Minimum multipliers for fishery offsets in marine habitats vary depending on the no net loss goal

Abstract

Offsetting impacts on fishery resources under losses of marine habitats due to development is important to help safeguard seafood security. However, the gains that marine habitat restoration can create for fishery resources remain largely unquantified. It is unknown what area of fish habitats must be restored to fully offset a hectare of development impacts. Here, we quantify minimum offset areas required to achieve different indicators of no net loss (NNL) goals: economic value, catch of each species, and catch of most-caught species. Simulations are conducted by using a case study of mangrove habitat offsets in Australia to infer the minimum restoration area needed to fully offset fishery losses caused by 84 development projects. We establish two offset policy rules: offsetting closest to the impact site, and offsetting to maximise benefit regardless of location. Achieving NNL for all fishery species requires a substantially larger offset multiplier than achieving NNL for most-caught species, or for economic benefits. For all NNL objectives, choosing optimal offset sites can minimise the area needed for restoration, but required multipliers still always >1. The amount of offset area required to achieve NNL goals for fisheries per unit area lost for each development project increases as development projects accumulated, since the suitable sites for offsetting become scarcer. These results reveal the large and increasing costs of fully offsetting marine impacts and the increasing risks, especially where suitable offset sites are likely to be scarce.