Ask yourself the question, did you do it, in reference to the alledged offence. There are two possible answers, yes or no. We already know the answer.
Answering, I didnt have my pen, my pen died, I lost my pen, I usually dont do this, I have never done anything like this is only trying to minimize and justify the situation. Intent need not be proved, this is not a criminal case. When it comes to Wildlife Act, Motor Vehicle act etc, intent does not matter, did you do it is what matters.
Its a small fine, it could have been worse. Man up, pay the fine, and learn from it.