Over the past few decades, it has become widely recognized that the management strategies of world fisheries must ensure sustainability of target species. The intervening years have seen many improvements to the concept of gear selectivity and methods for measuring the selectivity of fishing towed gears. Improved understanding of the principles of the selection of fish by gears has changed the list of parameters which are known to have a significant effect upon selection. The recent development of new mathematical models and the increased availability of powerful computers have resulted in improvements in the analysis procedures for the data produced to measure a gear’s selectivity. The catch of mackerel in the gulfofGuineahas steeply declined during the last two decades, and resource management is clearly required. Therefore, the need for evaluation of trawl codends used in this fishery and the potential improvements to their selectivity are of prime importance. In this paper, we use semi-empirical models to define selective properties of pelagic trawl codends targeting black mackerel (Trachurus spp) in theGulfofGuinea. These properties are determined using the experimental and theoretical methods of assessing the parameters of the selectivity curve, and by plotting the curve. Selection parameters were obtained by fitting a logistic equation using a maximum likelihood method. Trawl codend selectivity is estimated for 17 internal diamond mesh sizes in the range 47 - 79 mm. Using the basic selectivity equations, we determine the needed mesh size A = 58 mm for fishing mackerel in the gulf of Guinea. This nominal mesh size gives room for nj = 0.1 catch of juveniles, which not exceeds the allowable proportion [nj] = 0.1. To provide resource conservation, there is the need to make amendments in the fishery regulations for more rational exploitation of mackerel stoks, because the currently use nominal mesh size A = 56 mm is rather unselective.
Trawling constitutes the principal fishing method of most Gulf of Guinea countries in general and Cameroon in particular. Some of the pelagic fish populations in the region have been heavily exploited and fishing effort may be above optimum levels for many species. One of the main target specie in Gulf of Guinea is black mackerel (Trachurus spp) [
[4,5]. The selective properties of net codends are characterized by the selectivity curve S(l) and its parameters—selectivity coefficient, selectivity range and the fraction of fish not subject to the selective action of the net [
In this work, the determination of the selective properties of trawl codends is based on the use of semiempirical models to evaluate the selectivity curve. These models facilitate a qualitative assessment of the character and degree of influence on the selectivity of the sizecomposition of the target fish shoal, the biometric characteristics of the fish morphology, deformation of the meshes, quantity of the catch etc. [
The process of removing fish from a water reservoir is selective and much attention is given to the regulation of fishing gear selectivity. Selectivity is estimated from the part of the gear where most fish escape. Recent research has shown that the selectivity of trawl fishing gears is highly dependent upon the mesh size from the codend [
Some methods employed to determine the mesh-size of net codends are known:
The experimental methods are of partial character, laborious, do not give the generalities well [13,14]. Methods that use mesh size at the net codend equal to 60% - 80% of gillnet mesh size [15,16] give conflicting results because the working conditions and the characteristics of the gillnetting material and the codend netting are different. Instead, the methods using fish entanglement at the netting as a condition taking into consideration the biometric characteristics of the fish morphology to justify the mesh size are more justifiable [
Considering the above mentioned shortcomings, this work takes into consideration the method that uses the basic selectivity equations [
Most of the countries of the Gulf of Guinea are less developed. They do not possess enough means to carry out laborious and costly experimental works to study the selective properties and the selectivity of trawl codends as a whole, and based on this reason, the selected methods are the most applicable in these countries.
Following semi-empirical models where used to assess the selectivity curve and its parameters. Selectivity gives where generated using the logistic function fitted to plots of the probability of capture against size.
where S(l) is the function of the selectivity curve, K is the selectivity coefficient, D is the selectivity range, A is the internal mesh size and α is the proportion of fish not affected by the selective effect of the mesh. Qh is the catch in tons per a tow of one hour duration. Km is the coefficient of the body of the fish, Kcom is the coefficient that takes into consideration the deformation of the body of the fish as it passes through the mesh, Kcor is the coefficient that corresponds to the working shape of the mesh and the form of the mesh at the maximum cross section of fish’s body. εm is the relative work lengthening of the mesh. l is the fish length; Amin and Amax are the mesh sizes, which correspond to the minimum and maximum length of fish in the size composition of the targeted shoals.
The determination of the mesh size of the trawl codends was based on the condition that, the quantity of juveniles nj does not exceed the allowable [nj]. It is equally important to also know the relative quantity nt of mature fish that escape through the mesh codend. These characteristics of the selective action of trawl codends were determined using the following basic selectivity equations.
where Yj is the relative catch of juveniles, Yt is the relative catch of fish of mature size and Y0 is the relative total catch (Y0 = Yt + Yj). Nt is the fraction of mature fish in the targeted shoals and nem is the fraction of fishes entangled. g(l) is the function of the distribution density of the size composition of the targeted shoals, S(l) is the function of the selectivity curve of trawl codends and P(l) represents the function of the codend entanglement capacity (function of the selectivity curve of gillnets), lj is the mature fish length and αp is the ratio of fish that die against those that get out through the mesh.
The input data for determining the parameters and the function of the selectivity curve as well as the mesh size or dimension taking into consideration the allowable catch of juveniles are:
• Coefficient of fullness of fish’s body Km;
• Compression coefficient Kcom of fish’s body as it goes out via the mesh;
• Coefficient of correspondence Kсor between the working shape of the mesh and the maximum crosssectional area of the fish’s body;
• Relative working lengthening of the mesh εm;
• Catch per hour of trawling Qh;
• Minimum fish size in the targeted fish shoal l min;
• Maximum fish size in the targeted fish shoal l max;
• Mature fish size lj;
• Allowable catch of juveniles [nj];
• Allowable escape of mature fish through mesh of the trawl codend [nt];
• Seventeen internal mesh sizes whose selectivity curves cover the range of the size composition of fish in the targeted shoal;
• Variation series characterizing the size composition of the targeted shoal.
The value of the mature fish size lj and the allowable catch of juveniles [nj] are contained in the laws regulating fishing in the Gulf of Guinea [
The experimental fishing for the trawl codend selectivity was carried out on board the fishing trawler “Kelly Danielle” owned by the fishing company “Diamond Fish” from 2010 to 2011 in the Gulf of Guinea area within the territorial waters of Cameroon, Nigeria, Equatorial Guinea, Sao Tome and Principe and Gabon.
Experimental and statistical materials were obtained using the standard method [
The input data used to determine the mesh size of the trawl codend were:
• Material for the trawl codend—Polyamide, 3.1 mm double twine;
• Trawling speed υтр = 2.5 м/s;
• Tow duration—1 hour;
• Average catch per one tow Qh = 2 tons;
• Allowable mature fish size by the legislation lj = 220 мм;
• Allowable catch of juveniles [nj] = 0.1;
• Allowable quantity of fish of mature size, that escape via the mesh [nt] = 0.30.
The initial data for the calculated size composition was given in the form of a variation series and inserted in
Taking into consideration the input data, the calculated parameters were determined and inserted in
For each variant of 17 mesh sizes, we obtained the parameters of the selective curve: The quantity α of fish not affected by the selective action of the mesh, the selectiveity coefficient K and the selectivity range D (
Value of ordinates of selectivity curves for 13 fish sizes are given in
In this work, we use empirical models to define selective properties of a large range of diamond mesh codends of varying mesh sizes. In particular, the selective proper
li = fish size, mi = number of fishes.
Km = coefficient of fullness of fish’s body, Kcom = compression coefficient of fish’s body as it goes out via the mesh, Kсor = coefficient of correspondence between the working shape of the mesh and the maximum cross-sectional area of the fish’s body, εm = relative working lengthening of the mesh, Qh = catch in tons per hour of trawling, lj = mature fish size, lmin = minimum fish size in the targeted fish shoal, lmax = maximum fish size in the targeted fish shoal, [nj] = allowable catch of juveniles, [nt] = allowable escape of mature fish through mesh of the trawl codend, Aùin = minimum mesh size, Aùax = maximum mesh size, ΔA = step size.
Таble 3. Selectivity curve parameters α, K and D by mesh sizes (A1 to A17).
α = quantity of fish not affected by the selective action of the mesh, K = selectivity coefficient, D = selectivity range.
Таble 4. Catch parameters Y0, Yj, Yt and Yem by mesh sizes (A1 to A17).
Y0 = relative total catch (Y0 = Yt + Yj), Yj = relative catch of juveniles, Yt = relative catch of fish of mature size, Yem = relative quantity of entangled fishes.
Таble 5. Catch indicators nj, nt, nem and np by mesh sizes (A1 to A17).
nj = quantity of juveniles, nt = quantity of fish of mature size that escape via the mesh, np = post selection mortality, nem = quantity of entangled fishes.
Таble 6. Value of ordinates of the selectivity curve by mesh size.
ties of codends with mesh sizes in the range 47 - 79 mm and made from double braided PA twines of 3.1 mm thicknesses.
Many factors influence the selective properties of trawl codends. These could be biological or biometric, physic-technical, technical and exploitation [
Likewise, the mesh size also influences the selective parameters K, D and α.
For a mesh size of 47 mm, α approches 1 (0.811) and all fish caught is retained at the codend, while a meshsize of 79 mm, causes the value of α to get toward 0 (0.245), and all the catch might escape from the codend. The relationship between the mesh size A and the parameter α is complex, and tends to take an exponential shape
curve S(l). The relationship between the mesh size A and the selectivity coefficient K is shown in
fish to escape (A = 79 mm; A = 47 mm), while the values of D corresponding to intermediary mesh-sizes are minimal as presented in
The corresponding mesh-size for a variation in α close to 1 or to 0 also depends on the quality of the catch and on the size-composition of the target shoal g(l) as represented in
= f4(A).
In this work, such a justification is observed when the quantity of juveniles is limited as when as control is carried out for juveniles escaping via the meshes.
Using the data from the tables, we plotted graphs of functions nj = f1(A) and nt = f2(A) in one coordinate axes. By the curve nj = f1(A) and the given allowable catch of juveniles [nj] = 0.1, we determine the needed mesh size А = 58 mm (
For this mesh size A = 58 mm, nj = [nj] = 10% and nt = 28%.
The escaping of mature fish at this mesh size was 0.28 and did not exceed the allowable [nt] = 0.3, while the estimated catch of juveniles [nj] = 0.1.
The selectivity curve corresponding to the optimal mesh size А = 58 mm, is shown in
In this case, the selectivity coefficient K = 4.31 and the average inclination angle of the curve φ = 0.005.
The above used models (4-7) are linked between themselves by the regulatory indicators (lj, nj и A), the control indicator (nt) with the composition of the targeted shoal and the selectivity curve for the trawl codend. They do not contain assumptions and their exactness depends only on the exactitude of the initial data. The presented models will provide a better under standing of the selection process, permit a more targeted approach to codends selectivity experiments, and assist fishery managers to assess the impact of proposed technical measures that are introduced to reduce the catch of undersized fish. Currently, fishing regulations have adopted a mesh size of A = 56 mm as the standard for codends of pelagic trawl gears fishing mackerel in the Gulf of Guinea [
We are grateful to the ‘‘DIAMOND FISH’’ Company for permitting us carry out the experimental work onboard the fishing trawler “Kelly Danielle”.