Estimating the true impact of each parasite event, however, is complicated as costs may be affected by a diverse assortment of environmental and management factors and can range from direct losses in production to the more indirect costs of longer term control and management of infections and the wider, downstream socio-economic impacts on livelihoods and satellite industries associated with the primary producer.
Commonly encountered parasites of aquatic animals. Far left - Caligus elongatus. Middle top – The attachment organ of the monogenean Gyrodactylus salaris. Top right – A ciliate protozoan Trichodina sp. Second row middle – Anisakis nematodes. Second row right – The eggs of the turbellarian Bdelloura candida on the gills of the American horseshoe crab, Limulus polyphemus. Third row – Gyrodactylus salaris on the fin of an Atlantic salmon, Salmo salar. Bottom middle – The sessile peritrich Apiosoma sp. on the skin of a freshwater fish. Bottom right – The monogenean Dictyocotyle coeliaca from the body cavity of a ray.
Certain parasite infections may be predictable as they occur regularly, whilst others are unpredictable as they arise sporadically. In each case, 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.
In Table 1, for example, we provide some estimates of economic loss associated with some notable protistan and metazoan parasite events on some of the world’s leading finfish production industries.
The figures provided in Table 1 are extracted from a larger study in which the potential economic costs attributable to a range of key parasite pathogens using 498 specific events are detailed (see Shinn et al., 2015).
A significant proportion of stock losses, however, occur within the hatchery/nursery phases of production and in many industries these are factored into and accepted as part of normal operational practices.
Such fatalistic acceptance of their inevitability means that they are frequently under reported, hiding the severity and impact of certain parasites (e.g. omycete species belonging to the genera Aphanomyces and Saprolegnia; the dinoflagellates Amyloodinium ocellatum and Piscinoodinium pillulare, etc; ciliate protozoans such as Trichodina spp., Ichthyophthirius multifiliis and Cryptocaryon irritans, etc; and species belonging to the monogenean genera Gyrodactylus and Dactylogyrus).
In this article, we begin to estimate the global costs of parasitism by following production at four Thai commercial Oreochromis niloticus farms over the course of 12 months to determine average mortality rates in the earlier stages of production.
The data are presented in Figure 1, and from these the following survival rates can be determined: egg production (i.e., successful 77.5% hatch rate), swim-up fry (i.e., 77.8% survival from hatched eggs; 60.85% survival of starting egg number [s.e.n]), 21 day post-monosex fry (i.e., 78.9% survival from swim-up fry; 48.0% survival of s.e.n) and in one inch nursery sized fish (i.e., 83.3% survival from 21 d monosex stage; 40.0% survival of s.e.n).
Hatchery-based losses were then calculated using local production costs (i.e., 0.1 THB for each egg to swim-up stage; 0.2 THB for each swim-up to monosex fry; 0.3 THB for each monosex to nursery sized fish) and by assuming that 20% of the mortalities can be directly attributed to parasitic infection.
Given the broad diversity of aquaculture, food finfish species (c. 267 species/categories listed by FAO) and the vast spectrum of parasites that may impact on their production, it is almost impossible to ascribe a single value that captures all the losses induced by parasite activity in each industry.
Likewise, despite continuous health monitoring by on-site diagnosticians, it is technically impossible to determine the cause of mortality of every fish on site. From the figures provided above and in Figure 1, for example, c. 1.2 M 21-day post-monosex fry are lost each month (i.e., 40,000 day-1).
From, a parallel study conducted by some of the current authors, it would appear that parasites account for an annual loss of between 5·8 to 16·5% (i.e., US$ 62–175 M; assuming £1 =US$ 1·6) of the value of UK aquaculture production (across all species reared in freshwater, brackish and marine systems).
To begin moving towards an estimate of loss attributable to parasitism, a figure of 20% is applied here to estimate stage specific losses due to parasites within the hatchery phase of production and this is based on this latter study on aquaculture activities in the UK. It is important to stress, however, that this is not 20% of global harvest production.
In 2013, for which the last complete figures are available from FAO, the global production of finfish through aquaculture was 47.07 M tons. If we assume an average sale weight of harvest-sized fish of 0.4–0.5 kg fish-1, then the total number of harvest fish sold can be estimated at between 94.14 to 117.68 billion fish yr-1.
If this figure is adjusted by assuming a 10% loss of fish between nursery and harvest, then the number of post-nursery fish can be estimated at between 103.55 to 129.44 billion fish yr-1. If the same percentages of parasite-induced loss are applied for each stage of finfish hatchery production and assuming that US$ 1 = 32.16 THB, then the global loss of juvenile fish can be estimated at between US$ 107.31 M and 134.14 M per annum.
Using the annual production of all farmed tilapia species for 2013, for example, which was 4.822 M tons, we can estimate that there were 9.7–13.4 billion post-nursery fish produced and that the losses of juvenile tilapia to parasitic infection were US$ 4.84–6.66 M at the nursery stage, US$ 5.84–8.02 M at the monosex stage and US$ 5.13–7.05 M at the swim-up stage.
However, these estimates are for the direct losses due to parasitic infections and do not account for the role that parasites may play in facilitating secondary infections and the resultant losses.
Considering post-nursery losses, the total production in 2013 was 40.50 M tons of freshwater fish valued at US$ 1,641 t-1 fish and 6.57 M tons of brackish and marine fish valued at US$ 4,203 t-1. If we assume that parasites are responsible for the loss of between 1–10% of harvest-sized fish, then the value of these fish can be estimated at US$ 945 M (1%), US$ 2,362 M (2.5%), US$ 4,724 M (5%) and US$ 9,448 M (10%).
If the hatchery and grow-out figures are combined, then the global cost of parasites in finfish aquaculture can be loosely and tentatively estimated at between US$ 1,052 M and US$ 9,582 M p.a.
Moving towards an accurate estimation of the global cost of parasite-associated impacts, however, is dependent on detailed, high quality data and the resources necessary to undertake such studies.
However, as global aquaculture continues to grow and intensify, the prevalence and severity of parasite infections will similarly rise as will the attendant economic costs of parasitism. In addition, the increased trade in finfish and their products may facilitate the spread of parasites into new environments, while changing climatic conditions will place increased pressure (with consequential impacts) on aquaculture systems, current aquaculture production practices, and interactions between wild and farmed aquatic stocks, parasite life-cycles and transmission pathways.
Shinn, A.P., Pratoomyot, J., Bron, J.E., Paladini, G., Brooker, E.E. & Brooker, A.J. (2014) Economic costs of protistan and metazoan parasites to global mariculture. Parasitology, 142, 196-270. doi:10.1017/S0031182014001437.
Contact the Author:
Email: andy.shinn@fishvetgroup.com; Tel: +66 92 360 9119
