Trophic level-based indicators to track fishing impacts across marine ecosystems

Abstract

Trophic level (TL)-based indicators have been widely used to examine fishing impacts in aquatic ecosystems and the induced biodiversity changes. However, much debate has ensued regarding discrepancies and challenges arising from the use of landings data from commercial fisheries to calculate TL indicators. Subsequent studies have started to examine survey-based and model-based indicators. In this paper, we undertake an extensive evaluation of a variety of TL indicators across 9 well-studied marine ecosystems by making use of model-as well as surveyand catch-based TL indicators. Using detailed regional information and data on fishing history, fishing intensity, and environmental conditions, we evaluate how well TL indicators are capturing fishing effects at the community level of marine ecosystems. Our results highlight that the differences observed between TL indicator values and trends is dependent on the data source and the TL cut-off point used in the calculations and is not attributable to an intrinsic problem with TLbased indicators. All 3 data sources provide useful information about the structural changes in the ecosystem as a result of fishing, but our results indicate that only model-based indicators represent fishing impacts at the whole ecosystem level.

Ecological indicators to capture the effects of fishing on biodiversity and conservation status of marine ecosystems

“…In a previous study, an extensive evaluation was undertaken of a variety of trophic level indicators across nine well-studied marine ecosystems using model, survey and catch-based trophic level indicators (Shannon et al, 2014). Results highlighted that the differences observed between trophic level indicator values and trends depended on the data source and the minimum trophic level included in the calculations, and where not attributable to an intrinsic problem with these indicators.…”

“…This indicator had been selected in IndiSeas-phase I as it was simple and most readily available pressure indicator across the ecosystems examined at that time ) ( Figure S1). In IndiSeas-phase II, relative fishing effort and relative fishing mortality were also available for a subset of nine marine ecosystems (Shannon et al, 2014, Figure S2). Therefore, we used a non-weighted mean of the relative fishing effort across fleets and species and a non-weighted mean of the fishing mortality rate across species in order to test the correlations between our suite of pressure indicators of fishing pressure and our suite of ecological indicators.…”

“…As a result of this process, the additional biodiversity and conservation-based indicators chosen to supplement the initial IndiSeas-phase I indicators (Shin et al, 2010a,b) were: (i) mean intrinsic vulnerability index of fish in the landed catch ("Mean Vulnerability", or IVI) (Cheung et al, 2007), (ii) proportion of non-declining surveyed exploited species ("Non-Declining Exploited Species", NDES) (Kleisner et al, 2015), (iii) catch-based marine trophic index ("Trophic Index", MTI) (Pauly and Watson, 2005), and (iv) mean trophic level of the surveyed community ("Trophic Level of the Community", TLsc) (Shannon et al, 2014) ( Table 1 and Table S1). In addition, two extra indicators were chosen (for ecosystems with sufficient data): (v) mean trophic level of the modelled community ("Trophic Level of the Model", TLmc, calculated using Ecopath with Ecosim food web models) (Shannon et al, 2014);and (vi) proportion of discards in the fishery ("Landings/Discard", D) (Fulton et al, 2005;Link, 2005;Shannon et al, 2014).…”

“…In addition, two extra indicators were chosen (for ecosystems with sufficient data): (v) mean trophic level of the modelled community ("Trophic Level of the Model", TLmc, calculated using Ecopath with Ecosim food web models) (Shannon et al, 2014);and (vi) proportion of discards in the fishery ("Landings/Discard", D) (Fulton et al, 2005;Link, 2005;Shannon et al, 2014). Hereafter, we referred to this new proposed suite of six additional biodiversity-and conservation-based ecological indicators as IndiSeas-phase II indicators.…”

Meta-analysis review of fish trophic level at marine protected areas based on stable isotopes data

“…The uncertainty about species diet could limit trophic level estimation due to the fact that diet is a direct reflection on the trophic level. For this reason, the combined use of stable isotopes and stomach analysis could decrease this uncertainty (Ramsvatn, 2013;Shannon et al, 2014).…”