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. Data suggest that navigation locks have a potential to be operated as vertical slot fishways to provide increased access for a range of biota, including poor swimmers. Skilled predators, such as bears, bald eagles and fishermen 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 into. 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 Kermode bear, a morphed subspecies of the black bear. They found the Kermode 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. The spawning The term prespawn mortality is used to refer to fish that arrive successfully at the spawning grounds, and then die without spawning. Prespawn mortality is surprisingly variable, with one study observing rates between 3% and 90%. Factors that contribute to these mortalities include high temperatures, high river discharge rates, and parasites and diseases. However, "at present there are no reliable indicators to predict whether an individual arriving at a spawning area will in fact survive to spawn." The eggs of a female salmon are called her roe. To lay her roe, the female salmon builds a spawning nest, called a