I hope I'm not making too many assumptions in this rather long post, but I thought I'd give my impressions. Your husband’s defensiveness may be your biggest problem. It speaks to not only a willingness to erect barriers between you, but usually an unwillingness to face his own truths. Either one of these can destroy whatever relationship you have built. Yet I’m sure he doesn’t recognize those barriers.
I’ve always believed the primary reason people behave the way they do is social pressure. Humans are social animals, so by nature they are conformists. It may help you if he could understand his own urges well enough to have that honest conversation with you. Here is a possible conversation started that just popped into my head:
You: “You know I love you and want you to be happy. I can’t help but wonder - have you been craving fish for the 6 years that you were vegan, but I just never knew it? And now that you are developing friendships with men who fish, you don’t want to suppress that desire anymore? Since being a vegan is very important to me, I hope that at some time you can help me understand what changed that we can’t share this lifestyle now. Our relationship is also very important to me, and I’m afraid that this may become a problem between us if we can’t talk about it.”
Hopefully, at some time he will be able to admit to you and to himself that the desire to catch and eat fish isn’t the motivating factor, but the need to ease the awkward position he finds himself in at his new job. If he can even allude to that in a conversation, latch onto it with both hands! Tell him you TOTALLY get that, and you are absolutely willing to support his need to get along, when his comfort at work depends on it. We all have the need for acceptance, and one of the things that makes him a wonderful husband is his willingness to get along with others.
You might even suggest that you could see yourself “lapsing” from time to time if you had to spend your days at his workplace. Let him know you understand he is juggling competing interests: the need to get along at work, both for his job performance and for his mental sanity, and the desire to maintain your relationship at home. Make him understand that you are there to help - you are not the enemy. Then dream, together, of a different scenario, one where not only his own workplace, but the wider community were all vegan. How easy would it be then to stick to our principles!
(And if the taste of fish is still a craving for him, I hear there are some amazing recipes, and also processed vegan foods, that mimic this taste very well.)
I think what you want to know is, is your husband still really a vegan at heart? Is his belief that fish don’t suffer or feel pain a justification for his actions, or does he really believe that? These are more serious questions, that will have to come later. At first, just sympathize with his work situation, but don’t speak negatively about his new friends either. There are lots of lapsed vegans in the world - your husband is not the first. You need to believe he will come back to it someday, maybe bring a few of his new fishing buddies with him.
Another idea: ask him not to bring his fish home with him. He can give them to his friends, eat fish at their houses, or when you eat out. He can use you as an excuse. Let him put the burden on you (since you don’t have to work with them): “My wife is a committed vegan. I don’t want to hurt her by bringing the fish home.”
Personal confession: my husband and I don’t live together anymore. I am a committed vegan, he a committed omnivore. This and many other issues prevent us from getting along on a daily basis. We did try, for 30 years. We seem to get along better now that we don’t try to share living quarters, but I suspect we never had the kind of enriching relationship that the two of you have. I tried in many different ways to explain veganism to him, but he never wanted to hear me. I suppose I started to shut him out, too, not wanting to hear anymore about how the suffering his lifestyle causes others is not an issue for him - only his personal happiness is. I’d rather live alone than with someone I can’t talk to, except on a superficial level. So I’m 66 years old and starting a new life. Yet I have high hopes for your relationship with your husband.