Aggregation of wild fish around fish farms
During the last decades, coastal aquaculture has expanded due to the development of fish farms using net pens or cages, which represent submerged, suspended or floating animal confinement systems. Fish farms are now common artificial elements in coastal ecosystems, from cold to tropical regions, producing over 2 233 982 tonnes of fish annually (FAO, 2007), across thousands of sites. Fish may aggregate at a wide variety of natural objects and structures (Fish Attraction Devide, FAD) for example the floating debris formed by an estuarine oceanographic front (Kingsford, 1990), but also to artificial structures, like oil platforms, fish attraction devices designed for fisheries or artificial reefs (Dempster and Taquet, 2004).
Coastal fish farms may affect wild fish populations by representing large FADs, but with higher food availability compared to traditional FADs. The pen nets provide a structure in the pelagic environment, and the unexploited portion of food pellets loosed through the cages probably enhances the attractive effect (Bjordal and Skar, 1992). Therefore, fish farms may influence the presence, abundance, residence times and diet of fishes in a given area and can, therefore, have important effect on local fisheries.
Several studies have determined the abundance, biomass and species diversity of wild fish around fish farms, as well as spatio-temporal persistence of the aggregation, which is important to understand how farms can act as pelagic FADs. Around 160 fish species have been described at the moment around farms, bellowing to 60 families (Sanchez-Jerez et al. in press). Remark that commercially important fish species are also attracted in the Mediterranean Sea (Fernández Jover et al., 2009), around the Canary Islands (Boyra et al., 2004), off Scotland (Carss, 1990), Norway (Bjordal and Skar, 1992), in Australia (Dempster et al., 2004) and Indonesia (Sudirman et al. 2009). Consequently, such concentration of target species (e.g. carangids, mugilids, and sparids) in a determinate area may affect local fisheries in several ways.
Fish aggregation also are describe around bivalve aquaculture rafts; e.g. Morrisey et al. (2006) describe a low variety of fish, consisting of only five species of predominantly demersal carnivores typical of shallow reefs and benthic sediment in the area, and one pelagic species at New Zealand longline mussel farms. An experimental study has determined the use of oyster-rearing structures as resting sites during day time for Solea solea (Laffargue et al, 2006).
Fish settle to a wide variety of natural environments, but also to artificial structures like docks, oil jetties (Rilov and Benayahu, 2000), oil platforms (Love et al. 1994), fish attraction devices (FADs; Sinopoli et al., 2007) and artificial reefs (Beets, 1989). Settlement is a relevant aspect of fish life history because may affects the magnitude of post—recruitment processes (Jones, 1990). The majority of small juvenile fish that associate with artificial habitats only do so for a certain period of their life history and as such, spawning periods are thought to regulate the appearance of these species around FADs. Regarding to aquaculture facilities, information on their role as settlement habitat is very scarce. For Mediterranean fish farms, Fernandez-Jover et al. (2009) showed that there is an important settlement of post-larvae (individual less than 1 cm), with twenty juvenile fish species settle at farms throughout the year.
Fish farms attract predators
Fish farms, due to the high concentration of wild and rear fish and the relatively sort distance to the coastal line, suffer the interaction of numerous predator species around the world. Fin fish, sharks and marine mammals are attracted to the farm structures, mainly as a consequence of the increase on foraging efficiency. In USA and Canada, the Californian sea lion Zalophus californianus, the harbour seal Phoca vitulina and Steller sea lion Eumatopias jubatus all interact with coastal fish farms by predating upon salmonids inside the cages and damaging netting in the process.
On the Atlantic coast, harbour seals and grey seals Halichoerus grypus cause similar problems (Nash et al., 2000). In Chile, aggregation of sea lions (Otaria flavescens) with salmon farms have been described (Sepulveda and Oliva, 2005). Otters have also produced conflicts with production in specific regions (e.g. Freitas et al. 2007) and sharks are a common cause of cage damage and loss of fish in tropical and sub-tropical areas and, exceptionally, great white shark has been detected around tuna fish farms in Mediterranean Sea. In Norway, seals, otters (Lutra lutra) and dogfish (Squalus acanthias) are attracted to salmon farms, especially when there are dead fish left in the cages.
Along the Mediterranean coast, marine mammals and birds occur in relatively low numbers around fish farms since natural and human-induced disturbances have greatly reduced Mediterranean populations. However, in spite of this reduced abundance compared with other geographical areas, predatory interactions with aquaculture occur; bottlenose dolphins Tursiops truncatus aggregate around farms along the coast of Italy (Díaz-López and Bernal-Shirai, 2007), with some deaths as a consequence of accidental net entanglements.
Monk seals Monachus monachus have attacked fish at several marine fish farms in the Turkish Aegean Sea resulting in damage to both the net cages and, on most occasions, fish also escaped as a result of the attacks (Güçlüsoy and Savas, 2003). Regarding to fin fish, assemblages of associated wild fish concentrated in large numbers attracts predatory species, such as Coryphaena hippurus, Seriola dumerili, Pomatomus saltator, Dentex dentex and Thunnus thynnus and other pelagic species of also commercial interest (Dempster et al. 2002).
The attraction of Pomatomus saltatrix (bluefish) to Mediterranean fish farms is or particular interest (Sanchez-Jerez et al. 2008) because is an aggressive predator and have some economical interest. In some farms this predator intrudes into the cages, where it may kill or harm large numbers of farm fish. This is a serious problem for the farmers in terms of economic loss and technical difficulties in the production process. Bluefish may hence use farms as a new and productive feeding habitat, which may be related to a reduction in trophic resources for these predators due to overfishing of their normal pelagic fish prey stocks. As bluefish are widely distributed, increased development of marine net pen farms in coastal and offshore areas will most likely also involve an increasing level of interaction between fish farm and predators.
Despite the attention given to the interaction of predators with aquaculture, there is evidence of positive or negative interactions with local fishermen. Cetaceans have been facing serious problems owing to fisheries in the last half century (Reeves and Leatherwood 1994) but the question do not appears to be increasingly perceived because the predator aggregation produce by fish farms.
A higher concentration of predator as dolphins or bluefish can result on economic distress for fishers, however there are not many studies about the conflict between fishers and predator in areas where aquaculture has been developed. Díaz-López (2005) wonder if the interaction between the gillnet fisheries and bottlenose dolphins in Sardinia (Italy) may be increasing in the vicinity of the farm because the fish targeted as prey by dolphins and by fishers overlap.
Positive effects on fisheries
Regarding to trophic resources, fish farms may actually represent habitats of even higher quality than traditional artificial reefs. Great amounts of artificial food with high energy levels are introduced into the environment which supplies straight food to aggregated fish because of waste feed (Vita et al., 2004). The increase of particulate organic matter around fish farms is also remarkable (Sara et al., 2004), and can be directly used as food by plankton feeders, as Sardinella aurita, or can stimulate the growth of fouling communities.
Potential trophic resources for aggregated fish, as amphipods, molluscs or polychaetes, occurred in high numbers and/or biomass at the fish farm fouling (Greene and Grizzle, 2006). Because most fish species show great flexibility in their trophic ecology (Dill, 1983), these new resources seem to be perhaps the most important driving factor for explaining the huge aggregation of fish around fish farms.
Resource utilisation from fish farms has been described by relating the amount of artificial and natural feed items consumed by the fish assemblages (Fernandez-Jover et al., 2007a, 2008; Dempster et al., 2009). Fish aggregated around farms take advantage of lost food but the use of pellets rather natural items depends on the fish species, ranging from 20 per cent for cod up to 80 per cent for bogue.
This availability of food drive that foraging efficiency at fish farms is higher than in natural areas as show the fact that the vacuity coefficients is lower at farms compared to control areas. The consumption of food pellets by aggregated fish should reflected changes on biological conditions because the different composition compared to natural resources. Because of that, farm-associated fish usually had a higher biological condition. This better biological condition can be transformed into somatic growth or increase the reproductive spawning.
Negative aspects of the use of aquafeed by wild fish
The increasing utilisation of vegetal oils in the production of aquafeeds is causing that cultivated fish reflect higher levels of terrestrial FA in their tissues. Considerable research efforts are being applied to assess the maximum degree of substitution of fish oil by vegetal oil in the diet of cultivated species without compromising fish growth or health status.
The effects of this substitution are variable depending on the plant and fish species and sometimes controversial. However marine fish require determined amounts of highly unsaturated fatty acids for optimal growth, health status, reproduction behaviour and successful larvae and development. The essential fatty acids for fish mainly are docosahexaenoic acid, eicosapentaenoic acid and arachidonic acid (Sargent et al. 1999b), and those fatty acids are limiting factors for optimal production.
Replacement of fish oil by a high proportion of vegetal oil (up to 60-65 per cent) seem do not affect survival, growth rate or the overall health status of gilthead sea bream (Díaz-López et al. 2009), sea bass (Montero et al. 2005) and salmon (Bransden et al. 2003). However, health status can be affected. The important role of dietary fatty acids in modulation of immune responses is something that has been studied cultivated fishes (Waagbo 1994, 2006, McKenzie, 2001, Bransden et al.2003). Essential fatty acids are the precursors for several biochemical components having important roles in inflammation and regulation of immunity (Calder 2005).
Change in dietary fatty acid composition has been shown to influence non-specific defence related to specific immunity and disease resistant in fish (Waagbo et al. 1993, Fracalossi & Lovell 1994). Egg quality and larval development are correlated with consumption of essential fatty acids. Several works have demonstrated that deficiencies in these fatty acids cause decreased growth, increased developmental abnormalities and higher mortality (Sargent et al. 1993).
In addition, changes in the FA profile of fish may have consequences for human consumption since wild fish with FA profiles modified by aquafeeds are forming a significant component of the catch of artisanal fisheries in SW Mediterranean, reaching local markets, as demonstrated by Arechavala et al. (in press). Wild bogue aggregated at fish farms and those non-aggregated but captured within the same bay from trammel-nets presented higher percentages of 16:1 w7, oleic, linoleic and o-linolenic acids than control samples caught by trawlers 10s of km away from the nearest farm.
In contrast, values of DHA, ARA and o3/o6 ratio were lower in samples from fish farms and artisanal trammel nets which work near farms due to the increased vulnerability of aggregated species. The FA composition of wild bogue captured by artisanal fishing gears were always more similar to farm aggregated than to control samples.
Fisheries and escapes from sea cages
Escapes of fish from sea-cages have been reported for almost all species presently cultured across Europe, including Atlantic salmon, Atlantic cod, rainbow trout, Arctic charr, halibut, sea bream, sea bass and meagre. In comparison to Atlantic salmon, knowledge of the extent and effect of escapes of Atlantic cod is very limited. Also for other culture species, like sea bream, sea bass and meagre, knowledge regarding identification of escaped fish and how escapes might affect to local fisheries is virtually non-existent (Dempster et al., 2007).
Because the location of most fish farms in areas close to wild fish habitats, as happen for Atlantic salmon and seabream, it is probable that escapees would mix with their wild con-specifics. Consequently, the potential exists for escapees to interact negatively with wild populations, through competition, transfer of diseases and pathogens and interbreeding (Thorstad et al., 2008).
This last aspect is important because usually rear fish are genetically divergent from wild population (Triantafyllidis, 2007). The present level of escapes is regarded by many as a problem for the future sustainability of sea-cage aquaculture as escapees can have detrimental genetic and ecological effects on populations of wild conspecifics. For Atlantic salmon, escaped salmon in the wild have lower fitness, as measured by survival and reproductive success, than wild salmon, showing phenotypic differences because artificial selection (Weir and Grant, 2005).
In the case of the escapes produce negative effects, local fisheries can suffer a reduction of catches because the decline of local wild fish stocks (Svåsand et al. 2007). However, in spite of this risk at long term, in a short term local fisheries can take advantage of accidental escapes or deliberate release of fish by restocking programs (Sanchez-Lamadrid, 2004). In Canary Island, sea bass population, which is considered a non indigenous species, is increasing in coastal areas near of fish farms, but its abundance depends strongly on distance from fish farms (Toledo et al., 2009).
This abundance distribution could mean that escaped individuals show site fidelity or mortality rates are high because sport and professional fisheries and population is highly dependent on escapees. Capture rates of cod escapees in the local commercial and recreational fisheries is high (approximately 40 per cent), indicating that local fisheries capture may be improved by escapees.
Effects on diseases and parasites
Diseases and parasites are a principal constraint to the development of many fish species because the increased of intensification and commercialisation of aquatic production. Therefore, the aquaculture industry has been overwhelmed with its share of diseases and problems caused by viruses, bacteria, fungi, parasites and other undiagnosed and emerging pathogens (Bondad-Reantaso et al., 2005) and the transmission of disease between wild animals and farm animals is investigated. The most studied example is the infection of sea lice from farmed salmon to wild salmon (Krkosek et al. 2005).
At other regions cross infections are not so clear. In SW Mediterranean, it has been found that reared sea bass and sea bream did not share macroparasites with farm-associated bogue and Mediterranean horse mackerel and that there was no effect on the total parasite community when associated wild fish were compared to their non-aggregated counterparts. However, a farm effect on parasite species was detected with modified abundances and prevalences while parasitising farm-associated B. boops and T. mediterraneus (Fernandez-Jover et al., 2010).
At some regions, a public debate on the adverse impact of mariculture cage farm activity on environment because the increase in the incident of infections and diseases in local fish population has been produced. For example, on the northern Red Sea coast of Israel, mariculture industry is was investigated for determining if or not the cages farms were a responsible factor for spreading diseases to the surrounding wild fish fauna (Diamant et al., 2007).
Therefore, it is important the implementation of tools for the prevention, control, and eradication of infectious disease and the preservation of human, animal, and environmental health. For example, finfish farmers generally use strategies such as cleaning and disinfecting, health inspections to ensure pathogen-free stock, and immunisations. Biosecurity must be an important part of day-today aquaculture operations as well as national and regional planning and regulation, with prevention of disease introduction as primary objective (Lee, 2003).
Offshore fish farming and local fisheries
From an integrated coastal zone management perspective, incorporation of aquaculture into coastal space, so that ecological holding capacities are not exceeded, requires ecological-based management of the interaction of ecological impacts of aquaculture with those of other coastal users, as fishers, particularly concerning habitat modification and species populations that are important to fisheries. Using economical models, an ecological effect of aquaculture on the wild fish population, affecting either the habitat’s carrying capacity for a fish stock or the intrinsic growth rate of that fish stock is predicted. But also aquaculture affects fishing operations, technically by affecting the catchability coefficient of the harvest function (Mikkelsen, 2007).
FADs have been exploited by fishermen in many areas and, in these sense, fish farm can be used as fishery enhancement tools (Kinsgford, 1999). The effect of attraction seems to be higher around farms than around traditional FADs due to the availability of food, with up to 2800 times more wild fish in their immediate vicinity compared to areas without farms. A single Mediterranean sea-cage farm covering a sea surface area of just one hectare may have up to 40 tons of wild fish beneath it (Dempster et al., 2004).
Machias et al. (2006) estimated the effects of fish-farming on local landings in five regions of the Aegean and Ionian Seas (eastern Mediterranean basin) using data from time series of landings, fish farming production, fishing fleet, temperature and rainfall (1984–2001). Results suggested that increased fish-farming activity in enclosed, oligotrophic areas could imply an increase in fisheries landings. Pilot projects, using specific types of gear, have been developed for combined fish farming and fishing at the same area (Akyol and Ertosluk, 2009), trying to integrated both activities and avoid conflicts.
In Spain, increased fishing pressure, both commercial and recreational around the farms has been noticed, particularly through the escalating deployment of gillnets and purse seines around farms, capturing large quantities of wild fish when they move away from the farm, seasonally migrate or escapees. From local fish markets, several captures have been identified as aggregated to fish farms using fatty acid profiles as indicator (Fernandez-Jover et al., 2007). Fish farms seem to act as marine protected areas, but in a smaller scale, exporting biomass of exploited species (Forcada et al., 2009).
Settlement of larvae around fish farm facilities is actually shown (Fernandez-Jover et al., 2009) but future studies are necessary to understand the capacity of these facilities to enhance reproduction of aggregated wild fish and promoting nursery areas. If restrictions on fishing are applied within farm leasehold areas, it has been suggest that coastal sea-cage fish farms may act as small (up to 160 000 m2) pelagic marine protected areas (MPAs; Dempster et al., 2006).
In conclusion, coastal aquaculture activity directly affects coastal wild fish population working as a super FAD, with a significant input of outer food and, indirectly, can impinge on the local fisheries at a scale of km’s. Therefore, sea-cage aquaculture should be taken into account in fisheries management as they may affect the natural production and recruitment of a range of species.