Mechanisms a ecting the transport of early stages of Norwegian Coastal Cod - a fjord study

Abstract

An increasing concern in sheries management in Norway is how the Arcto-Norwegian Cod (ANC), with its feeding habitat in the Barents Sea, is separated from or mixed with the local populations of Norwegian Coastal Cod (CC) in the fjords and along the Norwegian coast. The fjord system of Sørfolda and Nordfolda in northern Norway is one of the spawning sites for the CC. The two fjords have a joint opening towards the ocean bay Vestfjorden, one of the main spawning sites for the ANC. The ANC eggs are lighter than the surrounding water masses in Vestfjorden resulting in a pelagic distribution, with exponentially increasing concentration towards the surface. The vertical distribution is sensitive to variations in wind-induced mixing. The CC eggs in Sørfolda and Nordfolda are heavier than the ANC eggs and within the low salinity water inside the fjord system they become mesopelagically distributed. A numerical model is used to investigate how the vertical distribution a ects the transport of eggs. The result enlightens the mechanisms separating ANC and CC. The simulation was set up for 1960 and 1989, representing a cold and dry year and a warm and wet year. The major di erence in forcing between the simulations of the two years is the fresh water input, which causes variations in the estuarine circulation. Changes in salinity structure a ect the vertical distribution of CC eggs, and might cause them to be transported out of the fjord system and being mixed with ANC eggs. A strong strati cation causes the eggs to be concentrated lower in the water column, and gives retention inside the fjord. The retention of CC eggs is subjected to variations within a season and between years.