This paper presents a non invasive, rapid and reliable way to quantitatively assess fin erosion in sea bass (Dicentrarchuslabrax). The method is based on a visual assessment of fin profile and area loss of all fins except the anterior dorsal, on a scale from 0 to 4 in comparison with a perfect fin. The effects of stocking density (SD) and temperature on fin damage were investigated under experimental conditions (100–250 g fish). Over a 4-month period, mean erosion index (mean erosion level of all fins) was 10 times higher at 120 than at 20 kg m−3, where most fins were undamaged. Damage was also dependent on time and oxygen concentration (35% lower at 53% O2 saturation than at 105%). Fin condition was also affected by temperature: mean erosion index was 0.22–0.25 at 13–16 °C, but five times higher at 25 °C. Caudal and dorsal fins were always the most eroded. Fin damage was
then monitored in a large land-based farm using high SD, and in a small sea
cage farm operating at low SD. At the first site, 6 batches of two market
size groups were examined: L (850–930 g, 50–80 kg m−3) and S (375–400 g, 42–60 kg m−3). Fin condition was good in all batches (mean erosion
index, 1.1–1.3) and lowest at the highest SD. At the second site, four
batches of large fish (350–890 g, 26–24 kg m−3) and four other groups
below market size (100–270 g, 8–16 kg m−3) were examined. Fin erosion
was the highest in large fish (mean erosion index, 1.1–1.2) and in the sea
cages most exposed to climatic disturbances. In both sites, the most eroded
fins were the caudal and dorsal. Differences in other external injuries were
also observed between the two sites (less necrosis and more scale injuries
in sea cages). The causes of fin damage are discussed in relation to
metabolic and/or behavioral adaptations to rearing conditions and the main
actions that could be taken to improve fin condition are discussed.