In Tunisia, the Ichkeul national park is located in the area of Menzel Bourguiba, north of Tunis.
Is it possible to access it by public transportation? It is OK to walk a few kilometers to access it, if needed.
Sounds like you're out of luck if you don't have private transport.
Only the French Wikivoyage seems to have anything on the site itself, and it's basically empty, so I went hunting for past reviews or blogs.
As a result, there's an excellent review on tripadvisor on the park. Unfortunately....
You may think that the park is difficult to reach, this is not true. Fact is there is no public transport to it but to get there from Tunis, it is an easy two to two and a half hour drive. Bonus is the road is excellent, well paved and the scenery is great too. You feel like driving in southern France.
So no, there's no public transportation, but walking is probably out of the question :( You're going to need your own or someone else's vehicle.