The collection: presentation, objectives and use of samples

Colisa is defined by its historical scope (more than 46 years), the nature and quantity of the samples (more than 480,000 samples of more than 40 species of fish and lampreys) , and their geographic origin (mainland France).

Origin of samples:

Long-term research and monitoring programmes conducted by INRAE's U3E Experimental Unit and DECOD Joint Research Unit in Rennes and ECOBIOP Joint Research Unit in Saint-Pée-sur-Nivelle as part of the ORE DiaPFC and INRAE's CARRTEL Joint Research Unit in Thonon-les-Bains as part of the Alpine Lakes Observatory (SOERE OLA).

Declarations of salmonid captures (angling and professional fishing) processed by the National Center for Interpretations of Salmonid Captures (CNICS).

Objectives:

From a scientific viewpoint, the collection contributes to current and retrospective description of  individuals or populations of fish based on life-history traits, changes in the genetic diversity of these populations and changes in the environmental conditions in which they evolve.

The collection also helps define conservation and management activities for exploited populations: defining quotas (total allowable catches), opening periods for fishing, etc. Similarly, the information contained and researched in the samples can be used to reconsider the management of populations, some of which are protected (IUCN red List) or considered to be heritage species.

Examples of use:

The collection car be used to study of life-history traits in populations (age, reproduction, growth), scales and otoliths are the main hard structures studied.

Scalimetry

Scales are valuable tissues, that make up most of Colisa. The ease with which they can be sampled, preserved and stored (in an envelope in a dry environment) make them an asset for long-term research on to aquatic environments and fish populations. They provide access to a wide range of information, about fish and the environment in which they live, using a variety of methods (structural, chemical, genetic, etc.).

Figure 1 : The principle of scale collection and reading for determining age and growth. Here is an example of a 5-year-old trout.

Studying scales determines relatively accurately a fish’s age, growth, age at and frequency of migration and age at first reproduction. Salmonid scales record periods of intensive feeding  as widely spaced growth rings (circuli), which correspond to the summer. Conversely, the circuli are narrower, overlapping and discontinuous in winter. Thus, counting the number of rings indicates the number of winters a fish experienced and how much it grew between them (Figures 1 and 2).

This characteristic also makes it possible to detect periods that a fish spent in lakes or the sea, which are more productive environments in which fish grow more rapidly (Figure 2). Thus, one can directly assess life choices of fish, starting with the age at migration. One can also identify reproduction events using spawning marks (deformed circuli) on the scales, which occur when growth resumes (Figure 2). The deformed circuli compensate for the scale erosion caused by mobilising minerals needed to produce gametes. Much of a scale’s surface can erode, to the point of destroying previous rings. A series of spawning marks can make it impossible to determine the age of older fish (more than 7-8 years old).

Thus, scales provide valuable and accurate information on the life-history traits of salmonids and other fish, but they may have limited accuracy for older individuals. Furthermore, spawning marks do not always form, which can result in underestimating the number of spawnings.

Figure 2: Scale of a sea trout that spent 3 years in freshwater and 3½ years at sea, with 3 overwintering periods in freshwater. A spawning mark can also be seen in the 6th year.

Otolithometry

Teleost fish have an inner ear that plays an important role in both hearing and static and dynamic balance. The well-developed inner ear of fishes, a membranous system located on each side of the midbrain behind the eyes, is composed of three semi-circular canals, each of which ends in a sac (Figure 3). These otic sacs contain small calcareous structures called otoliths that are composed of calcium carbonate (mainly as aragonite, but more rarely as vaterite) in a protein matrix. They play a mechanoreceptive role by influencing the perception of acceleration, pressure and vibration.

Unlike other hard structures, otoliths form during the earliest stages of fish development, grow continuously and are not resorbed, which enable them to record large amounts of information throughout a fish's life without being altered. Because otoliths grow radially from a central point (the primordium), cross-sectioning them produces an image similar to growth rings in the cross-section of a tree, making it possible to retrace the fish’s life history. Otoliths also contain several levels of temporal information (seasonal or annual macrostructures, daily microstructures) that provide information about various aspects of a fish's life.

Figure 3: Location and morphology of the inner ear of a fish. Photograph of a trout otolith.

Estimating age and life history traits from otoliths

Otoliths appear at the beginning of ontogeny : larval fish already have otoliths when they hatch, which continue to grow in concentric layers, whose structure and chemistry can vary depending on the fish’s physiology and environment. Dark layers of varying widths and spacing alternate with hyaline and opaque light layers.

During periods of faster growth (summer), aragonite crystals form more rapidly due to the higher calcium concentration in the environment. During periods of slower growth (winter), aragonite crystals form more slowly, and hyaline deposits form on the surface. A light and a dark layer are deposited each year, and counting these layers under transmitted light indicates the fish’s age in years and, using mathematical models, its growth curve (Figure 4).

Figure 4 : Number of growth rings on an otolith (x-axis, Age) and analysis of microchemical signals (y-axis, strontium:calcium ratio) to determine the age at migration to the sea.

Genetics

The DNA in these tissues can be used to genetically describe the individuals or populations studied and to reconsider these characteristics retrospectively (historical samples) in light of changes (global and local) at the sampling sites (introgression, pedigree, effective population size, etc.).

Microchemistry - Isotopy

Microchemistry and isotopy can be used to analyse these tissues to determine the origin and environmental conditions in which the fish lived, including water quality, trophic chains and living environments (lakes, rivers, sea, estuaries).

Microchemical methods are applied to otoliths mainly to reveal life-history traits of fish captured in different populations that are impossible to observe by other means. The main objective of many projects is to study and describe marine migration. The traits associated with the marine phase include the age at departure from the sea, the age at and frequency of return to freshwater as well as maternal ancestry (to determine whether a fish's mother was migratory or resident).