19/06/2026
For those regularly following our page, you will have seen a few weeks back we collected a live sample of carp from a fishery that had been losing some fish. (Go back to our post from May 28th if you want to see that!) A few of you asked us to give an update, and after speaking to the club who were happy for us to share the results, here they are!
In this instance it looks like the parasite Gyrodactylus sprostonae was the main issue. Below is an extract from the report which came back from our National Fish Lab with the full report details for those interested at the end of this post.
National Fisheries Laboratory Disease Diagnostics Report
Observations from histology support significant gill damage as a result of the overwhelming Gyrodactylus sprostonae infections. G.sprostonae has emerged in recent years as a cause of chronic losses in large specimen waters and can be a highly damaging pathogen of carp. The parasite is capable of rapid proliferation in weakened or stressed fish and will typically cause significant damage to gill tissue leading to lethargy, debilitation and mortality in badly affected fish. Whilst there are no glaringly obvious environmental triggers or fishery management related pressures at the fishery that are contributing to the debilitation of affected fish (and therefore allowing infections of G. sprostonae to proliferate), there are likely to be several environmental and physiological stressors at the fishery currently. With the extreme changes in air temperature around the time the losses were occurring, it is highly likely that there are significant fluctuations in water quality parameters (including water temperature, dissolved oxygen levels, PH etc.), and this could be a significant stressor, but cannot be confirmed. The DO was low on the morning of sampling (60% early morning) which may not be an issue alone but will have significant impacts in individuals with already compromised gill function due to the very heavy G. sprostonae infections.
So what is this prasite exactly?
G. sprostonae attaches firmly to the gills, where it feeds on the surface cells. Due to their tiny size, low level infections of monogeneans are seldom of concern and are easily tolerated by healthy fish. However, as infection levels increase, damage to the gills becomes more severe. In the mortalities we have investigated, some fish have harboured in excess of 100,000 parasites within the gills. This has caused severe gill pathology, resulting in respiratory distress, lethargy and mortality.
The lifecycle
G. sprostonae has a direct life cycle, giving birth to live young (left) that attach to the gills next to the mother. No other organisms are required for the parasite to complete its life cycle. At the time of birth, each parasite already has a juvenile within it, allowing rapid development of the population if conditions allow. Parasite reproduction is often temperature driven, increasing through spring and summer. When conditions allow, individual parasites may give birth every day and so infections can build up very quickly.
Thankfully the fishery didn’t lose too many fish in this instance. The lake is well run and isn’t overly stocked before you keyboard warriors jump at conclusions! We think a combination of spawning and fluctuating temperatures may have been the kick start in this instance. The club will continue to monitor and keep us in the loop of any changes.
Thanks to our national fish lab for the report, and to our analysis and reporting team for the algal analysis on this incident.
Remember, we have a fishery officer on call 24/7 so if you have any fishery incidents please call our incident line on 0800 807060.
Full report -
Diagnostic information and results
Full postmortem examinations including laboratory-based dissections, detailed microscopy, parasitology, virology, bacteriology and histopathology were conducted on the submitted fish.
Test Result Bacteriology
The kidney sample was negative, with no significant bacterial growth.
A lesion swab tested positive for Aeromonas sobria.
Virology
Negative for Koi Herpesvirus (KHV), Carp Edema Virus (CEV) and Spring Viraemia of Carp (SVC).
Histopathology
Significant changes were primarily noted in the gills and included increased presence of eosinophilic granular cells (EGCs; indicative of an immune response) and hyperplasia. These changes were considered to be associated with the very heavy Gyrodactylus sprostonae infection. Changes to other organs included presence of early nephrocalcinosis within the kidney (abnormal mineral deposition), and the o***y showed ova in a range of stages, and the presence of corpora atretica (degenerate ovarian follicles) and ‘compressed’ ova, suggesting that the fish had not spawned.
Algal Results
A water sample was taken by the Fisheries Duty Officer and examined by the Analysis and Reporting team at Brampton. There was a low abundance of colonial green algae and Ankyra sp. (a non-motile unicellular green algae).
External
The fish was generally in good condition externally, being a healthy size and weight. Localised colour loss and/or mottling to the flanks and opercular regions and ‘stretch mark’ type changes to both flanks were of note. Small lesions were identified along the ventral surface, but no obvious signs of ulceration were noted. A small area of abrasive damage to the right-hand side eye socket was present, and there were a number of scars that appeared to have originated from superficial scratches. Fins were considered in good condition, except for colour loss to their edges. The mouth was in good condition. Light infections of Ichthyobodo necator, Argulus sp., trichodinids (including Trichodina sp.), Gyrodactylus sp., and Piscicola geometra were present. Low to Moderate amounts of algae and detritus were also noted in the skin scrapes.
Gills
Gills were considered to be in poor condition, with changes including marked hyperplasia (excessive cell proliferation), including to the upper surfaces of lamellae leading to thickening and a white appearance. Diffuse haemorrhaging was noted across all lamellae, as well as congestion (abnormal flooding of vessels with blood) and focal haemorrhage. Low levels of lamellar deformity were observed, and considerable amounts of detritus were noted across all gill arches and in the gill tissue scrape.
Very heavy infections of Gyrodactylus sprostonae, light infections of Trichodinella sp., Dactylogyrus sp., and Argulus sp., were recorded.
Internal
High levels of connective tissue/adhesions were present within the body cavity. The o***y was large and appeared to be in poor condition and discoloured. The membrane was thickened and ovarian fluid discharged during measuring, evidencing that the fish had not spawned. The fish had not been feeding recently and there was evidence of haemorrhage and possible catarrhal enteritis in the gut. Light infections of Khawia sinensis were observed.
Age and Growth
The fish was aged at 13+. Growth was very good during the first 4 years but slowed to a more natural growth rate after. There was little evidence of any growth issues.
