Seasonal trends and spatial differences in marine mammal occurrence in Broadhaven Bay, north-west Ireland

Abstract

Long-term monitoring programmes of a comparatively small area complement larger scale, but temporally limited surveys and can provide extensive datasets on seasonal occurrence and fine-scale habitat use of multiple species. A marine mammal monitoring programme, involving year-round, land-based observations, has been conducted in Broadhaven Bay candidate Special Area of Conservation, north-west Ireland, during 2002, 2005 and 2008–2011. Nine cetacean and two seal species have been recorded, with grey seal, harbour porpoise, common and bottlenose dolphins, and minke whale present throughout the year. Generalized additive models, taking into account observer effort, sighting conditions (sea state) and interannual variation, did not reveal any significant seasonal patterns in the occurrence of grey seals, bottlenose dolphins and minke whales. On the other hand, common dolphin presence in Broadhaven Bay was highest during autumn and winter. Bottlenose dolphins could be separated spatially from both common dolphins and minke whales in a classification tree by their preferential use of the shallower inshore areas of the bay (<30 m depth). However, common dolphins and minke whales, which occurred mainly in the deeper outer section of Broadhaven Bay, could not be spatially distinguished from each other, and grey seals were distributed over the entire bay. Broadhaven Bay represents an important marine mammal habitat with respect to overall species diversity and the regular occurrence of bottlenose dolphin, harbour porpoise, grey and harbour seals (all listed under Annex II of the EU Habitats Directive).

Evidence for Distinct Coastal and Offshore Communities of Bottlenose Dolphins in the North East Atlantic

“…In Ireland, bottlenose dolphins are found in estuarine, coastal, continental shelf and oceanic waters [ 28 – 31 ]. To date, at least three genetically distinct populations have been identified [ 10 ]: a resident population inhabiting the Shannon estuary, a population inhabiting the coastal waters of western Ireland and a population identified genetically from stranding records of unknown origin, possibly representing an oceanic population [ 10 ].…”

Spatial Overlap of Grey Seals and Fisheries in Irish Waters, Some New Insights Using Telemetry Technology and VMS

“…While, much of the research on resource competition has focused on estimating biomass removal by seals and modelling the impacts of seal populations on species of high commercial value [ 1 , 4 , 22 , 23 ], Cronin et al . [ 14 ] developed a novel approach that employed tracking technologies to assess spatial overlap of grey seals with fisheries. They found a significantly low rate of spatial overlap, which implied that direct competition for the resource was far less than expected.…”

“…The tags were glued to the animals’ fur and therefore tagging was conducted in April to coincide with the completion of annual moult and to maximise the period of tag attachment. Seals were captured at the haul-out site using a combination of techniques including a seine net deployed at speed from a rigid inflatable boat directly in front of the site, and in hoop nets on land [ 14 ]. The captured seals were restrained in hoop nets throughout the administration of the anaesthetic and prior to the tagging procedure.…”

“…The metric is unbiased with respect to sample size and diversity since the numerator is rescaled by the summed inner products of p f and p s [ 14 , 21 ]. Low values of C MH f suggest low overlap.…”

“…They concluded that seals are not likely to be competing directly with the fisheries in southwest Ireland. Other approaches using tracking technologies investigated the spatial overlap in grey seal foraging effort and human fishing effort off the west coast [ 14 ]. This study demonstrated a significantly low spatial overlap between tagged grey seals and the offshore whitefish fishery on the Irish continental shelf.…”