Diet and heavy metal uptake by two top predator species in the Tees Estuary

Abstract

The key aim of the thesis was to estimate metal uptake and its seasonal variation from the diet by two predators from the Tees Estuary, harbour seals, Phoca vitulina and cormorants, Phalacrocorax carbo. The reproductive success of the colony of harbour seals, that has been re-establishing in the Tees Estuary since the late 1980ร, has been poor and metal loads maybe a potentially limiting factor. The diet of the predators was assessed and metal concentrations within the prey species were analysed. Median metal concentrations in Crustacea and fish species from the Tees Estuary were higher than reported in pristine estuaries. Maximum metal concentrations in some individuals suggested that hot spots still exist. There were differences in metal concentrations between species and season so the seasonal diet of the predator was important in determining metal intake rates. Metal concentrations tended to be highest in Crustacea, followed by pleuronectids and lowest in the gadids. The seals and cormorants were opportunist foragers and their diet reflected the seasonal availability of gadids. They appeared to switch to alternative prey when gadid numbers in the Tees Estuary declined. Metal burdens in the diet of these predators were expected to be lower in the winter because gadids were the dominant prey. Individual predators had different dietary preferences and hence, metal body burdens in predators would be expected to vary accordingly. Retention of metals in the Tees seals was estimated from daily metal burdens in the diet and the metal burden in the faeces. The estimated retention of metals was considered unlikely to cause an adverse effect on the seals. Further work is required however, to determine whether they bioaccumulate. Mercury concentrations were high in some body organs of two seal carcasses recovered from the Tees Estuary, although levels in predominant prey species were relatively low. Seal carcasses should be analysed where possible to measure metal concentrations, particularly mercury and organochlorine concentrations.