*I AM NOT A TEACHER OR NATIVE.*
One of the rules I've studied says: "Collective nouns such as team and staff may be either singular or plural depending on their use in the sentence." ("Rule 15", here on grammarbook.com).
In "1", we don't want to address every "ad" individually. In this context the set as a whole is emphasized. *
And in "2", considering its context, "are" is used because a key scene the speaker remembers is the group of fur seals and no matter if they've been a few or a lot in number, they have caught his/her attention.
* In my opinion both the cases are context-dependent and what a writer/speaker wants to say and emphasize on may play a key role in choosing the best verb. In a different story both the sentences above may employ the other verb.
Student or Learner