Mysid and fish zooplanktivory in Lake Ontario: quantification of direct and indirect effects

Abstract

Mysis relicta and planktivorous fish feed on zooplankton in Lake Ontario and form a trophic triangle that includes intraguild predation by fish on mysids. Thus, fish affect zooplankton both directly and indirectly. To evaluate the importance of alewife (Alosa pseudoharengus), rainbow smelt (Osmerus mordax), and mysids as zooplanktivores in Lake Ontario, we measured abundances and distributions, assessed diets, and computed mysid and fish consumption rates based on bioenergetics models. We further estimated indirect effects by comparing clearance rates given observed and potential mysid distributions. Estimated consumption rates varied widely with season and water depth and ranged between 2.6 × 10 -3 and 1.3 g·m -2 ·day -1 for mysids and between 1.4 × 10 -3 and 0.5 g·m -2 ·day -1 for fish, representing a daily removal of zooplankton of up to 10.2%·day -1 and 2.0%·day -1 by mysids and fish, respectively. Mysid planktivory exceeded fish planktivory in May and August, but fish planktivory dominated in October. Estimated mysid planktivory rates were 2-to 90-fold lower than the potential rate if mysids moved to temperatures that maximized their predation rates, suggesting an indirect positive effect of fish on zooplankton.Résumé : Dans le lac Ontario, Mysis relica et les poissons planctonophages se nourrissent de zooplancton et forment ainsi un triangle trophique qui inclut la prédation par des membres de la guilde des poissons sur les mysidacés. Les poissons affectent donc le zooplancton à la fois directement et indirectement. Afin d'évaluer l'importance de la consommation de zooplancton par le gaspareau (Alosa pseudoharengus), l'éperlan arc-en-ciel (Osmerus mordax) et les mysidacés, nous avons mesuré les abondances et les répartitions, déterminé les régimes alimentaires et calculé les taux de consommation des mysidacés et des poissons d'après des modèles bioénergétiques. Nous avons estimé, de plus, les effets indirects en comparant les taux de clearance, compte tenu des répartitions observées et potentielles des mysidacés. Les taux de consommation estimés varient considérablement en fonction de la saison et de la profondeur de l'eau: ils vont de 2,6 × 10 -3 à 1,3 g·m -2 ·jour -1 chez les mysidacés et de 1,4 × 10 -3 à 0,5 g·m -2 ·jour -1 chez les poissons, ce qui correspond à des retraits quotidiens du zooplancton respectifs de jusqu'à 10,2 % par jour et de 2,0 % par jour. La consommation de plancton par les mysidacés dépasse celle des poissons en mai et en août, mais celle effectuée par les poissons domine en octobre. Les taux estimés de consommation du plancton par les mysidacés sont 2-90 fois plus faibles que les taux potentiels que l'on obtiendrait si les mysidacés se déplaçaient vers les températures qui maximisent leur taux de prédation; il semble donc y avoir un effet indirect positif des poissons sur le zooplancton.[Traduit par la Rédaction] Gal et al. 2747

Importance of light, temperature, zooplankton and fish in predicting the nighttime vertical distribution of Mysis diluviana

“…1, Table 3). We assume that the fish in this deeper layer in September 2005 were primarily rainbow smelt, while fish caught above the thermocline (< 27 m) were primarily adult and juvenile alewives (see discussion in Gal et al 2006). Because so few alewives were caught in our trawls in September, 2005, we cannot draw conclusions about segregation of alewife age classes on this date.…”

“…Zooplankton densities are typically very low below the thermocline in Lake Ontario (Johannsson et al 1994, Gal et al 2006) and remain at constant, low-density levels below 30 m (see Benoit et al 2002 for discussion). Given (1) that the thermocline was shallower than 30 m in all 8 profiles, and (2) the similarity in zooplankton density estimates in the 40 to 50 m and 30 to 40 m stratified net tows in 2005, we do not think that there would have been a substantial peak in zooplankton density below 30 m that is correlated with the mysid peak; however, we cannot exclude this possibility, and future investigations may provide further insight into the prevalence and importance of deep zooplankton layers on mysid behavior.…”

“…We elected not to test a model based on mortality risk alone, since alewife densities drop markedly beneath the thermocline in the offshore part of Lake Ontario (Gal et al 2004), and this would ensure minimization of predation pressure at the deepest depth evaluated if foraging gains in shallower waters were not considered. We have already shown that this is an unrealistic prediction for mysids in Lake Ontario (Gal et al 2004(Gal et al , 2006. The objective of this study is to test whether mysid distribution can be predicted based on adult light and temperature preferences alone, or whether predator and prey distributions must be invoked to predict their distribution.…”

“…In Lake Ontario, the zooplankton prey of mysids is concentrated in the upper portion of the water column (Gal et al 2006) and their distribution may influence the nighttime habitat selection of mysids. Larsson (1997) demonstrated that a population of Daphnia pulex was distributed throughout different sections of an experimental chamber in direct proportion to the relative abundance of algal food resources, and argued that the distribution approximated an ideal free distribution (Fretwell & Lucas 1969).…”

Field assessment of condition indices (nucleic acid and protein) in Mysis diluviana

“…Historically, these species funneled the majority of zooplankton and phytoplankton production to the fish community in the offshore regions. In the 1990s, M. diluviana consumed as much, or more, of the offshore zooplankton production in Lake Ontario as the dominant planktivore, the alewife Alosa pseudoharengus (Gal et al 2006). In 1990, prior to the invasive species impacts, mysids composed ~20% of the biomass consumed by alewife in spring and fall (Rand et al 1995).…”