Creating a fish simulation in MATLAB

I want to create a catch fish simulation in MATLAB for my research studies and so far I have been able to create a fish and add it to a figure handler. However, I am only limited only by 2 fishes. I am intending to put in around 30 of them. How can I put more of them? Here are my current code

classdef Fish < handle

      %Constant properties
      properties(Constant)
          c_bounce=1;
          radius = 1;
          mass =  1;
          gamma = 10;
          loc = 5;
          c = 5
          d = 1;
      end
      %Private properties
      properties(SetAccess = private)
          fig;
          axs;
          pos;
          vel;
          accel;
          item;
          dt;
      end
      %Class method
      methods
          %Default constuctor
          function this = Fish(fig,axs,dt)
              this.fig = fig;
              this.axs = axs;
              this.pos = [randi([-50,50]), randi([0,50])];
              this.vel = [0,10];
              this.accel = [0,-10];
              this.dt = dt;

              this.item = rectangle('Position', [this.pos(1,1)-this.radius, this.pos(1,2)- this.radius, this.radius, this.radius],...
                        'Curvature',[1,1],...
                        'FaceColor', 'y',...
                        'Parent', this.axs);         
          end
          function move(this)
              this.pos = this.pos + this.vel* this.dt;
              this.vel = this.vel + this.accel* this.dt;
          end
          function draw(this)
              set(this.item,'Position',[this.pos(1,1)-this.radius,this.pos(1,2)-this.radius,...
                  this.radius,this.radius]);

                  axis(this.axs,[-50,...
                                 50,...
                                 0,...
                                 50]);
          end
          function animate(this,dt)
              while true
                  t_loopstart=tic();
                  move(this);
                  draw(this);           
                  el_time=toc(t_loopstart);
                  pause(dt-el_time);
              end
          end
          function collision(this)

          %  Set boundries
              bottom = 0;
              top = 50;
              left = -25;
              right = 25;
          %--------------------------------------------------------------
                  if this.pos(1,2)-this.radius < bottom
                   this.vel(1,2)=-this.vel(1,2)*this.c_bounce;
                  end
                   if this.pos(1,2)+this.radius > top
                      this.vel(1,2)=-this.vel(1,2)*this.c_bounce;
                   end
                  if this.pos(1,1)-this.radius < left
                      this.vel(1,1)=-this.vel(1,1)*this.c_bounce;
                  end
                  if this.pos(1,1) + this.radius > right
                      this.vel(1,1)= -this.vel(1,1)*this.c_bounce;
                  end

          end

      end
  end
  %%END OF FISH CLASS

function main()
    dt = 0.005;

      [mainfig, mainaxs] = initField();

      max = 5;

      for i = 1:max;
          school(i)=Fish(mainfig, mainaxs, dt);
      end
      %fish1 = Fish(mainfig, mainaxs, dt);

      tb=uicontrol('style','togglebutton','string','Start','callback',@go,'units','normalized','position',[.025 .05 .1 .05],...
          'backgroundcolor','w');
      function go(varargin)
          set(tb,'value',0)
          if strcmp(get(tb,'string'),'Start')
              set(tb,'string','Stop')
          else
              set(tb,'string','Start')
              return
          end
          %axis equal;
          while strcmp(get(tb,'string'),'Stop')  

              t_loopstart=tic();
              for i = 1:max;
                  fish = school(i);
                  fish.move();
                  fish.collision();

              end
              redraw(school);
  %             fish1.move();
  %             fish1.collision();
  %             redraw(fish1);

              el_time=toc(t_loopstart);
              pause(dt-el_time);
          end
      end
      function redraw(obj)          
          n = numel(obj);
          for i = 1:n
              fish = obj(i)
              set(fish.item,'Position',[fish.pos(1,1)-fish.radius,...
                  fish.pos(1,2)-fish.radius,...
                  fish.radius,fish.radius]);

              axis(fish.axs,[-50,...
                  50,...
                  0,...
                  50]);
           end
      end
  end
function [fig, axs] = initField()
    WinSize=[1,1,800,800];
    ScrSize=get(0,'ScreenSize');
    fig=figure('Name','Fish field simulation',...
                   'NumberTitle','off',...
                   'Menubar','none',...
                   'Units','pixels',...
                   'Position',[(ScrSize(3)-WinSize(3))/2,...
                   (ScrSize(4)-WinSize(4))/2,...
                    WinSize(3),WinSize(4)]);

       axs=axes('Parent',fig,...
                    'Units','normalized',...
                    'Position',[0.1,0.1,0.8,0.8]);

                axis(axs, [-50,...
                              50,...
                              0,...
                              50]);
  end

I split it into two classes one is the fish class and one is the main. I realise that I can't pass in the figure to the fish class and let it run the animation by itself since I am going to add a second class ('fish hunter') into the mix. So I have to do the other way round adding the fish class to the figure and make the figure as the container for all the objects.Is there any efficient way of doing this?

TL;DR I am trying to add more fish to my animation but my computer just stop redrawing the object.