Changes in the productivity base and fish populations of the Lower Fraser River (Canada) associated with historical changes in human occupation
Healey, M. C.; Richardson, J. S.
The Fraser River (mean discharge ca. 3700 m3/s) is one of the largest rivers in the Pacific drainage of North America. The lower 160 km flows across a broad alluvial valley which has gone through a number of transitions following the initiation of European colonization. During the latter half of the 19th century, the forest cover of mixed hardwoods and conifers was stripped off to provide lumber and fuel and to create farmland. This transition greatly altered the kinds of organic material contributed to the river by the floodplain. During the first half of the 20th century, extensive dykes were constructed to increase the available farmland and protect those lands from the seasonal floods. The dykes excluded the river from ca. 70% of the floodplain, further altering the allochthonous sources of carbon to the river. Losses of the natural sources of carbon and nutrients were partially replaced by artificial sources from agriculture and domestic wastes. During the latter half of the 20th century, inputs of agricultural and domestic carbon and nutrients have increased dramatically as the human population has grown and agriculture has intensified and industrialized. Inputs of industrial and domestic toxic wastes have also increased in this latter period. During the last quarter of this century and into the next the valley is being transformed into an urban landscape with associated changes in runoff and chemistry. In this paper we analyze how these ecological transitions have altered the energy dynamics of the lower Fraser and speculate on how these alterations have affected the resident and anadromous fish community.