Influence of tuna penning activities on soft bottom macrofaunal assemblages

Abstract

Aquaculture is an important food-producing industry that has often been criticised because of its potential adverse influence on water quality and benthic habitats present in the vicinity of a fish farm. A lucrative sector of the aquaculture industry is Atlantic Bluefin Tuna (ABT) ranching. The main source of pollution of the benthic environment at tuna farms is the uneaten feed-fish which accumulates on the seabed below the tuna pens, but the potential influence of ABT farming is expected to differ from those of other fish farm types such as those rearing sea bass and sea bream, because of the use of feed-fish instead of formulated feed and the large size of the farmed fish. Furthermore, differences in the characteristics of the tuna farms and of the receiving environment may result in varying levels of impact, if present. The present study investigated the influence of tuna penning activities on macrofaunal assemblages of the soft sediment habitat present in the vicinity of the fish pens. Grab samples for sediment physico-chemical attributes; namely mean sediment grain size (MSGS), and percent organic carbon content (POCC) and percent organic nitrogen content (PONC) in the sediment; and for macrofaunal studies, were collected from three tuna farms located c. 1 km off the northeastern to southeastern coast of Malta, at incremental distances from the sea cages (i.e., c. 0 m, 100 m, 1 km, and 2 km away) before initiation of the farming activities, and thereafter at six-monthly or annual intervals, over a period of ten years. The following study aspects were considered: (i) influence of the northeastern farm during its first year of operation on benthic habitat; (ii) use of polychaete, mollusc, amphipod and decapod taxocenes as indicators of the influence of ABT penning on macrobenthic assemblages; (iii) differences in the magnitude and spatial extent of influence of the three ABT farms that differed in size, stocking density, and location, on benthic habitat; (iv) spatial pattern in attributes of the macrofaunal assemblages present in the vicinity of a farm with incremental distance from the tuna pens; (v) suitability of benthic biotic indices (BBIs) AMBI, BENTIX, BOPA, BOPA-Fish farming (BOPA-FF) and M-AMBI, for monitoring the environmental impact of tuna farming; and (vi) temporal patterns in macrofaunal assemblages in the vicinity of three tuna farms over a ten-year period. Results from the study of the northeastern farm during its first year of operation indicated significantly elevated sediment POCC and PONC, and (albeit not significantly) higher abundance of capitellid polychaetes in the vicinity of the tuna cages, where uneaten feed-fish had accumulated on the seabed. The changes in benthic habitat were conspicuous in autumn towards the end of the tuna penning season, but some benthic recovery was observed after the fallow period. Of the considered taxoenes, polychaetes and amphipods appeared to be good benthic biotic indicators of the impact of tuna penning on macroinvertebrate assemblages. Results from the third study aspect indicated a higher magnitude of influence at the northeastern farm - the largest farm in terms of holding capacity - compared with the two southeastern farms, but a wider spatial extent of impact (1-2 km) was evident at one of the southeastern tuna farms. The spatial pattern in benthic macrofaunal assemblages was characterised by a high impact area directly below the cages, while a significant peak in diversity 100 m away from the cages was observed at only one of the investigated tuna farms. Of the considered BBIs, the BOPA-FF and M-AMBI indices appeared more sensitive to the environmental influence of tuna penning, but variation in Ecological Quality Status (EQS) assignment among BBIs showed the importance of including multivariate data analyses that are traditionally used in aquaculture environmental impact monitoring studies. Results from the sixth study aspect showed that the benthic EQS changed from ‘Bad’ and ‘Poor’ to ‘Good’/‘High’ categorisations at the northeastern farm after the first years of operation, but ‘Moderate’ EQS at the two southeastern farms towards the end of the study period was indicative of a ‘press’ disturbance. It was concluded that the seasonal nature of ABT penning and often offshore location of the farms, together with reduction of feed wastage, can mitigate the potential adverse benthic influence of these activities, while multiple tuna farms located close to one another result in added loading on the marine environment, hence highlighting the importance of good spatial planning for coastal aquaculture activities. The high spatio-temporal variation in the influence of tuna penning on benthic macrofaunal assemblages in the vicinity of a farm showed the importance of including multiple impacted and reference areas, as well as replicated sampling times in environmental monitoring of tuna farms. The overall findings are discussed in light of: (i) current knowledge on the influence of aquaculture, in particular ABT ranching, on soft bottom macrofauna present in the vicinity of the activity; and (ii) implications for environmental monitoring and mitigation strategies of tuna penning activities in the Mediterranean, and, in a more local context, the Maltese Islands. Finally, proposals are made for potential further research on aspects of the environmental effects of tuna penning.