Behavior and ecology of overwintering sprat Sprattus sprattus

Abstract

Sprat Sprattus sprattus is a marine, forage fish of high ecological and economical value. This thesis addresses the ecology and behavior of sprat during overwintering, a phase that is poorly studied in marine pelagic fish. Stationary upward-facing echosounders were used to study individual behavior as well as collective behavior of sprat throughout four separate winters in a high-latitude, hypoxia-impacted fjord. The acoustic studies were supplemented by field campaigns. Long-term, continuous coverage of several winters provided unique information on sprat behavior in relation to different environmental conditions, for example ice-free versus ice-covered waters and hypoxic versus welloxygenated conditions. Feeding behavior of sprat in relation to size, as well as abundance, composition and distribution of potential prey and predators could furthermore be established. The physical and biological properties at the overwintering fjord-habitat varied considerably between winters, as also did length-distribution and feeding activity of sprat. Generally, the largesize classes of sprat had empty stomachs, particularly in one winter. The apparent lack of feeding suggests an inactive overwintering mode where predator avoidance is prioritized at the expense of feeding. The winter diet of sprat otherwise varied according to prey availability, yet with a positive selection towards calanoid copepods, especially the larger-sized Calanus spp. Sprat was shown to have a flexible behavioral repertoire, displaying different behaviors with changing conditions. For instance, ice conditions promoted a response where sprat moved to shallower depths after the ice covering, and oxygen content proved to be an important factor governing their vertical distribution and diurnal behavior. Solitary swimming in near-bottom waters prevailed in moderate hypoxia (~ 30 % O2 saturation) as opposed to schooling in mid-waters when deep waters were oxygen depleted (0-7 % O2 saturation). Two different anti-predation strategies were likely displayed: “hiding at depth” and “hiding in schools”. Highly varied, and mixed diel vertical migration (DVM) patterns were always observed. The dynamic behavior of sprat was likely shaped by individual strategies involving optimized feeding and predator avoidance, as well as relating to temperature, hypoxia and negative buoyancy. Negative buoyancy appears as common in sprat, as implied by frequent “rise-and-sink” swimming while not schooling.

Stationary, upward-facing echosounders proved to be a powerful tool for assessment of individual behavior of sprat, and novel information on several behavioral traits is provided in this thesis. A surfacing behavior was always prevalent at nighttime with individual sprat carrying out rapid excursions to- and from the surface. Associated gas release suggested that the physostome sprat gulped air when at the surface, presumably to refill their swimbladders. The results further indicated that this behavior affects the acoustic properties of sprat. These findings are important considering the potential implications this can have for reliable stock assessments of forage, physostome fish. The discovery of brief dives into severely oxygen depleted waters by individual sprat revealed that sprat can tolerate very low oxygen levels (< 0.45 mL O2 L-1 at 8 ºC) for a couple of minutes. The results further indicated that sprat exhibit certain tactics for dealing with what can be a typical challenge in hypoxia-impacted habitats; reduced overlap with potential prey Overall, this study provides a unique insight into the dynamic conditions that sprat may encounter while overwintering and shows that the overwintering strategies of sprat are flexible and varied. It also contributes with novel information on a scarcely described phase in the life history of fish at high latitudes.