The Geographical Expansion of Fisheries conceals the decline in the Mean Trophic Level of Iran's catch

Abstract One of the most consistent impacts observed in the analysis of commercial catch globally is the phenomenon known as ‘fishing down marine food webs', wherein fisheries increasingly target small fish in low-trophic levels as a result of the depletion of large, high-trophic level fish. Detecting and mitigating fishing down effects are critical to avoid undermining the long-term sustainability of marine ecosystems; yet the widespread geographical expansion of fisheries in response to declining coastal catches has concealed these effects. Iran is the most important fishing country in the Persian Gulf and Oman Sea region, with ever-growing artisanal fisheries that provide food and employment. In this paper, using catch time series spanning over 66 years (1950–2016) and biological data for 67 marine species/groups, we estimate mean trophic level (MTL), fishing-in-balance (FiB) and region-based MTL (RMTL) indicators to examine the effects of fishing down marine food webs while considering the geographical expansion of Iran's fisheries in the Persian Gulf and Oman Sea. We demonstrated that when not accounting for the geographical expansion of fisheries, MTL did not change much, and primarily reflected the presence of large fish (trophic level > 3.5). However, the climbing trend of FiB suggested that the increasing pattern of Iran's catch is potentially driven by the spatial expansion of the fishing fleet, which is concealing the fishing down effects. By considering the nonstationary nature of fishing fleet and fish stocks over time and area, results from RMTL bolster those obtained by FiB: the actual (observed) catch exceeded the potential catch calculated from the observed trophic structure, indicating that fleets have likely moved from nearshore to deeper, offshore waters to gain access to new resources. Once this is accounted for, our study reveals a downward trajectory of the coastal RMTL, providing insights into the condition of the coastal ecosystem in the Persian Gulf and Oman Sea.