Mercury accumulation by the eelpout (Zoarces vivparus L.) in the Forth Estuary, Scotland

Abstract

The Forth Estuary, in eastern Scotland, has received inputs of the toxic metal, mercury, from an industrial discharge for many decades. This has led previously to accumulation of relatively high levels of mercury in the environment and biota of the estuary, although both inputs and levels in biota have fallen sharply since the early 1980s. The general literature on mercury in the marine environment is reviewed, including the human health implications of mercury in aquatic food chains. A review is also presented of current knowledge of mercury in the Forth Estuary environment and biota, as carried out by the statutory water quality authority, the Forth River Purification Board (FRPB) and others. Studies by the FRPB have indicated that a resident teleost fish species, the eelpout (Zoarces viviparus L.) may be a suitable candidate for use as a bioaccumulation monitoring species for mercury in the Forth. Its potential and demonstrated use in other pollution studies in North Sea and Baltic waters are reviewed. In order to investigate the use of this species for bioaccumulation monitoring of mercury, the variability of mercury concentrations in a number of tissues was investigated, in relation to biological, temporal (both seasonal and annual) and spatial factors, during the period November 1989 to March 1992. Although skeletal muscle and liver were investigated in most detail, kidney, testes, ovary, and tissues of the viviparous brood were also studied. Mean mercury concentrations in all tissues were relatively low, although skeletal muscle, liver and kidney tended to have higher values than the reproductive tissues.

The log of mercury concentration in skeletal muscle increased in a statistically significant linear fashion with length, weight and age of the fish, in almost all samples. There was no statistically significant difference in the rate of increase (i.e. the slope of the linear regression of log concentration on length) between the sexes. There was a clear seasonal pattern in the rate of increase of mercury concentration with length in both years, with highest values in the Spring period, March to May, followed by a sharp decrease in the rate in Summer, June to August. Differences between seasons were not, however, statistically significant in either year. The rate was negatively correlated with the allometric condition factor. The total mass of mercury in the skeletal muscle of a standard fish showed little significant variation between seasons although there was considerable seasonal difference in total muscle mass. It is suggested that seasonal variability of mercury levels in eelpout skeletal muscle in the Forth Estuary is related principally to seasonal changes in skeletal muscle mass, rather than to changes in mercury burden. Both liver and testes showed a seasonal peak in relative weight (somatic index) in Summer months, with a seasonal minimum mercury concentration in the same period. There was little difference between seasons in the total mercury burdens of these organs, with respect to fish size. Thus, the seasonal variability of mercury concentrations in the liver and testes appears to be due to seasonal growth and regression patterns. Mercury concentrations and burdens of the female reproductive tissues were studied in relation to the viviparous reproduction of the eelpout. The mercury concentration of the intra-ovarian brood is significantly correlated with that of the maternal skeletal muscle. The increase of brood mercury burden through the brood development period was slow initially, followed by a rapid increase which was closely related to a period of rapid increase in brood weight. The distribution of mercury in the ovary was described, and a high concentration of mercury was found in a cellular fraction of the ovarian fluid, relative to the supernatant fluid. Larval eelpout are known to consume this cellular fraction, including red blood cells, which is released from the inner wall of the ovary. It is possible that this matrotrophic form of nutrition may provide the main route for accumulation of mercury, at least in older larvae. Uptake, internal dynamics and elimination of methylmercury by eelpout was investigated under laboratory conditions by the administration of »'Hg-labelled methylmercuric chloride, by oral dosing or intra-peritoneal (i.p.) injection. Patterns of tissue redistribution of mercury with time suggest that the dynamics of i.p. administered methylmercury are related closely to the structure and biological functions of the tissues. The transfer of a portion of an i.p. administrated dose of methylmercury, from the peritoneum to the intra-ovarian tissues of the eelpout (including brood and ovarian fluid), was demonstrated for the first time in a limited investigation of mercury transfer between generations. Elimination of orally administered methylmercury, measured by whole body counting of live fish was, initially rapid, with 20% of the dose lost over two days, with a slower loss of a further 20% over the following two weeks. The initial period of rapid loss coincides with the loss of mercury from the intestine of i.p. dosed fish suggesting a two compartment loss. More detailed analysis of biological half-times of elimination was not permitted by the restricted duration of the experiment. The literature and concepts of trend monitoring and Environmental Capacity for mercury in the marine environment were reviewed. Linking of archive data, collected by the FRPB between 1978 and 1988, with results from the present study, allowed an analysis of temporal trends of mercury in Forth Estuary eelpout. There has been a clear trend of decreasing mercury concentrations in eelpout from the Forth Estuary since the early I980’s although levels in fish from the Firth of Forth, where environmental mercury levels are lower, have remained relatively unchanged. In the early I980's, mean concentrations in fish from the site closest to the main mercury discharge were in excess of 0.3 mg.kg the acceptable limit established by the European Community Environmental Quality Standard (EQS) for mercury in fish muscle Mercury concentrations in eelpout from the Firth were lower in than those from the Estuary in all years. Using this data, an aposteriori estimation was made of the Environmental Capacity of two Forth Estuary sites to receive mercury inputs. The meeting of the EQS was taken as the acceptable endpoint of the calculation. The estimated values of Environmental Capacity, of I8.58 kg per year at the site closest to the discharge would now be in excess of the currently permitted inputs of mercury to the Forth Estuary.