Recruitment studies of herring (Clupea harengus L.) in Lindaaspollene, Western Norway, 1-3
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This study was conducted between 1977 and 1980 on the herring stock of a landlocked fjord, Lindaaspollene in western Norway. It describes the early life history from time of spawning to about two months after hatching. Paper 1 deals with the major spawning grounds, Bjørnøy and Syslakvåg. A general study of the spawning areas and the distribution of eggs in relation to substrate and depth, as well as mortality of eggs at different stages of development, is presented. The spawn was primarily deposited on hard bottom substrates from high-tide water level down to about 10 m depth. Little spawn was observed on fine sand or mud. The mortality was generally low during the pre-hatching period, averaging less then 10%. Egg densities of more than one million eggs per square meter were accompanied by reduced survival rates. Diving ducks (Somateria mollissima) were estimated to have a predator potential of 1/3 of the total herring egg production in 1977. Fish predators were estimated to remove up to 2/3 of the total egg production in years with light spawn (1978), but this was considered negligible in years of heavy spawning. On the basis of egg abundance, seasonal estimates of spawning stock size were obtained. Paper 2 describes hatching, distribution and mortality of herring larvae. A method for estimating larval production based on the abundance of larvae at an early stage of development and the rate at which the larval numbers decline, is presented. Different methods to estimate larval production were compared and discussed in relation to sampling, environmental and biotic factors. Hatching begins during the latter half of April and the early part of May, and the hatching season lasts for some 3-4 weeks. Hatching success fluctuated widely in relation to estimates of egg production with an average of 50%. Dispersal of larvae from the hatching centres was traced intensive sampling aided by information about the surface water circulation. Vertical distribution of herring larvae was examined in relation to stage of development, time of the day, and surface illumination. Newly hatched larvae rose to surface waters shortly after hatching and showed small variations in amplitude of migration. The vertical migration seemed to increase with increasing age. Density-dependent mortality between hatching and an age of about 2.5 months was found to have mean rates of 8 to 16% per day. The impact of predation by planktonic invertebrates on the mortality of newly hatched herring larvae is discussed in relation to in situ observations. Paper 3 describes growth, condition and feeding characteristics in relation to abundance and mortality of herring larvae. Growth and survival were better in 1977 and 1979 than in 1978 and 1980. Mean growth in 1978 was observed to be comparatively slow (0.13 mm/day in lenght and specific growth rate of 1.1%/day) during the first two weeks after hatching, coinciding with the first-feeding stage. This was consistent with high mortality which probablywas linked with environmental conditions. Only prey items of less nutritive value to fish larvae, such as bivalve larvae, were available at the feeding stage. Only a small fraction of herring larvae survived the first-feeding stage. Their growth recovered when the feeding conditions improved in May and June with a new peak production of zooplankton, but their numbers were not sufficient to support a good year-class. Beyond 20 mm in standard length, corresponding to an age of 50 days after hatching, net avoidance influenced the size distribution of herring larvae. A feeding period approximately 18 hours, with peak feeding incidence between 0500-1200 hrs, and 1500-2100 hrs, was recorded over a wide range of larval stages. Mean feeding incidence was observed to be low in herring larvae younger than two weeks (19%), increasing to 65% in 4-7 weeks old larvae. The diet of herring larvae consisted primarily of copepod nauplii, while copepodites constituted the main food of larger larvae.
SerieFiskeridirektoratets skrifter, Serie Havundersøkelser
vol 18 no 4