Shortfin Scad, Decapterus macrosoma
Shortfin Scad, Decapterus macrosoma. Fish caught from coastal waters 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, California.
The Shortfin Scad, Decapterus macrosoma, is a member of the Jack or Carangidae Family, that is known in Mexico as macarela alicorta. There are twelve global members of the genus Decapteurs, of which five are found in Mexican waters, two in the Atlantic, two in the Pacific, and one in both the Atlantic and Pacific Oceans.
The Shortfin Scad has a “mackerel-like” elongated slender cylindrical body 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 has a short base with 2 standalone spines followed by 1 spine and 27 to 31 rays; their caudal fin has a slender base and is deeply forked; their first dorsal fin has 8 spines; their second dorsal fin 1 spine and 33 to 39 rays and a long base; and, their pectoral fins are slightly longer than half the head length and are typically 16.5% of standard length. They have 10 to 13 gill rakers on the upper arch and 27 to 31 gill rakers on the lower arch. Their lateral line has a long and low arch anteriorly. Keys to identification include the 110 to 126 scales and 24 to 40 scutes on the straight portion of their lateral line and the lack of scales on top of their head.
The Shortfin Scad is a pelagic species found in both coastal and oceanic waters at depths up to 213 m (700 feet). They reach a maximum of 35 cm (14 inches) in length. As of January 1, 2023, the International Game Fish Association world record stood at 1.00 kg (2 lbs 3 oz) with the fish caught in coastal waters off Japan in June 2014. The Shortfin Scad is poorly studied with very limited information available about their lifestyle and behavioral patterns including specific details on age, growth, longevity, movement patterns, diet, habitat use, and reproduction.
The Shortfin Scad is wide-ranging and found in both the Indian and Pacific Oceans. In Mexican waters they are a resident of the Pacific but have a limited range being found from Guerrero Negro, Baja California, 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). All four of these fish are known as Chi-willie by local Mexican fishermen. 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. Two of the three (the Mackerel Scad and the Shortfin Scad) can only be identified conclusively via a microscopic examination to determine the shape of their jaw bone and the scale count within the flat section of their lateral line. The Amberstripe Scad, it is normally bigger than the other two and has a caudal fin with a yellow upper lobe.
From a conservation perspective the Shortfin Scad is currently considered to be of Least Concern with stable, widely distributed populations. They are caught by commercial fishermen utilizing gillnets, haul seines, purse seines, bottom trawls, and hook-and-line. That are utilized by some cultures as subsistence food. 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 90 m (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, Striped Marlin, and Yellowfin 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 and cut bait for bottom fishing.