## 1 Introduction  
Tilapia (_Oreochromis_ spp.) is one of the important farmed fish species in Taiwan, accounting for approximately 70,000 metric tons from the annual report for fisheries statistics, Fisheries Agency,Council of Agriculture, Taiwan [1]. The demand for tilapia from domestic markets as well as from international markets has increased two-fold since the last decade. Tilapia can tolerate adverse water quality and other stressors better than most other commercial aquaculture species. Tilapia is one of the fastest growing fish, increasing its compatibility for intensive farming. The intensive farming of tilapia with high stocking density in Taiwan has increased immensely, allowing the adaptability effects of climate change and food shortages. However, high stocking rates make intensive and semi-intensive farms more susceptible to infectious diseases [2].  
The first report on streptococcal infection in Taiwan can be traced back to 1985 [3] in cage-cultured tilapia, while the first case report on streptococcus was recorded in 1954 from a Japanese rainbow trout farm [4]. After the first case was announced in Japan in 1958, streptococcus gradually spread to other parts of the world, infecting almost all major cultured species, and thus came to be recognized as a major fish disease. In streptococcus, a family of Gram-positive bacteria, pathogens isolated from diseased fish include mainly _S. agalactiae, S. iniae, S. dysgalactiae_, and _Lactococcus garvieae_, and some minors such as _L. lactis, L. piscium_, and S. paranuberis.  
Recently, some reports revealed that _S. agalactiae_ cause serious economic loss in aquaculture in tilapia and other fish species in Taiwan with a host range from freshwater to seawater fish, including tilapia, ayu, eels, mullet, bass, catfish, ornamental fish, and wild fish, as well as shrimp and bullfrogs [5; 6; 7]. Due to economic concerns, tilapia is usually cultured at high density, increasing stress levels, which may lead to decreased immunity and a decreased survival rate [8]. _Streptococcus_ spp. are ubiquitous in water and can also be isolated from healthy fish; leading to false diagnosis of infectious strains. Tilapia may become infected by streptococcus due to several reasons, including stress. The symptoms of infection in tilapia include weakness, loss of appetite, abnormal swimming behaviour, exophthalmia, intra-ocular haemorrhage or opacification, convex and bleeding eyes, haemorrhage muscle, anus and fin base [9], lesions including gill telangiectasia, haemorrhage, ascites, and kidney, spleen, and liver enlargement or whitish pinpoint necrosis [9; 10].  
The impact of climate change attracts people's attention recently, but the related studies were very limited in the aquatic animals. In Finland, the increasing of water temperature was found to be correlated to the high parasite infection rate, and considered to affect the diversity of aquatic animals, including their breeding, migration, growing behaviour, feeding, and metabolism [11; 12]. In an experimental study, it was also concluded that a sudden increase or decrease in environmental temperature can lead to a significant decrease in the number of white blood cells in tilapia fish, indicating the cause for higher susceptibility to streptococcal infection [13].  
The aim of this study was to collect farmed tilapia disease data from different Taiwan veterinarian aquatic animal inspection stations from 2006 to 2015, and examine the occurrence of streptococcus with respect to temperature fluctuations and other parameters. These findings will fulfil the needs for an early disease warning and management model for tilapia farms and help reduce the economic losses to tilapia aquaculture farms in Taiwan.