Blue runner

Appearance

The blue runner is moderately large in size, growing to a maximum confirmed length of 70 cm and 5.05 kg in weight, but is more common at lengths less than 35 cm. The blue runner is morphologically similar to a number of other carangids, having an elongated, moderately compressed body with dorsal and ventral profiles of approximately equal convexity and a slightly pointed snout. The posterior section of the eye is covered by a moderately well developed adipose eyelid, and the posterior extremity of the jaw is vertically under the center of the eye. The dorsal fin is in two parts, the first consisting of 8 spines and the second of 1 spine followed by 22 to 25 soft rays. The anal fin consists of 2 anteriorly detached spines followed by 1 spine and 19 to 21 soft rays. The pectoral fins become more falcate with age, having 21 to 23 rays, and are slightly longer than the head. The lateral line has a pronounced but short anterior arch, with the curved section intersecting the straight section below the spine of the second dorsal fin. The straight section contains 0 to 7 scales followed by 46 to 56 very strong scutes, with bilateral keels present on the caudal peduncle. There are a total of 86 to 98 scales and scutes over the entire lateral line. The chest is completely scaled. The upper jaw contains an irregular series of outer canines with an inner band of small, regularly spaced teeth, while the lower jaw contains a single band of small teeth. The species has 35 to 42 gill rakers in total; 10 to 14 on the upper limb and 25 to 28 on the lower limb, with this the only feature that differs between ''C. crysos'' and ''C. caballus''. There are 25 vertebrae present.

The blue runner's colour varies from bluish green to olive green dorsally, becoming silvery grey to brassy below. Juveniles often have 7 dark vertical bands on their body. Fin colour also varies, with all fins ranging from to dusky or hyaline to olive green. The species also has a dusky spot which may not be distinct on the upper operculum.

Naming

The blue runner is classified within the genus ''Caranx'', one of a number of groups known as the jacks or trevallies. ''Caranx'' itself is part of the larger jack and horse mackerel family Carangidae, a group of percoid fishes in the order Perciformes.

The species was first scientifically described by the American ichthyologist Samuel L. Mitchill in 1815, based on a specimen taken from the waters of New York Bay, USA which was designated to be the holotype. He named the species ''Scomber crysos'' and suggested a common name of 'yellow mackerel', with the specific epithet reflecting this, meaning "gold" in Greek. The taxon has been variably placed in either ''Caranx'', ''Carangoides'' or ''Paratractus'', but is now considered valid as ''Caranx crysos''. The species has been independently redescribed three times, first as ''Caranx fusus'', which is still incorrectly used by some authors , and later as ''Caranx pisquetus'' and ''Trachurus squamosus''. These names are considered invalid junior synonyms under ICZN rules. The species has many common names, with the most common being 'blue runner'. Other less commonly used names include 'bluestripe jack', 'Egyptian scad', 'hardtail jack', 'hardnose', 'white back cavalli', 'yellow tail cavalli', as well as a variety of broad names such as 'mackerel', 'runner' and 'crevalle'.

There has been suggestions that the blue runner may be conspecific with the eastern Pacific species ''Caranx caballus'' , although no specific studies have been undertaken to examine this relationship. Both species were included in a recent genetic analysis of the entire Carangidae family, with results showing both species are very closely related, although the authors did not comment on genetic distance between the two.

Distribution

The blue runner is extensively distributed throughout the tropical and temperate waters of the Atlantic Ocean, ranging widely along both the eastern American coastline and the western African and European coastlines. In the western Atlantic, the species southernmost record comes from Maceio, Brazil, with the species ranging north along the central American coastline, and throughout the Caribbean and the numerous archipelagos throughout. From the Gulf of Mexico its distribution extends north along the U.S. coast and as far north as Nova Scotia in Canada, also taking in several north-west Atlantic islands. The blue runner is also present on several central Atlantic islands, making its distribution Atlantic-wide.

In the eastern Atlantic the southernmost record is from Angola, with the blue runner distributed extensively along the west African coast up to Morocco and into the Mediterranean Sea. The blue runner is found throughout the Mediterranean, having been recorded from nearly all the countries on its shores. The species is rarely found north of Portugal in the north east Atlantic, although records do exist of isolated catches from Madeira Island and Galicia, Spain. The furthest north it has been reported is southern Great Britain, where two specimens were taken in 1992 and 1993. There has been a trend of having this and other tropical species found further north more often, with publications indicating the blue runner has recently established stable populations in the Canary Islands, where it previously was rarely sighted. Some authors have attributed this northward migration to rising sea surface temperatures, possibly the result of climate change.

Behavior

The blue runner normally moves either in small schools or as solitary individuals, although large aggregations of up to 10,000 individuals are known in unusual circumstances. Throughout some parts of its range, it is one of the most abundant species; for example statistics from Santa Catarina Island indicate it is the third most abundant species. The biology, particularly reproductive and growth biology has been quite extensively studied in the blue runner due to this high abundance in the Atlantic, and its importance to fisheries and the ecology of its environment.

Habitat

The blue runner is primarily an inshore fish throughout most of its range, however it is known to live on reefs in water depths greater than 100 m. Throughout much of its Central American range, it is quite rare inshore, instead more commonly sighted on the outer reefs. The blue runner is primarily a semi-pelagic fish, inhabiting both inshore reefs and the outer shelf edges, sill reefs and upper slopes of the deep reef. Those individuals on shallower reefs often move between reef patches over large sand expanses. Juvenile fish are also known to inhabit the shallow waters of inshore lagoons, taking refuge around mangroves or in seagrass amongst coral reef patches. Fishermen have also taken the species in the Mississippi delta, indicating it can tolerate lower salinities in almost estuarine environments.

Blue runner are easily attracted to any large underwater or floating device, either natural or man made. Several studies have shown the species congregates around floating buoy-like fish aggregating devices , both in shallower waters, as well as in extremely deep waters, indicating the species may move around pelagically. In these situations, blue runner always form small aggregations at the water surface, while other larger species tend to congregate slightly deeper. A number of investigations around oil and gas platforms in the Gulf of Mexico have found blue runner congregate in large numbers around these in the warmer months, where they modify their feeding behavior to take advantage of the structure. Purpose built artificial reefs and marine aquaculture cage structures are also known to attract the species, with the former having the added benefit of dispersing wayward food scraps.

Reproduction

The blue runner reaches sexual maturity at slightly different lengths throughout its range, with all such studies occurring in the west Atlantic. Research in northwest Florida found a length at maturity of 267 mm, a study in Louisiana showed the species reaches sexual maturity at 247–267 mm in females and 225 mm in males, and in Jamaica lengths of 260 mm for males and 280 for females were estimated. Spawning appears to occur offshore year round, although several peaks in spawning activity have been found in different areas through the species range. Peak spawning season in the Gulf of Mexico occurs from June to August, with a secondary peak in spawning during October in northwest Florida. Elsewhere, peaks in larval abundance indicate spawning in the warmer summer months between January and August. Each female releases between 41,000 and 1,546,000 eggs on average, with larger fish producing more eggs. Both the eggs and larvae are pelagic.

The blue runner's larval stage has been extensively described, with distinguishing features including a slightly shallower body than other larval ''Caranx'', and a heavily pigmented head and body. During this early juvenile stage, there are several dark vertical bars clearly present on the side. Larvae and small juveniles remain offshore, living either at depths of around 10 to 20 m, or congregating around floating objects, particularly Sargassum mats and large jellyfish. As the fish grow, they often move to more inshore lagoons and reefs, before slowly making their way to deeper outer reefs at the onset of sexual maturity. Absolute growth rates are not well known, but the species has all the adult characteristics by a length of 59.3 mm. In all cases studied, there are more females in the adult population than males, with female to male ratios ranging from 1.15F:1M to 1.91F:1M. Annual mortality rates for the population in the Gulf of Mexico range from 0.41 to 0.53. The oldest known individual was 11 years old based on otolith rings.

Food

The blue runner is a fast swimming predator which primarily takes small benthic fishes as prey in inshore waters. Studies on the species diet on both side of the Atlantic have shown similar results. A Puerto Rican study found the species supplements its fish dominated diet with crabs, shrimps, copepods and other small crustaceans. More detailed research in Cape Verde found as well as fish, blue runner take shrimp, prawns, lobsters, jellyfish and other small invertebrates. The diet of juveniles is more zooplankton dominated, with young fish predominantly taking cyclopoid and calanoid copepods, and gradually moving to a more fish based diet. Adult blue runner living offshore or aggregating around oil and gas platforms tend to have less fish in their diet, foraging extensively on larger zooplankton during the summer months, with larval decapods and stomatopods, hyperiid amphipods, pteropods, and larval and juvenile fishes also taken.

Studies around these platforms has found blue runner feed with equal intensity during both day and night, with larger prey such as fish taken preferentially at night, with smaller crustaceans taken during the day. Blue runner are one of a number of carangids known to forage in small schools alongside actively feeding Spinner Dolphins , taking advantage of any scraps of food left by the feeding mammals, or any organisms displaced while they forage. The species is also known to eat the dolphins excrement. As well as being important predators, they are also important prey to many larger species including fishes, birds and dolphins.