In a context of high occupation of the coastal area, we study this occupation impact measuring developmental stability of post-larvae and juveniles (Oblada melanura in nursery zones and Dicentrarchus labrax in aquarium). We intend to provide information for the management of aquaculture and seaside resort zones. We expect to create a new mid/long-term stress indicator using otolith Fluctuating Asymmetry (FA).

Figure 1 : 79 days old D. Labrax sagittae.
Photo: Pierre Vandenbussche 2015

FA is used to measure bilateral individual developmental stability. Departure from symmetry for a character is described by frequency distribution of right minus left part. In case of FA this frequency distribution should be a Normal curve centred around 0 (Palmer 1994). If stress occurs, this Normal curve tends to present an increase of the kurtosis (a perfect Normal law presents a kurtosis of 0).

Figure 2: 3 kinds of FA level, kurtosis vary in the same way of FA.
In this example, Kurtosis: Green < Blue < Red so we can suppose that FA vary in the same way: Green < Blue < Red

In order to obtain FA information, we use an intern-ear hard structure called otolith. Fish present 3 pairs of otolith. Those otoliths are used by fish for earing and space recognition. They are called sagittae, lappili, asterisci. We use the sagittae pair in our work.

We chose this structure for 3 reasons:

First, we use otolith because it is an internal structure, meaning there is less risk of deterioration during sampling time. This precaution is really important because of the use of post-larvae and juveniles. At this early life stage fish are really fragile. As sampling method could damage the external part of fish the utilisation of external symmetric traits is impossible.

Second, otolith presents, like a pearl, a successive ring structure which is easily recognisable on microscopy. This is due to the way it is formed. Every day a new ring, or increment, is deposited (Chambers and Miller 1995). So if a change occurs in the fish environment, traces of this change will appear on the concerned ring (Arai, Ohji et al. 2007).

Thrird, the sagittae pair is generally the biggest of the three otolith pairs and so can be collected more easily. It also reduces the risk of misinterpretation during analyses.

Arai, T., M. Ohji, et al. (2007). "Trace Metal Deposition in Teleost Fish Otolith as an Environmental Indicator." Water Air Soil Pollut(179): 255-263.

Chambers, R. C. and T. J. Miller (1995). Evaluating Fish Growth by Mean of Otolith Increment Analysis: Special Proprieties of Individual-level Longitudinal Data. Recent Development in Otolith Research, Columbia, South Carolina, United States of America, University of South Carolina Press.

Palmer, A. R. (1994). Fluctuating asymmetry analyses: A primer. Developmental Instability: Its Origins and Evolutionary Implications, Tempe, Arizona, Springer Netherlands.