Temporal and spatial patterns of reproductive indices of European hake (Merluccius merluccius) in the northern North Sea and Norwegian coastal areas

Spatial and temporal patterns of reproductive maturity were examined for European hake (Merluccius merluccius) in the northern North Sea, the Skagerrak and the Norwegian mid-west coast. Maturity stages based on macroscopic examination of gonads and Gonadosomatic Index data were collected from annual scientific surveys conducted in winter, summer, and autumn 2011-2015. Our observations show that spawning activity is highest in July in the central and northern North Sea. Very few hake in spawning condition were found in January and February in the northern North Sea and Skagerrak. Macroscopic staging showed high numbers of post-spawning hake in October/November along the Norwegian west coast north of 62°N, suggesting that main spawning occurs in this area in August/September. Fish in spawning condition occurred mainly in depths from 80-120 m at stations with sea surface temperatures between 12 and 15 °C and bottom temperatures of 6.5-8.5 °C. Our results confirm that European hake has an extended spawning season with a peak later in the year in more northern waters. Occurrence of spawning north of 62°N along the Norwegian coast and an intra-annual variation of length frequency distributions indicate migrations and more complex population dynamics of hake in its northern range than previously assumed.