Orcas interact with other marine species such as harbour seals, harbour porpoises, California sea lions, elephant seals, herring, grey whale calves, and Chinook salmon. Orcas are seen as predators to these marine species. Depletions of the Chinook salmon population has been a factor in the declining population of orcas of the South Resident area. In the mid-1990s, the Chinook salmon abundance remained well below the average until 2002-2003. During this time, the Southern Resident population dropped by 17% and the Northern Residents by 8%. Orcas tend to travel in groups which makes it easier for them to catch prey. They swim near the shoreline in order to catch sea lions and seals that can be found in land or water. Also, sometimes they surround grey whales travelling with their calves and separate them so they can prey on the grey whale calves. Orca predation is thought to be one of the major factors affecting populations of multiple marine mammal species because it reduces the abundance of their food source resulting in a reduction in the population of orcas. In the below image it shows the orcas food chain and how the predators and prey are connected. For example, if the phytoplankton population were to decrease there would be nothing for the krill to eat leading to a reduction in their population as well. Since krill is an important food source for animals like crab eater seals and small fish, it will lead to a decrease in the population of animals higher up on the food chain including orcas (killer whales) at the top of the food chain. Orcas interact with leopard seals because they both compete for food such as different types of seals and penguins. This could lead to a reduced abundance of food for orcas since leopard seals eat their food. The greater the population of leopard seals the less food is available for orcas which could lead to a decrease in their population.
Orca Food Web
The graph below shows an Orcas food sources and the proportion of the different species they eat compared to each other. According to the graph, they eat NF seals the most behind Chinook salmon, their main food source. If the NF seal population were to somehow decline, it would reduce the abundance of orcas food source resulting in a decrease in the orca population.