Planktivore vertical migration and shoaling under a subarctic light regime
Summary (4 min read)
Introduction
- Light is important for visually oriented predators as darkness provides cover for their prey, and behavioural responses to changes in light intensity are often associated with predator-prey interactions (Blaxter 1975; Helfman 1993; Pitcher and Parrish 1993) .
- A predator increasing its foraging activity will simultaneously increase its predation risk, and there is therefore a tradeoff between foraging gain and predation risk (Gilliam and Fraser 1987; Lima and Dill 1990) .
- In some planktivorous fish species, swimming activity has been observed to be highest in crepuscular light (Batty 1987; Iida and Mukai 1995; Gjelland et al. 2004) .
- By contrasting day and night samples from June, August, and September, the authors investigated how behavioural traits relate to the changing light regime, i.e. both within the diel cycle and during the ice-free season.
Methods
- In order to evaluate DVM and shoaling patterns in planktivorous coregonids, the authors combined echosounding techniques with gillnetting for planktivores, planktivore diet analysis, and zooplankton sampling in a high latitude lake at periods of contrasting differences in the diel light cycle.
- Published literature on coregonid reactive distance and salmonid piscivore reactive distance in relation to light intensity were used to evaluate the influence of light level on the foraging opportunity and predation risk for the studied planktivores.
Study site and fish community
- The pelagic fish community of the oligotrophic Lake Skrukkebukta was sampled around the 20 th of June, August, and September 2000.
- The ice-free season in the watercourse lasts from the end of May or beginning of June to October -November.
- Two morphs of whitefish have been described: a pelagic densely-rakered (DR) morph, which forages predominantly on zooplankton, and a larger benthic-dwelling sparsely-rakered (SR) morph, which forages on benthic prey (Amundsen et al. 2004; Østbye et al. 2006) .
- DR whitefish and vendace are the dominant pelagic fish in the Pasvik watercourse (Bøhn and Amundsen 2001; Gjelland et al. 2007) , with brown trout being the dominant pelagic predator (Bøhn et al.
- About 1000 of these fish are released into Skrukkebukta.
Reactive distance relative to light
- In order to develop a reactive distance model of visual foraging in coregonid planktivores, the authors analyzed data on Coregonus artedi reactive distance in relation to prey size (Link 1998 ) and light intensity (Link and Edsall 1996) .
- The authors found that there was a constant relationship.
- The full coregonid reactive distance model as a function of light and prey size can now be given (Fig. 2d , Eq. 6, prey length and reactive distance in m).
- The species-specific reactive distance may differ at a given light intensity, but the shape of the reactive distance to light relationship is remarkably similar and there is no further increase in the reactive distance above approximately 18 lux for any of the species.
- The shape of the reactive distance model was little influenced by the turbidity.
Light measurements
- The light extinction coefficient k was estimated from light profiles sampled in 0.5 m intervals during June and August.
- Surface illumination (unit lux) was estimated using hourly averaged global irradiation data (Wm -2 ) from Bioforsk Soil and Environment Division, Svanhovd research station, situated about 10 km from the study lake.
- The exact conversion between Wm -2 and lux depends inter alia on weather and sun elevation.
- For the August and September nights, when light level was too low for global irradiation measurements, the Janiczek and DeYoung (1987) model was used to estimate the surface illumination in lux.
Zooplankton sampling
- Zooplankton samples were collected using a 30 l Schindler-Patalas trap with 65µm mesh size.
- In the laboratory, all crustacean individuals in the daytime samples were counted and identified to species or genus, other prey taxa were identified to family level.
- Only cladocerans were counted in the night-time samples.
Biological sampling
- The relationship between target strength TS (the logarithmic domain of acoustic backscattering area, positively related to fish size) and fish length normally use total length LT of the fish (Simmonds and MacLennan 2005) .
- LT was found by multiplying LF with 1.08, a conversion factor found from subsamples of both coregonid species in the catches.
- The age of the coregonids was read from whole otoliths (Skurdal et al. 1985) .
- Prey items in the coregonid stomachs were categorized as Bosmina, Daphnia, Cyclopoida, Calanoida, benthic invertebrates, insect pupae, or surface insects.
- The stomach fullness was subjectively determined on a scale from 0 to 100 % (full), and the contribution of each prey category to the total volume of the stomach content was likewise determined.
Acoustic sampling
- To monitor and evaluate swimming behaviour of pelagic fish, sampling was performed using acoustics with a combination of mobile vertical (down-looking beam, oriented 90° from surface) and horizontal (side-looking beam, oriented approximately 5° from surface) techniques around midnight and mid-day (Fig. 1b ).
- The down-looking acoustics were used to quantify fish depth distributions, depth of shoals, and fish density estimation.
- Only transects parts covering depths greater than 15 m were used, with a degree of coverage c = 3 (Aglen 1983) for each of the side-and downlooking surveys.
- The length of the acoustic pulse (0.44 m) was subtracted from the lower range.
Reactive distance
- The reactive distance relationship to light produced by the planktivore coregonid reactive distance model (Eq. 6) differed somewhat in shape from that of the piscivore salmonids reactive distance model (Eq. 7) (Fig. 2d ).
- But the two models also share a similarity in that.
Gillnet catches and fish density
- A total of 330 fish were caught in the pelagic gillnets.
- Of these, 10 SR whitefish and 1 pike was excluded from the further analyses.
- DR whitefish dominated the catches in all months.
- The pelagic coregonid fish community consisted of small individuals with modal lengths of approximately 10 cm for both vendace and DR whitefish (Table 2 ).
- The pelagic fish density in September was estimated to 1799 fish ha -1 (range 801 to 3197 for the 95 % lower and upper confidence intervals, respectively) by the Sv/TS scaling method.
Zooplankton distribution and coregonid diet
- The highest daytime zooplankton densities were found close to the surface in all sampling months.
- Around midnight, the vertical distributions of Bosmina and Daphnia were relatively even, whereas the depth distribution of both these species was skewed towards the surface during mid-day (Fig. 3 ).
- This indicated that there was a tendency towards reversed DVM in these two zooplankton species.
- The order of importance of the prey categories found in coregonid stomachs was Bosmina, chironomid pupae, Daphnia, surface insects, Cyclops scutifer, Leptodorea kindti, and with benthic prey items such as Chydorus and chironomid larvae as the least important of included prey items (Fig. 3 ).
- The coregonid stomach fullness was least in June, and increased towards September (Fig. 3 ).
Diel vertical migration and shoaling patterns
- There was a consistent pattern of vertical migration, with day vertical fish distributions being deeper than midnight distributions in all months (Fig. 4 ).
- The difference in the centre of gravity Dcg between day and midnight depth distributions in June was only 1.2 m and not significant (Tukey test, P=0.77), as seen with the down-looking surveys (Fig. 4 ).
- The increased depth of the day vertical fish distributions towards August and September was stronger than the increased light penetration (Fig. 4 ).
- The distribution peaks were below the 0.4 lux light level in August and September (Fig. 4 .
- This pattern was seen both with the down-looking and side-looking surveys.
Discussion
- The authors findings show that DVM behaviour in coregonids consistently varied with changes in the day-night light cycle.
- Deeper day-time than night-time distributions of the fish were observed in all months, and the range of the DVM increased with increasing differences in light levels between night and day from June to September.
- The pattern of a more extensive DVM as differences between day and night light levels increased supports the hypothesis that DVM is strongly influenced by the light level (Blaxter 1975) .
- The optimal trade-off between foraging and predation risk is thus argued to be state dependent (Lima and Dill 1990; Milinski 1993; Lima 1998b) , although field evidence is sparse.
- The planktivore coregonids avoided light levels below the light threshold for visual foraging inferred from foraging experiments in other coregonids, suggesting a preference for a visual foraging mode.
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Citations
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Cites background from "Planktivore vertical migration and ..."
...For example, diel vertical migration is a behavioral strategy observed in many fish (Brett 1971; Gjelland et al. 2009; Hrabik et al. 2006), with diel shifts often linked to changes in diet and habitat use (Nunn et al. 2010)....
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Cites background from "Planktivore vertical migration and ..."
...Similar light-mediated shoaling behaviours have been shown for coregonids (Gjelland et al. 2009)....
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104 citations
References
7,461 citations
"Planktivore vertical migration and ..." refers background in this paper
...A predator increasing its 59 foraging activity will simultaneously increase its predation risk, and there is therefore a trade-60 off between foraging gain and predation risk (Gilliam and Fraser 1987; Lima and Dill 1990)....
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...foraging activity will simultaneously increase its predation risk, and there is therefore a trade60 off between foraging gain and predation risk (Gilliam and Fraser 1987; Lima and Dill 1990)....
[...]
3,945 citations
"Planktivore vertical migration and ..." refers background in this paper
...The reactive distance can be defined as the distance at which an animal reacts to and 141 initiates an attack on a prey (Holling 1959)....
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...139 Reactive distance relative to light 140 The reactive distance can be defined as the distance at which an animal reacts to and 141 initiates an attack on a prey (Holling 1959)....
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3,129 citations
"Planktivore vertical migration and ..." refers background in this paper
...According to the μ/g-rule (Werner and Gilliam 1984), animals with continuous 549 growth up to a minimum reproductive size should choose behaviours that minimize mortality 550 (μ) per unit increase in growth (g)....
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1,906 citations
1,691 citations
"Planktivore vertical migration and ..." refers background in this paper
...…on its prey population 45 may be as important as the consumptive effects (i.e. removal of individuals) in population 46 regulation, and are often transmitted through dynamic traits such as behaviour of individuals 47 in the prey population (Lima 1998a; Preisser et al. 2005; Pangle et al. 2007)....
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...The optimal 568 trade-off between foraging and predation risk is thus argued to be state dependent (Lima and 569 Dill 1990; Milinski 1993; Lima 1998b), although field evidence is sparse....
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...This simple relationship has been extended to other 551 animals in the μ/f-rule (Lima 1998b), were f denotes foraging rate (Gilliam and Fraser 1987; 552 Clark and Levy 1988)....
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