Tilting behaviour of the Atlantic mackerel, Scomber scombrus, at low swimming speeds

Negatively-buoyant Atlantic mackerel, Scomber scombrus L., (fork length 30–39 cm) tilt their bodies with the head up while swimming at speeds below 0.8 body length per second (B.L. s⁻¹). This behaviour is quantitatively described by the body attack angle and swimming speed measured from film records. The maximum recorded body attack angle was 27° in a 32 cm-long fish swimming at 0.45 B.L. s⁻¹ while its nose followed a course close to the horizontal. In general, larger body attack angles were shown at lower swimming speeds and were associated with denser bodies at each speed. We consider that this behaviour pattern allows the fish to maintain a chosen swimming depth while its body creates lift by acting as a hydrofoil. Lift from the fins is insufficient at low swimming speeds.     

Characterization of swimming muscle in the Atlantic mackerel, Scomber scombrus L.

Both red and white muscle were removed from juvenile and adult Atlantic mackerel, Scomber scombrus L., for histochemical characterization of the muscle fibre types. Staining of white muscle for myosin ATPase, SDH, NADH diaphorase, GPDH and LDH revealed that these fibres are homogeneous. Red muscle was shown to be heterogeneous, of at least two fibre types recognizable on the basis of myosin ATPase staining with preincubation at a pH of 9·8. These two red types are dispersed throughout the red muscle and are present in both juveniles and adults. Red muscle is located both deep within the myotomes and as a superficial layer of muscle fibres. A third group of muscle fibres, intermediate in nature, was distinguished at the apex of the red muscle 'triangle,' between the epaxial and hypaxial muscle, using NADH diaphorase and myosin ATPase stains. This paper discusses the possibility that functionally different muscle fibres occur in the red swimming muscle of the Atlantic mackerel.     

Batch fecundity of Atlantic mackerel, Scomber scombrus L.

The total annual fecundity of mackerel, Scomber scombrus L., during a spawning season is produced by release of discrete batches of eggs. Total fecundity is then the product of the number of batches times the batch fecundity ( F _bw). An extensive trawl survey of the Western mackerel stock in the NE Atlantic was undertaken in the 1989 spawning season. Batch fecundity was estimated for 298 fish by counting hydrated eggs in ovaries just prior to ovulation. F _bw, expressed as eggs g⁻¹ body weight, varied with latitude: south of 51°N, F _bw= 55.49, s.e. = 2.04, n = 227; between 51°N and 55°N, F _bw= 45.72, s.e. = 3.41, n = 52 and north of 55° N, F _bw= 41.33, s.e. = 5.52, n = 19. It is suggested that as spawning fish migrate northwards the batch size decreases with progress of the spawning season.     

Endurance at intermediate swimming speeds of Atlantic mackerel, Scomber scombrus L., herring, Clupea harengus L., and saithe, Pollachius virens L.

Endurance and swimming speed were measured in mackerel, herring and saithe when they were induced by the optomotor response to swim at prolonged speeds along a 28-m circular track through still water in a 10-m diameter gantry tank. The maximum sustained swimming speed ( U _ms was measured as body lengths per second ( b.l.s ⁻¹) for each species and for saithe of different size groups. Herring with U _ms of 4.06 b.l.s ⁻¹ (25.3 cm, 13.5°C) were the fastest, mackerel U _ms was 3.5 b.l.s ¹ (33 cm, 11.7°C) and saithe (14.4°C) showed a size effect where U _ms at 25 cm was 3.5 b.l.s ¹ and at 50 cm 2.2 b.l.s ¹. When swimming at speeds higher that U _ms, all three species showed reduced endurance as speed increased. How the curved track reduces the swimming speed is discussed.     

Effect of caudal fin height on swimming kinematics in the mackerel Scomber scombrus L.

Geometry of the caudal fin of the mackerel Scomber scombrus was modified to examine the effect of the height of the fin on swimming kinematics. Mackerel with reduced caudal fin height had significantly shorter stride length and higher tail beat amplitude compared with the control having an intact caudal fin ( P  < 0·005).     

Presumptive mycobacteriosis in North-east Atlantic mackerel, Scomber scombrus L.

A total of 9470 mackerel aged from 0 to 20 years was examined for a disease condition whose causative agent has not been identified but which resembles mycobacteriosis. The study area extended from off Portugal to the northern North Sea. Most mackerel over 2 years old snowed evidence of gross infection. Mean intensity increased with age, indicating a chronic disease of low pathogenicity. There is statistical evidence of an adverse effect on growth, and it is suggested that this may delay first spawning of some young mackerel for at least 1 year. Western stock mackerel appear to be more heavily infected than North Sea stock, and the lowest levels of infection were found off Portugal. The general level of infection decreased in the most recent year classes examined–those of 1982, 1983 and 1984.     

Bands against stripes on the backs of mackerel,Scomber scombrus L.

A thin band of reflecting platelets overlies the central parts of the light and dark stripes found on each side of the dorsal surfaces of the body of the mackerel (Scomber scombrus L.). When this fish has its antero-posterior axis horizontal and its mid-dorsal and mid-ventral lines in the same vertical plane, V, the surfaces of the reflecting platelets in these bands are within a few degrees of being vertical. These surfaces are, however, tipped about 17 degrees from plane V towards the tail. In the angular distributions of radiance commonly found in the sea, the reflections from these bands can mask parts of the pattern of light and dark stripes seen by neighbours in ways that depend on the orientation of the fish in the external light field and the position of the fish relative to its neighbours. With this arrangement, when the fish changes its orientation and/or its velocity with respect to neighbouring fish, this is signalled to the neighbours as changes in the patterns of brightness of its dorsal surfaces. Relatively small changes in roll, pitch and yaw can produce large changes in appearance and, as vision is a most important sense in the mackerel, it seems likely that these changes are important for signalling.     

Protein variation in the Atlantic mackerel Scomber scombrus

Twenty enzymes and three non-enzymatic proteins were examined by electrophoresis in samples of the Atlantic mackerel Scomber scombrus. The observed phenotypes were attributed to genes at thirty-nine loci. Seven loci were variable giving a mean heterozygosity of 0.049 ± 0.033. Heterozygosity estimates for marine teleosts are discused.     

Early development and growth of the laboratory reared north-east Atlantic mackerel Scomber scombrus L.

The early development, growth and morphological changes of mackerel Scomber scombrus were investigated at different incubation temperatures (8, 10, 13, 15 and 18° C). Details on the early life history are illustrated with special reference to morphological transformations. Culture techniques to rear larval mackerel stages are described using laboratory cultured foods. Artificially fertilized eggs were hatched after 80·6 h at 18·4° C and 256·8 h at 8·7° C. The standard length ( L _S) of the individuals at first feeding was 4·71 ± 0·18 mm. Four mortality critical periods and cannibalistic behaviour were identified. A maximum average larval size of 37·5 ± 4·41 mm L _S was attained 30 days post-hatch (dph) at 18·4° C. Development and growth were affected significantly by temperature during both endogenous and exogenous feeding periods. Larvae grew more rapidly at high, than at low temperatures. Daily specific growth rate (in mass) ranged from 2·4% at 10·6° C to 16·9% at 18·4° C. Likewise, average growth rate (in length) ranged from 0·05 mm day⁻¹ at 8·4° C to 0·37 mm day⁻¹ at 18·4° C. The allometric relationship of L _S, with several body measurements was not affected by temperature. Comparison with larvae collected in the Bay of Biscay did not show any significant difference in the dry mass and L _S relationship; conversely, the growth rate in length differed significantly between both laboratory and field conditions. The trends observed in the laboratory are described in relation to some aspects of the year-class strength regulation.     

Spawning behaviour and diel periodicity of egg production in captive Atlantic mackerel,Scomber scombrus L.*

Spawning was observed in groups of captive Atlantic mackerel and occurred when a few fish swam away from the main school during a brief burst of high speed swimming. Counts of eggs from aquarium and field studies show that, in contrast to most pelagic fish species, spawning by Atlantic mackerel can occur at any time ofday or night and is not group-synchronized. Implications of the observed die1 periodicity of egg production to stock size estimation by the Daily Egg Production Method are discussed.     

A light intensity threshold for schooling in the Atlantic mackerel, Scomber scombrus

The schooling behaviour of Atlantic mackerel was studied in a large tank at different light intensities in the range 12.6–1.8 × 10⁻¹⁰μEs⁻¹ m⁻². Variable light intensity was produced by accurately controlling the current to a green light-emitting diode (LED) 3 m above the experimental tank. Under high light levels (1.8 × 10⁻⁶μEs⁻¹ m⁻²) mackerel always formed a single school, whereas at lower levels (1.8 × 10⁻⁸μEs⁻¹ m⁻²) they swam as individuals. At light levels down to 1.0 × 10⁻⁶μEs⁻¹ m⁻² the mean nearest neighbour distance in a school remained relatively constant (0.3–0.9 body lengths), and individual mackerel swam along a path which deviated from the position of their nearest neighbours by less than 14°. As light dropped below 1.8 × 10⁻⁷μEs⁻¹ m⁻², both nearest neighbour distance and heading angle between nearest neighbours increased, with mean values of 1–1.8 body lengths and 23–92°, respectively, at 1.8 × 10⁻⁹μEs⁻¹ m⁻². The results are discussed in terms of ambient light conditions in the sea.     

The estimation of a mackerel (Scomber scombrus L.) spawning stock size by plankton survey

The International Council for the Exploration of the Sea (ICES)Mackerel Working Group recognises two mackerel stocks in Europeanwaters, the North Sea stock and the Western stock. From Marchto July 1977 an Anglo-French plankton survey was carried outin the shelf waters of the Bay of Biscay, Celtic Sea and westof Ireland to estimate the size of the Western stock. Sampleswere collected with a 76 cm Lowestoft pattern high-speed planktonsampler fished to a maximum depth of 100 m. The number of eggsm^–2 at each of five development stages were raised todaily production estimates with development rate coefficientsmeasured with live material collected in March. The mean numberof stage I eggs m^–2 d^–1 was raised to a total dailyproduction estimate for each month by the area within the minimumcontour level, and a production curve was drawn for the spawningseason. The total egg production was estimated at 1.98 x 10¹⁵eggs, + 30%, – 20%. With a mean fecundity of 361 000 anda sex ratio of 0.64 males per female this is equivalent to 8995 million fish.     

Ultrastructure of atrial and ventricular myocardium in the pike Esox lucius L. and mackerel Scomber scombrus L. (Pisces)

Summary Atrial and ventricular muscle in the pike and mackerel hearts consists of narrow, branching cells. The atrial cells in the two species are similar whereas the ventricular cells differ. The sarcolemma is attached to the Z and M lines of the sarcomere. Intercalated discs are common, and the transverse parts display desmosomes and intermediate junctions. Nexuses are uncommon and only occur in the longitudinal parts of the intercalated discs. The sarcoplasmic reticulum forms a regular hexagonal network on the myofibrillar surface. Subsarcolemmal cisternae form peripheral couplings at the I-A level. Junctional processes are usually inconspicuous, but an electron dense substance is present between the sarcolemma and the Junctional sarcoplasmic reticulum. Specific heart granules are common in atrial cells of both species and in ventricular cells of the pike, but are very scarce in mackerel ventricular muscle.This work was supported by grants from the Norwegian Research Council for Sciences and the Humanities     

Routine respiration rates of Atlantic mackerel, Scomber scombrus L., and herring, Clupea harengus L., at low activity levels

Once adapted to the captive environment, mean minimum respiration rates were 118 mgO₂ kg⁻¹ h⁻¹ for mackerel, body length ( b.l ) range 290 to 380 mm, at 11.1o C at a swimming speed of 0.6 b.l. s¹ and 93 mgO₂ kg⁻¹ h¹ for herring, length range 255 to 310 mm, at 9.3° C at a swimming speed of 0.3 b.l. s¹.     

Long-term changes in growth and age at maturity of mackerel, Scomber scombrus L., from the North Sea

Mean length and mean weight at age of Atlantic mackerel in the North Sea increased significantly between 1960 and 1979, then, decreased markedly during the 1980s. In contrast, median age at maturity decreased (c. 1-1.5 years) from 1960. Median length at maturity remained stable until the 1980s, when it declined sharply. The significance of these changes is discussed in relation to variation in sea temperature, food availability and stock size. It is speculated that the changes during the 1980s resulted from immigration of western stock mackerel.     

Development of serotonergic neurons in the brain of the mackerel, Scomber scombrus. An immunohistochemical study

The ontogenetic development of serotonin immunoreactivity (5-HTir) neurons was investigated by immunocytochemistry in the brain of the mackerel, from fertilization to the complete resorption of the yolk sac (201 h). 5-HTir appears first in cell bodies in the diencephalon, on both sides of the third ventricle. At hatching time 5-HTir perikarya are also present in the mesencephalon and the inferior raphe but in small numbers. During larval stages, immunoreactivity appears consecutively in the area thalamo piaetectalis, the nuclei raphe pallidus and obscurus, the nuclei tegmenti dorsalis lateralis, recessus posterioris, lateralis hypothalami and paragigantocellularis lateralis. These results, together with previous findings, suggest a common pattern of distribution of the different 5-HTir neuronal populations in the brain of a teleost during ontogeny but a different sequence of appearance of initial 5-HTir in these populations between species.     

Thermal effects on growth and time to starvation during the yolk-sac larval period of Atlantic mackerel Scomber scombrus L.

The effect of incubation temperature (8·6, 11·1, 13·2, 15·1 and 16·8° C) on north-east Atlantic mackerel Scomber scombrus development, growth and age at starvation during the yolk-sac larval period was investigated. Standard length at hatch was found to be inversely proportional to incubation temperatures within the natural thermal ranges of this species; it ranged from 3·76 mm at 11·1° C to 3·30 mm at 17·8° C. Following hatch, however, larval growth rate was positively related to temperature. Individual logistic models, as a function of temperature and age, were fitted to the development processes of gape, eye pigmentation, jaw mobility and yolk exhaustion. Thereafter, development was classified into different ordered stages and an extended continuation model was fitted to the multinomial ordered stage classification. In all cases, there was a difference of >23 h between the first and the last individual developing in certain stage. The probability of survival decreased with age and was inversely related to temperature. Yolk utilization varied from 4·5 to 8·6 days and individuals died between 7·9 and 12·2 days from 17·8 to 11·1° C. The study demonstrated the significant impact that temperature has on development, growth and survival rates, during the early life history.     

Predation on Benthosema glaciale (Myctouhidae) by spawning mackerel (Scomber scombrus)

During spawning on the outer Dingle Bay Ground mackerel feed on myclnphids which migrate to the surface at night.     

Differential population structuring of two closely related fish species, the mackerel (Scomber scombrus) and the chub mackerel (Scomber japonicus), in the Mediterranean Sea

Population genetic structures of the mackerel (Scomber scombrus) and chub mackerel (Scomber japonicus) were studied in the Mediterranean Sea. Fragments of 272 bp (S. scomber) and 387 bp (S. japonicus) of the 5'-end of the mitochondrial control region were sequenced from spawning individuals collected off the coasts of Greece, Italy, Spain, and Portugal. High levels of mitochondrial control region haplotypic diversity (> 0.98) were found for both Scomber species. Nucleotide diversity was higher in the mackerel (0.022) than in the chub mackerel (0.017). Global F(ST) values were also higher and significant in the mackerel (0.024, P < 0.0001) as opposed to the chub mackerel (0.003, P > 0.05). Molecular variance analyses showed differential genetic structuring for these two closely related species. There is extensive gene flow between Mediterranean Sea and Atlantic Ocean populations of chub mackerel, which are organized into a larger panmictic unit. In contrast, Mediterranean Sea populations of mackerel show some degree of genetic differentiation and are structured along an east-west axis. The analysed eastern Mediterranean Sea mackerel populations (Greece, Italy) are clearly separated from that of the western Mediterranean Sea (Barcelona), which forms a panmictic unit with eastern Atlantic Ocean populations. The genetic structures of both species showed asymmetric migration patterns and indicated population expansion.