Water clarity, or the transparency of water, is a fundamental indicator of marine ecosystem health, particularly in the Salish Sea of the Pacific Northwest. Clarity is reduced by turbidity, which is the cloudiness caused by suspended solid matter. This matter includes fine particles such as silt, clay, organic materials, and microscopic organisms like phytoplankton. For the salmon-eating Resident orcas, the clearness of the water column directly influences their ability to secure the food required for survival.
Orca Hunting Strategies and Sensory Reliance
Resident orcas, which depend on Chinook salmon for the majority of their diet, have evolved a sophisticated set of sensory tools to locate prey in the often dark and deep ocean. Echolocation is their primary hunting mechanism, relying on the generation of high-frequency clicks that travel through the water. These clicks bounce off objects, and the returning echoes allow the whale to form a detailed “sound picture” of its surroundings. Orcas can use this bio-sonar to detect the air-filled swim bladder of a salmon at distances of up to 500 feet.
This acoustic hunting ability is especially useful at night or in deep water where light penetration is low. The whales use rapid click sequences, known as buzzes, when closing in on prey to target the fish precisely before capture. While sight is a secondary sense for hunting, it is used in shallower, well-lit waters to visually track fast-moving salmon. The effectiveness of the orca’s foraging strategy rests on the unimpeded function of both its acoustic and visual systems.
Sources of Reduced Water Clarity (Turbidity)
Turbidity in the Salish Sea results from both natural processes and human activities that introduce fine suspended particles into the water column. Sediment runoff occurs when rainfall washes loose soil from exposed land, such such as construction sites, logging operations, or river discharge. This runoff carries fine clay and silt particles directly into marine environments.
Nutrient pollution, often from agricultural or wastewater runoff, introduces excess nitrogen and phosphorus. These nutrients trigger massive growths of microscopic algae, known as algal blooms, which cloud the water and drastically reduce clarity. Vessel movement also causes the resuspension of fine bottom sediments, stirring up material that has settled on the seafloor and increasing water cloudiness.
The Functional Interference with Prey Acquisition
Increased turbidity interferes with the orca’s ability to hunt by degrading the efficacy of both their echolocation and visual systems. The dense concentration of suspended particles causes sound attenuation, meaning the echolocation clicks lose energy as they travel. This occurs through two primary mechanisms: the particles scatter the sound waves in multiple directions and they absorb the acoustic energy.
This scattering and absorption reduce the effective range and resolution of the orca’s bio-sonar, forcing the animals to search at closer distances and with higher energy expenditure. The whales must produce louder clicks or increase the frequency of their clicks to compensate for the interference, which consumes valuable energy reserves.
Reduced water clarity impairs the orca’s secondary visual hunting method. Turbidity shortens the distance at which an orca can visually identify and track a salmon, especially when the prey attempts to evade capture near the surface. The combination of impaired echolocation and degraded vision means the orcas must spend more time and energy searching for and pursuing the same amount of food, directly lowering their hunting success rates.
Population Health and Conservation Implications
Reduced water clarity contributes to chronic nutritional stress in the Resident orca population. When hunting success declines, the whales are unable to consume the 10 or more Chinook salmon required daily, leading to an energy deficit. This nutritional stress forces the whales to metabolize their own blubber reserves to meet caloric needs.
The mobilization of fat reserves is harmful because orcas accumulate lipophilic toxins, such as persistent organic pollutants, in their blubber over time. Burning this fat releases the stored toxins back into the bloodstream, which is linked to immune system suppression and reproductive failure. Studies show that up to two-thirds of pregnancies in the Southern Resident orca population have failed, with nutritional stress identified as a major contributing factor. This cycle of poor hunting success, energy deficit, and toxin release ultimately hinders population growth and threatens the species’ long-term survival.