Reduction from 8 degrees of latitude to 4 doesn't tell us the area reduction, because the area depends on the absolute distance from the pole. It's also not linear because if the outer radius changes, the change should be proportional to the square of the radius, so potentially more than 50%, or potentially no change or even growth, if the ideal thermal area moved outwards
Or you could click on the article and see for yourself, under the safe presumption that the researchers would do a more thorough job than the lay person. From the article, figure 2:
> Mean spatial abundance of euphausiids per decade in the North Atlantic from 1958–2017 split into six time-periods 1958–1967; 1968–1977; 1978–1987; 1988–1997; 1998–2007; 2008–2017.
