| Summary: | While some marine mammals haven't recovered from historic hunting, others have recovered rapidly to presumed pre-exploitation levels. Also harbour sea ls in the Wadden Sea grew at a rapid rate of 12% p.a. following the ban on seal hunting. As a consequence, - 40,000 seals are currently residing in the international Wadden Sea, and collectively, they might act as an important top-down regulatory force. The objective of this study was to estimate the potential impact of predation by harbour seals in The Netherlands on the f ish community in the Wadden Sea and nearby coastal waters. Hard fish remains in faecal samples and estimates on daily energy requirement were used to estimate prey selection and the magnitude of seal predation. GPS tracking data provided information on where they most likely caught their prey. Estimates of abundance and growth of demersal fish species, derived from fish surveys, provided estimates on total prey availability. Harbour seals in the Wadden Sea were found to feed predominantly on flatfish, flounder, sole, plaice and dab, sandeel, fivebearded rockling, whiting, cod, dragonet and bullrout. Given their high daily food requirement, the study suggests there is insufficient food available in the Wadden Sea to sustain the entire harbour seal population. Although harbour seals only spend 10-20% of their time foraging in the Wadden Sea, they may potentially reduce the demersal fish biomass by 50% in the period Sept-June. There are however large sources of uncerta inty, e.g. t he catchability of the fishing sampling gear, particularly for the larger fish specimens, and movement of f ish between the North Sea and Wadden Sea. These resu lts suggest it is important to take the harbour seal prey consumption into account when understanding the functioning of the Wadden Sea ecosystem, which acts as an important nursery area for both seals and several fish species.
|
|---|