Fish out of (tropical) waters

And some ID help from Facebook.

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A juvenile spotfin butterflyfish. — Photo by Matt Pelikan

On a brilliant, sunny day in early October, my work took me and two Nature Conservancy colleagues out onto the Tisbury Great Pond. Isaiah Scheffer, the Chilmark shellfish warden, boated us out to Town Cove to check the progress of an oyster reef that the Conservancy and the Chilmark and West Tisbury shellfish departments created in 2013–14.

The oysters seemed fine, but the high point of the trip came when Isaiah passed me a bucket containing four fish he had found in some oyster cages he maintains for the Town of Chilmark. I’m no fish expert, but I know enough to recognize oddities when I see them! Two of the fish, compressed and brightly colored, looked like refugees from someone’s aquarium. The other two, one mud-brown with pale, vermiculated markings, and one pale pink with reddish spots, looked suspiciously like young groupers.

Yes, groupers — those massive tropical or subtropical sea bass one dines on in Florida seafood restaurants or sees while snorkeling over Caribbean reefs. Their presence in the Tisbury Great Pond is actually less surprising than you might imagine, as I’ll explain; but first, there’s the question of what, exactly, Isaiah found.

Once I got my photos of the fish next to a field guide, the two more colorful specimens were easy to identify: they were juvenile spotfin butterflyfish, Chaetodon ocellatus.

The two groupers, in contrast, illustrated why fish can be frustratingly hard to ID. Many kinds of fish show considerable individual variation, and in many species, adults and juveniles look dramatically different. The resulting confusion carries over into Internet sources — I suspect a good percentage of grouper photos on the Internet are misidentified — and field guides generally only illustrate one or two examples for each species. The traits biologists use to ID fish, such as the number of spines in a particular fin or the number of scales along the fish’s lateral line, often aren’t visible in a photo. And adding to the confusion, many of the common names used for fish are imprecise, with one name applied to any of several species, depending on where you are.

Still, there are a lot of experienced fishermen out there, and for the spotted grouper, a Facebook plea for ID assistance quickly returned a compelling answer: the oddly grouped spots on the fish’s side, suggesting cat footprints, marked it as a juvenile scamp grouper, Mycteroperca phenax.

The muddy brown fish was the tough one. Several knowledgeable folks pronounced it a black grouper (which usually means Mycteroperca bonaci, but “black grouper” is one of those names that can refer to multiple species). My own research convinced me that juvenile black grouper and gag grouper (Mycteroperca microlepis, sometimes also called black grouper) are so similar and variable as to be almost impossible to distinguish. But one structural feature, a subtle lobe on the margin of the preopercle (a sort of secondary flap on the gill cover), is usually present in gag but reliably absent in black grouper. The lobe is visible in the full-resolution version of the photos I took, so I’m pretty sure the fish Isaiah found was a gag.

The valuable book “Island Life,” a Vineyard biological catalog compiled from available sources by Allan Keith and Stephen Spongberg, lists more than 200 bony saltwater fish known on Martha’s Vineyard (including a surprising number of tropical ones). Neither gag nor scamp appears on the list, though the book notes that scamp has been found on Nantucket. (The butterflyfish is described as “uncommon” here.) So this column may constitute a first published record for the Vineyard for two fish species. That is not to say these species have never occurred here: For every one actually found, hundreds surely arrive. And it’s not to say that nobody has seen these species here — but if they did, their observations didn’t turn up in the exhaustive search of the literature carried out by Allan, Steve, and their collaborators.

The presence of tropical fish here has a good explanation: Warm-water fish of many species ride north of their core range in surprising numbers on the Gulf Stream, sometimes drifting inland toward southern New England in eddies that spin off the main current. This past season, the Tisbury Great Pond was open to the sea for much longer than usual, creating ample opportunity for eddy-borne fish to be sucked into the pond by tidal flow. Once inside the pond, groupers and butterflyfish, which associate with underwater structures such as reefs, would inevitably gravitate to oyster gear, since that’s about the only kind of structure to be found in the nearly featureless pond.

So Isaiah’s discovery of these fish fits an established pattern, and indeed represents the tip of the iceberg in terms of vagrant fish on the Vineyard. The episode is a good reminder that the Island is a link in hemispheric-scale routes for the dispersal and migration of wildlife. And it shows that for the attentive observer who’s familiar with the wildlife expected on the Vineyard, every outing into the field creates an opportunity to find something new.