Distribution and habitat preferences of five species of wrasse (family Labridae) in a Norwegian fjord

Abstract

Wrasse (Labridae) are used widely as cleaner fish to control sea lice infestation in commercial farming of Atlantic salmon (Salmo salar) and rainbow trout (Oncorhynchus mykiss) in Norway. As a result, there is an intense fishery for wrasse along the Norwegian coast. Little is known of the population ecology of wrasse and, therefore, an evaluation of their distribution, demographics, and habitat preferences was required as a baseline from which to assess the impact of the fishery. We analysed experimental catch data from a 3-year survey carried out in 1997-1999 (before the fishery began) during and after the wrasse spawning season in a Norwegian fjord. Corkwing wrasse (Symphodus melops) was always the most abundant species, ranging from 52 to 68% of the catches. Goldsinny (Ctenolabrus rupestris) and rock cook (Centrolabrus exoletus) were the second most abundant species (up to 30%). Ballan (Labrus bergylta) and cuckoo (Labrus mixtus) wrasse represented <2% of the catches. Rock cook was relatively more abundant at more exposed stations, while corkwing wrasse was characteristic of more sheltered stations. Goldsinny and ballan wrasse both occupied mainly intermediate stations in the more protected areas. Smaller fish were found at the most protected areas, while larger fish were found at the more exposed stations. Sex ratio in goldsinny and rock cook varied significantly from year to year. In corkwing, the sex ratio remained the same each year, and females were always in larger proportion (61-66%). Spawning occurred mainly in June for goldsinny, rock cook, and corkwing wrasse. The fact that species composition and/or size distributions vary considerably over very small distances must be considered in monitoring programmes. Erroneous conclusions about shifts in species composition can easily be made simply because sampling gear are set in slightly different areas.