A landmark salmon fishing pact announced yesterday between Canada and the U.S. will set fishing quotas based on the abundance of fish rather than fixed annual limits, likely reducing the catch of some stocks by 5 to 50 percent. The agreement settles years of rancorous fighting between the two nations over fish. Two funds for salmon conservation and restoration will be jointly administered by Canada and the U.S. The Clinton administration will ask Congress for $140 million for the funds, while Canada will not be required to contribute, a tacit admission that Canada deserves compensation because American fishers, particularly Alaskans, have taken more than their fair share in recent years.