engleski

Uncommon behaviour of some benthic animals below tuna farms in the Adriatic – a response to organic enrichment

Tuna, Thunnus thynnus (L.), mariculture has notable environmental impact on benthos below the cages. In the oligotrophic Adriatic Sea we noted change of composition and abundance of benthic megafauna due to the increased sedimentation of particulate organic matter (parts of tuna faeces and excessive food, mostly defrosted sardines and anchovies). As a result aggregation of scavengers and opportunists were mostly present. During all-season, ten year monitoring of ten tuna farms in the Croatian Adriatic we sometimes noticed uncommon behaviour of some benthic animals. At occasions numerous adult opisthobranchiate gastropods were present: Aplysia depilans Gmelin, Pleurobranchus testudinarius Cantraine, Pleurobranchaea meckelii (Blainville) and Cratena peregrina Gmelin. A. depilans, P. meckelii and C. peregrina were seen copulating and aggregates of eggs of A. depilans, P. testudinarius and C. peregrina were noted. Seastars Astropecten aranciacus (Linnaeus) and sea anemone Calliactis parasitica (Couch), living in symbiosis with Dardanus arrosor (Herbst) were seen swallowing the whole sardines (leftover of tuna food). Hermit crabs, Pagurus prideaux Leach, were seen in the lasting aggregations, more that 500 individuals per m2. Sea urchins Psammechinus microtuberculatus (Deblainville) Heller were seen in the aggregations over more then 100 individuals one over the other grazing on Ulva sp. on depths over 30 m. Holothurians Holothuria tubulosa Gmelin were seen with Ulva growing on its dorsal side. Urchins Cidaris cidaris (Linnaeus) were seen feeding on the surfaces covered with Beggiatoa. The mentioned observations are not usually common to see in surrounding sea and indicate ability of noted animals to adapt to changed environmental conditions.

Tuna fish mariculture; environmental impact; benthic organisms

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