private void killEnemy(EnemyShip enemy) {
        remaining--;
        if (enemy.wasKilledByPlayer()) {
            score += enemy.getScoreValue();
            killedInLevel++;
            ScorePopup popup = new ScorePopup(screen, enemy);
            popup.setWorld(this);
            if (level.isLastWave() && killedInLevel == spawnedInLevel &&
                    spawnedInWave == wave.getNumEnemies())
                spawnPickup(enemy);
        }
        ships.remove(enemy);
    }