Mean Trophic Index Research Articles

Reference levels of ecosystem indicators at multispecies maximum sustainable yield

Abstract We investigate reference points for ecosystem indicators in support of an Ecosystem Approach to Fishery. In particular, we assess indicator capacity to detect when the Multispecies Maximum Sustainable Yield (MMSY) is reached, under a wide range of multispecies fishing strategies. The analysis was carried out using a simulation approach based on the ecosystem model OSMOSE in the southern Benguela. We show that the 13 ecosystem indicators have reference points at MMSY that are highly variable across fishing strategies. The state of the ecosystem at MMSY is so variable across fishing strategies that it is not possible to set reference points without considering the fishing strategy. However, strategy-specific reference points were found to constitute robust proxies for MMSY in more than 90% of the simulated fishing strategies. For instance, under the current fishing strategy in the southern Benguela, robust reference points at MMSY could be identified for the following indicators: mean length of fish, mean lifespan, biomass over catch ratio, trophic level of the surveys, mean trophic index, proportion of predatory fish, intrinsic vulnerability index, and mean maximum length.

Reconciling Trends of Mean Trophic Index and Mean Temperature of the Catch in the Eastern Mediterranean and Black Seas

The mean trophic level, i.e., the Mean Trophic Index (MTI) and Mean Temperature of the Catch (MTC) were estimated for Turkish waters in the Eastern Mediterranean and the Black Sea based on the reconstructed marine fisheries catches (consisting of N = 88 species and/or higher taxa) of in Turkish waters for the year 1950 to 2010. The MTI decreased in both regions of Turkey, following expectations, but the MTC showed different trends in these two regions. It increased (by 0.48 °C·decade-1), along with the SST in the Eastern Mediterranean, while it fluctuated in the Black Sea. This fluctuating behaviour, however, was similar to the fluctuating tendency of SST in the Black Sea. This suggests that the MTI and MTC can be relied upon as indicators of fisheries impacts on ecosystems, and of the impact of changing temperatures on fisheries, respectively.

Mean trophic level of coastal fisheries landings in the Persian Gulf (Hormuzgan Province), 2002–2011

Fishing activities can alter the structure of marine food webs by the selective removal of some species. The changes in the marine food webs of the Hormuzgan waters of the Persian Gulf, Iran were assessed, based on estimates of the mean trophic index (MTI) and Fishing in Balance index (FiB), and on landing profile of the exploited marine community (49 species) during the period, 2002–2011. The total landings (Yt) (R=0.88, P<0.001) increased gradually while the Yt of carnivores has slightly declined, and the Yt of herbivores, detritivores and omnivores has increased. Consequently, the MTI significantly decreased (R =-0.69, P<0.05) at a rate of 0.11 during this decade. The MTI showed a decreasing trend, which indicates exploitation of marine resources. The FiB index also showed a downward trend and negative values from 2002 to 2009, which may be associated with unbalanced structure in the fisheries, but an upward trend from 2009 to 2011. The time variation of the landing profile showed two periods with significant diff erences in their species composition (R=0.88; P =0.005), and based on analysis of similarity, species have been identified as discriminator species, namely Thunnus albacores and Benthosema pterotum. Results indicate that changes in MTI reflected changes in the Hormuzgan landing structure. The examination of the MTI, FBI, and landing profile (LP) temporal pattern suggests that the status of fishery resources in Hormuzgan inshore waters is overexploited, and provides evidence of the probability that a fishing down process is occurring in this area, and that this trend may continue in the long-term. Therefore, environmental fisheries management and conservation programs should be prioritized for these valuable resources.