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Newport Laboratory: Fisheries Behavioral Ecology Program

Thermal Influences on Walleye Pollock Behavior and the Bering Sea Food Web

Figure 1, see caption
Figure 1.  Scope for activity of an 80 mm juvenile walleye pollock as a function of temperature.  Scope for activity is expressed as the difference between maximum swimming speed measured in recirculating flume and routine swim speed measured in 2.9 m diameter arenas.

 
 

Temperature represents the most pervasive aspect of the environment affecting ectotherms and varies markedly on a variety of spatial and temporal scales. However, our understanding of the impact of temperature on marine food webs remains insufficient to address the consequences of long-term climate variability. This is in part because knowledge of the behavioral responses of fishes to temperature variation lags well behind our understanding of physiological responses. This has occurred despite the fact that behavior represents the link that allows (or prevents) physiological processes from scaling up to population and ecosystem-level responses.

Research at the Fisheries Behavioral Ecology Program (FBEP) continues to address the responses of Alaskan resource species to temperature variation and how these relate to food-web energetics and habitat quality. The approaches to these studies range from detailed observations of activity patterns of fish in small groups, to statistical analysis of the Bering Sea food web.

In one experiment, Thomas Hurst examined the swimming characteristics (routine swim speed, path sinuosity, and group cohesion) of juvenile walleye pollock in large arenas at temperatures between 2 and 9C. Routine swim speed and maximum swim speeds (measured in a recirculating flume) had contrasting responses to temperature, demonstrating a behavioral rather than physiological regulation of activity level.

Contrary to most assumptions, routine swim speed was higher at the low temperatures, whereas maximum swim speed displayed the expected increase with temperature. Hence, at low temperatures, walleye pollock use a much larger fraction of their swimming capacity in routine activity, leaving a smaller scope for activity (Fig. 1). Furthermore, these observations have important implications for bioenergetic analyses of growth and production of pollock. Specifically, they suggest that current models may be markedly underestimating the metabolic expenses and energetic requirements for growth of juvenile walleye pollock and species with similar thermal responses at low temperatures.

Another result of that study was that juvenile walleye pollock schooled more tightly at low temperatures than they did at higher temperatures, potentially serving to reduce predator encounters or increase survival following an encounter. Predator encounter rates reduce as the inter-fish distances and areal extent of the group decline. The reduction of distances between fish may also enhance the effectiveness of group responses when faced with an attack. Following up on the latter possibility, Hurst is working with FBEP technician Rich Titgen and University of Washington colleague Danny Grunbaum on analyzing the responsiveness of juvenile pollock to attacks by larger pollock.

  Figure 2, see caption
Figure 2.  Representation of computerized analysis of thermal effects on walleye pollock predation vulnerability.  Figure shows the digitized paths of six pollock prey (avg. 78.7 mm TL) and one pollock predator (232 mm TL) in a 2.9-m diameter area over 14 seconds (420 frames) at initiation of attack.  The digitized fish path output is overlaid on a still-frame from overhead video.
 

They have been adapting a computerized fish tracking system developed by Grunbaum for the difficulties of working with small targets (prey) in large experimental arenas. After stereotyped predator attacks are isolated on the video, the software identifies and tracks the position of each prey fish through the encounter with the predator (Fig. 2). The responsiveness of the prey group is described by metrics such as the distance at which their escape behavior is elicited and the time required for the prey to reform a cohesive group.

From a different perspective, the thermal forcing of marine food webs is being examined through an analysis of the AFSC food habits database with REFM colleague Troy Buckley. The researchers are looking at relationships between (spatial and interannual) thermal variation observed in the summer groundfish survey and the diet composition of Pacific cod.

Of particular interest is the predatory effect of cod on walleye pollock. Preliminary results indicate that temperature has a significant impact on the size classes of pollock consumed by cod in the southeast Bering Sea. At low temperatures (especially below 0.5C), cod predation is focused almost exclusively on pollock less than 150 mm standard length (SL). At higher temperatures (over 3.5C), over 15% of pollock consumed by cod were larger than 450 mm SL (representing 47% of pollock biomass). Alternative explanations for the observed shift in vulnerability of pollock to predatory cod are changes in encounter rates associated with distributional overlap or changes in relative physiological performance between predator and prey.

By approaching the question of thermal effects on food web dynamics from multiple perspectives, and combining laboratory and field studies, the research at the FBEP has the potential to improve empirically-based assessments of trophic control of managed species as well as the mechanistic understanding of these responses.

By Thomas Hurst
 

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