Abstract:Stage IV ovaries of plaice were obtained at sea from five spawning areas of the North Sea and English Channel (Eastern English Channel, Southern Bight, Central Southern North Sea, German Bight and Flamborough) in the years 1977, 1979 and 1980. Additional samples were provided from Dutch landings at IJmuiden for 1972. The numbers of eggs were counted from 790 fish. After a logarithmic transformation, analysis-of-covariance techniques were used to describe the relation between fecundity, given by the number of e… Show more
“…It has been seen that subpopulations of European plaice (Pleuronectes platessa) in the Irish Sea that have a higher surplus production have a higher reproductive investment (Nash et al 2000). Variation in fecundity of plaice has been shown to occur on both spatial (Bagenal 1966;Nash et al 2000) and temporal scales (Bagenal 1966;Horwood et al 1986;Rijnsdorp 1991) and can be affected by food level (Horwood et al 1989). However, Rijnsdorp (1994) stated that relative fecundity (number of follicles per unit of body weight) is constant over a wide range of population abundances and levels of surplus energy, with a decrease in relative fecundity only occurring at high population levels.…”
Abstract:The fecundity of European plaice (Pleuronectes platessa) in the Irish Sea between 2000 and 2004 was estimated during the spawning season for fish in the three main spawning areas (Liverpool Bay, the Cumbrian coast, and the western Irish Sea) and one small spawning group on the west coast of the Isle of Man. Fecundity was also estimated during September of 2003 and 2004. The aim of this was to assess the variability in fecundity between areas and years in the Irish Sea and also to identify when differences in fecundity become apparent in the maturation cycle. There were variations in fecundity on both the temporal and spatial scales. The greatest variation in fecundity between years occurred in the western Irish Sea, whereas there was no variation between years in the southeastern Irish Sea (Liverpool Bay). There was no difference in fecundity between areas or years during September. The maximum fecundity in plaice is determined by the total weight of the fish at the end of follicle recruitment in the ovary, and differences in the fecundity of each population are the result of different levels of down-regulation in the period between the end of follicle proliferation and spawning.
“…It has been seen that subpopulations of European plaice (Pleuronectes platessa) in the Irish Sea that have a higher surplus production have a higher reproductive investment (Nash et al 2000). Variation in fecundity of plaice has been shown to occur on both spatial (Bagenal 1966;Nash et al 2000) and temporal scales (Bagenal 1966;Horwood et al 1986;Rijnsdorp 1991) and can be affected by food level (Horwood et al 1989). However, Rijnsdorp (1994) stated that relative fecundity (number of follicles per unit of body weight) is constant over a wide range of population abundances and levels of surplus energy, with a decrease in relative fecundity only occurring at high population levels.…”
Abstract:The fecundity of European plaice (Pleuronectes platessa) in the Irish Sea between 2000 and 2004 was estimated during the spawning season for fish in the three main spawning areas (Liverpool Bay, the Cumbrian coast, and the western Irish Sea) and one small spawning group on the west coast of the Isle of Man. Fecundity was also estimated during September of 2003 and 2004. The aim of this was to assess the variability in fecundity between areas and years in the Irish Sea and also to identify when differences in fecundity become apparent in the maturation cycle. There were variations in fecundity on both the temporal and spatial scales. The greatest variation in fecundity between years occurred in the western Irish Sea, whereas there was no variation between years in the southeastern Irish Sea (Liverpool Bay). There was no difference in fecundity between areas or years during September. The maximum fecundity in plaice is determined by the total weight of the fish at the end of follicle recruitment in the ovary, and differences in the fecundity of each population are the result of different levels of down-regulation in the period between the end of follicle proliferation and spawning.
“…Whereas it was previously thought that density-dependent control of fish populations only occurs during the early years of life (Ricker, 1954;Beverton and Holt, 1957;Shepherd and Cushing, 1980), it is more and more recognized that adult stages may also be regulated by density-dependent mechanisms (Ware, 1985;Horwood et al, 1986;Rijnsdorp, 1994;Trippel, 1995;Rochet, 1998). As population density decreases, growth rate increases, maturity is achieved earlier, fecundity at a given size is higher, and as a result of the younger age of mothers, egg size decreases.…”
Density dependence means that exploited fish populations exhibit earlier maturity, a faster growth rate, increased fecundity and reduced egg size. Here, the consequences of these effects on population dynamics, the estimation of spawning biomass per recruit and associated biological reference points are examined by a simulation model. The model is a self-regenerating model in which the population parameters (age at maturity, growth, fecundity, egg size) vary according to three classes of population abundance. Early life stages are characterized by a size-dependent growth and mortality model. It is concluded that spawning per recruit is an ambiguous concept because, if density dependence occurs in the adult population, the spawning biomass of a cohort is not proportional to the number recruited. This leads to significant level of uncertainty in the estimates of spawning biomass per recruit and the associated biological reference points such as F low , F med , F high and VSPR.2000 International Council for the Exploration of the Sea
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