Northern vs. Southern Resident Habitat

Today, as I was reading a chapter in Dolphin Societies: Discoveries and Puzzles, I came across a fact I had never thought about before.

There are two groups of "resident" fish-eating killer whales around Vancouver Island. One is the Southern Residents, made up of J-, K-, and L-Pods, and they spend most of their time off the south end of Vancouver Island, where we see them near the San Juan Islands here in Washington. The other group is the Northern Residents, made up of pods A-I, plus R and W Pods. They live mostly off the northern end of Vancouver Island. The two ranges are shown in the picture on the left, with the light gray representing the Northern Resident Killer Whales (NRKW) and the darker gray representing the Southern Resident Killer Whales (SRKW). The two populations aren't known to interact with one another.

In the chapter I was reading, it mentioned that the division between the Northern and Southern resident habitat corresponds, on the east side of Vancouver Island, to "two distinct tidal regions: a northern region with southward-flowing flood currents and northward-flowing ebb currents, and southern region with opposite ebb and flood current directions".

I don't know why this seems so revolutionary to me, but it never dawned on me that the barrier between Northern and Southern Residents was more or less drawn by the tidal region. Of course its not an absolute barrier, with Southern Residents sometimes coming down the inside passage to the inland waters of Washington state, but this still seems pretty significant. It doesn't seem unreasonable that salmon runs may also be divided by the tidal exchange regions, and maybe there is some sort of agreement that the two different areas and the salmon they contain "belong" to either of the two resident communities.

Whenever I come across something that causes me to think about the local whales in a new and different way, I get excited by it. This little fact was one of those moments for me, so I thought I'd share it, however minute it might be.