Impact of fish and pearl farming on the benthic environments in Gokasho Bay: Evaluation from seasonal fluctuations of the macrobenthos

ABSTRACT: In order to clarify the influence of mariculture on the benthic fauna, samples of the macrobenthos were collected from Gokasho Bay, where intensive fish culture and pearl oyster culture have been carried out. Monthly samples collected from the fish farm and pearl farm sites during June 1995 to July 1996 revealed that the community structure of the two sites showed distinct differences with seasonal fluctuations. At the fish farm site, azoic conditions were found from July to November; after December, the diversity increased markedly through successive recruitments of small-sized species such as the polychaetes Capitella sp. and Pseudopolydora paucibranchiata , and the amphipods Aoroides spp.; macrofaunal density, biomass and species richness peaked from March to April. At the pearl farm site, a higher diversity, including larger-sized species, and no clear seasonal fluctuations in abundance was found, and the community structure was similar to that at the control site. These results show the large impact by fish farming on the macrofauna, whereas pearl farming causes less effect on the benthic fauna. It is suggested that the difference in the level of organic input between the two sites results in the differences in the dissolved oxygen content of the bottom water, sulfide content of the sediments and, subsequently, the macrobenthic assemblages.     

Effects of fish farming on nutrients and benthic community structure in the Eastern Aegean (Turkey)

The impact of a sea bass (Dicentrarchus labrax L., 1758) farm on water quality and benthic community structure was investigated at a fish farm site in Engeceli Bay (western part of Izmir Bay) between April 2001 and February 2002. The characteristics of the water column in the fish farm were investigated in terms of physical and chemical parameters. Concentrations of nitrate, phosphate and ammonium ions in all sampling stations within the Bay were compared with the water quality parameters measured at the outer part of Izmir Bay (Eastern Aegean Sea). While there were significant differences from season to season of the levels of ammonium, orthophosphate, total phosphate and nitrite, there were no significant differences among stations. However, nitrate values and organic carbon contents showed significant differences among sampling stations. A reference station was selected to compare benthic groups. While the reference site is characterized by a sandy bottom with the lowest carbon and highest diversity values, the other stations with silty‐clay bottoms showed higher organic enrichment and lower diversity with increasing abundance of polychaeta. Organic enrichment and particle size of sediments were closely associated with faunal groups particularly with polychaeta and mollusca.     

The effect of demand feeding on swimming speed and feeding responses in Atlantic salmon Salmo salar L., gilthead sea bream Sparus aurata L. and European sea bass Dicentrarchus labrax L. in sea cages

Abstract Using underwater cameras, data were collected on the feeding behaviour and swimming speeds of Atlantic salmon Salmo salar L., gilthead sea bream Sparus aurata L. and European sea bass Dicentrarchus labrax L. in sea cages. Comparisons were made between the behaviours of fish fed on demand using interactive feedback systems and those of fish fed under the standard feeding practice of each farm (control). In all three species, swimming speeds were similar before feeding , but they were significantly higher in the control regimes during feeding. When fed on demand, sea bass had reduced swimming speed just before and during feeding compared with that observed during the non‐feeding periods. Higher proportions of feeding fish were observed in the control regime cages than in fish fed on demand for all three species, indicating a greater feeding intensity during meals in the control regimes. This was further supported by observations of an increase in the density of sea bass in the upper water in the control cages during feeding. The results suggest decreased levels of competition between the on demand‐fed fish during feeding, which might be hypothesized to lead to improved growth and production efficiency in aquaculture.     

Polychaetes as indicators of marine fish farm organic enrichment

Marine fish farms cause environmental disturbance of sediment due to uneaten food and fish faeces that accumulate on the sea bottom. These products give rise to organic enrichment followed by degradation processes; reduction of level of dissolved oxygen in the bottom water; generation of high levels of sulfides; changes of benthic assemblages and even azoic conditions. The soft bottom benthic communities and, above all, the polychaetes are considered good indicators of environmental perturbations considering their relatively fast changes in diversity and abundance and because their trophic flexibility and life-history traits are considered a pre-adaptation to the condition of disturbed habitats. The aim of the present study is to evaluate the polychaete population changes as a result of organic enrichment produced by a marine fish farm. The study was carried out on a fish farm located in the coastal water of Porto Ercole (Tyrrhenian Sea) with two submersible sea cages and a year production of 180 tonnes. The macrobenthos samples were collected during four campaigns (July and December 2001, May and October 2002). The results showed: 1) a rapid change of the assemblage located beneath the cages related to the fish biomass reared and a high dominance of the opportunistic Capitella spp.; 2) limited sediment disturbance close to cages (20–50 m); 3) no effect in the area 50 m beyond the cages. Two months after the fish harvest and thus reduction of organic enrichment, perturbated assemblages showed signals of recovery like species richness increase and reduction of opportunistic species.     

Nile tilapia (Oreochromis niloticus) invasions disrupt the functional patterns of fish community in a large subtropical river in China

Although non‐native species can modify ecosystem function, their effects in southern China have not been investigated, despite their ecological importance. This study assessed how Nile tilapia Oreochromis niloticus (L.) invasions impact the functional patterns of fish communities in the large subtropical Pearl River by analysing the relationships between invasion and body size over a 9‐year period. The relative abundance of Nile tilapia increased significantly over time. In particular, the established Nile tilapia significantly undermined the relative densities and the body size of the native fishes. Importantly, this study provided empirical evidence that functional indices (such as body size) are more sensitive when evaluating the effects of non‐native species on a community than richness. Understanding processes such as those should be the basis of controlling alien fish species and fisheries management in the Pearl River.     

Assessment of rocky reef fish assemblages close to seaweed farming

The seaweed Kappaphycus alvarezii (Doty) Doty ex P. C. Silva, a red algal species, is the main global source of Kappa carrageenan. The introduction of such exotic species in regions outside their original locale can change the community structure of the areas into which they are introduced. The possible influence of seaweed farming on the rocky reef fish assemblage was assessed in Paraty, Brazil. The reef fish assemblage in the seaweed farming area was compared before and after the commencement of farming with two undisturbed control areas. Among the ten species ranked as the most frequently occurring in each area, eight were the same. The fish community structure close to the seaweed farming area did not change from that of the control areas, over the months of the study. Neither the fish diversity and richness indices, nor the total average abundance among areas was significantly altered. The average abundance of the different trophic groups varied over time in both the farm and control areas, revealing similar patterns. These results suggest that K. alvarezii can be cultivated up to at least 50 m from a rocky coast without altering the fish community structure of the surrounding waters.     

Assessing long‐term fish responses and short‐term solutions to flow regulation in a dryland river basin

Water resource development and non‐native species have been cited as primary drivers associated with the decline of native fishes in dryland rivers. To explore this topic, long‐term trends in the fish community composition of the Bill Williams River basin were studied over a 30‐year period (Arizona, USA). We sampled 31 sites throughout the basin that were included in fish surveys by Arizona Game and Fish in 1994–97 and the Bureau of Land Management in 1979–80. We found that non‐native species have proliferated throughout the entire basin, with greater densities in the lower elevations. Native species have persisted throughout most of the major river segments, but have experienced significant declines in frequency of occurrence and abundance in areas also containing high abundances of non‐native species. Next, we assessed the short‐term response of the fish assemblage to an experimental flood event from the system's only dam (i.e. Alamo Dam). In response to the flood, we observed a short‐term reduction in the abundance of non‐native species in sites close to the dam, but the fish assemblage returned to its preflood composition within 8 days of the event, with the exception of small‐bodied fish, which sustained lower postflood densities. Our findings demonstrate the importance of natural flow regime on the balance of native and non‐native species at the basin scale within dryland rivers and highlight minimal effects on non‐native fishes in response to short duration flood releases below dams.     

Associations of fish with various types of littoral habitats in reservoirs

Fish associations with different types of littoral habitats were studied in four canyon‐shaped reservoirs in the Czech Republic in years 2010 and 2011 by gillnets. Two to three habitats per reservoir–beaches (former meadows), stump fields (former forest) and rubble slopes–were defined and sampled along the longitudinal axis of reservoirs. Effects of reservoir, habitat and locality (position along longitudinal axis) on fish biomass, abundance and species structure were tested for juvenile and adult fish separately. Hierarchical analysis of variance revealed that habitats differed significantly in fish biomass and abundance. Redundancy analysis showed that analysed environmental variables had significant influence on fish community structure. Most variability in community structure was explained by reservoir and then by combination of habitat and slope steepness. Locality position had the smallest influence on community structure. For both adult and juvenile fish total abundance and biomass, the most inhabited habitat was beaches; rubble slopes were the least inhabited. Habitat associations differed among species. Among adults, bream Abramis brama, white bream Blicca bjoerkna and roach Rutilus rutilus were associated with beaches and stump fields, whereas perch Perca fluviatilis, ruffe Gymnocephalus cernuus, asp Aspius aspius and pike Esox lucius were associated with rubble slopes. Bream, white bream, bleak Alburnus alburnus, roach, ruffe and pikeperch Sander lucioperca were associated with beaches among juveniles, whereas the only juvenile associated with rubble slopes was perch. We showed that most common species are associated with distinct habitats and also that utilisation of various littoral habitats differs in general.     

Response of different benthic habitats to off‐shore fish cages

Off‐shore fish farming can increase the organic load of nearby coastal marine ecosystems due to the deposition of fish food and faeces on seabeds. Seagrass meadows are particularly affected by aquaculture activities but there are few empirical data showing differential effects of the same farming activity on multiple habitat types. Here, we assessed over a 2‐year period whether macrofaunal assemblages inhabiting sandy bare seabeds and Cymodocea nodosa meadows varied in their macrofaunal community structure to the fish farming activities. We observed high spatial and temporal variability in macrofauna composition and dynamics among seabed habitats and a limited impact of fish cages in their area of influence as compared with control areas. Seagrass meadows showed a higher abundance in macrofauna communities than sandy bare bottoms. Local marine currents could partially explain some results because of their influence on grain size composition. Differences in grain size resulted in higher abundances of the tanaid Apseudes talpa beneath fish cages and the absence of the sensitive amphipod Ampelisca brevicornis. Differences of resilience of seabeds (seagrass meadows and sandy bare bottoms) should be taken into account for environmental monitoring studies of off‐shore fish cages. Our results suggest that hydrodynamics are a key factor to determine buffer areas between fish cages and seagrass meadows.