Maturity and growth of the Spring Spawning herring in the Kattegat-Skagerrak

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Abstract
The two mainindigenous herring (Clupea harengus L.) stocks in the Skagerrak and the Kattegat, the Skagerrak Spring Spawners (SSS) and the Kattegat Spring Spawners (KSS) have been studied regarding percentage number spawning at age, biomass and length increment, and gonad maturity cycle for the years 1969, 1971 and 1973–1976. The SSS were significantly (p < 0.01) longer and had a larger biomass at a fixed age than the KSS. Both stocks showed a strong seasonal variation in biomass with minimum in the first quarter and maximum normally in the third quarter of the year. The maturation cycle was different between the sexes: the recruited males of both stocks passed the winter in pre-maturity Stage IV, whereas the gonads of the females successively developed from Stage III to Stage IV during that time. The proportion of females and males were similar in both stocks. Normally, a small percentage of both stocks spawned at 2 years of age, but in 1971 it was 20 %. Almost all of the SSS spawned at 3 years of age and older, whereas among the KSS the percentage of non-spawners at the age of 3 years varied between 6 and 46 between years. There seem to be good and bad years with respect to spawning, especially among the KSS. The recruitment of 2 and 3 years old herring to the spawning stock could not be related to fish length, and it is hypothesized that rather a critical fat level has to be reached and maintained for some time for a subsequent successful gonad development of spring spawners. Different strategies for gonad development of spring spawners, summer spawners, autumn spawners and winter spawners are briefly discussed. Based on biological considerations it is suggested that spring spawning herring should preferably be fished in the second half of the year.