Temperature-sensing telemetry - possibilities for assessing the feeding ecology of marine mammals and their potential impacts on returning salmonid populations

Adult salmonids returning to the River Tees (north-east England) tagged with temperature-sensing acoustic transmitters provided some unexpected and novel information on the possible impact of seal predation at a tidal barrage 16 km from the sea. Predation events of tagged fish by seals were inferred by an increase in the temperature transmitted by the acoustic tags. Subsequent events of feeding or drinking by the seals were also inferred from further changes in temperature recorded by tags whilst in their stomachs. Rates of inferred predation on tagged individuals were high (47%). This is the first time that temperature-sensing transmitters deployed inside wild salmonids have revealed instances of predation by, and subsequent feeding ecology of, marine mammals. The results are discussed in relation to the use of such technology in future studies attempting to address the interactions between marine mammals and their fish prey species.