Does effort regulation of mixed fisheries really "work"?

Disclaimer! Here I am highlighting my own paper from 2019 that I quite like but that few have seen. It probably happens to all of us that our work does not receive the attention we believe it deserves. Well, maybe a bit of blatant self promotion could help?

Many of the world's fisheries are "mixed" – multiple species are caught at the same time. This poses a dilemma for the management: how can fishing effort be driven towards species that have more catch potential, away from those that have less potential and are perhaps  already overexploited? One way is setting catch quotas for all species, banning discarding, strictly enforcing both measures, and letting the fishers to figure out how to avoid catching species for which catch quotas become limiting (the "choke species"). In addition to being potentially quite complicated to set up and manage, and a hassle for the fishers to follow, it is a "stick" approach that may find limited approval among the fishers.

As an alternative, effort regulation is sometimes promoted. The idea is that if the fishers are limited by the effort they can use, then they should direct that effort towards fishing activities that are the most profitable. This could mean targeting abundant species that give high catch-per-unit-effort (CPUE), and as a consequence, fishing less those (rare) species that give low CPUE. But how likely is that to happen?

The complication is that fishers do not care about CPUE as such, but by net revenue per unit effort. This may not be closely correlated with the abundance of the particular species, the metric that the managers want to control, in a sense of avoiding too low abundances. The fishers may also not be able target the most profitable species to a degree that would help.

To get quantitative insight into this problem, Xiaozi Liu (then Academia Sinica (Taiwan) and Future Oceans Lab (Spain), now Institute of Marine Research) and I set up a simple two-species, single-season model that included three key parameters affecting profitability of fishing: species-specific catchability and price, and separability, here meaning the ability of fishers to catch the favoured species separately from the less-favoured species.

The results are quite simple. Effort regulation can work, in a sense of not resulting in very large difference in species' abundances by the end of the fishing season, when the species have parameters that make them similarly attractive. This could happens when the species are quite similar overall, or they mix positive and negative attributes in a fortuitous way (valuable species has low catchability, and cheap species have high catchability). Separability has a multifaceted role. A certain degree of separability is needed for the fishers' targetting decision to play a role. But very high separability raises the risk that targetting drives the more attractive species to low levels.

In summary, there are conditions when effort regulation could lead to fisheries that target the common species and save the rare ones, but this outcome is by no means guaranteed.

Reference:

Liu, X., and Heino, M. 2019. Evaluating effort regulation in mixed fisheries: a Monte Carlo approach. ICES Journal of Marine Science, 76: 2114–2124. doi: 10.1093/icesjms/fsz155.