Sea Lion Lake

On the LakeGrade rubric, Sea Lion Lake pulls an A: clarity at 68.9 ft and 12 µg/L of phosphorus put it in the top bracket for Wisconsin. Sub-grades cluster within a single letter of each other, which usually means the lake is in stable trophic balance rather than fighting one specific stressor.

A TSI value of 28 puts Sea Lion Lake in the oligotrophic range, where biological productivity stays low and the water column remains clear. At 82 ft of maximum depth, Sea Lion Lake stratifies reliably through the summer — the hypolimnion stays cooler and clearer than the surface layer suggests. The lake's 114 acres and partial shoreline records put it in the mid-range bucket — large enough for varied use, small enough that watershed inputs reach the whole basin. Within Florence County's 39 graded lakes, Sea Lion Lake ranks 20 — below the local median, though not at the bottom.

Zebra mussels have been documented at Sea Lion Lake, which alters the filter-feeding balance and can paradoxically increase water clarity while disrupting the food web. The fishery includes walleye alongside the lake's other 5 documented species — a notable draw for Wisconsin anglers. A documented public access point at Sea Lion Lake makes the lake usable for shore fishing, paddle craft, and trailered boats. The grade is based on limited monitoring — fewer than three independent measurement years contribute, so future updates may shift the letter.