Bone is a highly anisotropic, viscoelastic material that has the ability to continually adapt to changes in its physiological or mechanical environment. The capacity of bone to resist mechanical forces and fractures depends not only on the quantity of bone tissue but also on its quality.
Bone is a composite material, made from a collagenous matrix and from minerals. The collagenous matrix provides toughness (fracture resistance) and the minerals increase the bone’s stiffness (bending resistance). By itself, the mineral phase is brittle and fractures easily. Bone strength largely depends on the non‐mineralized matrix, specifically on the orientation of collagen fibres that are arranged according to the direction of mechanical load. Alterations of collagen properties can therefore affect the mechanical properties of bone and increase fracture susceptibility. Consequently, collagenous bone matrix without minerals can be tougher than mineralized bone
Electrofishing has been used for many years as a sampling technique employing electric currents and electric fields to control fish movement and/or immobilize fish, allowing their capture. Salmonids are more susceptible to electrofishing injuries than other species. The most commonly reported serious injuries to fish from electrofishing are spinal dislocations and, in extreme cases, vertebral fractures that are apparently caused by strong muscle contractions. Internal haemorrhaging is also seen, as is skin discolouration, referred to as “branding”. A large proportion of spinal injuries evident on X-rays cannot be seen from external examination.
Besides electrofishing methods, spinal injury of fish due to electricity can be seen following electrical storms (lightning strikes), or in hatcheries, where equipment such as feeders are improperly earthed.
Especially severe spinal injuries or muscular haemorrhages can be represented externally by brands (these are in fact bruises) bent backs, punctures, or abnormal swimming, but in most fish even severe injuries are not externally obvious.
The principal cause of spinal injuries appears to be muscular convulsions (myoclonic jerks or seizures) induced by sudden changes in field intensity or, more specifically, in voltage differential across the fish or affected tissues at or above a relatively low threshold in magnitude of change for twitch. It is interesting to note that in salmonids, the major location for these spinal changes is usually at the level of the dorsal fin; possibly this is the “fulcrum” for the muscle contractions.
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