To eat or be eaten

By Arina Favilla, SRC intern

When we are hungry, all we have to do is open up the fridge and decide what we want to eat. On the other hand, when fish are hungry, they must leave the safety of their home to forage in areas where there are likely predators awaiting them. They must balance their decision based on hunger and risk factors: is this next meal worth the possibility of becoming prey?

All organisms must eat to meet certain metabolic and energetic demands to survive. The metabolic rate of a fish is influenced by temperature because most fish are ectothermic, meaning their body temperature depends on the surrounding water temperature. As water temperature increases, their metabolic rate increases, which consequently increases how often they forage to meet their energetic requirements. However, foraging also means putting themselves at risk for predation. To investigate how temperature and the impact of predation risk affects foraging activity, Pink & Abrahams (2015) set up controlled experiments with fathead minnows and a likely predator, yellow perch.

Methods

 First, they observed foraging and activity rates of minnows exposed to different temperatures in aquarium tanks (39ºF, 59ºF, 75ºF). After food was dispersed by a feeder at the water’s surface, they recorded the number of times the fish would swim to the surface to eat as well as their activity level both before and after feeding for 30 min. They then added the risk of predation to assess how temperature impacts the level of risk fish are willing to take in order to eat. For 15 minutes, they observed the foraging activity of the fathead minnows and recorded whether they used the high-risk feeder closer to the perch’s tank or the low-risk feeder (experimental set-up is shown in Figure 1). These foraging behaviors were compared for minnows at different temperatures (41ºF, 59ºF, 73ºF).

Figure 1. The experimental set up of temperature effect on predator-prey interactions is depicted in the diagram. Fathead minnows were kept in one tank, which was placed close to a separate tank with the yellow perch. Two feeding devices were used: the one closer to the yellow perch tank was considered the high-risk feeder and the one further way, the low-risk feeder. For trials where no predator is present, a divider between the tanks was used to hide the predator.

Figure 1. The experimental set up of temperature effect on predator-prey interactions is depicted in the diagram. Fathead minnows were kept in one tank, which was placed close to a separate tank with the yellow perch. Two feeding devices were used: the one closer to the yellow perch tank was considered the high-risk feeder and the one further way, the low-risk feeder. For trials where no predator is present, a divider between the tanks was used to hide the predator.

Results

These experiments showed that temperature significantly influenced the foraging and activity rates of the fathead minnows. The fish exposed to warmer temperatures were more active and foraged more frequently (Figure 2). Although the presence or absence of the predator did not influence foraging rates, it did influence which feeder the fish chose, with more fish choosing the low-risk feeder when the predator was visibly present (Figure 3). However, as the temperature increased for the different treatment groups, more minnows were observed foraging at the high-risk feeder (Figure 3).

Figure 2. This graph shows that in warmer water the fathead minnows fed more often and were more active.

Figure 2. This graph shows that in warmer water the fathead minnows fed more often and were more active.

Figure 3. The graphs compare the response of the fathead minnows exposed to different temperatures to the presence and absence of the yellow perch. The top graph (a) shows that temperature did not affect whether the fathead minnows fed at the high-risk feeder or at the low risk feeder. The bottom graph (b) shows that with the predator visibly present, the fathead minnows preferred to feed at the low-risk feeder at cooler water, but in warmer water, they discriminated less between the feeders.

Figure 3. The graphs compare the response of the fathead minnows exposed to different temperatures to the presence and absence of the yellow perch. The top graph (a) shows that temperature did not affect whether the fathead minnows fed at the high-risk feeder or at the low risk feeder. The bottom graph (b) shows that with the predator visibly present, the fathead minnows preferred to feed at the low-risk feeder at cooler water, but in warmer water, they discriminated less between the feeders.

Outcomes

This study shows that temperature influences how much risk of predation is weighed in a fish’s decision to forage. At higher temperatures, fish are hungrier due to higher metabolic rates and are more likely to risk predation in order to feed because they don’t want to die of starvation. Predators have the potential to affect the structure of ecosystems both through their foraging as well as their effects on the behavior of their prey. Better understanding these prey-predator interactions and how environmental factors, such as temperature, influence them is critical in understanding ecosystem dynamics and predicting ecosystem responses to future environmental changes.

Reference

Pink, Melissa, and Mark V. Abrahams. “Temperature and Its Impact on Predation Risk within Aquatic Ecosystems.” Canadian Journal of Fisheries and Aquatic Sciences 73.6 (2016): 869-76.

Fish living in the “twilight zone” have a greater biomass than previously thought.

By James Keegan, RJD Intern

Mesopelagic fish, fish living at depths between 200 and 1000 meters in the ocean, reside in water with very low levels of light. Although they are typically small, mesopelagic fish constitute the largest biomass of fish in the world because of their immense numbers. Previous estimates state that there are about 1,000 million tons of mesopelagic fish worldwide. However, using data collected on the Malaspina 2010 Circumnavigation Expedition, Irigoien et al. 2014 show that there are about 10 times more mesopelagic fish than previously estimated. Such an increase in an already massive fish community alters how we determine the role mesopelagic fish play in ocean food webs and chemical cycling.

Lampfish25

A man holding the mesopelagic species Stenobrachius leucopsaurus. It belongs to a family of fish commonly known as the lanternfish. (Occidental College. url: http://www.oxy.edu/sites/default/files/assets/TOPS/Lampfish25.jpg)

Previously, scientists pulled nets behind their boats in a process called trawling in order to capture fish and estimate their populations. This process is not efficient in catching mesopelagic fish and leads to an underestimation of their numbers. Instead of trawling, scientists aboard the Malaspina 2010 used an echosounder, a type of SONAR, to determine the biomass of mesopelagic fish. In this method, the echosounder emits a pulse of sound into the water and records the sound that returns after bouncing off an object. Using sound to weight ratios previously determined in other studies, Irigoien et al. 2014 were able to estimate the mesopelagic fish biomass from the recorded acoustic data. Irigoin et al. 2014 then used food web models to corroborate the estimate given by the acoustic data. Their estimates determined the mesopelagic biomass to be about 10-15,000 million tons, about 10 times higher than previous estimates.

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Caption: Acoustic data collected on the Malaspina 2010. The top of the figure represents the surface of the ocean, and the bottom of the figure represents a depth of 1000 meters. The colors in the figure show where sound bounced off marine organisms and returned to the echosounder. Between 200 and 1000 meters, the organisms are mostly mesopelagic fish. The black triangles indicate the border between ocean basins. AT stands for Atlantic Ocean, IO for Indian Ocean, WP for Western Pacific, and EP for Eastern Pacific. (Irigoien et al. 2014)

Irigoien et al. 2014 also found that mesopelagic biomass is closely tied to the plankton, miniscule, floating organisms of the ocean, that undergo photosynthesis. These photosynthetic plankton form the base of the marine food web, and other, larger plankton consume them. Mesopelagic fish then feed on these herbivorous plankton.

Diatoms_through_the_microscope

A photo of diatoms, photosynthetic plankton, under microscope. (Wikipedia. url: http://en.wikipedia.org/wiki/File:Diatoms_through_the_microscope.jpg)

In the open ocean, where nutrients are poor, herbivorous plankton do not efficiently capture photosynthetic plankton. This implies that fish will not efficiently obtain their energy, which ultimately comes from the photosynthetic plankton. However, Irigoien et al. 2014 contest that the transfer of energy to the mesopelagic fish is more efficient in the open ocean because the water is warm and clear, allowing the visual fish to more easily capture their prey. Considering this argument, Irigoien et al. 2014 determined that mesopelagic fish may be using about 10% of photosynthetic plankton for energy.

Irigoien et al. 2014 showed that the biomass of mesopelagic fish, as well as their usage of energy in the open ocean food web, is much greater than previously thought. Due to the impact these two findings would have on ocean ecosystems and chemical cycling within them, scientists must make further and more accurate investigations regarding the mesopelagic fish community.

 

References:

Irigoien, Xabier, T.A. Klevjer, A. Røstad, U. Martinez, G. Boyra, J.L. Acuña, A. Bode, F. Echevarria, J.I. Gonzalez-Gordillo, S. Hernandez-Leon, S. Agusti, D.L. Aksnes, C.M. Duarte, S. Kaartvedt (2014) Large mesopelagic fishes biomass and trophic efficiency in the open ocean. Nature Communications 5, Article number: 3271 doi:10.1038/ncomms4271