The Gulf Stream Brings Visitors to Rhode Island

Late in the summer and in early fall the Narragansett Marine Laboratory held open house for marine transients--that is, non-resident finny forms which stray into Rhode Island waters and end up in laboratory nets or the trawls and creels of local fishermen.

More than 75 kinds of typically tropical fish have been reported between Rhode Island and the south shore of Cape Cod, and at least 100 subtropicals. Many of these may be expected yearly while others are known only as occasional stragglers.

A John Dory (Zenopsis ocellata) put in an appearance, its flat sides resplendent in dark polka-dots and forward dorsal spines prolonged into- filamentous streamers reaching to the tail. Normally living in depths of 70 to 200 fathoms on the seaward slope of the continental shelf between Chesapeake Bay and Nova Scotia, its presence near Block Island was exceptional. Indeed, records of John Dory capture so far inshore can be counted on one hand.

But the majority of strangers hereabouts are emigrants from shallow southern seas rather than deep northern waters. At the end of September our experimental aquaria boasted a young barracuda (Sphyraena barracuda), too small to menace swimmers but large enough to exhibit the out-size mouth and knife-like teeth which make it the most feared of West Indian inhabitants.

Other cosmopolites of tropical seas mingling amicably in the tanks were a group of jacks: the threadfish (Alectis ciliaris), trailing its finrays above and below; the moonfish (Vomer setapinnis), which resembles a silver dollar, and two dozen blue runners (Caranx crysos). Since members of this family (Carangidae) are possessed of well-developed soundmaking mechanisms, NML bioacousticians have welcomed them particularly, and made repeated auditions to study the significance of their sounds. One experiment showed, for instance, that the blue runner could not be induced to "talk" in solitary confinement, but, when a group was netted together, an individual alarmed cry often set off a frenzied chorus of schoolmates. The communicative value of sound cannot be denied.

Outstanding among the -visiting chorusers were members of the drumfish family; a young Atlantic croaker (Micropogon undulatus) and a number of spot (Leiostomus xanthurus), natives of more southly coasts to Texas. Along with their northern cousins the squeteague (Cynoscion regalis), they are still thriving captivity, eating ravenousy and growing rapidly. By means of a hydrophone in their shoreside tank, with cable ending in the laboratory several hundred feet a continuous live broadcast is enjoyed. Winterquarters are shared, too, with the soundproducing orange filefish (Alutera schoepfi), a common coastal species from Florida to Brazil, in the region of the West Indies, Canaries, Madeira and the East Indies.

How and why do so many different fishes stray so far from their tropical homes? The dramatic story, of the Gulf Stream, a mighty river-within-the-sea, is the answer.

Imagine a great highway paralleling U.S. Route 95 from Florida Keys to Massachusetts but, since it lies some distance offshore, surfaced by lukewarm water rather than tar and concrete. Traffic on this watery road is apt to be dense because great quantities of seaweed (e.g. Sargassum bacciferum), broken off by storms in the Gulf of Mexico and Caribbean Sea, float at its surface. Since hundreds and thousands of tiny fish and shellfish are indigenous to the weed, they too are carried along.

Larger fish, hiding beneath logs and bits of wreckage and feeding, upon the Sargassum weed community, follow its northern progress and thus unwittingly join the march. For some hundreds of miles they travel away from their normal habitat, surrounded by conditions of temperature, neighbors and possibilities of food similar to those experienced in the West Indies.

The path of the Gulf Stream, however, is something of a winding road, its inner margin varying from less than 100 to 200 nautical miles from shore. Sometimes it meanders in ox-bow fashion, so that varying-sized masses of warm water are pinched off and carried shoreward by prevailing southerly winds. Consequently masses of Sargassum weed are detached, along with their large animal population, and carried into shallow water. If tropical forms are blown from their course in the Gulf Stream during the colder months, they immediately perish but in late summer when coastal waters are sufficiently warmed, temporary survival is expected. Protected coves and harbors along, the southem shores of Cape Cod, Rhode Island. Connecticut and Long Island form natural collecting grounds.

Although it is the more slowly changing temperature of the waters over the broad continental plateau separating the Gulf Stream from the coast which, governs the time of appearance, it is the temperature of the immediate waters inshore upon which their disappearance depends. The minimum for survival of Gulf Stream forms seems to be about 12.5° C. When Narragansett Bay reaches that temperature no further wanderers will be encountered until next year. In the meantime, it is hoped that the 1966 immigrants to Rhode Island may continue to thrive in the artificially heated tanks of the marine laboratory.

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