Analysis of a Fishery Model with two competing prey species in the presence of a predator species for Optimal Harvesting

. A harvesting fishery model is proposed to analyze the effects of the presence of red devil fish population, as a predator in an ecosystem. In this paper, we consider an ecological model of three species by taking into account two competing species and presence of a predator (red devil), the third species, which incorporates the harvesting efforts of each fish species. The stability of the dynamical system is discussed and the existence of biological and bionomic equilibrium is examined. The optimal harvest policy is studied and the solution is derived in the equilibrium case applying Pontryagin's maximal principle. The simulation results is presented to simulate the dynamical behavior of the model and show that the optimal equilibrium solution is globally asymptotically stable . The results show that the optimal harvesting effort is obtained regarding to bionomic and biological equilibrium.


Introduction
Introducing red devil fish species in freshwater ecosystems has threatened the presence of commercial fish (such as, tilapia and goldfish) in the habitats. In the freshwater ecosystems, red devil (Amphilophus Labiatus) growth highly and abundantly. Red devil is as one of the major threats to global biodiversity [1,2]. The red devil species introduced in the ecosystems through spreading the seeds of the commercial fish. The red devil species is from Nicaraguan lakes. The presence of a red devil fish in the ecosystems has become the leading threat of the survival of other fish in the habitats. Red devil has preyed on other commercial fish, such as carp and tilapia. Red devil fish is a type of predator that interferes the survival of other fish in which as the predator, red devil fish has low commercial value.
A number of the investigations regarding to fisheries resources have been conducted. The fishery model of two competing species was addressed by Clark et al. to discuss the aspect bionomics and the policy of optimal harvesting policy [2]. The combined harvesting of two competing species from the aspect of bionomic was also studied and discussed for the optimal equilibrium policy [3][4][5][6][7].
Das et al. [8] and Chaudhuri et al. [9] proposed the model one prey -one predator with combined harvesting. They analysed the dynamical behaviour of the model and the optimal harvesting policy. They assumed that the growth of both prey and predator are formulated in the logistic terms. Das et al. [8] used the functional response of predator that it is limited by density of prey. Rao et al. [10] investigated analytically the dynamical behaviour of the model for three species with one prey and two competing predators. They also discussed the policy of optimal harvesting. .
Kar et al. [11] analyzed a harvesting model for two competing species taking into account the presence of a predator species which feeds on the two competing species, as the third species which was not harvested. They also analyzed bionomic equilibrium solution and the policy of optimal harvesting.
In this paper, we propose and analyze an ecological model of multispecies harvesting of two competing prey species and a predator species. We consider a combined harvest effort that imposes the exploitation of each species. The growth model of each species is modeled by logistics terms and we consider that the functional response of both competitor and predator of other species is assumed in bilinear term. The existence of the possible equilibrium states of the model and the stability of nontrivial equilibrium state by using a Lyapunov function is discussed. Next, we discuss the possibilities of the existence of a bionomic equilibrium. The optimal harvesting policy is studied and the optimal solution is derived in the nontrivial equilibrium case by using Pontryagin's maximum principle. Finally, some numerical examples are discussed.

Model formulation
We consider an aquatic ecosystem, there are two prey species of fish that competes each other for using a common resource. In the presence of a red devil fish, an invasive species (as predator) threat the survival of these prey species. Besides competing for the use of same resources, red devil fish also eats the small other fish. All these species are imposed continuously harvesting. We propose the logistic growth function for both two prey species and the predator (that is, the population of each species compete for the same resource), in addition to the third species increases due to in the presence of prey populations. The model equations for three species in which two competing prey species and a predator species with harvesting on all species is given by the following, x t x t and 3 ( ) x t are the biomass densities of the first prey, the second prey and predator populations with the natural growth rates 1 2 , r r and 3 r respectively. The parameters 1 2 , K K and 3 K are the carrying capacities of the first prey, the second prey and predator populations, respectively. Parameter 1 2  is rate of decrease of the first prey due to the competition with the second prey, 1 3  is rate of decrease of the first prey due to inhibition by the predator, 2 1  is rate of decrease of the second prey due to the competition with the first prey, 2 3  is rate of decrease of the second prey due to inhibition by the predator, 3 1  is rate of increase of the predator due to successful attacks on the first prey, 3 2  is rate of increase of the predator due to successful attacks on the second prey, i q for 1, 2 , 3 i  are catch ability coefficient of the first prey, the second prey and predator species respectively. E is the harvesting effort and are the catch-rate functions based on the catch-per-unit-effort hypothesis. In the analysis of the system, we assume that

Model analysis
In the section, we analyze the existence and the stability of equilibria.

Existence of equilibrium
We determine the conditions for the existence of the equilibrium points of the system (1). By equating the right hand side of the system (1) to zero, we obtain the equilibrium states. The possible equilibrium states are given as follows, The state in which only the predator survives, two prey species are washed out. The equilibrium state is The state in which the first prey and predator species survive and the second prey species extinct out, is given by The state in which the second prey and predator exist, while the first prey extinct out. It is given by

The stability of the equilibrium state
We analyze the global stability of the system (1) by constructing a suitable Lyapunov function. The global stability of the system (1) for the co-existence equilibrium state is given in the following theorem. , , ln ln We see that V is definite positive. The time derivative of V along the solutions of the system (1), is given by, can be rewritten as, where, , and   This completes the proof.

Bionomic equilibrium
The concept of bionomic equilibrium is the concepts that are related to the biological and economic equilibrium. The biological equilibrium is derived from the right hand sides of the system (1) equal zero. The economic equilibrium is obtained by equaling the total revenue that is obtained by selling the harvested biomass with the total needed efforts for harvesting. Let c is the fishing cost per unit effort, i p for 1, 2 , 3 i  are the price per unit biomass of the first, the second and the third species respectively. The net economic revenue at any time t is given by We can obtain the bionomic equilibrium that are given in the following equations, (4), we obtain the nontrivial biological equilibrium. The possibility of the bionomic equilibrium is determined in the following cases. Case I. p q x p q x p q x c    , the fishing cost per unit effort is less than the total revenue, so the biomass can be harvested. The economic equilibrium is given by x x x is obtained by solving both the equations (4) and (5) simultaneously.

Optimal harvesting policy
Our objective is to maximize the present revenue value J of a continuous time that is given by where  represents the instantaneous annual rate of discount. In this problem, we maximize J subject to the state equations (1) by applying Pontryagin's maximal principle [6,11].
are the adjoint variables. By using Pontryagin's maximal principle, we write the adjoint equations regarding to optimal equilibrium solution as 1 3 The solution of the equation (10) is

 
In our optimal problem, we have an equilibrium solution that satisfies the necessary conditions of the maximum principle. For the parameter values as is given, we find that both the biological equilibrium R exist and achieve to the optimal equilibrium solution. We find that the optimal harvesting effort is 5 .6 E  units, regarding to the optimal equilibrium (3 6 , 3 5 ,1 2 6 ) . The simulations are presented in the following figures.