Abstract
The present study assesses swimbladder dynamics in natural occurring sardine, Sardina pilchardus, populations with the aim to examine whether this is affected by bathymetric positioning and the physiological state of the individuals. To do so, swimbladder size and shape were modeled in relation to catch depth and the size of various visceral compartments such as gonad, liver, fat and stomach. Swimbladder size was shown to be related to depth in a way that individuals with smaller swimbladders occurred at larger depths. Moreover, evidence is provided that the swimbladder in sardine might have a functional relationship both with the reproductive and the feeding state of individuals, since none of the fish with hydrated gonads and/or large stomachs displayed distended swimbladders.
Highlights
In most marine teleosts the swimbladder acts primarily as a hydrostatic organ, while in a few species it may function as an organ for sound production
In order to examine whether swimbladder dynamics in the Atlantic sardine are affected by bathymetric positioning and the physiological state of individuals, we modeled the relationship of swimbladder size and shape with depth and the size of various visceral compartments
Macroscopic assignments of swimbladder size were validated with image analysis-based measurements of cross-sectional areas; both relative swimbladder size, R, and circularity, SBc, increased significantly in all four size levels of the macroscopic scale (Fig. 3) which validated its use in subsequent analyses
Summary
In most marine teleosts the swimbladder acts primarily as a hydrostatic organ, while in a few species it may function as an organ for sound production (e.g., oyster toadfish; Fine, McKnight Jr & Blem, 1995). The swimbladder provides a dual function, acting as a buoyancy regulating organ and as a gas reservoir for the acoustico-lateralis system (Nero, Thompson & Jech, 2004). The fish swimbladder is the main reflector of acoustic energy, being responsible for up to 90–95% of the backscattered sound intensity which is of primary importance in acoustic estimates of fish abundance (Foote, 1980). As a corollary, mapping the swimbladder structural morphology and understanding factors that may affect its size and shape is essential in quantifying its contribution to several biological functions such as buoyancy regulation and in improving accuracy in estimates of fish biomass from acoustic surveys.
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