1. Introduction

The Arctic char, a historical inhabitant of Lake Geneva, faces a decline in population and catch rates. This study investigates various hypotheses, including inadequate fish management, reduced reproductive success, increased interspecific competition, reduced immunoresistance, and the emergence of Bacterial Kidney Disease (BKD). The study focuses on the last hypothesis, given BKD’s potential impact on fish mortality [1, 2].

While the correlation between declining char catches and population reduction is not direct, the study considers BKD as a potential factor, especially amid the observed increase in catches during the 1990s. BKD’s role in inducing stress, particularly during reproduction, is explored, suggesting a possible regulatory function depending on population density [3, 4].

BKD, caused by Renibacterium salmoninarum (RS), affects salmonids but not other fish families. The disease can induce kidney dysfunction, limiting osmoregulation and causing mortality, especially in smolts. The study aims to answer questions regarding BKD’s presence, proportion, and its potential to induce significant mortality in the wild char population [5].

2. Material and Methods

Laboratory Detection Methods

The study evaluates three diagnostic methods for BRD [6]:

• selective agar (SKDM)
• non-selective agar (KDM)
• immunohistology using monoclonal antibodies

Immunohistology proves effective in diagnosing BKD.

Sampling Campaigns

Two sampling campaigns are conducted in 2004 and 2005, involving 331 and 142 Arctic char, respectively. Clinical signs and immunohistological analysis reveal a low prevalence of BKD (0.6 percent in 2004, 0 percent in 2005) [7].

Laboratory Analysis

Histological sections of kidney samples from potentially infected fish reveal BKD presence in two individuals (0.6 percent infection rate) [8].

3. Results

Laboratory Detection Results

The bacterial strain proves difficult to cultivate, but immunohistology demonstrates BKD presence effectively.

Sampling Campaign Results

While some individuals show clinical signs, only two cases (0.6 percent) confirm BKD infection in 2004. No clinical signs are observed in 2005, indicating a minimal infection rate.

4. Discussion

Presence of BKD

BKD is confirmed in Lake Geneva, but the extremely low infection rate suggests it is not the primary cause of the observed decline in char population.

Other Hypotheses

The study rejects the hypothesis that BKD is the main contributor to the decline. Further investigations into fish management and pike predation are suggested to understand the ongoing decline in char catches [9].

References

1. BANNERCR, ROHOVECJS, FRYERJL.1983. Renibacterium salmoninarum as a cause of mortality among Chinook salmon in salt water.
2. Journal of the World Mariculture Society, 14: 236-239.
3. BRUNOD.W., M.UNROALS.1986. Observations on Renibacterium salmoninarum and the salmonid egg. Diseases of Aquatic Organisms, 1:83-87.
4. VSHAMBERSE.2004. An investigation into the prevalence and interaction of BKD in wild and farmed fish in selected river systems in the UK. Trout News, 38:20.
5. EVELYNTPT, PROSPERI-PORTAL,KETCHESONI.1986. Experimental intraovum infection of salmonid eggs with Renibacterium salmoninarum and vertical transmission of the pathogen with sucheggs despite their treatment with erythromycin. Diseases of Aquatic Organisms, 1:197-202.
6. FORELF.A.. 1904. The origin of the ichthyological fauna of Lake Geneva. In: F Red(ed.) Lake Geneva, limnological monograph,
7. Flight. III, pp. 343-354, Lausanne.
8. FRYERJL, SANDERSI.1981. Bacterial Kidney disease of salmonid fish. Annual Review of Microbiology, 35: 273-298.
9. HAMELBONE. 2001. The dynamics and effect of Bacterial Kidney Disease in Snake River spring Chinook Salmon (Oncorhynchus tshawytscha). PhD thesis, University of Washington, Washington.