Physiological response and mortality caused by scale loss in Atlantic herring

Slipping of crowded herring (Clupea harengus) from purse seines may lead to high mortalities. We suggest that scale loss during crowding may be one reason for the mortalities. To test this hypothesis, herring were transferred to 5 m tanks and subsequently de-scaled on 25% or 50% of the body surface. Recovery was monitored for mortality (over 7 days), behavior (days 1 and 2) and blood metabolites (days 0, 1, 2, 4 and 7). Most mortalities occurred between 1 and 4 days post treatment and increased with the degree of de-scaling reaching 60% in worst cases. Larger fish with good condition had better survival than smaller fish. Many de-scaled fish increased their swimming speed significantly. Some also deviated from the normal schooling behavior swimming with the current rather than against it. De-scaling led to loss of osmoregulatory ability and presumably water loss as seen through increases in plasma ions and osmolality. Plasma glucose was reduced in de-scaled fish while plasma levels of cortisol and lactate were increased. An apparent bimodal response was observed in the blood data. While some fish recovered and returned to pre-treatment levels for most parameters within days, others were unable to recover. It is concluded that de-scaling may lead to mortality in herring. The mortality appears to be related to fish condition, loss of osmoregulation and increased stress response and energy expenditure. Altered behavior may contribute to the mortality.