conjunctions - Use of the word "that" in formal tone, technical writing

I need help settling a disagreement. I have read many posts about the word "that" — probably too many, since I have gotten myself confused! In this first example, there are two subordinate clauses, and I believe the first sentence (below) is correct. Which of these is preferable, and why?

Marine biologists at the University of Utah examined the louse genes and determined that their hosts split into three species 5–6 million years ago, and that these species were all equally abundant before whaling began in the 11th century.

vs:

Marine biologists at the University of Utah examined the louse genes and determined their hosts split into three species 5–6 million years ago, and these species were all equally abundant before whaling began in the 11th century.

Here's a different example in which I'm again of the opinion that the word "that" is necessary, but once again, I know not why:

During the winter months (July–October), southern right whales come so close to the shoreline that visitors can watch whales from strategically placed hotels.

vs:

During the winter months (July–October), southern right whales come so close to the shoreline, visitors can watch whales from strategically placed hotels.

I might also note that we are aiming for a formal (encyclopedic) tone. Any help would be appreciated. Thanks!

Answer

In both cases, that is the That-Complementizer, a marker for a tensed Noun Clause, or Complement.

In the first case, both that's are proper, since the repetition marks the two tensed complements that are conjoined by and, thus avoiding ambiguity, which is always a problem in a clause like this.

In the second case, the construction so Adj that S/such a NP that S uses the same complementizer that to mark the complement clause.

That complementizers can be deleted if they are obvious, as in sentences like

• She thinks (that) Mary likes him.


• I'm so tired (that) I can't sleep.

but not if they mark a subject complement and come first in a sentence

• That he arrived late is really his own fault.


• *He arrived late is really his own fault.

because the complementizer is needed as a flag for the clause.

However, the longer a sentence is, and the more grammatical complexities it has, the more it is usually not a good idea for a writer to delete every marker (article, preposition, pronoun, complementizer, etc.) that one can, just because it is possible.

Generally there is a function that these markers serve, to clarify a sentence, and one should always be aware that they're potentially there, whether one deletes them or not. Putting them back in is the first thing a writer should do when a sentence is getting troublesome.