Modelling trophic structure and energy flows in the coastal ecosystem of the Persian Gulf using Ecopath with Ecosim

Abstract

In the present study, the trophic structure for some species of the coastal ecosystem of south of the Qeshm Island was developed using the mass balance modeling software Ecopath (Version 6.5.1). In this model, 33 functional groups including fish, benthos, phytoplankton, zooplankton, seaweed and detritus were simulated. In total, 3757 samples of stomach contents were analyzed based on the weight and numerical methods. Bony fish and crustacean were found to be the main prey in most of the stomach contents. The mean trophic level in the study area was calculated to be 3.08. The lowest trophic level was belonged to Liza klunzingeri, (2.50) and the highest belong to Trichiurus lepturus (4.45). The range of total mortality varied from 1.11 per year for T. Lepturus to 3.55 per year for Sillago sihama. The lowest and highest Consumption/Biomass value was belonged to Trichiurus lepturus (2.94) and Rastrelliger kanagurta (11.60) respectively. In this study, the role of ecological groups in the food web was determined using Keystone Index. The results showed that benthos and subsequently phytoplankton and zooplankton are among the dominant ecological groups in the southern ecosystem of Qeshm Island. The Connectance Index and the System Omnivory Index of this ecosystem were estimated to be 0.14 and 0.15 respectively. Population dynamics studies especially the exploitation rate (E) showed that Pomadasys kaakan, Rastrelliger kanagurta and Sillago sihama are under fishing pressure. Overall, the indices of ecological relationships with the population dynamics parameters showed that the exploitation of the fisheries resources in the study area is not in a good condition. The results of Ecosim model using time series of mean biomass from 2004-2016, showed a decrease in the biomass of Nemipterus japonicus, Otolithes ruber and Rastrelliger kanagurta. In general, the results of this study showed that the current fish harvest on the population of carnivorous species is very high which in long term could decrease the number of fish recruitment and consequently the capacity and sustainability of Qeshm Island fishing. Moreover, the high exploitation of fishing resources in this area could have negative effect on top-down food web and gradually reduce the top predator species, and eventually resulted in changing the trophic level of the species and reduction of the top of the food web species, which could be resulted in damaging of the ecological balance and endangering long-term sustainable utilization.