Swimbladder volume and body density in an armoured benthic fish, the streaked gurnard

Body density of streaked gurnards Trigloporus lastoviza is anomalously low (mean 1·043 g ml⁻¹) for a benthic fish and overlaps with that of pelagic fish such as cod and lumpfish. It is clear that the heavy armour of gurnards is offset by buoyancy provided by the swimbladder. Swimbladder volume is a mean 3·88% of gurnard volume. Evidence is given to indicate that this is quite large for a marine teleost: data are also presented to show that the '5% of fish volume for marine fish, 7% for freshwater fish' rule for swimbladder volumes is unreliable.