The rocklings, of which this is the only common local representative, differ from their near relatives, the hakes (genus Urophycis), in the facts that their ventral fins are short, with 5 to 7 rays, and that the first section of their dorsal fin consists of only one ray, which is nearly as long as the head, and which stands over the upper corners of the gill openings, followed by a series of about 50 very short, separate, hair-like rays without connecting membrane, which can be laid down in a groove on the back. Thus there is only one well-developed dorsal fin. Rocklings differ further from all other gadoids in the presence of long barbels on the top of the nose as well as on the chin, the number of these being the most obvious specific character among the several species of rocklings. In the present species there are a pair of these barbels close in front of the nostrils, a third and somewhat shorter barbel standing alone on the tip of the snout, and there is a fourth barbel hanging from the chin.
Rocklings, remind one of young hake in their slender bodies tapering back from the shoulders; [page 235] and (hakelike) they are rounded in front of the vent but flattened sidewise behind it. Their upper jaw is longer than the lower and their teeth are smaller than in the hakes, while their noses are shorter and blunter; their eyes are smaller, and the dorsal profile of their heads is more rounded than it is in any of the hakes. The pectorals are rounded and the narrow pointed ventrals are situated well in front of the latter. The second dorsal fin (45 to 53 rays) originates over the mid length of the pectorals, runs back nearly to the base of the caudal fin, and is equally high from end to end with a rounded rear corner. The anal fin is similar to the second dorsal in shape, but it is shorter (39 to 43 rays). The caudal fin is oval when it is spread.
The color of this rockling is comparatively constant by all accounts and this is corroborated by our own experience. Its back is dark yellowish olive or dusky brown, its sides are paler, and its belly is white dotted with brown. On some individuals the sides behind the vent are more or less clouded with a darker shade of the general body hue. The first dorsal ray, the posterior edges of the second dorsal fin and of the anal fin, the lower half of the caudal fin, and the pectoral fins are sooty or bluish black. Otherwise the vertical fins are grayish or bluish brown. The ventral fins are pale, and the lining of the mouth is dark purplish or bluish.
This rockling has been described as growing to a length of 16½ inches in Scandinavian waters, but about 12 inches is the longest recorded from the Gulf of Maine, where they average only about 6 to 10 inches.
Rocklings are bottom fish, like hake. Occasionally they have been found in very shallow water, on Nahant Beach in Massachusetts Bay, for example; in water only a few feet deep at Woods Hole; in 6 to 7 fathoms, both in St. Mary's [page 236] Bay, Nova Scotia, and in Buzzards Bay on the south coast of Massachusetts. But they appear to be more plentiful in depths of 25 to 30 fathoms or more; there are rocklings in the deep gully off Halifax, and also in the deep trough of the Gulf of St. Lawrence. They have been taken on the continental slope off southern New England to a depth of 724 fathoms. And there is no reason to suppose that the adult fish ever rise far above the bottom, unless by accident.
The occasional appearance of adult rockling in very shallow water in winter near Woods Hole suggests that some may work inshore and into shoal water in autumn, to work offshore again and deeper in spring, for the summer. Beyond this they seem to be year-round residents wherever they are found.
The name "rockling" is a misnomer for this fish for it is found most often on soft bottom in the Bay of Fundy, while those that we have trawled in Massachusetts Bay and in Ipswich Bay from the Grampus were on smooth muddy sand between the hard patches. And most of the rockling living in the deep sinks and channels in the western side of our Gulf, and on the continental slope, are on soft smooth ground.
Judging from the stomach contents of Scandinavian and British fish (their stomach contents have not been examined on this side of the water so far as we know) they feed chiefly on shrimps, isopods, and other small crustaceans, less often on fish fry. On the other hand, rockling have been found in cod stomachs in Massachusetts Bay, and no doubt all fish of prey devour them on occasion.
The eggs are buoyant, described (we have never seen them) as 0.66 to 0.98 mm. in diameter. When newly spawned the oil is in small droplets, most of which soon coalesce into one globule of 0.14 to 0.25 mm., often with one or two smaller ones close to it. The danger of confusing them with squirrel-hake eggs is discussed in the account of that fish (p. 226). And Battle has found that they develop normally at temperatures ranging from 55° to 66°.
Newly hatched larvae are a little more than 2 mm. long. The yolk is absorbed at about 3.6 mm. and the later larval stages, up to about 10 mm. long, are characterized by the very large black ventral fins shown in the illustrations (fig. 114); by the presence of only one post anal band of black pigment; and by the short stocky body-form. Young hake are more slender and have scattered pigment; young cusk have two post anal bands; and all other Gulf of Maine gadoids have short ventral fins. After the rockling is 17 to 20 mm. long the structure of the first dorsal fin serves to identify it.
These larger fry are silvery, awaiting their descent to bottom before assuming the dull colors of the adult. In British waters they are sometimes called "mackerel midges" because they suggest little mackerel remotely, in their general appearance. In European waters, where there are more plentiful populations of the silvery fry of one or the other species of rockling they are often cast ashore. And one such instance is described for our Gulf by Storer who writes that many were picked up on Nahant Beach during one tide in the summer of 1860; and others found in the surf at West Beach, Beverly.
Rockling fry, like those of other gadoids, drift at the surface for their first few months. How long they do so in our waters is not known, but analogy with cod, haddock, and other species suggests three months at most. And it may be assumed they seek the bottom at a length of about 2 inches for our largest pelagic fry were 40 to 45 mm. long. During this pelagic stage they drift with the current like any other fish fry, and are at the mercy of mackerel and other fish. But they are not plentiful enough in the Gulf of Maine to be as important an article in the diet of the mackerel as the fry of the far commoner European [page 237] rocklings are, on the other side of the Atlantic. Nothing is known of their subsequent rate of growth.
Both sides of the North Atlantic. The American range is from the northern part of the Gulf of St. Lawrence and the northeastern coast of Newfoundland (perhaps even farther north) to Narragansett Bay and Long Island Sound in coastal waters, and to the latitude of Cape Fear (N. C.) in deep water along the continental slope. The Arctic three-bearded rockling (Gaidropsarus ensis Reinhardt), otherwise known only from Greenland, has been trawled on the lower part of the continental slope in the offings of southeastern Nova Scotia, of Cape Cod, of Martha's Vineyard, of New York and of New Jersey at depths of 858 to 1106 fathoms, by the Fish Hawk and Albatross I, but this is not shoal enough to bring it within our limits.
There are several other species of rockling in north European waters, but none of them have been recorded from our side of the Atlantic.
The little rockling is of no commercial value, and it seldom comes up into very shallow water where it would force itself on the notice of seaside visitors. But it is a common bottom fish in the deeper parts of Massachusetts Bay as Goode and Bean remarked long ago, while our experience, corroborated by Huntsman for the Bay of Fundy, is that this applies to the entire Gulf. Definite Gulf of Maine records for adult rocklings are from St. Mary Bay (Nova Scotia); various localities in the Bay of Fundy including Passamaquoddy Bay; Jonesport; off Mount Desert; off Pemaquid; near Seguin Island; mouth of Casco Bay; the deep gully to the westward of Jeffreys Ledge; Ipswich Bay; Gloucester; Nahant; various stations in the deeper parts of Massachusetts Bay; Provincetown; the deep open basins of the Gulf; and Georges Bank. And we have taken its young fry rather frequently in our tow nets in season.
Huntsman and Battle have found the eggs of this rockling in Passamaquoddy Bay throughout the summer, commencing in May and most abundantly at the time the bottom water warms to 9° or 10° C. And its breeding season probably continues from spring to early autumn in the western Atlantic as it does in the eastern, for Dannevig (1919) records rockling eggs (probably this species) as early as the end of May near Halifax, while we have taken rockling larvae only 5.5 mm. long as late as September and October in our tow nets in Massachusetts Bay.
It is probable that the rockling spawns all around the peripheral belt of the Gulf, with Massachusetts Bay as an important nursery, to judge from our repeated captures of its larvae there. And we have taken the pelagic fry in our tow nets at the various localities marked on the accompanying chart (fig. 109) from the first week in July until October; seldom, however, more than half a dozen in any one haul (the largest catch was 18 specimens). Huntsman, similarly, describes the fry as common in the center of the Bay of Fundy, and they have been taken in the tow nets at Woods Hole in April. But we have taken neither the eggs, the larvae, nor the pelagic fry in any of our tow nettings in the central parts of the Gulf, which perhaps justifies the assumption that the spawning grounds of the rockling within our Gulf are limited mostly to depths less than 75 fathoms, though it may spawn much deeper than that on the continental slope.
To the west of Cape Cod, the rockling is now known to occur in coastal waters as far as Narragansett Bay, and in Long Island Sound, where it was found generally in 5½ to 9 fathoms, and abundantly at 21 fathoms by the Fish Hawk in the summer of 1914. And it has been trawled by the Fish Hawk and by the Albatross I at many stations in deeper water offshore along the shelf and slope, southward to the offing of Cape Hatteras (lat. 35° 40' N.).[page 238]
Eastward and northward from our limits, the rockling is said to be rather common in Nova Scotia waters in general, coastwise as well as on the fishing banks. The Albatross trawled it at three stations along the continental edge between the offing of southwestern Nova Scotia and of Sable Island, at 93 to 134 fathoms; and while Huntsman describes it as characteristic of the deep channels of the Gulf of St. Lawrence, Dannevig points out that the stations within the Gulf of St. Lawrence where the Canadian Fisheries Expedition took rockling eggs and larvae in any number, rather generally distributed in the southern part, a few in the northeastern part, were all "close to land or above the more shallow banks." Pelagic rockling fry are listed under this name in the Reports of the Newfoundland Fishery Research Commission also, from many stations in the Grand Banks region, and around the coast of Newfoundland to the Northern Peninsula on the east and to the inner end of the Strait of Belle Isle on the west. But it would not be astonishing if the fry of the three-bearded rockling (p. 237) should prove to be represented in these collections, together with those of our four-bearded species. Dannevig, indeed, has suggested that part of the rockling eggs taken by the Canadian Fisheries Expedition in Nova Scotian waters and south of the Grand Banks in May and June belonged to some species other than cimbrius.
The rockling is neither large enough nor plentiful enough to be of importance commercially, or of interest to the angler.
 Storer credits it with 48 rays, but subsequent students have not found so many.
 Huntsman (Trans. Roy. Soc. Canada, 3, vol. 12, Sect. 4, 1918, p. 63) and further information contributed by him.
 Goode and Bean, (Smithsonian Contrib. Knowl., vol. 30, 1895, pp. 384-385) give a long list of locality records for the rockling on the shelf and slope between the offings of eastern Nova Scotia and of North Carolina (lat. 35° 40' N.).
 Sumner, Osburn, and Cole, Bull. U. S. Bur. Fish., vol. 31, Pt. 2, 1913, p. 771.
 Battle (Contrib. Canadian Biol., N. Ser., vol. 5, No. 6, 1930) has made a careful study of the effects of extreme temperatures and salinities on the development of the eggs of the rockling.
 Fishes of Massachusetts, 1867, p. 279.
 These fry, and one recorded at Nahant earlier by Gill (Proc. Acad. Nat. Sci., Philadelphia, (1863) 1864, p. 241) were reported as an Arctic 3-bearded species (Gaidropsarus argentatus Reinhardt) which was described originally from Greenland and which has been found widely distributed in Denmark Strait; on the north coast of Iceland; and in the Norwegian Sea from the Faroes north to Bear Island. But there is no reason to suppose that the Nahant specimens were anything other than the fry of our common four bearded rockling. For a recent account and discussion of the species argentatus, with excellent illustrations, see Jensen, Spolia Zool. Mus. Hauniensis, Copenhagen, vol. 9, 1948, pp. 167-173, pl. 4, fig. 4.
 A specimen trawled by the Albatross II in 12 fathoms off the mouth of Chesapeake Bay on February 10, 1930, is the only one recorded in shallow water so far southward.
 Goode and Bean (Smithsonian Contrib. Knowl., vol. 30, 1895, p. 381) give a list of these localities. For a recent account of G. ensis, with illustrations, and list of Greenland localities see Jensen, Spolia Zool., Mus. Hauniensis, Copenhagen, vol. 9, 1948, p. 167, pl. 4, fig. a.
 Bull. Essex Inst., vol. 11, 1879, p. 9.
 The Atlantis trawled it both in the Jeffrey bowl, and in the open basin of the Gulf, August 1936; and we trawled it in the central basin in July 1931.
 Contrib. Canadian Biol. (1921) 1922, p. 69.
 Contrib. Canadian Biol., Fish., N. Ser., vol. 5, No. 6, 1930, p. 13 .
 It spawns from the end of January until August in the Baltic.
 Canadian Fisheries Exped., (1914-1915) 1919, p. 53, table 1C.
 Nichols and Breder, Zoologica, N. Y. Zool. Soc., vol. 9, 1927, p. 172.
 For list of early stations, see Goode and Bean (Smithsonian Contrib. Knowl., vol. 30, 1895, pp. 384-385). They also report a specimen apparently of this species from the offing of Cape Fear, N. C. (lat. 34° 01' N., long. 76° 11' W.). But it was in poor condition, hence of doubtful identity.
 Trans. Roy. Soc. Canada, Ser. 3, vol. 12, Sect. 4, 1918, p. 63.
 Canadian Fisheries Exped. (1914-1915) 1919, p. 27: charts figs. 18, 19.