Mean temperature of the catch index can be masked by changes in catch composition unrelated to ocean warming

Oceans are increasingly warming through climate change. Fish and invertebrate ectotherms respond to ocean warming through poleward and depth-related migrations, a consequence of which is disruption of fisheries catch compositions. Mean temperature of the catch (MTC) is an index of change in catch composition, from colder to warmer water species. MTC is widely applied as an easily parameterised variable using readily available data (catch and species preferred temperature), but few studies underscore situations that might mask the “true” MTC trend. Here, we use fisheries catch in the Arabian-Persian Gulf (“Gulf”) to highlight, for the first time, how abrupt changes in market demand can strongly influence catch composition and thereby mask a trend in MTC, and discuss the implications of the unmasked MTC trend to fisheries in the region. We found that a recent sharp decline in MTC from 27 to 26°C, despite a gradual increase in sea surface temperature, coincided with an escalated demand for the largehead hairtail (Trichiurus lepturus), a relatively cold-water species in the Gulf, that caused catch to dramatically increase for export to overseas markets. Our findings suggest that the change in MTC reflected a fishery response to satisfy increased international market demand, rather than reflecting warming-driven changes in catch composition. When excluding the effect of T. lepturus catch, the Gulf MTC trend was stable over time and consistent with a trend in many tropical and subtropical waters. Our findings highlight that an MTC change can be masked by factors unrelated to warming-driven changes in catch composition, and that catch-only MTC trends should be examined cautiously.