Biodiversity Disease Threat to European Fish

The study examines the ecological impact of the invasive Asian fish Pseudorasbora parva on the native European fish Leucaspius delineatus.
The focus is on how P. parva might carry a deadly infectious pathogen that inhibits spawning and affects mortality rates in L. delineatus, posing a threat to biodiversity.

Infection Prevalence:
- In moribund or dead L. delineatus, the prevalence of the pathogen was 67% (sample size, n=12).
- Subclinical infections in the treatment group showed a lower prevalence of 28% (n=32).
- Other cyprinids, such as Pimephales promelas (fathead minnow), were also found to be susceptible with a prevalence of 20% (n=5).
- No Pseudorasbora parva specimens (n=10) tested positive for the pathogen, likely due to low concentrations in healthy carriers.
Research Methodology:
- Cohabitation studies were used to detect carrier states in fish for various pathogens, demonstrating they are reliable for detecting healthy carriers.

The introduction of P. parva, a non-native species, has significant implications:
- Threat to Aquaculture Trade: It serves as a healthy host for a non-specific pathogen, threatening the aquaculture trade, particularly for salmonids.
- Challenges in Identifying Carriers: There is difficulty in identifying fish populations that serve as reservoirs for pathogens.
- Biodiversity Threats: The pathogen may threaten the conservation of European fish diversity, particularly for endemic species like L. delineatus.

Malicious Effects of Introduced Species:
- The introduction of new species is known to have unintended negative consequences, as demonstrated by the interaction between P. parva and L. delineatus.
Species On the Virulent List:
- L. delineatus: Only representative of its genus and a unique nest-guarding fish among European cyprinids.
- Once widespread, now critically endangered, especially noted by its decline over the past forty years.
- P. parva: Introduced in 1960 in Romania, rapidly spreading throughout Europe and correlated with the extinction of L. delineatus in many locales.

Laboratory Findings:
- P. parva water inhibited spawning of L. delineatus completely: 0 eggs produced in P. parva water versus 1,596±840 eggs in clean water control.
- Increased mortality rates: 69.3% mortality in L. delineatus exposed to P. parva versus 16.2% in controls, statistically significant at P<0.05 using Mann-Whitney U-test across 4 experiments.
Natural Pond Observations:
- A noted decline of 96% in L. delineatus populations in a natural pond over three spawning seasons (2002-2004).
- Noteworthy increase of 13% in population immediately before P. parva introduction in 2001.

The pathogen affecting L. delineatus is characterized as an intracellular eukaryotic organism, similar to Sphaerothecum destruens, known to infect salmon species
Infection Evidence:
- Histological examination of moribund L. delineatus revealed extensive infection in visceral organs and reproductive tissues with the intracellular pathogen.
- DNA analysis confirmed the presence of the pathogen in exposed L. delineatus and not in control groups, using polymerase chain reaction to amplify a specific segment of ribosomal DNA from S. destruens.

Figure 1:
- Population estimates of L. delineatus from 2001 to 2004, showing a clear decline after the introduction of P. parva.
- a) Population estimates displayed as red bars (showing standard error of mean):
- 1,200 specimens in April 2001.
- Decrease to 600 specimens by April 2002 after mixing with P. parva.
- b) High-power light microscopy images of L. delineatus liver sections showing rosette-like agents.
- Slide stained with hematoxylin and eosin, scale bar at 50 micrometers.

Notable references include works by Faraday, Chladni, and Rayleigh cited throughout the study, as well as relevant ecological and biological literature regarding fish diseases, biodiversity, and invasive species.