Unfortunately, no single factor has emerged as a cause for lower Chinook returns on the Copper River, the Kenai, the Kuskokwim or the Yukon. Excessive harvest through commercial trawling, inshore commercial fishing, degraded spawning habitat, rising sea temperatures, and increasing predation by protected sea mammals are all factors that could be playing a role. While every fishery has different upriver conditions, each share common ground when it comes to the ocean portion of the salmon life-cycle.