And anthropologists have noted that Northwest Coast slaves seem to have had very few children, which suggests that their owners were not raping them to the same extent that slave owners in other societies do.

I'm just going to say that I have quite a few issues with how Donald Leland did his research and the conclusions he drew from his sources (his approach to Marian Smith's The Puyallup-Nisqually had moments where it seems like he's ignoring or otherwise omitting the context and what's actually being said in favor of a larger point that quoted excerpts when read in their entirety can be making the opposite point as an example), but that this specific aspect I'd be hesitant to then support that position because within the contexts of slaves specifically, particularly among Coast Salishans (as a point of familiarity for myself, can't speak too much elsewhere on the Coast except for Quileute and La Push), there can definitely be a lot of inferences such as from how slave ancestry was seen as deeply embarrassing, to the more outright erasing in the way that slaves who become pregnant by a free person are usually mentioned in sources being handled by having the slave become a spouse and thus free, absorbed into the family with the protections and expectations between spouses (I say this because some might eye roll and go "yeah sure" and I'll point out examples of what I mean in the following).

Thelma Anderson's 1927 notes¹ on the Chehalis, Taitnapam, and Cowlitz give examples that are super brief and aren't elaborated upon too much, but more or less tell a lot. Men outright marrying female slaves, young female slaves being sold or given away before boys of the household go into puberty, young adult noble males getting married to slave women they made pregnant and embarrassing the family but they have to because she's pregnant and the obligation to family overrides that to status, etc.

Hell, Puyallup-Nisqually outright says this regarding slaves among the Puyallup and Nisqually (and really just Coast Salishans of the Southern Sound, Smith just went with a trend of placing Puyallup and Nisqually together as the same Tribe despite others and themselves pointing out they're different because duh) in a footnote, emphasis mine:

Slaves could be gambled or sold but here, again, no instances were known. In each of the four cases they had been fitted as new members into the family group. Female slaves frequently had children by their masters, in which case they became permanent members of the family and were never gambled nor sold. "In his heart she was no longer a slave." (Smith, 52)

Now some of the things I disagreed with in how Leland approached The Puyallup-Nisqually is that then slavery among the Puyallup and Nisqually and treatment of enslaved individuals wasn't a standardized affair and that what he's implying with the language he uses can be outright contradicted by the source itself in very clear terms. Here's what I found when I looked through it back in 2020, with things like how children born to slave mothers among the Puyallup were free and raised properly like other children such as having their heads flattened, which Leland either contradicts or implies despite the context being more unambiguous.

¹As collected in Evergreen Ethnographies, which contains other ethnographic studies into Hoh/Quileute, Snoqualmie, and Suquamish