There is evidence of specialized fishing adaptations in the archaeological record dating from Paleolithic times, which parallel technological developments in the exploitation of terrestrial resources. Fishery resources are in places abundant and reliable. Year-round livelihoods are possible with minimum technical elaboration. Shellfish may be picked off the rocks at low tide, artificial pools built of rocks to trap fish as the tide flows out; streams and rivers may be dammed with sticks and stones. Middens at coastal, riverine and lacustrine sites demonstrate long, continuous occupation with strata of shells and fish bones several metres deep.