| Artificial reefs (AR) are generally designed and constructed in specific body of water to aggregate marine organisms, by providing habitats and improving the biological environment, and to enhance and conserve fishery resource. When conducting evaluation of the ecological performance of artificial reefs, it is one of the key factors to consider the energy transfer between primary carbon source and secondary production. Furthermore, on a small spatial scale the changes of ecosystem structure resulting from the deployment of artificial reef are closely associated with the performance of ecosystem function, simultaneously, it also normally implys if ecosystem function is healthy. Consequently, in the process of biological resource and environmental restoration, for the effective evaluation of ecological performance of artificial reefs, maintenance of the balance of water ecosystem and sustainable exploration of fishery resources, it is essential and important to monitor and evaluate the stability of ecosystem structure, energy transfer and flow of food web.This study was conducted in the representative Lidao AR ecosystem located in Yellow Sea coast of China from 2009 to 2013. The biological and environmental data were collected and analyzed, with the help of knowledge of community structure analysis, feeding ecology, ecological modelling. The pattern of benthic fish and macroinvertebrate community structure in Lidao AR zone was examined. The differences of trophic niches among three dominant rocky fish were compared. The EwE model was used to evaluate the structure and function of the ecosystem, and further to explore the issues of fishery management strategies in the current ecosystem. The specific results were as follows:1. Analyses of benthic fish and macroinvertebrate community structure and its relationship with main environment factors in Lidao AR zonesThe species composition of benthic fish and macroinvertebrate community in the Lidao artificial reef (AR) and control zones of Shandong province between May 2009 and February 2010 were analyzed. Variations in temporal-spatial distribution patterns and community structure of benthic fish and macroinvertebrate community were determined by a series of diversity and richness index. The relationships between abiotic environmental variables and benthic fish and macroinvertebrate community pattern were investigated with gradient analysis, combining Monte Carlo tests, to determine limiting factors affecting benthic fish and macroinvertebrate community structure of AR zones. A total number of 18 fish species belonging to 15 families and 8 orders were collected during the surveys. Sebastes schlegeli, Hexagrammos otakii and Hexagrammos agrammus were the most abundant species. S. schlegeli was the absolutely dominant species in weight composition of catch (41%). The species number and catch of benthic fish and macroinvertebrate at the AR were higher than that at the control site, but the difference was not significant (P>0.05). The highest average catch per unite effort (CPUE) by season of investigation zones appeared in the nearshore AR zones (NA) and reached 830.24 g/(net.d) during the surveys. It was noted that the parameters of community structure were highest in spring or autumn. However, no significant differences were found in community structure parameters between the AR zones and the control zones (P>0.05). Cluster analysis showed that similarities among quadrats of different zones were relatively high in spring, summer and winter, while similarities among quadrats between the reef and control zones offshore in autumn and all zones in spring were higher than those of NA and control zones.37.1% of the variation in composition of benthic fish and macroinvertebrate community was explained by water temperature in the redundancy analysis (RDA), which suggested water temperature was the main explanatory factor (Monte Carlo test.P=0.002). The water temperature and depth were key environmental variables for the temporal-spatial variation of benthic fish and macroinvertebrate community structure in the Lidao AR zones.2. A comparison of food partitioning among the three dominant rocky reef fishesStable isotope (δ13C and δ15N) and stomach contents analyses were applied to examine the feeding habits and food partitioning among three most abundant and co-occurring rocky fish species, H. otakii, H.agrammus and S. schlegeli inhabiting coastal Lidao waters in summer 2013. The relative importance of potential food sources in supporting production of these benthic reef-associated fish of nearshore waters were also determined using the Bayesian mixing model in stable isotope analysis in R (SIAR). The stomach contents analysis indicated there fish mainly consumed a variety of polychaeta, seaweed, crustaceans and fish, but presented different preferences for prey items. In terms of mass contribution percentage (M%), H. agrammus mainly fed on polychaeta (M%=33.7%), seaweed (M%=23.73%) and crustacean (M%=29.5%), while H. otakii has switched to dominantly prey on crustacean (M%=82.21%) regardless of co-occurring of polychaeta (M%=2.24%) and seaweed (M%=0.70%). S. schlegeli exhibited significant carnivorous diet with a domination of fish (M%=79.67%) and crustacean (M%=20.31%). In offshore site, H. otakii and S. schlegeli mainly consumed decapods. Kruskal—Wallis comparison tests showed δ13C and δ15N values differed significantly (P<0.001) among three fish species and potential food sources collected in nearshore waters in summer,2013, with more enriched δ15N for H, otakii (15.31‰), followed by S. schlegeli (14.98%o) and H. agrammus (14.47%o). SIAR analysis showed three fish were all carnivorous and the main food sources to support nearshore three fish species were from fish, shrimp and crab with a respectively varying proportion. The stable isotopic signature of H. otakii were contributed by shrimp, fish, crab and polychaeta, the mean contribution proportion summed up to 50.9% for the crustacean, which was consistent with the result of stomach content analysis showing M% (82.21%) in its diet composition. Shrimp were dominant contribution components in the isotopic signatures of H, agrammus, accounting for 41.6% of mean contribution, followed by crab, polychaeta and macro algal. It was worthy to note that macroalgal had similar importance to the diet of H. agrammus in terms of M% of diet composition and proportional contribution of potential food sources of stable isotope. Shrimp, crab, polychaeta and fish respectively contributed 31.1%,19.6%,14.8% and 14.8% to the isotopic signatures of S. schlegeli. Stable isotope and stomach contents analysis co-verify the significant diet difference and food partitioning existing among the three rocky fish.3. An evaluation of system structure and function for Lidao AR ecosystem using Ecopath.A Lidao AR Ecopath model was constructed using Ecopath with Ecosim (EwE) software based on the survey data from 2009 to 2012. The mean trophic levels (TL) of the functional groups ranged from 1.0 for the primary producers (phytoplankton, benthic algae and seagrass) and detritus to 4.135 for Type Ⅲ fishes (fishes found in the water column above the ARs, Scomberomorus niphonius). Nearly half of the total system biomass (48.9%) in the system, excluding detritus, (620.20 t km-2 year-1), was comprised of benthic finfish and invertebrates. The total fishery yield from all fisheries (86.82t km-2 year-1) was dominated by low TL herbivorous and detritivorous species such as the sea cucumber Apostichopus japonicas (TL=2.1,46.07%), other echinoderms (TL=2.1,34.6%) (sea urchin Asterias amurensis; Strongylocentrotus nudus) and abalone Haliotis discus hannai (TL=2.0,18.4%), and as a consequence the mean trophic level of the catch was low (2.1). The mean transfer efficiency was 11.7% through the whole system. The ecosystem had a relative low maturity, stability and disturbance resistance, remaining at a developing stage, indicated by the related indexs such as total primary production/total respiration (TPP/TR=1.82), connectance index (CI=0.32), Finn’s cycling index (FCI=5.46%) and mean pathway length (MPL=2.69).4. A preliminary exploration of fishery management strategies for Lidao waters based on EwE.The ecological carrying capacities for sea cucumber A. japonicus and abalone H. discus hannai were estimated to be 309.4 t km-2 year-1 and 198.86 t km-2 year-1 in the Lidao artifical reef zone, respectively accounting for 31.72% and 52.80% of their standing crop, which meant the further potential for stock enhancement.Ecosim was developed in the basis of balanced Ecopath model of the Lidao AR ecosystem to explore three scenarios, including two alternative fishing practices and evaluating the effect of surrounding aquaculture activities. The scenario 1 is to close all fishery (keeping the original state for the first 3 years) and then run the model projections for 17 years. The scenario 2 is to reduce fishing effort to the current 50% level (keeping the original state for the first 3 years) and run the model projections for following 17 years. Both scenarios resulted in an obvious increase in the relative biomass of the main exploited species, sea cucumber A. japonicas and abalone H. discus hannai, but did not cause any predicted cascading effects on benthic macroalgal and seagrass group and detritus group, the simulation suggested that this ecosystem is characterized more by bottom-up process. In addiditon, the predicted relative biomass of Type I, Type II fish and H. otakii decreased by 30-40% at the end of simulated period because of absolutely dominant species S. schlegeli feeding on more preys small benthic fish compared with other benthic predators. Both fishing scenarios indicated fishery practices have a significant impact on the structure of current ecosystem.The third scenario of gradually removing kelp farms over 10 years and then projecting forward for 10 years resulted in an increase of approximate 2 fold in the biomass of type III fish and 1.2 fold in its main prey (small pelagic fish), while the relative biomass of detritivore A. japonicas and heterotrophic bacteria decreased by 31.37% and 12.73%, respectively. The result indicated nearshore kelp cultivation might restrict the trophic flow of grazing food chain, rather strengthen the benthic food web, resulting in a significant change of trophic flow mode in nearshore waters. It is likely that kelp cultivation activities would provide energy subsidies for the stock enhancement of benthic organism. |
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