Evaluating the histological-based condition of wild collected larval fish: A synthetic approach applied to common sole (Solea solea)

In the present study, the histological-based condition of common sole (Solea solea) larvae in the English Channel was evaluated to assess the relevance of experimentally-calibrated indices for wild collected specimens. Based on topographical observation of foregut, midgut, hindgut, liver and pancreas, each of the 202 larvae analysed was attributed one of the six grades of a condition index that was developed for sole in the nineties. In parallel, a synthetic table of 20 criteria historically used as condition indicators was created from an extensive review of the literature. These criteria were related to tissular integrity, cells height, nuclei, presence of prey and vacuolisation of the same five target organs. For each larva, each criterion was scored on a scale from one to three. Multiple correspondence analyses coupled to clustering and indicator values were used to explore the relationship between the condition index and criteria as well as to highlight which complementary information the latter could provide to the former. Results showed that some adjacent grades were related to the same set of scored criteria, suggesting that these grades are not as distinguishable in the field as they can be in controlled conditions. Some criteria like vacuoles in the enterocytes, size of the hepatocytes, amount of zymogen, stain of acini, presence of prey in the digestive tract and goblet cells in the foregut, were independent of the condition index and depicted more recent food intake than a true condition status. Conversely, criteria related to tissular integrity as well as presence of energetics reserves like vacuoles in the liver appeared more relevant to evaluate the condition status of fish larvae since their increasing scores were associated to different grades. It is concluded that the grading system obtained from experimental observations may not be adapted to reflect the different larval condition states that could be encountered in the field. A simplified grading system, based on three grades and a reduced set of relevant criteria is then proposed.