Extending single-species maximum sustainable yield and precautionary approaches to account for species interactions

Many jurisdictions have a legal requirement to manage fish stocks at maximum sustainable yield (MSY), while following the precautionary approach (PA). Generally, MSY is calculated on a single-species basis; however, due to species interactions, there is no guarantee that fishing multiple stocks at their precautionary single-species MSY level will lead to all stocks being precautionary or even at fished MSY. This suggests the necessity of a multispecies MSY (MMSY). Although there are several definitions of MMSY, there is no agreed best definition. At least two approaches have been suggested, firstly a community maximum yield, either with or without integrated risk measures designed to prevent stock-depletion, and secondly, defining MMSY in terms of a set of simultaneous single-species MSYs. Here we adopt the second interpretation which allows for the implementation of MMSY without requiring new concepts when combined with multispecies PA to safeguard stock structure. We extend single-species definitions of MSY and PA to give two potential definitions of precautionary MMSY. In the first we constrain the search for MMSY to fishing mortalities that follow the multispecies PA and in the second we find the closest fishing mortality that follows the multispecies PA to the unconstrained MMSY. We demonstrate these definitions using a case study of 9 stocks in the North Sea and compare them with single-species and non-precautionary equivalents. We propose that the second definition be precautionary MMSY as it is more robust than the first. • We define multispecies MSY (MMSY) that follows the precautionary approach (PA). • We define MMSY as the closest precautionary point to MMSY calculated without PA. • The proposed approach is a generalisation of the current single-species management. • We demonstrate our definition of MMSY for nine stocks in the North Sea.