“Fish bodies on the Lagoon” is the sort of email subject line that gets my attention. This particular email, sent on the evening of January 20th by my friend Jan Hyer, described large numbers of dead fish that had washed up on Oak Bluffs side of the Lagoon. Jan walks her dog to the shore of the Lagoon almost every day, so the beaching of the dead fish must have happened quite suddenly, likely on just one or two tides.
I stopped by the site early the next morning and found the narrow beach littered with dead scup. The view south was obstructed, but running my binoculars over the beach heading north, I made out fish for as far as I could see. Lanny MacDowell later reported more dead scup at the head of the Lagoon, and I surmise that dead fish were driven by west winds onto most of the Oak Bluffs shore. Thousands, conceivably tens of thousands, of scup, must have perished.
It’s a short list of things that typically cause such mass mortality in fish, and most of that list could be ruled out by season or location. A toxic spill large enough to poison the whole Lagoon would be hard to overlook, and would have killed other species as well. And winter, with cold temperatures putting the brakes on biological activity of all sorts, is not the usual season for either “hypoxic events” (when all the oxygen in the water is hogged by decaying organic matter) or outbreaks of toxic microbes or diatoms (“red tides”).
That left a sudden onset of cold weather, trapping fish in water too cold to tolerate, as the only possible culprit I could think of. Recent weather patterns — an incredibly mild December followed by a sudden turn onto Winter Street in January — certainly fit the description. The windy conditions around the time of the fish kill may have contributed, cooling the surface through evaporation and turning the water column over until the chill reached the depth of the bottom-loving scup — fish that shouldn’t have been here, but were snookered by mild weather into a lethal mistake.
Such incidents — mass scup mortality caused by sudden cold in early winter — turn out to be a well-known phenomenon (though rare enough so I, at least, have never seen one before). Researching such kills underscored my profound ignorance of scup, a homely fish that I promise never again to take for granted.
For one thing, though I’ve always assumed scup stay in our waters year-round, the little devils turn out to be quite migratory. Multitudes of scup summer around the Vineyard, in a variety of habitats, (though, like many fish, most often around underwater rocks or structures). These fish support vigorous recreational and commercial fisheries, but we are near the northern limit of their range and as weather cools during the fall, the population shifts southward and into deeper water (some reportedly winter as deep as 100 fathoms, flirting with the edge of the Continental Shelf). I would never have imagined these stubby fish could travel that far.
Moreover, this fish that I imagined was quite hardy turns out to be, in fact, rather delicate. A temperature of 45 degrees, multiple sources agree, represents a pretty precise lower limit that scup try very hard to avoid passing. They’re evidently happy up to a relatively warm 77 degrees, a temperature at which even I might consider entering the water.
Like many kinds of schooling fish, scup generally congregate in groups of similar age and size. The ones that died last week in the Lagoon were, accordingly, quite uniform, and quite hefty as this small-to-middlin’ species goes: The random specimen I examined closely spanned about 15 inches from nose to tail-tip and weighed a pound and a quarter. It would have weighed a bit more, but like 100 percent of the dead scup I looked at, its eyes had been eaten. This helpful service was probably performed by fish crows (a recent Wild Side column, mvtimes.com/2015/12/29/something-to-crow-about) summarizes the recent establishment of this compact crow species, which loves to forage on marine debris, on the Vineyard).
A scup species profile
The state’s Division of Marine Fisheries (DMF) website (bit.ly/DMFscup) reveals that the current scup fishery is barely more than a tenth of the size of the (evidently unsustainable) harvests of 30 years ago, and scup numbers have presumably declined considerably from historical highs. Even with their numbers depleted, scup remain numerous in our waters, but DMF considers them to be “currently being harvested at the maximum level their populations can withstand.”
Scup are important for ecological as well as commercial reasons, as predators of invertebrates and as a “forage fish.” Ospreys love them, for instance, and bluefish cheerfully mince scup into oil slicks. So one hopes that our increasingly volatile climate does not increase the frequency of early winter kills of this valuable fish. By reducing the stock that breeds the following spring, each such event represents unwanted additional pressure on a valuable species.