Depth-dependent swimbladder compression in herring Clupea harengus observed using magnetic resonance imaging

Area of Science:

• Marine Biology
• Acoustic Oceanography
• Fish Physiology

Background:

• The swimbladder is a key organ influencing fish buoyancy and acoustic properties.
• Accurate assessment of fish populations using acoustics relies on understanding how fish morphology changes with depth.
• Atlantic herring (Clupea harengus) are commercially important, making their acoustic biomass estimation critical.

Purpose of the Study:

• To investigate the morphological changes of the Atlantic herring swimbladder under varying hydrostatic pressure.
• To determine how these morphological changes influence the fish's acoustic backscattering properties.
• To assess the impact of swimbladder compression on acoustic biomass estimation methods for herring.

Main Methods:

• Magnetic resonance imaging (MRI) was used to visualize swimbladder morphology.
• A dead Atlantic herring specimen was subjected to controlled pressure changes in a specialized chamber.
• Swimbladder volume and compression patterns were analyzed at different pressure levels.

Main Results:

• Swimbladder volume decreased with increasing pressure, consistent with Boyle's Law.
• Swimbladder compression was anisotropic, with greater reduction in lateral dimensions compared to dorsal dimensions.
• Uneven compression resulted in a less significant alteration of acoustic backscattering than symmetrical compression would.

Conclusions:

• The anisotropic compression of the Atlantic herring swimbladder mitigates the expected increase in acoustic backscattering with depth.
• This morphological response has implications for the interpretation of acoustic data used in fisheries stock assessments.
• Further research should incorporate these findings to refine acoustic biomass estimation models for Clupea harengus.