Morphometrical characters of Atlantic horse mackerel Trachurus trachurus (Linnaeus, 1758), in the North Atlantic zone of Morocco

. The Atlantic Horse mackerel Trachurus trachurus (Linnaeus, 1758) is subject to considerable fishing pressure. under strong fishing pressure, the objective of the research is to establish the species morphological characterization: the growth model, sex-ratio and the body condition factor, using standard morphometric methods and truss network analysis. 1494 samples were collected from the North Moroccan Atlantic coast, sampling was carried out from July 2021 to July 2022 at the port of Tangier and Larache (35°47, 33°89), indeed individuals total length measured between10 and 41 cm and the body weight presented between 35 and 500 g, All morphometrical analyses showed similar results, The values for males, females and the total sample showed that the growth of T. trachurus was minor, which that mean the weight grows less quick than the length, The condition factor was calculated indicating the smallest values were observed in the range of Total length between 10 and 19 cm.


Introduction
Morocco's North Atlantic zone is home to a wide range of fishery resources, including pelagic stocks, which account for over 70% of the zone's marine resources, and demersal stocks, which come in second place.Or small pelagic stock includes five main species: sardines, sardinella, mackerel, anchovies and horse mackerel, which the Atlantic Horse mackerel, Trachurus trachurus (Linnaeus, 1758), is among the most important fishing fund in Atlantic seacoast [1], in fact Trachurus trachurus is fished by a national fishing fleet.trawlers and seiners actively exploit this species, where annual catches have fluctuated from 2004, reaching its peak in 2015/2016 [2].
The horse Mackerel is a teleost's species found in the North Atlantic, the Mediterranean and the Black Sea.It is particularly frequent in the Adriatic Sea, off the coast close to the continental shelf at depths of between 10 and 500 m.It is a Migratory species and the distribution of stocks in the West and North Seas partly overlap during wintering in the Atlantic Ocean [3].
Morphological studies of marine fish have revealed morphological divergences in the bodies of different species.These changes are linked to the specific characteristics of their aquatic environment [4].
Phenotypic variation within a species may result from isolation, environmental variations, thermal responses, local adaptation or a combination of these factors.These variations can influence the overall dynamics of the species, including growth, maturity, reproduction, recruitment, abundance and survival of individuals.Different environmental factors such as temperature, salinity, food availability and swimming patterns can lead individuals to adopt different adaptive strategies, resulting in distinct morphometric characteristics from one population to another [5 ,6].
This species has been overexploited in the last few years, and it is important to carry out investigations in order to propose suitable methods of stock management, there is some studies is focused in biological aspect or growth aspect of the horse mackerel in Atlantic coast.
Accordingly, the aim of this study is to identify the population of horse mackerel exploited on the Moroccan Atlantic coast and the living conditions of this species through the length-weight relationship, and variation of condition factor in relation to the total length (Tl).
The characteristics of the seabed and the bathymetry are important factors in determining the abundance and availability of fishery resources, The shelf is generally divided into coastal and deep areas by a coral layer, the depth and width of which ICCR'2 varies from region to region.In most areas, the continental shelf consists of an area of undulating seabed, divided into rocky, sandy, and muddy areas.
The zone is influenced by the closeness of the Strait of Gibraltar, which determines the winds circulation, which means that the intense winds are coming from the west, while the moderate winds are from the north-west and the northeast, also by Canarian winds who's came from the south [7].
The area is also known as the "upwelling".This phenomenon boosts the nutrient content of coastal waters, enabling the development of high biological productivity [7].

Sampling methods
The Samples are collected at the port of Tangier and Larache over 13 months between July 2021 and July 2022, A Total of 1494 specimens provided to the laboratory of fisheries at the regional centre of the Institute of Fisheries Research of Tangier for examination of measurements (total length , weight) and also biological examination (identification of the sex), indeed The total length (Tl) was taken to the nearest centimetre using a graduated woody broad or all the samples was measured approximately 10 and 41 cm and the body weight measured between 35 and 500 g ( with precision scale d=1g), for the samples collected in the port Tangiers and Larache.The gender was determined macroscopically

Length-Wight Relationship
The length-weight relationship is generally used to estimate the weight of catches in fisheries biology and to identify stocks [3].
This allometric relationship is used to convert measured lengths into weight data.It has the following form:

P= a × L (b).
P: Individual Weight (grams), L: Individual length (centimeters), a: Constant of proportionality, b: Factor of condition, it translates the allometry.
These types of isometries describe, depending on the height and weight considered: • If b= 3: weight grows proportionally to height, a perfect isometry, • If b > 3, weight grows faster than length and the allometry is majorant, • If b < 3, weight grows less quickly than length and the allometry is minor.

Sex-Ratio
The sex ratio (SR) reflects the rate of masculinity or femininity in the population under consideration, it is expressed either as the percentage of females in the population or as the percentage of males in the same population.The distribution of males (M) and females (F) can be calculated in separate ways: (1) SR = (f/n) x 100 (2) m: males' number, f: females' number

Condition Factor K
The variations of condition were calculated using indices of the weight-length measurements.The condition factor (K) was calculated using the following formula K = (W/L 3 ) × 100.
W: weight in g, L: Length in cm

Results and discussion
Of a total of 1,485 T. trachurus specimens taken during the study period from port 570 were males, 735 females.The total length of all samples (males, females) ranged from 10 to 41 cm and the weight from 35 to 500 g.In tangier, the total length of the males was between 10 and 39 cm, and the weight between 45 and 477 cm, or the total length of the females was between 13 and 41 cm and their weight between 53 and 500 g, however in port of Larache, the individual's males measure between 11 and 37 cm and they weight 34 to 402 g, or the females measure between 14 and 36 cm, and weights between 40 and 385 g.
The frequency of total length (Tl) shows that classes of size between 20-30 cm are well identified with an average 26.4 cm in Tangier and 26.17cm in Larache (Fig. 2,3).The study of the morphometric characteristics of T. trachurus shows that the size distribution of the horse mackerel is like that of the common horse mackerel, these results is comparable with a study in north Atlantic Sea [3] who's reported that the sizes is between 15 and 45cm, also in other study in the Portuguese water ranges from 8 to 56 [8], however the size of Trachurus trachurus caught in Turkish Mediterranean Sea is between 10 and 18,9 cm [9], which means that the size varies with the zone where it is caught.

Fig. 4. Length-weight relationship
The same observation was made in Portuguese waters showing that b is less than 3 [11].In general, therefore, it can be concluded that T. trachurus grows faster in length than in weight.These findings differ from those obtained for the same species in the Adriatic Sea [10].This difference observed between the localities is possibly explained by certain environmental factors such as the availability or non-availability of food.This is closely related to the upwellings that generally occur in the Mediterranean.Upwellings favour secondary production (zooplankton), which is the horse mackerel's main food source.

Sex ratio
The sex ratio for horse mackerel is in the Females advantage 52,80% or a masculinity rate of 47,19 in Tangier, whereas the findings from Larache indicate a sex ratio in favour males with 53,71% or females' sexratio proportion 46.28 % (Table 1), in fact, these results were not identic This depends on feeding, which varies according to the quantity and quality of the food, and environmental factors, which make males more vulnerable to fishing gear, as in February, when almost all the males are present, with an absence of females at the Larache specimens, superiority of the females can also be a result of their great longevities, earlier development of the ovaries [12][13].

Condition factor K
The calculated condition factor K indicates that the smallest values of condition factor for both of point were recorded in small sizes which is considered as immature and prematurely individuals (lengths between 10 and 19 cm), or the highest values of condition factor were recorded in the lengths between 21 and 30cm (Fig. 5,6).The monthly evolution of this coefficient (K) in the Atlantic horse mackerel the lowest value showed in loss noted during the month of September, and the highest values in March, which coincide with final period of spawning witch that was showed in Mediterranean Sea [13], or where the fish quickly recover their weight during the sexual rest period (Fig. 4).

Conclusion
In conclusion, in the North Atlantic Moroccan Sea, the horse mackerel T. trachurus grows faster in length than in weight, while the differences in allometric parameter in other regions, that explained by environmental factor, or food availably, in other hand the sex ratio results shows that there is difference between the both port so that is significatively in relation with fishing gear or the fishing period, and finally the condition factor K high values was between 21-30 whose considered matures.

Fig. 5 .
Fig. 5. coefficient of condition in port of Larache

Fig. 6 .
Fig. 6. coefficient of condition in port of Tangier