Salmon Run - Obstacles To The Run

Obstacles To The Run

Salmon start the run in peak condition, the culmination of years of development in the ocean. They need high swimming and leaping abilities to battle the rapids and other obstacles the river may present, and they need a full sexual development to ensure a successful spawn at the end of the run. All their energy goes into the physical rigours of the journey and the dramatic morphological transformations they must still complete before they are ready for the spawning events ahead.

The run up the river can be exhausting, sometimes requiring the salmon to battle hundreds of miles upstream against strong currents and rapids. They cease feeding during the run. Chinook and sockeye salmon from central Idaho must travel 900 miles (1,400 km) and climb nearly 7,000 feet (2,100 m) before they are ready to spawn. Salmon deaths that occur on the upriver journey are referred to as en route mortality.

Salmon negotiate waterfalls and rapids by leaping or jumping. They have been recorded making vertical jumps as high as 3.65 metres (12 ft). The height that can be can achieved by a salmon depends on the position of the standing wave or hydraulic jump at the base of the fall, as well as how deep the water is.

Fish ladders, or fishways, are specially designed to help salmon and other fish to bypass dams and other man made obstructions, and continue on to their spawning grounds further upriver.

Skilled predators, such as bears, bald eagles and fisherman can await the salmon during the run. Normally solitary animals, grizzly bears congregate by streams and rivers when the salmon spawn. Predation from Harbor seals, California sea lions, and Steller sea lions, can pose a significant threat, even in river ecosystems.

Black bears also fish the salmon. Black bears usually operate during the day, but when it comes to salmon they tend to fish at night. This is partly to avoid competition with the more powerful brown bears, but it is also because they catch more salmon at night. During the day, salmon are very evasive and attuned to visual clues, but at night they focus on their spawning activities, generating acoustic clues the bears tune in to. Black bears may also fish for salmon during the night because their black fur is easily spotted by salmon in the daytime. In 2009, researchers compared the foraging success of black bears with the white-coated spirit bear, a morphed subspecies of the black bear. They found the spirit bear had no more success catching salmon at night time, but had greater success than the black bears during the day.

Otters are also common predators. In 2011, researchers showed that when otters predate salmon, the salmon can "sniff them out". They demonstrated that once otters have eaten salmon, the remaining salmon could detect and avoid the waters where otter faeces was present.