report survey tarbert razor

Marine Scotland has published a report from surveys conducted on Tarbert Bank to assess razor clam densities and sizes

In 2023, a comprehensive survey was conducted on Tarbert Bank to assess razor clam densities and sizes, particularly focusing on Pod Razor (Ensis siliqua) and Razor Shell (Ensis magnus).

This initiative was part of the Scottish Government’s electrofishing scientific trial.

 

Key Findings:

 

Survey Methodology:

– The survey utilised a combination of commercial electrofishing rig and towed video on the fishing vessel ‘Skye.’

– Video recordings were analyzed to estimate the number and sizes of razor clams observed on the seabed.

– The study covered three size classes and converted count data to area densities (numbers of razor clams per square meter).

 

Survey Details:

– Ninety-five tows were completed down to 22.5 meters water depth.

– The survey was divided into north and south sectors, with different fishing regulations.

– Commenced on February 16, 2023, and concluded on March 3, 2023, with additional catchability experiments on March 4, 2023.

– Water temperatures ranged from 8.4 to 8.9°C, and salinities ranged from 33.7 to 34.2.

 

Razor Clam Densities:

– The average density of E. siliqua across all sizes was 1.57 individuals per square meter, with a maximum of 6.68 individuals per square meter.

– Northern areas (fished) had an average density of 1.87 individuals per square meter, while southern areas (unfished) had 1.16 individuals per square meter.

– Larger E. siliqua dominated in the unfished area, with different size distributions noted between north and south sectors.

– Spatial patterns indicated higher densities of medium and small-sized E. siliqua in the presently fished area.

 

Spatial Distribution and Hydrodynamics:

– Small-sized E. siliqua were found at higher densities around the northern periphery of the bank, suggesting a possible link to local hydrodynamics.

– No clear trends were observed relating E. siliqua densities to water depth.

 

Other Observations:

– Video recordings also captured other marine life, including crabs, hermit crabs, starfish, brittlestars, juvenile flatfish, and sandeels.

– Eider ducks, observed in previous surveys, were not spotted during this survey.

 

Efficiency of Electrofishing Equipment:

– Depletion experiments suggested an average catchability of at least 82%, with potential for improvements by adjusting towing speeds.

– Catchability may vary in cold water, and further studies during summer months are recommended.

 

Conclusion:

– The data collected establishes a baseline for future surveys, revealing significant differences between presently fished and unfished sectors.

– Local hydrography may play a crucial role in razor clam recruitment dynamics, as indicated by spatial patterns.

– This comprehensive report contributes valuable insights into razor clam populations, informing future conservation and fishery management efforts on Tarbert Bank.

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