve system like that."
William Reville is associate professor of biochemistry and public awareness of science officer at UCC- www.understandingscience.ucc.ie