Food deprivation affects vertical distribution and activity of a marine fish in a thermal gradient:potential energy-conserving mechanisms

Abstract

The effects of reduced food availability on the behavior of juvenile walleye pollock Theragra chalcogram~na were examined in laboratory experiments designed to test for potential energyconserving responses. Groups of juvenile fish were held on 1 of 6 ration treatments ranging from ad hbltum to near starvation, and thenvertical distribution and activity levels were quantified in a 2.5 m deep water column under isothermal and thermally stratified conditions. Stratification resulted in a general sh~ft to the upper, warmer layer in the 2 experiments employing a sharp thermocline at middepth, but the occurrence of fish in the colder bottom layer varied with different ratlon treatments. Movement lnto cold water increased in intermediate ration groups compared to high ratlon groups. Since reduced temperatures should reduce metabolic costs, this behavior is consistent with our hypothesis that food deprivation should invoke energy-saving behaviors. However, activity levels increased for fish held on intermediate rations, suggesting that the greater movement into cold water was a corollary result of increased searching for food. Fish in the lowest ration treatments had decreased activity levels, but also decreased their movement into cold water when a sharp thermocline was present, negating potential bioenergetic benefits. In the third experiment, there was a gradual thermal gradient from surface to bottom rather than a sharp thermocline. Temperatures associated with vertical positions of the fish were determined. In this experiment, clear energy-conserving responses to temperature were displayed by food-deprived fish; the average temperatures occupied by fish on starvation rations were 3 to 4°C colder than those of the higher ration groups. Based on the high Qlo for metabolic rates of juvenile pollock, these reduced temperatures potentially conferred energy savings of up to 34 %, relative to the metabolic expenditures of fish on high rations. The contrast in behavior for the lowest ration groups between sharply stratified and gradually stratified conditions suggested that the severity of the temperature gradient influenced the fishes' ability to take advantage of cold water as an energetic refuge. The behavior of fish in the laboratory was consistent with prior observations in the Bering Sea, where juvenile walleye pollock remained in surface waters if food availability was high, but initiated vertical migration into deeper, colder water with reduced prey densities. Results of this study demonstrated a broad flexibility in the behavioral mechanisms used by walleye pollock to deal with declining food levels. The initial response to food limitation was increased activity, indicative of greater searching behavior With extended food deprivation, a switch to energy-conserving behavior was evident. The temperature responses of fish experiencing severe food limitation provided support for a bioenergetic hypothesis of die1 vertical migration.

In situ observations of the association between juvenile fishes and scyphomedusae in the Bering Sea

“…3). It is not known why these fish do not utilize the shelter afforded by the medusae, but perhaps they have found it more beneficial to migrate further down in the water column for trophic or energetic reasons (see Bailey 1989, Sogard & Olla 1996b. The video observations also document occurrences of pollock aggregations close to the surface during night deployments.…”

Recruitment of coastal fishes and oceanographic variability in central California

“…Lastly, the relative lack of significant correlations between re cruitment and event scale oceanographic processes may also be explained by the adequate swimming ability of these fishes and their behavioral adaptations to exploit preferred food sources and avoid predation (Parrish et al, 1981;Lenarz et al, 1991;Sogard and Olla, 1996). Late stage juvenile pelagic rockfish are strong swimmers and can actively migrate nearshore (Moser and Boehlert, 1991), thus they may follow food sources to the nearshore envi ronment or respond to a physiological cue to settle during a certain developmental window regardless of the nearshore transport processes occurring.…”

Influence of starvation on the critical swimming behaviour of the Senegalese sole (<i>Solea senegalensis</i>) and its relationship with RNA/DNA ratios during ontogeny

“…The two main mortality agents acting upon marine fish larvae are predation and starvation (Bailey and Houde, 1989). These factors are not independent, as starvation leads to a decreased growth rate (Yin and Blaxter, 1986), slower development (Kamler et al, 1990) and changes in behaviour (Sogard and Olla, 1996;Chick and Van den Avyle, 2000). Larvae with a low nutritional status will consequently be smaller, weaker and less developed with regard to sensory and locomotory capacities than well-fed larvae of the same age, thus being more susceptible to predation (but see Billerbeck et al, 2001).…”