Considering Covid has been present for a year and the drop happened not this whole time but specifically after Brexit went into effect, this is not convincing.  These drops are specifically Brexit-related, not Covid-related.
Perhaps fishing isn't overall a large part of the UK economy, but any industry that drops 83% is going to decimate the businesses in it.  In 2019, UK exported 2 billion pounds worth of fish.  An 83% drop in exports will prove devastating if it doesn't recover throughout the rest of the year.