Shortfin Scad, Decapterus macrosoma
The Shortfin Scad, Decapterus macrosoma, whose common Spanish name is macarela alicorta and who is locally known (incorrectly) as the chi willie (Pacific Jack Mackerel, Trachurus symmetricus), is a member of the Jack or Carangidae Family, known collectively as jureles and pámpanos in Mexico. There are twelve global members of the genus Decapteurs, five of which are found in Mexican waters, two in the Atlantic, two in the Pacific, and one species in both oceans.
The Shortfin Scads have “mackerel-like” elongated slender cylindrical bodies with a depth that is 16 to 20% of standard length. Their upper jaw bone is concave on top and at the end and rounded on the lower corner. They are metallic blue dorsally and silvery ventrally. They have a small black blotch on the upper edge margin of their gill cover. Their anal fin base is relatively short and their second dorsal fin base is long. Their pectoral fins are slightly longer than half the head length. They have 34 to 38 gill rakers and 24 to 40 scutes. Their lateral line has a long and low arch anteriorly. Keys to identification are the 110 to 126 scales and scutes on the straight portion of their lateral line and the lack of scales on top of their head.
The Shortfin Scads are a pelagic species found in both coastal and oceanic waters at depths up to 700 feet. They reach a maximum length of 35 cm (14 inches). They are a poorly studied species and very little is known about their behavior patterns.
The Shortfin Scads are wide-ranging and found in both the Indian and Pacific Oceans. In Mexican waters they are found from Guerrero Negro southward along the central and southwest coasts of Baja, in the southern two-thirds of the Sea of Cortez, and along the coast of the mainland south to Guatemala.
The Shortfin Scad can be easily confused with the Amberstripe Scad, Decapterus muroadsi (scales on head), the Jack Mackerel, Trachurus symmetricus (numerous scutes; arched lateral line), and the Mackerel Scad, Decapterus macarellus (scales on head). Note: There are three Scads of the Decapterus genus residing in Mexican waters of the tropical Eastern Pacific that are exceedingly difficult to separate: the Amberstripe Scad, Decapterus muroadsi, the Mackerel Scad, Decapterus macarellus, and the Shortfin Scad, Decapterus macrosoma. They can only be identified conclusively via microscopic examination to determine the shape of their jaw bone and the scale count within the flat section of their lateral line.
The Shortfin Scads show up in the San José River basin in the greater Los Cabos area during the cold water periods between December and February when they can be caught in abundance on Sabiki type rigs out of 300-foot water. They survive bait tanks well and are utilized as live fly-lined or troll bait to target the larger big game fish such as dorado, marlin, and tuna. If retained past the daily outings in the Los Cabos area, they are frozen and used later as dead fly-lined or trolled bait.
Shortfin Scad, Decapterus macrosoma. Fish caught from coastal waters off off Point Palmilla, Baja California Sur, February 2010. Length: 23.5 cm (9.3 inches). Fish identification courtesy of H.J. Walker, Jr., Scripps Institution of Oceanography, La Jolla, CA.