The extent to which temperature, temperature gradients, predator smell, and prey availability influence the migratory behaviors and vertical distribution of the opossum shrimp, Mysis relicta, was explored through controlled laboratory experiments and comparisons with field distributions of mysids in Lake Ontario. By varying environmental conditions in 2-m tall experimental columns in a temperature-controlled room, we determined that mysids prefer temperatures between 6uC and 8uC with limited movement into waters of 12uC or higher. No mysids moved into waters above 16uC in the absence of prey. However, a higher proportion of mysids moved into temperatures of 14uC and 16uC (but not 18uC) when densities of Daphnia pulicaria exceeding 120 L 21 were present at those temperatures. Mysis avoided waters with kairomones from a primary mysid predator, the alewife (Alosa pseudoharengus). The rate of temperature change with depth did not restrict mysid movements. A temperature preference function based on the experimental data was applied to an existing model of mysid vertical distribution. The modified model predicted the depth of maximum mysid density to within 1 m and yielded high percentage overlap index values when compared with published mysid vertical distributions in Lake Ontario. Our approach may be used to model how diurnal, seasonal, and larger climactic changes can impact both the vertical position and feeding ecology of mysids, a keystone species in many deep-water pelagic food webs.

Light and temperature strongly influence the vertical distribution of the mysid shrimp, Mysis relicta . We monitored the vertical movements and depth selection behavior of mysids exposed to different light intensities and light–temperature gradients in the laboratory and derived a mysid light preference function in units relevant to mysid vision: “mylux”. Mysids preferred light levels between 10−8 and 10−7 mylux (∼10−6 to 10−5 lux) and rarely moved into waters of 10−3 mylux (∼0.1 lux) and greater. A model that assumed equal weight and independence of mysid light and temperature preference functions successfully predicted the proportion of mysids found in two different temperature–light combinations in the laboratory. This model also predicted the depth of maximum mysid density to within 2 m on two spring nights and within 5 m on two summer nights of varying moon phase and thermal conditions in Lake Ontario. This study provides novel insights into how temperature and light interact to influence the vertical distribution of mysids. Our model may be used to predict mysid vertical distribution in any deepwater system inhabited by mysids in which the primary mysid predators are visual feeders.

The opossum shrimp Mysis diluviana (formerly M. relicta) performs large amplitude diel vertical migrations in Lake Ontario and its nighttime distribution is influenced by temperature, light and the distribution of its predators and prey. At one location in southeastern Lake Ontario, we measured the vertical distribution of mysids, mysid predators (i.e. planktivorous fishes) and mysid prey (i.e. zooplankton), in addition to light and temperature, on 8 occasions from May to September, 2004 and. We use these data to test 3 different predictive models of mysid habitat selection, based on: (1) laboratoryderived responses of mysids to different light and temperature gradients in the absence of predator or prey cues; (2) growth rate of mysids, as estimated with a mysid bioenergetics model, given known prey densities and temperatures at different depths in the water column; (3) ratio of growth rates (g) and mortality risk (μ) associated with the distribution of predatory fishes. The model based on light and temperature preferences was a better predictor of mysid vertical distribution than the models based on growth rate and g:μ on all 8 occasions. Although mysid temperature and light preferences probably evolved as mechanisms to reduce predation while increasing foraging intake, the response to temperature and light alone predicts mysid vertical distribution across seasons in Lake Ontario.

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An understanding of the effect of light on predator-prey interactions in aquatic systems requires the integration of sensory physiology, behavioral ecology, and spatial distributions of predator and prey in the field. Here, we present such an integrative approach to a study on the interactions between the alewife, Alosa pseudoharengus, and the mysid shrimp, Mysis diluviana, (formerly M. relicta) in Lake Ontario at night, when it is unknown whether visual feeding is possible. Visual pigment analyses of alewife rod photoreceptors were used to derive an alewife-specific unit of brightness-the 'alelux' (wavelength of maximum absorbance, l max 5 505 nm)-which formed the basic unit of light intensity in alewife feeding-rate experiments and field applications. At light levels of 10 27 alelux (, 10 24.1 lux) and greater in the laboratory, alewives engaged in visual search and strike behaviors and fed at rates that were significantly higher than those under completely dark conditions. Field observations from Lake Ontario showed that light levels at the upper edge of the mysid distribution were within the range of those required for visual feeding in the laboratory on a full moon night, but not on a new moon night. These increased light levels translated into feeding rates that were . 30 times higher on the full moon night, despite a larger degree of spatial separation of the two trophic levels. We hypothesize that observed increased water clarity in Lake Ontario in recent years has led to increased consumption of mysids by alewife at night and associated food-web changes.

We investigated growth rate, nucleic acid (DNA, RNA) and protein indices and respiration in juvenile (8.5 to 12 mm total body length, 7 to 20 mg wet wt) and young adult (12 to 14 mm, 20 to 30 mg wet wt) Mysis relicta, as a function of temperature, body mass and molt stage in order to develop methods to assess condition or growth in the field. Mysids were exposed to either a preferred temperature (6.5°C) and 3 ration levels, or a range of constant and dielly-cycling (DC) temperatures with ad libitum feeding. Mysid growth parameters (specific rates of growth [SGR], respiration [M O 2 ], and RNA content cell -1 ) integrated the DC temperature experienced as averaged responses weighted by the time spent at each temperature. M O 2 peaked at 12.7°C on acute temperature exposure from 4.2°C. M O 2 compensation with prolonged temperature exposure occurred at mean diel temperatures ≤ 8.5°C. Mysids could not survive at 16°C even for 5 h d -1 . These results confirm behavioral observations of temperature preferences. RNA concentration in M. relicta increased with ration and decreasing temperatures. Protein:DNA ratio, %protein and SGR increased with ration and then plateaued. Protein:DNA ratio, %protein and DNA:weight ratio did not change with temperature with unlimited feeding. Forward, stepwise, multiple regression models for each experiment and the combined data accounted for 31 to 72% of variability in SGR. Our experimental data provide guidance, a preliminary temperature-correction factor for RNA, and benchmarks for use of nucleic acid and protein indices in assessing growth or condition of M. relicta in the field.

Mysis relicta can be observed on echograms as a sound scattering layer when they migrate into the water column at night to feed on zooplankton. However, quantitative measures of mysid abundance with hydroacoustics requires knowledge of mysid target strength (TS), a method of removing fish echoes and contribution from noise, and an understanding of the effect of range on the ability of hydroacoustics to detect mysids (the detection limit). Comparisons of paired net data and acoustics data from July 7, 2005 yielded a mysid TS of −86.3 dB (9 mm animal) and a biomass TS of −58.4 dB (g dry wt) −1 . With ambient noise levels (S v of −125 dB at 1 m depth) and this TS, we can detect a mysid density of 1 m −3 at 60 m depth with a signal to noise ratio of 3 dB. We present a method to remove backscattering from both noise and fish and apply this method and the new TS data to whole lake acoustic data from Lake Ontario collected in July 25-31, 2005 with a 120 kHz echosounder as part of the annual standard fish survey in that lake. Mysis abundance was strongly depth dependent, with highest densities in areas with bottom depth >100 m, and few mysids in areas with bottom depth <50 m. With the data stratified in five bottom depth strata (>100 m, 100-75 m, 75-50 m, 50-30 m, <30 m), the whole-lake average mysid density was 118 m −2 (CV 21%) and the whole-lake average mysid biomass was 0.19 g dry wt m −2 (CV 22%) in July 2005. The CVs of these densities also account for uncertainty in the TS estimates. This is comparable to whole-lake density estimates using vertical net tows in November, 2005 (93 m −2 , CV 16%).

We measured acoustic backscattering from Mysis relicta , a common invertebrate in northern lakes, using five frequencies (38, 120, 200, 430, and 710 kHz). Acoustic backscattering from mysids was highest at 430 kHz and lowest at 38 kHz (19 dB lower). Maximum difference between the four other frequencies was 5.2 dB. Mysid target strength (TS) ranged from –80.1 dB at 430 kHz to –99.4 dB at 38 kHz (12 mm average length, range 5–21 mm). A theoretical scattering model (Stanton’s fluid-like, bent-cylinder model) predicted TS within 0.3–1.9 dB of observed TS for the different frequencies. The detection range was lowest at 38 and 710 kHz and greatest at 120 and 200 kHz. Fish were common above the mysid layer and produced higher acoustic backscattering at 38 kHz than at the other frequencies. A combination of 38 kHz and 120 or 200 kHz provides a strong contrast between mysid and fish acoustic backscattering that would help separate these groups using acoustic data.