Fish farm cages are commonly used as case studies for the assessment of the effects of organic enrichment, occasionally inducing hypoxic conditions below the cages as a source of disturbance for benthos. As a consequence, this might alter ecosystem services and biogeochemical processes, as well as the structure of benthic communities and result in loss of diversity (Quero et al., 2020; Keeley et al., 2013 ). However, as the distance from the fish cages increases, the concentration of organic matter and other pollutants decreases and the benthic communities gradually recover from disturbance , as is evident in the successional model of benthic enrichment responses for macrofaunal communities (Karakassis et al., 1999). The commonly used Before-After Control-Impact (BACI) methodology is a useful approach for evaluating fish farming effects through observational studies. By incorporating both time and control sites in the analysis , it minimizes the effects of unmeasured factors on the observed outcomes of the study (Seger et al., 2021, McDonal d et al., 2000) . Th e main objective of this study was to employ the BACI methodology to examine the environmental impacts resulting from the relocation of a fish farm’s cages, which involved dividing and transferring a portion of the cages to a different, nearby, location.
Materials and Methods
A fish farm located in the northern Evoikos Gulf, operating for more than 20 years planned in 2020 to make a major rearrangement of the fish cages. Until summer 2020, the f arm had five large clusters of cages located 50m apart from each over resulting in high precipitation of organic matter in a relatively small area . In 2020, two of the clusters were moved to an adjacent area 500m away, therefore creating two “parks” with the same total fish load as the original . Thus, a BACI study was conducted. Specific sampling stations were designated to assess changes in the environmental impacts of fish cages resulting from the reconstruction process. Benthic samples were collected on three sampling occasions: before the transfer in 2020, immediately after the transfer in 2021, and one year after the transfer in 2022. The sampling stations included the old and new cage locations, as well as control stations without any fish cage s. Additionally, a sampling station was positioned near the cages that were transferred, but remained unchanged across time. This station was included to assess the impact of the two cage locations being in close proximity prior to the transfer, and to evaluate the subsequent recovery. Sampling at this location can provide an insight into whether the reduced impact due to the transfer of some of the cages affected the environmental conditions for the remaining cages in the vicinity. From these stations, benthic macrofaunal samples were collected by means of a Van Veen grab (0.025m2), sieved through a 0.5mm mesh size sieve and preserved in 4% formalin. Furthermore, three replicates of every sample were collected from each station. Several environmental variables were also measured . These include temperature, depth, Redox potential (Eh), Organic Matter (OM) with the Loss of ignition (LOI) test, granulometry and Chlorophyll-a .
Prior to the 2020 transfer, sediment redox potential values indicated hypoxic conditions at the impacted site, leading to low benthic diversity and poor ecological status. While there was some improvement in environmental conditions after the transfer, the ecological status at the old site remained moderate throughout the second year (2021), and the relocation of cages did not appear to have a significant effect. However, signs of recovery were observed after 2022 in both the old site and the station with unchanged cages, with a gradual reduction in organic matter, improvement in sediment redox potential values, and an increase in ecological status and benthic diversity. In contrast, pre-transfer, the new establishment area showed higher values of diversity and good ecological status, which was similar to the control station, but the new impact site’s environmental conditions deteriorated post-transfer due to increasing impact.
Based on the results of the study, it is concluded that the trans location of fish cages has a significant impact on the environmental conditions and benthic communities of the affected areas. It is evident that the relocation of cages can have both positive and negative effects on the environmental conditions and benthic communities of impacted areas. While dispersion of the cages can improve environmental conditions locally, relocation can also impact a new, undisturbed area. Further analysis is required to fully assess these effects and assess if the movement of cages improved in total the conditions of the fish farm area.
Overall, this study emphasizes the need for regular monitoring and evaluation of fish cage operations and their impact on the environment to ensure sustainable aquaculture practices as even a cages relocation event can significantly alter the biodiversity and environment of an area.
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Quero , G.M., Ape, F., Manini, E., Mirto, S., & Luna, G.M. (2020). Temporal Changes in Microbial Communities Beneath Fish Farm Sediments Are Related to Organic Enrichment and Fish Biomass Over a Production Cycle. Front. Mar. Sci. 7:524.
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