Histological analysis of intestine in Atlantic Halibut (Hippoglossus hippoglossus) larvae – looking for clues to explain larval mortality

Abstract

The value of the Atlantic halibut (Hippoglossus hippoglossus) has increased a lot over the last years, as well as the Atlantic halibut aquaculture industry. The aquaculture industry has always dealt with problems, and one of the biggest is mortality. A larval mortality in the early days post start feeding in Atlantic halibut larvae occurred at an aquaculture facility with unknown reasons. The larvae were feeding, but still died, so there was a suggestion that intestinal issues were the reason. This study aims to investigate possible reasons for an intestinal issue that can explain or help us understand the reason of the larval mortality, using methods that focus on the histology of intestine and possible pathogens. The methods included staining of histological sections and gram staining of samples gathered at the aquaculture facility. Analysis of the samples showed that the mortality started to occur 3-4 days post start feeding, peaked at day 6-7 post start feeding, and then decreased. Analysis of the intestinal folds showed that the average length of intestinal folds seemed to decrease at the same time as mortality peaked. The gram staining showed results of bacteria in the intestine of the larvae in the same period. The results may suggest that the reason for the mortality may be a developmental issue of the intestine folds of the Atlantic halibut larvae, or maybe a disease caused by bacteria. However, this is impossible to conclude, and there should be further studies investigating possible factors of the possible developmental issues and studies to identify the bacteria and a possible disease it may cause.