Nature News: How to identify a mystery fish (or anything in nature)

originally published January 15 2022 in the York Weekly, Portsmouth Herald, etc and online at seacoastonline.com

A dead fish washed up on the beach might seem like an unsavory thing to write about, sad and tragic, but sometimes these encounters give us a glimpse into the unknown.  What lives off our coast?  Who dwells in those depths?

A friend recently posted a photo of a fish she found washed up on the beach, asking for help with the identification…. “We found this fish washed up on Ogunquit Beach when we took a walk there last week. We have been trying to identify it with no luck. We’d love to know what it is since we haven’t seen anything like it before. It’s about 2 feet in length from nose to tail, dark shaded scales, tail like a tuna, big eye socket, and upturned mouth. Any ideas are greatly appreciated!”   Those of us who responded to her post were definitely not fish experts–our guesses ranged from young tuna to piranha to something called an Atlantic pomfret.  It turns out that the Atlantic pomfret was the closest to the actual identification, one that had come from a Google image search…but it took some time to corroborate the ID.  

An Atlantic pomfret washed up on Ogunquit Beach last week. The size of a small tuna!

The wonders of a dichotomous key!

While I don’t really know much about the myriad of fish to be found in the Gulf of Maine, I do have two methods to help identify any given fish-ask an expert  or use a guide book/taxonomic key.  I did both. I sent the photo off to friends who know more about fish than I do, but at the same time I scoured my bookshelves for relevant guides (found none) and the internet, where I had better luck.  While, as I mentioned before, I don’t know much about fish, I do know how to use a dichotomous key.  A dichotomous key is usually set up as a series of two choices (often phrased as questions) that lead you to the correct identification.  In the case of this fish, the key I found to Gulf of Maine fish started with these two choices: is the mouth soft, with no firm jaws, no pectoral fin and in form is eel-like, or, does the mouth have firm jaws and are there pectoral fins even if the form is eel-like? Each choice sent me to another set of choices (always 2 choices since this was a dichotomous key), which continued until I arrived at my fish. The key took me to the big-scale pomfret (Taractichthys longipinnis), a member of a group of fish known as sea breams or pomfrets.

One reason old-fashioned dichotomous keys are better than apps…..they make you think.

While there are a number of apps available to help instantly identify whatever you see out there in nature, there is something to be said for using an old-fashioned dichotomous key to identify fish or trees or flowers or birds.  Unlike an app, keys force us to think.  For example, while using a key we might have to look up words we don’t know–in my case, I had to look up what the little bumps between the dorsal fin and the caudal (tail) fin were called–and discovered that they are finlets and are thought to improve swimming performance.  And, by using a key I looked more carefully at this fish…where I had thought it looked very tuna-like, after attempting to ID it with the key I realized it’s mouth and forehead shape looked nothing like a tuna (more like a piranha!). I also noticed how large those scales on its side looked. When my more expert friends responded, they agreed on the big-scale pomfret ID, noting the steep, rounded forehead, the shape of the scales, the long fins (hence the species name ‘longipinnis’)  and more…we had our fish!  

It is nice to still have mysteries in the world.

The big-scale pomfret (also called a long-finned sea bream) is a rarely-caught fish, probably because they tend to be solitary and live at depths of over 1,500 feet.  While this was a big fish at 2 feet in length, they can reach 3 feet.  The world record weighed almost 21 pounds! From studies of their stomach contents it looks like big-scale pomfrets tend to feed close to the bottom on squid and shrimp. 

I didn’t find many fun facts or extensive background information about this fish.  I love this!  It is nice to know there are mysteries out there in the deep.  It is nice to be able to walk our beautiful shoreline and occasionally get a glimpse into that deep ocean world that is so different from our own.

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