Studies on the biology of the ballan wrasse, Labrus bergylta

Abstract

An ever-increasing demand for ballan wrasse, for use as cleaner-fish on salmon farms, has revealed a dearth in the knowledge and literature regarding the early life history of the species and also its general population structure. Original contributions to knowledge are made in the early life history and phylogeography of ballan wrasse. The research undertaken is very relevant to the aquaculture industry and fisheries management as it addresses egg and larval growth, including otolith micro- and marco-structure analyses, as well as population genetics. In examining embryonic development, it was possible to assign eight primary stages, using morphological landmarks, following standardised nomenclature. In general, embryonic development is typical of demersal marine finfish species with a short egg stage (approximately five days at ambient temperatures). In addition, hatch-time data, derived from temperature dependent developmental data from developing ballan wrasse eggs, revealed a significant negative (exponential) relationship between increasing temperature and time to hatch. Larval data, from three different hatcheries, were compared in this study, to describe typical growth. Growth was positive among all batches of larvae, with varying changes in growth rates at different points in the development of different batches. Two statistical modelling techniques were compared with one another. The analyses favour the use of the non-linear, Generalised Additive Mixed Models (GAMMs). Again, looking at larval development, the relationship between otolith diameters, standard length and larval age was examined. Lapillar diameter was the best predictor of age. Micro-incremental counts of the sagittal otoliths, from laboratory reared larvae, were undertaken as a means to validate the number of rings deposited daily. The findings indicate that a single ring is deposited daily. There was an ontogenetic shift in the relative values of somatic and otolith micro-incremental growth, which corresponded to larval stage transition points. Using samples from Ireland, Scotland and Norway, mitochondrial DNA (control region) data was used to investigate the genetic diversity and population structuring in waters around the British Isles and southern Norway. Ballan wrasse in southern Norway showed lower levels of genetic diversity, than those from Ireland and Scotland, which appear to be the result of a population bottleneck followed by expansion. Two highly divergent clades suggest distinct recolonisation patterns in these two regions. These results have important implications for conservation and management.