Background crashing

import greenfoot.*;  // (World, Actor, GreenfootImage, Greenfoot and MouseInfo)

/**
 * Write a description of class Background here.
 * 
 * @author (your name) 
 * @version (a version number or a date)
 */
public class Background extends World
{

    // variables
    private GreenfootImage bgImage;

    // Breedte en hoogte van de background-image
    private int imageHeight, imageWidth;

    // breedte en hoogte van het canvas
    private int canvasHeight, canvasWidth;

    // Startpositie van het image
    private int xStartImage, yStartImage;

    // x en y-positie van de background-image
    private int xPosImage, yPosImage;

    // x and y path over which the background image moves
    private int xPathImage, yPathImage;
    private double xPathFish, yPathFish;

    // Snelheid waarmee background beweegt.
    private double xSpeedImage, ySpeedImage;

    // Temp vars

    private Fish frenzy;
    /**
     * Constructor for objects of class Background.
     * 
     */
    public Background()
    {    
        // Create a new world with 600x400 cells with a cell size of 1x1 pixels.
        super(900, 600, 1); // Catch the width and height of the canvas with getWidth() and getheight().

        frenzy = new  Fish(5);

        bgImage = getBackground();

        // Breedte en hoogte van de background-image
        imageHeight = bgImage.getHeight();
        imageWidth = bgImage.getWidth();

        // breedte en hoogte van het canvas
        canvasHeight = getHeight();
        canvasWidth = getWidth();

        // Starting position of image
        xPosImage = 0;
        yPosImage = canvasHeight - imageHeight;

        addObject(frenzy, getWidth()/2, getHeight()/2);
    }

    public void act(){
        moveWithSpriteAnimation();   
    }

    public void moveWithSpriteAnimation(){

        if (Greenfoot.isKeyDown("right")){
            moveBackgroundToSprite();
        }
        if(Greenfoot.isKeyDown("left")){
            moveBackgroundToSprite();
        }
        if(Greenfoot.isKeyDown("up")){
            moveBackgroundToSprite();
        }
        if(Greenfoot.isKeyDown("down")){
            moveBackgroundToSprite();
        }

    }

    public void moveBackgroundToSprite(){
        // Movement speed over X        
        xPathFish = ((double)3/4 - (double)1/4)*canvasWidth;
        xPathImage = canvasWidth - imageWidth;

        xSpeedImage = xPathImage / xPathFish;

        if(frenzy.getX() > ((double)1/4)*getWidth() && frenzy.getX() < ((double)3/4)*getWidth()){
            int newX = frenzy.getX()*(int)xSpeedImage;
            int thisY = frenzy.getY();
            getBackground().drawImage(bgImage, -newX, thisY);
        } else {
            // getBackground().drawImage(bgImage, 0,0 );
        }

        // Movement speed of Y
        yPathFish = ((double)3/4 - (double)1/4)*canvasHeight;
        yPathImage = canvasHeight - imageHeight;

        ySpeedImage = yPathImage / yPathFish;

        if(frenzy.getY() > ((double)1/4)*getHeight() && frenzy.getY() < ((double)3/4)*getHeight()){
            int thisX = frenzy.getX();
            int newY = frenzy.getY()*(int)ySpeedImage;
            getBackground().drawImage(bgImage, thisX, newY);
        } else {
            System.out.println("Vis is buiten");            
        }
    }
}   

import greenfoot.*;  // (World, Actor, GreenfootImage, Greenfoot and MouseInfo)

/**
 * Write a description of class Fish here.
 * 
 * @author (your name) 
 * @version (a version number or a date)
 */
public class Fish extends Actor
{
    private int speed;
    private int xValueFish;
    private int yValueFish;
    // The empty fish constructor
    public Fish(){

    }

    // The empty fish constructor
    public Fish(int speed){
        this.speed = speed;

    }

    public void act() 
    {
        moveTheFish();
    }   

    public void moveTheFish(){
        if(Greenfoot.isKeyDown("left")){
            move(-speed);
        }

        if(Greenfoot.isKeyDown("right")){
            move(speed);
        }

        if(Greenfoot.isKeyDown("up")){
            int up = getY();
            up = up - speed;
            setLocation(getX(), up);
       }

        if(Greenfoot.isKeyDown("down")){
            int down = getY();
            down = down + speed;
            setLocation(getX(), down);
        }
    }

    public void setSpeed(int speed){
        this.speed = speed;
    }

    public int getSpeed(){
        return speed;
    }
}