// postcondition: returns the minimum number of fish that need to be 
  //                added to make the population density greater than
  //                minDensity
  private int numUnder()
  {
    int current = theEnv.numObjects();
    int cells = theEnv.numRows() * theEnv.numCols();

    int needed = 0;
    while ((double)(current + needed) / cells  <= minDensity)
      needed++; 1

    return needed;
  }

1. This solution is preferred if you really want to save mental energy and play it safe.  Efficiency does not matter on the AP exam, unless specifically requested.  Perhaps a little more elegant would be:

       needed = (int)(minDensity * cells) + 1 - current;
       if (needed < 0) needed = 0;

  //  postcondition: returns a random location within the bounds of theEnv
  private Location randomLocation()
  {
    Random gen = RandNumGenerator.getInstance(); 1
    int rows = theEnv.numRows();
    int cols = theEnv.numCols();
    return new Location(gen.nextInt(rows), gen.nextInt(cols));
  }

1. Using

       Random gen = new Random();

   or using Math.random() is likely to cause lost points.

  // precondition:  0 <= numToAdd <= number of empty locations in theEnv
  // postcondition: the number of fish in theEnv has been increased
  //                by numToAdd; the fish added are placed at
  //                random empty locations in theEnv
  public void addFish(int numToAdd)
  {
    while (numToAdd > 0)
    {
      Location loc = randomLocation();
      if (theEnv.isEmpty(loc))
      {
        new Fish(theEnv, loc); 1
        numToAdd--;
      }
    }
  }

1. The Fish constructor adds the fish to theEnv.  If you write

      theEnv.add(new Fish(theEnv, loc));

   theEnv.add will throw an IllegalArgumentException, because loc will have been already occupied.