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Historical fishing effort has resulted, in many parts of the ocean, in increasing catches of smaller, lower trophic level species once larger higher trophic level species have been depleted. Concurrently, changes in the geographic distribution of marine species have been observed as species track their thermal affinity in line with ocean warming. However, geographic shifts in fisheries, including to deeper waters, may conceal the phenomenon of fishing down the food web and effects of climate warming on fish stocks. Fisheries-catch weighted metrics such as the Mean Trophic Level (MTL) and Mean Temperature of the Catch (MTC) are used to investigate these phenomena, although apparent trends of these metrics can be masked by the aforementioned geographic expansion and deepening of fisheries catch across large areas and time periods. We investigated instances of both fishing down trophic levels and climate-driven changes in the geographic distribution of fished species in New Zealand waters from 1950–2019, using the MTL and MTC. Thereafter, we corrected for the masking effect of the geographic expansion of fisheries within these indices by using the Fishing-in-Balance (FiB) index and the adapted Mean Trophic Level (aMTL) index. Our results document the offshore expansion of fisheries across the New Zealand Exclusive Economic Zone (EEZ) from 1950–2019, as well as the pervasiveness of fishing down within nearshore fishing stock assemblages. We also revealed the warming of the MTC for pelagic-associated fisheries, trends that were otherwise masked by the depth- and geographic expansion of New Zealand fisheries across the study period.

Abstract: The worldwide crisis of fisheries, which are usually managed on a single species basis, has led to calls for ‘ecosystem management’, along with the development of various ecosystem indicators. The Marine Trophic Index (MTI) and the related Fishing-in-Balance (FiB) index are two such indicators, which can be used to draw inferences on the sustainability of fisheries, notably those targeting high-trophic level species, in an ecosystem context. These indices are used here to evaluate the status of marine fisheries in India, based on taxonomically and spatially disaggregated time series of catches covering the years 1950, when 0.6 million tonnes were landed to 2000, when 3.3 million tonnes were landed. We show that the MTI is steadily decreasing in all 13 Indian States and Union Territories, at rates averaging 0.058 trophic level per decade, about the same as in other parts of the world. This decline, however, is not due to the sequential addition of newly exploited species of low trophic level to the multi-species catch from which mean trophic level is calculated. Rather, the MTI values were computed after exclusion of species with trophic levels lower than 3.25. Notably, this excluded Indian oil sardine and penaeid shrimps, the catch of which grew enormously in the 1980s. What has to date maintained the landings of higher trophic level fish in India has been the geographic expansion of the fisheries, which, until the early 1970s, exploited only waters immediately under the coast, while they now reaches to the edges of the continental shelf and beyond. This expansion is quantified here through a ‘spatial expansion factor’, based on a re-interpretation of the Fishing-in-Balance index. This index was proposed earlier to analyse, in an ecosystem context, the interrelationship between mean trophic level and magnitude of the catch, and the trophic transfer efficiency among trophic levels of the food web. The FiB index is shown here to allow, under some specific assumptions about productivity of the exploited areas, inferences on the spatial behavior of fisheries. Based on the newly formulated spatial expansion factor, it is suggested that the Indian shelf fisheries, covered by 2000 about 4 times the area they covered in 1970. However, this expansion had apparently met its natural limits, and catches can be expected to stagnate and ultimately decline, with serious consequences for the marine fisheries sector and consumers in India. [Copyright &y& Elsevier]