Region-based MTI: resolving geographic expansion in the Marine Trophic Index

The Marine Trophic Index (MTI), which tracks the mean trophic level of fishery catches from an ecosystem, adequately tracks changes in mean trophic level of an ensemble of exploited species in response to fishing pressure. However, one of the disadvantages of this indicator is that declines in trophic level can be masked by geographic expansion and/or the development of offshore fisheries, where higher trophic levels of newly accessed resources can overwhelm fishing-down effects closer inshore. Here, we show that the MTI should not be used without accounting for changes in the spatial and bathymetric reach of the fishing fleet, and we develop a new index that accounts for the potential geographic expansion of fisheries, called the region-based MTI (RMTI). To calculate the RMTI, the potential catch that can be obtained given the observed trophic structure of the actual catch is used to assess the fisheries in an initial (usually coastal) region. When the actual catch exceeds the potential catch, this is indicative of a new fishing region being exploited. The MTI of the new region can then be calculated, and subsequent regions are determined in a sequential manner. This method improves upon the use of the Fishing- in-Balance (FiB) index in conjunction with the original MTI calculated over the whole time series because assumptions of fleet and stock stationarity over the entire time series and geographic area are removed. We illustrate the utility of the RMTI with simulated examples and actual data, and generalize these results.

For detailed explanation of  Marine Trophic Index, refer to the Marine Trophic Index, Fishing in Balance Index & Region-based MTI methods.

Publication: Kleisner, Mansour and Pauly (2014) MEPS 512:185-199
Download RMTI code  from GitHub in R or Python
Download sample input data (India)