Anthropogenic disturbance to marine mammals is an escalating conservation concern, particularly in regions where coastal tourism intersects with critical wildlife habitats. The UK supports a globally significant population of grey seals (Halichoerus grypus), which rely on terrestrial haul-out sites for rest, socialisation and pup rearing. However, the rapid expansion of commercial wildlife tourism (“tripper” boats) and recreational watercraft presents a chronic source of stress.

Sue Sayer MBE (2026) The Impact of Seaborne Activities on Grey Seal (Halichoerus grypus) Haul-Out Behaviour in West Cornwall. Ann Mar Biol Res 9(1): 1037.

Anthropogenic disturbance to marine mammals is an escalating conservation concern, particularly in regions where coastal tourism intersects with critical wildlife habitats. The UK supports a globally significant population of grey seals (Halichoerus grypus), which rely on terrestrial haul-out sites for rest, socialisation and pup rearing. However, the rapid expansion of commercial wildlife tourism (“tripper” boats) and recreational watercraft presents a chronic source of stress.

While the Department for Environment, Food and Rural Affairs (Defra) recommends that powered vessels maintain a 100-metre buffer zone from marine wildlife, real-world compliance and the efficacy of these guidelines remain understudied. This study evaluates the frequency, drivers, and severity of seaborne disturbances at a key offshore grey seal haul-out site in west Cornwall, utilising predictive modelling to identify safe operational thresholds.

This study employed a non-invasive, land-based observational framework designed to monitor human wildlife interactions without altering natural seal behaviour. A specifically designed range- and angle-finder was used to accurately assess the distance from craft and seals

Study Site & Timeline: Field observations were conducted over a seven-month period from April to October 2025 at an offshore, open-water seal haul-out site in west Cornwall.

Data Collection: The land-based observer used a digital camera with an effective 900mm focal length from a fixed cliffside vantage point. This layout eliminated observer-induced disturbance. Monitoring occurred during peak daylight hours, totalling 184 hours of active observation. Baseline control periods (no vessels present) were recorded alongside active vessel encounters.

Metrics & Variables Logged: For every watercraft interaction, observers recorded the vessel type, minimum approach distance and behavioural compliance using the Wildlife Safe (WiSe) scheme guidelines.

Seal responses were classified using a standardized three-tier disturbance scale:

• Level 1 (Vigilance): Head raises, disrupted sleep, or sustained looking at the vessel.

• Level 2 (Displacement): Seals actively shifting positions on the rocks.

• Level 3 (Flushing): Catastrophic panic resulting in seals stampeding or sliding into the sea.

Statistical Analysis: A Generalised Linear Mixed-effects Model (GLMM) was utilized to analyse the simultaneous, compounding effects of multiple environmental and anthropogenic variables (distance, vessel behaviour, time of day, and group size) on the probability of seal disturbance.

Seaborne disturbances were found to be chronic and widespread, with overall disturbance recorded in 71.4% of all interactions. On average, a disturbance event occurred every 7.5 minutes, translating to an individual seal being disturbed every 4.1 minutes. Severe Level 3 flush events occurred once every 61 minutes.

Anthropogenic Drivers of Disturbance

The GLMM isolated four variables that statistically drove seal stress:

1) Vessel Proximity: Distance was the strongest predictor of disturbance (p<0.001). At a close proximity of 25 metres, the probability of disturbance exceeds 80%. At the Defra-recommended 100-metre mark, the probability remains high at 46.8%. The probability only drops below the standard 25% “safe” threshold at a distance of 141 metres.

2) Behavioural Code Non-Compliance: WiSe code violations occurred in 71.5% of interactions. The most common infractions were stopping to look (54.9%), erratic reversing or drifting under power (18.9%), speeding (15.0%), and direct head-on approach angles (11.6%). Committing just two behavioural infractions spikes the probability of disturbance past 60%, even if the vessel maintains a 100-metre distance.

3) Time of Day: Disturbance followed a steep decay curve throughout the day (p<0.05). The highest probability of disturbance and all recorded Level 3 flush events occurred within the first 65 minutes of the first morning vessel’s arrival.

4) Group Size: Larger haul-out groups exhibited lower stress thresholds (p<0.05), significantly increasing the risk of collective vigilance and chain-reaction stampedes.

Vessel Comparison & the Silent Craft Phenomenon

Commercial wildlife tourism (“tripper”) boats accounted for the vast majority of interactions and routinely breached guidelines, maintaining an average approach distance of just 48.2 metres (compared to the 100m guideline). Conversely, commercial fishing vessels maintained an average distance of 141 metres and caused zero recorded disturbances.

Crucially, while non-motorized recreational craft (kayaks and stand-up paddleboards/SUPs) were significantly less frequent, they caused disproportionately severe reactions due to their silent, stealthy nature. Across all non-powered craft interactions, 59.6% resulted in catastrophic Level 3 flushing. This included a single event where an SUP appearing suddenly behind a rock 24 metres from the seals, causing 19 of them to stampede into the water.

Ecological Implications

The findings demonstrate that Cornwall’s grey seals are experiencing severe, chronic disruption to their natural behavioural budgets. The frequency of disturbance—every 4.1 minutes per seal—indicates that these animals face near-constant sleep deprivation and heightened metabolic costs while hauled out. Higher levels of disturbance during the early morning, including the concentration of Level 3 flush events, suggests that the first vessel interaction of the day breaks the seals’ overnight rest, inducing a state of heightened vigilance or weary “tolerance” for the remainder of the day.

Furthermore, the extreme reactivity of seals to silent craft (SUPs and kayaks) highlights a critical flaw in current education. Standard wildlife guidelines focus heavily on motorised vessels, yet the sudden, visual “startle” effect of a silent paddleboarder mimics natural predator behaviour, triggering catastrophic stampedes. These stampedes pose severe physical risks, including pup separation, skin abrasions, and internal injuries on jagged rocks during falls.

Future Research and Education

This study confirms the widely accepted view that seal disturbance is closely related to vessel distance. It is possible that the relationship is both stronger and has greater reach than previously thought.

The model highlighted 3 other factors that have a significant bearing on the level of disturbance

The strength of this relationship within the context of the high proportion of Level 1 disturbance events has implications for future research and messaging:

1) - a greater and more precise understanding of the impact of lower-level disturbance is needed at both individual and population levels.

2).-.there is need for greater awareness of lower-level disturbance amongst all water users.

3) – messaging to boat operators in general and, quite specifically, to tripper boat operators needs to be more comprehensive, encompassing not simply distance but including factors such as boat behaviour, time of day and the number of seals hauled-out (group size).

4) – kayak and paddleboard users should be made aware of the specific risks that their activities can pose to seals.

Policy Recommendations

The data clearly demonstrates a massive “conservation deficit” between policy design and field reality. The current Defra 100-metre guideline is already a compromise, as the GLMM shows that a 46.8% disturbance probability remains at this distance. Compounding this, commercial tripper boats operate at less than half of this recommended buffer zone (48.2m), and over 70% actively violate the WiSe code of conduct.

The 100 metre guideline promoted by Defra and other organisations should not be seen as a safe “buffer” but as the absolute minimum distance, where significant behavioural and energetic costs are already being incurred.

Additionally, marine spatial planning should consider the implementation and enforcement of “no-stop” and “no wake” buffer zones,

Policies should support the educational initiatives outlined above.

Study Limitations

While robust, the study’s conclusions are bound by certain limitations.

1) The data is geographically isolated to a single offshore site in west Cornwall, meaning exact distance thresholds may shift depending on localized topography or the frequency and type of human activities.

2) Making observations involved close concentration on a specific haul-out group in order to take photographs, measurements and notes. The data should not, then, be taken as a comprehensive assessment of site-wide disturbance.

3) The resulting data was heavily skewed toward commercial tripper boats, limiting the ability to run a separate, highly powered GLMM specifically for non powered craft.

4) Finally, because this study relied on external behavioural observations, it cannot account for internal physiological markers—such as elevated cortisol or spiked heart rates—meaning the true metabolic cost of “tolerant” seals may be significantly underestimated.

This table summarizes the core outputs of the Generalised Linear Mixed-effects Model (GLMM), highlighting how heavily each statistically significant variable drives seal disturbance.

This section translates the raw field tracking data into clear operational metrics, comparing established conservation guidelines against real-world observations.

Table A: Guideline Compliance vs. Observed Field Reality

Table B: Standardized Levels of Marine Wildlife Disturbance

To objectively quantify seal reactions, the study utilized a standardized three-tier disturbance scale: