.png)
A Field Guide to Great Lakes Sharks: Identifying the Species That Shouldn't Be Here
- Gannett Outfitters
- 2 days ago
- 3 min read
Every field guide begins with a premise. For birds, it's that you want to know what you're looking at. For wildflowers, it's that the right name unlocks the right understanding.
This guide operates on a slightly different premise: that if you are looking at a shark in the Great Lakes, you will want to know — quickly, clearly, and without having to think very hard — which one it is.
What follows is a working identification guide for the four shark species considered most likely to appear in Great Lakes freshwater, ranked by probability of encounter. We have written it in good faith, consulted available documentation, and reserved our own doubts for the footnotes.
1. Bull Shark (Carcharhinus leucas)
Probability of encounter: Low but non-zero.
The bull shark is the first and most credible candidate in any freshwater shark discussion. It is stocky, grey-brown above and pale below, and typically ranges from 7 to 11 feet in length at adulthood. Its snout is short and broadly rounded — blunter than a great white, wider than a mako. The first dorsal fin is large and triangular; the second is notably smaller. Bull sharks do not hold their bodies rigid when swimming; they tend toward a rolling, side-to-side motion that can look unsettling in calm water.
What distinguishes bull sharks from all other shark species is their osmoregulatory capability — the ability to regulate internal salt concentrations across radically different water salinities. They have been confirmed in landlocked freshwater lakes on multiple continents. They have been documented in the Mississippi River as far north as Illinois. If any shark is in the Great Lakes, it is almost certainly this one.
Identifying features: Broad snout, no markings, second dorsal fin approximately half the size of the first. If seen near shore, note whether it circles or moves in a direct line. Bull sharks tend toward the latter.
2. Greenland Shark (Somniosus microcephalus)
Probability of encounter: Extremely low.
The Greenland shark is not a warm-water animal. It prefers the deep, cold waters of the North Atlantic and Arctic, where it moves slowly — almost imperceptibly, at times — through near-freezing depths. It is massive, reaching lengths of up to 21 feet, and is rarely seen near the surface.
Its inclusion here is not entirely speculative. Lake Superior, the largest and coldest of the Great Lakes, maintains deep-water temperatures of approximately 34 to 38 degrees Fahrenheit year-round — within the Greenland shark's comfort range. The lake reaches a maximum depth of 923 feet, which is surface territory for a species that has been caught at depths exceeding 7,000 feet.
The Greenland shark is a scavenger and slow predator. It is almost certainly not responsible for any reported shoreline sightings. But it is the species that would most plausibly inhabit Lake Superior's cold, dark midwater column without ever being seen at all.
Identifying features: Very large, grey, with a small, rounded snout and tiny eyes. If you are close enough to identify its eyes, you are too close.
3. Spiny Dogfish (Squalus acanthias)
Probability of encounter: Unknown.
The spiny dogfish is a small shark — rarely more than four feet — with a distinctive spine in front of each dorsal fin. It is common in both Atlantic and Pacific coastal waters and is one of the most abundant shark species in the world. It tolerates a wide range of temperatures and salinities.
Its presence in the Great Lakes has never been confirmed. However, Asian carp, sea lamprey, round gobies, and several other species reached the Great Lakes through intentional or accidental introduction, and the spiny dogfish is a commercially traded fish with significant presence in bait supply chains. The pathway for introduction, while not obvious, is not impossible.
Spiny dogfish are not dangerous to humans. They are, however, interesting, and their presence in any Great Lakes ecosystem would represent a significant and previously undocumented incursion.
4. The Unclassified Candidate
Every credible field guide includes a catch-all entry. This is ours.
A number of reported Great Lakes sightings — particularly from Lake Superior and northern Lake Huron — describe animals that do not match the profile of any known shark species. Lengths estimated at 15 to 20 feet. Slow surface movements. No visible dorsal activity. Dark coloration. No tail fin observed.
These reports may describe misidentified lake sturgeon, which can reach extraordinary sizes and have a prehistoric silhouette. They may describe debris, shadows, or optical distortion on open water. Or they may describe something that does not yet have a name in any field guide.
The Great Lakes Freshwater Shark Search Project is particularly interested in this category of report. If you have seen something in the Great Lakes that does not appear in the entries above, we want your account.
Comments