Ecological habitat partitioning and feeding specialisations of coastal minke whales (Balaenoptera acutorostrata) using a recently designated MPA in northeast Scotland

Abstract

In the design of protected areas for cetaceans, spatial maps rarely take account of the life-history and behaviour of protected species relevant to their spatial ambit, which may be important for their management. In this study, we examined the distribution and feeding behaviours of adult versus juvenile minke whales (Balaenoptera acutorostrata) from long-term studies in the Moray Firth in northeast Scotland, where a Marine Protected Area (MPA) has recently been designated. Data were collected during dedicated boat surveys between 2001 and 2022 inclusive, from which 784 encounters with 964 whales of confirmed age-class (471 juveniles and 493 adults) were recorded from 56,263 km of survey effort, resulting in 238 focal follows. Adults and juveniles were occasionally seen together, but their distributions were not statistically correlated, and GIS revealed spatial separation / habitat partitioning by age-class―with juveniles preferring shallower, inshore waters with sandy-gravel sediments, and adults preferring deeper, offshore waters with greater bathymetric slope. GAMs suggested that the partitioning between age-classes was predominantly based on the differing proximity of animals to the shore, with juveniles showing a preference for the gentlest seabed slopes, and both adults and juveniles showing a similar preference for sandy gravel sediment types. However, the GAMs only used sightings data with available survey effort (2008 to 2022) and excluded depth due to collinearity issues. Whilst adult minkes employed a range of “active” prey-entrapment specialisations, showing inter-individual variation and seasonal plasticity in their targeted prey, juveniles almost exclusively used “passive” (low energy) feeding methods targeting low-density patches of inshore prey. These findings corroborate the need to incorporate demographic and behavioural data into spatial models when identifying priority areas for protected cetacean species. Not all areas within an MPA have equal value for a population and a better knowledge of the spatial preferences of these whales within the designated Scottish MPAs, appointed for their protection, is considered vital for their conservation.

Environmental DNA reveals fine scale spatial and temporal variation of prey species for marine mammals in a Scottish marine protected area

“…We analysed temporal trends in the community through partitioning the data by sampling month, and spatial trends by dividing the samples into categories based on their distance from shore; near (<1.2 km), middle-near (between 2.5 and 7 km), middle-far (between 7 and 10 km), and far (> 10 km up to 16.1 km) (Drummond et al, 2021;Fraija Fernández et al, 2020). These categories were defined based on previous observations that juvenile minke whales were most frequently sighted inshore (<2.5 km), whilst adults more frequently occurred further offshore (>10 km) (Robinson et al, 2023).…”

“…The dominant minke whale prey species, such as sandeels and clupeids, were the most abundant vertebrate species detected with our primer sets (Figure 3). Abundances vary throughout the foraging season which would account for the dietary plasticity exhibited by these whales (Robinson et al, 2023). Sandeels were most abundant during June and July, when they are foraging in the water column, but declined from August onwards, when they return to burrows in the sediment (Henriksen et al, 2021) (Figure 2.5).…”

“…The early peak in abundance coincides with the main spawning period of sprat, whilst the latter peak likely represents the arrival of juvenile sprat and herring to overwinter in the firth (Thompson et al, 1991). Juvenile minke whales target yearling sandeels throughout the foraging season, whilst adults target larger sandeels before switching to sprat and juvenile herring as they become more abundant (Robinson et al, 2023). Juvenile minkes are also found at shallower depths, while adults prefer deeper, offshore waters, corresponding with the distance from shore that their targeted prey species are found (Robinson et al, 2023).…”

“…Juvenile minke whales target yearling sandeels throughout the foraging season, whilst adults target larger sandeels before switching to sprat and juvenile herring as they become more abundant (Robinson et al, 2023). Juvenile minkes are also found at shallower depths, while adults prefer deeper, offshore waters, corresponding with the distance from shore that their targeted prey species are found (Robinson et al, 2023). Accordingly, while sandeels were detected across all clupeids were detected in greatest abundance between 7 and 10 km offshore, at depths between 50 and 120 metres (Figure 7).…”

“…One of the biggest limitations of using eDNA metabarcoding to explore predator-prey dynamics is the inability to distinguish between different age classes (Hansen et al, 2018), as predators often preferentially target prey of certain sizes (Robinson et al, 2023;Visser et al, 2021). In particular, spawning events have been observed to increase the abundance of read counts retrieved in eDNA studies, but larval forms are unlikely to be suitable prey for piscivorous marine mammals (Di Muri et al, 2022;Ratcliffe et al, 2021).…”