Influence of diseases on the wild eel stock: The case of Albufera Lake

In recent years, eel stocks in the European Union have declined dramatically. This decline has been attributed to habitat loss, over fishing, and more recently, pathogens, but little data exist on bacterial diseases of wild eels.This study reports on the bacterial and parasitic diseases affecting the wild-eel population of the Albufera Lake. Over a 3-year period, 122 eels were analysed after having been randomly chosen from those caught in the lagoon via traditional fishing procedures. This collection of individuals comprised eels at different growth stages as well as some individuals showing pathological signs, such as haemorrhagic fins, petechiae on the belly and ulcers. Microbial isolation was performed by culturing ulcers, kidney and liver samples on TSA agar plates. Isolates were identified and their virulence assessed, with results showing that the microbial diseases affecting the wild-eel population were: vibriosis [non-motile Vibrio vulnificus biotype 2 serovar A; LD50 inferior to 3.6 × 103 cfu/fish], edwardsiellosis [Edwardsiella tarda; LD50; 1.5 × 104 to 7.4 × 105 cfu/fish], aeromonosis [Aeromonas hydrophila, Aeromonas bestiarum and Aeromonas jandaei; LD50 3.3 × 106 to 2.3 × 107 cfu/fish], and candidiasis [Candida sorbophila; LD50 4.6 × 106 to 3.7 × 107 cfu/fish], with an overall prevalence of 34.4%. Chronic bacterial infections presented an overall prevalence of 18.9%; mainly caused by mesophilic Aeromonas, producing tail-rot ulcers, and avirulent bacteria isolated from internal organs [Pseudomonas/Alcaligenes, Plesiomonas shigelloides; LD50 > 108 cfu/fish]. Swim-bladder nematode parasites [Anguillicola crassus] were also collected from most of these individuals. Thus, prevalence of anguillicolosis was 11.5% in the wild-eel population, but increased to 20% among individuals affected by a chronic bacterial disease. The incidence of each pathology differed among individuals depending on their size [Pearson χ2 (12 d.f.) = 23.772; p = 0.013].The results of our study highlight three facts: 1) pathogenic bacteria could play a leading role in the decline of the eel population in the Albufera Lake; 2) the youngest eels may be the most susceptible to acute bacterial diseases in this natural habitat; and 3) elderly eels suffer from chronic illness, which may reduce the survival of these breeding individuals during their downstream migration to the sea.