Population dynamics of cod, Gadus morhua, in Porsangerfjord, Northern Norway - Analysis of back-calculated length at age from otoliths in 1992 and 2009

Abstract

The thesis investigates growth of Atlantic cod (Gadus morhua) caught in 1992 and 2009, in Porsangerfjord North-Norway, with use of back-calculated length at age based on cod otoliths. The cod population in Porsangerfjord ecosystem have experienced major changes in abundance during the last decades: a dramatic decline in the costal cod population, and environmental influences.

A back-calculation method estimating length at previous age was done on cod otoliths. The otoliths were embedded in black resin and transversely cut before photographed in a stereomicroscope. Growth zones in otoliths were measured and growth was back-calculated for previous length at age.

In the outer areas (A - B) adult cod, above 5 years of age, were longer at age in the second time period (1998 - 2009) than cod from the first time period (1980 - 1992). Adult cod grew faster in outer areas than in inner areas (C - D) in 1992. In outer area cod had better growth for 1 year old, and 5 - 7 year old, than in inner area (C) in 2009.

In conclusion, increasing sea temperature seem to play maybe the most important role in increased mean length at age for adult cod, above 5 years of age, caught in 2009, compared to cod caught in 1992. In addition, a shift from abundance of cod in inner areas with cold sea temperature in 1992, towards outer warmer areas in 2009 has probably also contributed to increased length at age for older cod.