Estimating impacts and offsets for the death of fish

Abstract

The Fish and Fish Habitat Protection Program (FFHPP) has requested science advice on the potential consequences and how to quantify impacts from works, undertakings or activities (WUAs), other than fishing, that cause the death of fish, and ways that the death of fish can be offset. To support this request, this document is organized into four main sections. First, the available metrics for quantifying impacts and offsets for fish mortality are reviewed. There are differences in the data requirements and equivalency objectives among metrics; total biomass lost is the recommended metric for most cases. Second, modelling approaches that could be used to evaluate the community or food web consequences of fish mortality are briefly reviewed. An example of simple community models is provided, demonstrating that the outcomes for fish communities from fish mortality events are likely to be more than the sum of single species responses. Third, an overview is provided of the considerations that should be included in management decisions about fish mortality, providing information on the components that could be included in a risk management framework for WUA-related residual mortality. An adaptation of a precautionary fisheries management framework is presented as an example of a risk management framework for decisions about fish mortality impacts and offsets. Finally, the literature on current practices for offsetting fish mortality events is reviewed. There is evidence that habitat creation, habitat restoration and enhancement, and biological and chemical manipulation can all provide approaches to offsetting fish mortality, but consideration of the specifics of the offset application and sufficient monitoring, including pre-impact assessment, are needed to effectively achieve an offset for fish mortality.