Review of cetacean and basking shark distribution data in relation to advice on static gear for relevant Marine Protected Areas

1. Background to NatureScot advice

NatureScot’s advice to Marine Directorate for the following cetacean and basking shark features of Marine Protected Areas is provided within the Conservation and Management Advice documents for each site, compiled by NatureScot (Table 1). Advice was provided for the protected mobile species features of the MPAs considering their sensitivity to pressures caused by a range of fishing gear types including static gear (drift nets and set nets) and is summarised Annex 1. 

The following sections focus more specifically on our advice relating to static gear (drift nets and set nets) and the new data available to support management. 

2. Updated modelling

In 2021, a NatureScot commissioned report (Paxton et al., in publication) was produced which aimed to provide an updated picture of the density distribution of the above listed (Table 1) mobile species within Scottish MPAs, except for bottlenose dolphin (see section 3 for available data). This modelling analysis considered all known spatially relevant existing aerial and boat-based effort-related survey data (1994 – 2019) to generate modelled predictive point density estimates with associated confidence intervals. The mapped outputs are intended to be incorporated into the Geodatabase of Marine features adjacent to Scotland (GeMS). 

Overall, the new adjusted and modelled densities show that the features are still using the areas of the MPAs, which were identified as having persistently higher than average densities in Paxton et al., 2014, albeit each species is showing some inter-annual variation in the areas being used and, in some cases, a slight northward shift. Each feature is discussed below. It is important to note when interpreting the results that the 2014 modelling produced persistence above mean density, whereas the 2021 modelling produced point density estimates. 

Minke whale (east coast)

The Southern Trench MPA was selected for minke whales on the basis of habitat modelling work showing it consistently supports above average densities of minke whales, backed up by effort corrected sightings data (Paxton et al., 2014). Sightings of minke whales within the MPA are highest during the months of June to October, however there is evidence that minke whale are present throughout the year, albeit in lower numbers (Robinson et al., 2009). 

The latest 2021 predictive modelling results continue to show that minke whale observations on the east coast are highly seasonal, with observations limited to the summer months between April and October. The survey data are relatively sparse on the east coast compared to the west coast, with higher densities found on the outer reaches of the Moray Firth and Moray coast (though with high levels of uncertainty). It is also important to understand that the band of higher densities extending through the Moray Firth and north of Sutherland is driven by the lack of smoothing around the coast. Nevertheless, this most recent mapping of the minke whale feature on the east coast does show the continued presence of minke whales but that the density reduced for a few years before increasing again in 2019, showing high inter-annual variability. 

An animation showing the predicted change in minke whale (point) density distribution over time annually between 2000 and 2019 along the east coast is available to support this advice. It is important to note that the predicted densities in this animation show point estimates without upper and lower confidence.

The new modelling data supports our current management advice recommendation for minke whales in the Southern Trench MPA. The modelling does not show areas of the site that are more or less important for minke whales.

Minke whale (west coast)

The Sea of the Hebrides MPA was selected for minke whales on the basis of habitat modelling work showing it consistently supports above average densities of minke whales, backed up by effort corrected sightings data (Paxton et al., 2014). Sightings of minke whales within the Sea of the Hebrides MPA are highest during the late summer months, however, there is evidence that minke whales are present throughout the year, albeit in lower numbers (Anderwald and Evans, 2007, Paxton et al., 2014). 

The 2021 predictive modelling results point to potential shifts in distribution across years with a change from higher density areas in the Small Isles and south of the Isle of Skye in 1996 and 2004, to higher densities in the northern Inner Hebrides (between north Skye and Lewis and Harris) in 2019. This suggests that given inter-annual changes in distribution, there may also be high densities of minke whales that occur within the Inner Hebrides beyond the extent of the Sea of the Hebrides MPA.

There is also continued evidence supporting that minke whales on the west coast are highly seasonal reflecting that minke whales move into coastal Scottish waters for the summer foraging season. 

An animation showing the predicted change in minke whale (point) density distribution over time from March to November 1994 – 2019 along the west coast is available to support this advice. It is important to note that the predicted densities in this animation show point estimates without upper and lower confidence.

The new modelling data supports our current management advice recommendation for minke whale in the Sea of the Hebrides MPA.

Harbour porpoise

The Inner Hebrides and the Minches SAC was selected for harbour porpoise on the basis that the area came out in the highest 10% of modelled density areas for the whole of the UK in Heinänen and Skov (2015) and another model by Booth et al. (2013) selected it as the ‘top 20%’ (which used acoustic data as well as sightings data). The results from both models supported each other. Above average densities of harbour porpoises, persistent over several years, are found within the site. 

The 2021 modelling agrees with the previous modelling results that the distribution of harbour porpoise within the west coast of Scotland remains relatively stable across years. This most recent analyses indicates that porpoise are primarily located throughout the Hebrides and the in the Minch, mainly concentrated off Lewis and Skye. However, there are areas around Mull to Islay and along the coast of the Outer Hebrides that do not come out as strongly in the recent Paxton et al., 2021 modelling. 

While harbour porpoise are detected year round on the west coast, there continues to be an increase in abundance in the summer months between mid-June and late October.

An animation showing the predicted change in harbour porpoise density distribution over time monthly from January to December between 1994 and 2019 along the west coast is available to support this advice. It is important to note that the predicted densities in this animation show point estimates without upper and lower confidence.

This new modelling data supports our current management advice recommendation for harbour porpoise in the Inner Hebrides and the Minches SAC.

Risso’s dolphin

The North-east Lewis MPA was selected for Risso’s dolphins on the basis of habitat modelling work showing it consistently supports above average densities of Risso’s dolphins, backed up by effort corrected sightings data (Paxton et al., 2014).

As with previous analyses (e.g., Paxton et al., 2014), an area of greater Risso’s dolphin density was also identified in the updated 2021 modelling off the east coast of Lewis that is evident across years between 1996 to 2019, though to varying degrees. Additionally, there is some evidence of higher densities between Coll and the southern Outer Hebrides in 1996 and 2019, though the densities predicted here are considerably lower than those predicted for the north-east of Lewis.

The latest 2021 modelling showed no evidence of seasonality. However, this differs to the findings of Waggitt et al., 2020 who identified seasonal differences in Risso’s dolphins throughout the Hebrides, with higher estimated densities in the summer months (concentrated in the continental shelf but also reflected in the more coastal waters within the Inner Hebrides). Additionally, while vessel surveys, photo-identification studies, and shore-based surveys conducted in the North-east Lewis MPA have recorded Risso’s dolphins year-round, they did identify a seasonal increase in sightings rates between May and October (Weir et al., 2019). 

An animation showing the predicted change in Risso’s dolphin density distribution over time annually from 1994 – 2019 along the west coast is available to support this advice. It is important to note that the predicted densities in this animation show point estimates without upper and lower confidence.

This new modelling data supports our current management advice recommendation for Risso’s dolphins in the North-east Lewis MPA.

Basking shark

The Sea of the Hebrides MPA was selected for basking sharks on the basis of habitat modelling work showing it consistently supports above average densities of basking sharks, backed up by effort corrected sightings data (Paxton et al., 2014). Higher numbers of shark present are particularly notable during the months of June to October when they spend most of their time close to the surface feeding and often in aggregations (Speedie, 2009, Doherty et al., 2017).

The results presented in the 2021 modelling are generally in agreement with previous analyses of basking shark data (Witt et al. 2016, Paxton et al. 2014) with strong evidence for a concentration of basking sharks around Coll and Tiree and a summer peak in abundance.

An animation showing the predicted change in basking shark density distribution over time from May – November 2000-2019 along the west coast is available to support this advice. It is important to note that the predicted densities in this animation show point estimates without upper and lower confidence.

This new modelling data supports our current management advice recommendation for basking sharks in the Sea of the Hebrides MPA.

3. Bottlenose dolphin data availability

The latest published data available for bottlenose dolphin within the SAC and wider East Coast population was reported in Cheney et al., 2018. The most recent Site Condition Monitoring data, from the last 6-year survey period between 2017 and 2022, shows a decline in the estimated number of dolphins using the Moray Firth SAC (report currently in review by NatureScot and due to be published in Q1 2024/25). However, numbers using the SAC are stable over the long-term (last two decades) with inter-annual variability. Between 2001 and 2022 there has been an increase in the number of calves born and identified each year, in both the SAC and Tayside and adjacent waters. Additionally, passive acoustic monitoring continues to show there is inter-annual and seasonal variation in the amount of time dolphins spend at sample sites within the SAC, but the frequency of use of these sites appears to be stable long-term. The data also suggest that dolphins use certain areas of the SAC outside the summer months more than previously thought. For context, the estimated number of dolphins using the Tayside and adjacent waters increased by an estimated 4.8% per year between 2009-2022, with some inter-annual variation. Overall, the Coastal East Scotland management unit bottlenose dolphin population is continuing to increase. 

The latest SAC population estimate and site usage information supports our current management advice recommendation for bottlenose dolphins in the Moray Firth SAC.

4. Summary of review of NatureScot advice for static gear fishing in relation to cetacean and basking shark MPAs

In light of our updated understanding of the predicted density distributions of the mobile species features - Risso’s dolphin, minke whale, harbour porpoise and basking shark - within each site, and the population estimate and site usage information available for bottlenose dolphins in the Moray Firth (Table 1), it is our view that our advice as provided in the Conservation and Management Advice documents for these sites remains appropriate. 

It is recommended that the following points relating to the usage of the MPAs are considered when developing measures:

• Harbour porpoise: The whole Inner Hebrides and the Minches SAC area is considered important in a UK context for harbour porpoise.
• Risso’s dolphin: Similarly, the whole area around the north-east coast of Lewis is a persistent high density area for Risso’s dolphins.
• Minke whale (west): Overlap of the northern part of Sea of the Hebrides MPA with the Inner Hebrides and the Minches SAC means that this area is of importance for both minke whale and harbour porpoise. There is inter-annual variability in use of areas in the MPA by minke whales.
• Minke whale (east): There is consistent evidence that minke whales use the Southern Trench area during the late-summer months, with some inter-annual variability. However, the modelling does not show areas of the site that are more or less important for minke whales.
• Bottlenose dolphin: The SAC is considered important for >50% of the population in any one year, with inter-annual and seasonal variation in use of the area.
• Basking shark: There are persistent high density areas within the Sea of the Hebrides MPA, particularly around Coll and Tiree.