Walden, a game

You can do this achievement on the same playthrough you are using for the Hermit achievement. However, you must wait until after you have received the Hermit achievement to unlock this one as this requires you to go into Concord.

You will receive letters in your Concord mailbox asking you to retrieve different specimens for Dr. Agassiz. There are 3 total tasks. If the letters are not appearing, try picking up arrowheads in and around the post office to trigger them.

The first task will have you catching fish. You can find these at most of the fishing spots around the lake. I caught them all at the fishing spot nearest to Thoreau's cabin. You will need:

5 Common Shiners
3 Yellow Perch
1 Walleye

The second task will have you finding animals. You will need the following:

2 Hares - Can be found year-round. You've probably seen a bunch around your house as they usually appear there.
3 White-Footed Mice - Can only be found in the Summer and Fall seasons. I found them most commonly on both sides of the General Store as well as behind it.
1 Red Fox - Can be found year-round. Mostly seen in the south of the map both next to the shoreline and farther down. Can also occasionally be seen along and around the railroad tracks.
1 Mink - Also can only be found during the Summer and Fall seasons. Most commonly found swimming along the shoreline of the lake and around the various tiny ponds on the map.

The third and final task will have you finding turtle eggs. There are 7 nests in total from which you must collect eggs. Refer to the map below for the locations of the turtle nests, indicated by the orange circles:

https://m.imgur.com/a/lfM0iV6

Credit goes to eGeorge_ on PS for the map.

If the map above is not loading for you, you can find the nests on your own without much difficulty. Start from the original fishing spot southeast of your cabin and follow the shoreline clockwise around the lake.