Journal Article

The trophic fingerprint of marine fisheries

Watson, Reg, Pablico, Grace T., Fulton, Elizabeth A., McGilliard, Carey R., Tracey, Sean R., Branch, Trevor A., Ricard, Daniel, Jennings, Simon

2010

Overview

Catch-based fisheries data can mislead It is often claimed that industrial fisheries are 'fishing down marine food webs' by depleting top predators (such as tuna) before targeting their prey species (plankton feeders such as oysters and sardines). But new global data reveal little evidence for this pattern of sequential depletion, working downwards through the trophic levels of the marine ecosystem. Rather, comparison of model predictions of the widely adopted marine indicator, mean trophic level (MTL) derived from reported catches, with actual ecosystem MTL suggests that fishing has intensified throughout all levels of marine food webs. The trend can be masked by the use of data based on catches, and if we are to accurately monitor future fisheries collapses — and recoveries — we may need to shift focus from catch-based indicators to tracking true abundance trends using scientific surveys and models. The health of marine ecosystems is traditionally assessed by measuring the mean trophic level (MTL) of fishery catches. These authors model catch MTL and actual ecosystem MTL, and show that the former is not a good measure of the latter. They then show that MTLs have actually been increasing in recent years, but that fisheries are still at risk of collapse because all trophic levels have been similarly affected. Biodiversity indicators provide a vital window on the state of the planet, guiding policy development and management 1 , 2 . The most widely adopted marine indicator is mean trophic level (MTL) from catches, intended to detect shifts from high-trophic-level predators to low-trophic-level invertebrates and plankton-feeders 3 , 4 , 5 . This indicator underpins reported trends in human impacts, declining when predators collapse (“fishing down marine food webs”) 3 and when low-trophic-level fisheries expand (“fishing through marine food webs”) 6 . The assumption is that catch MTL measures changes in ecosystem MTL and biodiversity 2 , 5 . Here we combine model predictions with global assessments of MTL from catches, trawl surveys and fisheries stock assessments 7 and find that catch MTL does not reliably predict changes in marine ecosystems. Instead, catch MTL trends often diverge from ecosystem MTL trends obtained from surveys and assessments. In contrast to previous findings of rapid declines in catch MTL 3 , we observe recent increases in catch, survey and assessment MTL. However, catches from most trophic levels are rising, which can intensify fishery collapses even when MTL trends are stable or increasing. To detect fishing impacts on marine biodiversity, we recommend greater efforts to measure true abundance trends for marine species, especially those most vulnerable to fishing.

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: 704/158/2445 / 704/158/2446 / 704/158/670 / Animal and plant ecology / Animal, plant and microbial ecology / Animals / Applied ecology / Aquatic Organisms - isolation & purification / Aquatic Organisms - metabolism / Biodiversity / Biological and medical sciences / Biological diversity / Biomass / Commercial fishing / Databases, Factual / Ecosystem / Ecosystems / Environmental changes / Environmental Policy / Estimates / Exploitation and management of natural biological resources (hunting, fishing and exploited populations survey, etc.) / Fisheries / Fisheries surveys / Fishes - metabolism / Fishing / Food Chain / Food chains / Food webs / Fundamental and applied biological sciences. Psychology / Human Activities / Humanities and Social Sciences / Invertebrates - metabolism / letter / Marine ecosystems / Marine fish / Models, Biological / multidisciplinary / Plankton / Plankton - metabolism / Predators / Science / Science (multidisciplinary) / Sea water ecosystems / Synecology / Threatened species / Trawling / Trends / Trophic levels