Introduction
Pre-harvest mortalities in marine aquaculture result from the complex interplay of a broad range of factors that include stock source/genotype, developmental defects, predation and cannibalism, impaired nutrition, physical damage, sub-optimal/hostile environmental conditions and disease.
Economic losses accrue not only from mortalities but also from impacts on growth and food conversion, post-harvest downgrading or rejection of carcasses and derived products, fish escapes, management decisions that impact on profitability, e.g. protracted decisions to treat, grade or harvest, and the costs and effects of particular husbandry and management practices, e.g. fallowing, grading, vaccination, treatment and stock handling.
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Parasitic diseases attributable to obligate or opportunistic eukaryotic pathogens continue to have a major impact on global finfish and shellfish aquaculture, and in many regions they represent a key constraint to production, sustainability and economicviability.
Although robust data can often be generated concerning the general patterns of stock loss within a typical production cycle, obtaining accurate figures for the impacts of disease can be more problematic due to a number of considerations, which include production scale, available resources, difficulties in making rapid or accurate parasite identifications at the farm level and poor record keeping.
The frequent association of disease with other pre-disposing factors, such as poor water quality and the broad variety of precipitating events that may act to stress the farm population, also means that untangling the impacts of disease from those attributable to other causes may be difficult or impossible.
Thus, in many cases, the economic impact of parasitic diseases can only be estimated. Parasite-induced impacts in marine aquaculture can be divided into two broad categories: unpredictable/ sporadic and predictable/regular. For both, there may be costs in treating and managing infections once established, but for predictable infections there will also be costs associated with prophylactic treatment/ management.
For example, the management costs associated with controlling infections of caligid copepods, e.g. Lepeophtheirus salmonis and Caligus spp. in farmed Atlantic salmon (Salmo salar) can largely be predicted within a production cycle asthese parasites pose a perennial threat to captive reared stocks.
The infection dynamics of these species are well understood, and they can be controlled through the employment of an integrated pest management strategy (IPMS) involving the use of a broad range of management tools in addition to direct treatment intervention. The global Atlantic salmon production industry is well established (> 40 years) and can draw upon the long-term, shared experiences of parasite control and management that have led to the development of effective strategies to minimise mortalities, damage and loss of profit (see Frenzlet al. 2013, 2014).
For many other new or less-established industries, however, particularly those restricted to a small number of production sites, the parasite threats may be largely unknown and emerging, and new infections can have a devastating impact. The impact of Paramoeba perurans (syn. Neoparamoeba perurans), the causative pathogen of amoebic gill disease (AGD), on the early Tasmanian production of rainbow trout, Oncorynchus mykiss, serves as an appropriate example (Munday et al. 1990).
Many parasite infection events are complicated by the complex interplay of numerous factors making it difficult to calculate the precise costs attributable to the parasite. The Chilean crash in national salmon production from 385 086 tonnes (t) in 2006 to 230 678 t in 2010 (FAO FishStatJ, 2013), for example, appears to have been multifactorial with the key pathogens involved being ISAv (infectious salmon anaemia virus) and the caligid copepod Caligus rogercresseyi.
Major contributing factors included a large number of marine farms in production, the high stocking densities employed, a concentration of farms within a small area (*40% of the salmon production around Chiloé Island), a lack of biosecurity measures, weak disease surveillance, poor sanitary control and a failure to employ zone management (Ibieta et al. 2011).
Many farm sites were subject to infection from other disease agents, such as salmonid rickettsial septicaemia (SRS) caused by Piscirickettsia salmonis (see Olivares and Marshall, 2010), infectious pancreatic necrosis (IPN) caused by the pancreatic necrosis virus (IPNV) (Ibieta et al. 2011), P. perurans (see Bustos et al. 2011; Rozas, 2011) and rising C. rogercresseyi infections (Rozas and Asencio, 2007), which were suggested to predispose salmon stocks to the ISAv infection.
By the end of 2008, 105 Chilean sites were confirmed as ISAv positive with a further 44 suspect sites; a quarter of the positive sites were owned by a single company who declared losses of US$ 81·2 billion for the second half of 2007 (Marine Harvest, 2007). As can be seen from the above, the calculation of the proportion of this loss that could be deemed to be attributable to C. rogercresseyi is not possible.
Although immediate losses to production can often be estimated, it is usually difficult to calculate thefull magnitude of the downstream socio-economic effects of major disease events on the livelihoods and associated industries centred around primary producers.
While insurance claims may provide some guidance as to losses, these may be overinflated, based on ‘best price’ or on an estimated loss of trade/ income. In addition, the costs of remedial action (e.g. treatment, disposal and monitoring) and/or changes to management practices and infrastructure also need to be considered.
The resilience of the Chilean salmonid aquaculture industry, which rapidly implemented improved infectious disease control measures and was able to fall back on well-established coho salmon, Oncorynchus kisutch, and rainbow trout industries (24 and 38% of national salmonid production in 2011, respectively), arguably minimised the full potential economic impact of the 2007 crisis (Alvial et al. 2012a, b).
Although there are a number of studies that have attempted to estimate the full economic consequences of parasite infections, both through the documentation of a specific disease event (Roberts et al. 1994; Torgerson and MacPherson, 2011; Charlieret al. 2012) and through the estimation of the potential impact of disease introduction (Paisley et al. 1999; Riddington et al. 2006; Voort van deret al. 2013), the data and resources required to undertake such studies generally preclude the accurate estimation of the cost of parasite-associated impacts.
For example, teasing out the role and the precise economic impact attributable to parasitic agents, e.g. P. perurans and C. rogercresseyi, from that due to the other contributing factors leading to the Chilean 2007 crisis, is a near-impossible task, and therefore, the costs can only be speculated upon.
For this reason, the economic impact of these parasites in the Chilean 2007 crisis, is not included in this summary. In this review, we provide an overview of the world’s major marine and brackish aquaculture production industries and assess the impacts of the major parasite species that affect production or otherwise impose an economic cost. As discussed above, it is not possible to provide a comprehensive review of all parasite-related losses in aquaculture, but we provide estimates for some of the more serious loss-related events and provide brief details relating to each.
For each event resulting in a notable loss, i.e. either mortality or deviation from projected revenue, we cite either figures given in the original publication or have applied a simple formula to determine the likely loss to the stock only at the point in the production cycle when the disease event occurred. In the absence of details within the original report, many of the costs provided here are assumptionbased and so a degree of caution should be exercised.
It is anticipated that this study will prove informative for risk assessment by new aquaculture-based enterprises and will aid an appreciation of the sporadic nature and impact of some parasite-induced infections, and can assist in the development of more robust risk assessments and biosecurity practices, which can help mitigate against and/or minimise the potential impacts of parasite-mediated disease in aquaculture.
January 2015
