Abstract
This report summarises the current scientific understanding of how marine geophysical and geotechnical site investigation surveys impact fish and shellfish in Irish waters. It is intended to support evidence-led decisions for preparing and assessing Maritime Usage Licence applications.
Key impact pathways
Underwater noise and physical disturbance of the seabed are the main potential impact pathways. Most survey activities have a small spatial footprint, and effects are typically localised and short term, particularly for mobile adult fish and shellfish. Impacts are more likely where surveys overlap with sensitive habitats, vulnerable life stages, or occur repeatedly in the same area.
Noise-related effects
The evidence base is strongest for underwater noise. Behavioural responses are the most consistently observed effect, including startle responses, short-term displacement and altered foraging or schooling behaviour. For eggs, larvae and some shellfish, intense impulsive noise can cause injury or mortality under certain conditions. Continuous noise is more often linked to stress responses and behavioural disruption, particularly where exposure is prolonged.
Physical disturbance of the seabed
To our knowledge, there have been no studies on the physical impacts of geotechnical surveys on fish or shellfish. Potential effects are inferred from studies on dredging, drilling, aggregate extraction and bottom fishing. Seabed disturbance can cause localised benthic mortality, alter habitat structure and reduce prey availability. While geotechnical survey footprints are highly localised, significant impacts are most likely near sensitive habitats or during key fish spawning periods and spawning areas which are species specific.
Cumulative effects
Cumulative risk increases where surveys are clustered, repeated over time, or overlap with other marine activities. Species responses are likely driven by a combination of noise, vibration, physical presence and sediment disturbance rather than any single pressure acting alone. This is poorly documented in the literature and requires further research attention.
Knowledge gaps
Key knowledge gaps include the lack of field studies on geotechnical survey impacts on fish and shellfish, limited data on behavioural and sub-lethal effects for many species, uncertainty around early life stage sensitivity, and a lack of international best practice for managing impacts of site investigation surveys on fish and shellfish (or associated habitats).
Recommendations
Mitigation should be proportionate and precautionary where uncertainty remains. Key measures include avoiding sensitive habitats and life stages such as spawning periods, minimising repeated disturbance through coordinated planning, selecting lower-impact methods where feasible, managing noise and sediment disturbance, and ensuring robust survey planning, data sharing and communication with stakeholders.