It's the age group of fish that have not yet completed a year of life. Groups are named 0-group, 1-group, etc. with respect to the years completed in their life, according to otolith reading. So 0-group includes fish that are younger than one year, group-1 consists of fish that have completed their first year but not their second etc.

(After a specific age -different in each species- the different ages cannot be distinguished in otoliths due to the asymptotic form of the growth pattern, so it can only be deduced that fish are at least x-years old or older than that age. Then they are characterized with the plus sign, e.g. the 10+ group is composed of fish that are 10 years old or older, but we cannot define their exact age...)

Thank you @Evangelos!