The predator-prey interactions in the offshore food web of Lake Superior have been well documented, but the sensory systems mediating these interactions remain unknown. The deepwater sculpin, (Myoxocephalus thompsoni), siscowet (Salvelinus namaycush siscowet), and kiyi (Coregonus kiyi) inhabit low light level environments. To investigate the potential role of vision in predator-prey interactions, electroretinography was used to determine visual sensitivity for each species. Spectral sensitivity curves revealed peak sensitivity at 525 nm for each species which closely corresponds to the prevalent downwelling light spectrum at depth. To determine if sufficient light was available to mediate predator-prey interactions, visual sensitivity was correlated with the intensity of downwelling light in Lake Superior to construct visual depth profiles for each species. Sufficient daytime irradiance exists for visual interactions to approximately 325 m for siscowet and kiyi and 355 m for the deepwater sculpin during summer months. Under full moon conditions, sufficient irradiance exists to elicit ERG response to light available at approximately 30 m for the siscowet and kiyi and 45 m for the deepwater sculpin. Visual interactions are therefore possible at the depths and times when these organisms overlap in the water column indicating that vision may play a far greater role at depth in deep freshwater lakes than had been previously documented.
Lake Superior is the largest of the Laurentian Great Lakes and home to 38 fish species, including 19 nonnative species , with the majority of these fishes inhabiting the shallow, nearshore waters or surrounding watersheds. The cold deep, oligotrophic offshore waters of Lake Superior are relatively depauperate with fish density less than 6.9 kg/ha . Although many invasive aquatic species have disrupted and/or become integrated into shallow water community, the deep waters of Lake Superior remain dominated by native species . Piscivores such as burbot (Lota lota) and siscowet lake trout (Salvelinus namaycush siscowet) dominate the highest trophic levels and feed predominately on deepwater sculpin (Moxocephalus thompsonii) and/or kiyi (Coregonus kiyi) . The deepwater sculpin and kiyi, along with the cisco (Coregonus artedi) form the second trophic level, and prey on a wide variety of zooplankton such as mysis (Mysis diluviana), scuds (Diporeia spp.), cladocerans, and copepods [3,5]. Thus, energy transfer in the deep, oligotrophic water of Lake Superior is mediated through a relatively simple food web (Fig. 1).
The diel vertical migrating (DVM) zooplankton, Mysis diluviana, is the primary conduit for energy flow from benthic waters to the surface, as it feeds diurnally on benthic detritus and switches to midwater phytoplankton and zooplankton during its nightly ascent [6,7]. Two planktivorous fish, deepwater sculpin and kiyi, prey primarily on the mysis, with the deepwater sculpin also consuming benthic amphipods (Diporea spp.) [3,8]. The siscowet is the most abundant piscivore in the lake [2,9] and its feeding habits are dictated by diurnal vertical migrations of the planktivores . During the day, the siscowet remain in deepwater (>140 m) and prey primarily on benthic sculpin, while at night, they vertically migrate to consume kiyi which are following the migration of mysis [2,3,4,10,11].
Despite detailed information on the food web and diet of dominant species, little is known about the role that vision plays in mediating deepwater predator-prey interactions. While olfactory and auditory cues may be used for long range detection of prey, short range interactions usually are mediated by the mechanosensory lateral line or visual input . Vision is often the main sensory modality in shallow, sunlit waters while the lateral line may be dominant in turbid and/or low light environments. To understand the role of vision, it is important to determine the visual and spectral sensitivity of the organism, and the intensity and spectral composition of downwelling irradiance. Predator-prey or population models often contain little sensory information and by incorporating sensory physiology, future models will better predict population structure and dynamics.
The fishes that comprise the deep water food web spend the majority of their time in a low light level environment. Following a pelagic larval stage, deepwater sculpin transition to the benthos and remain at depths ranging from 15 to 407 m [13,14,15,16] with the majority of the Lake Superior population inhabiting depths below 70 m. In contrast, siscowet and kiyi are midwater water fish that undergo diel vertical migration (DVM) with siscowet depth distributions ranging from the surface (night) to 407 m (day), while kiyi are found between 25 m (night) and 325 m (day) [7,10,17]. Therefore all three species spend the majority of their life in light limited environments.
The visual pigment sensitivity hypothesis  suggests that fish visual sensitivity corresponds with its light environment due to the adaptation of visual pigments. The spectral sensitivities of numerous marine species support this hypothesis [19,20,21,22,23,24,25,26]; marine organisms exhibit peak sensitivity to blue light as these are the predominate wavelengths at depth due to the filtering properties of seawater . Marine fish contain the rhodopsin visual pigment, based on vitamin A1, which is well adapted for the detection of blue wavelengths. However, freshwater systems favor the transmittance of green light due to the high concentration of chlorophyll and other particulate matter in the water column [27,28]. The visual pigment porphyropsin, based on vitamin A2, is present in freshwater fish with its absorption spectrum matched to the predominant green downwelling light . Freshwater fish utilize porphyropsin, or in conjunction with rhodopsin for detection [21,30].
Historically, deep sea fishes received more attention for their visual ability at depth than freshwater fish, creating a gap in the knowledge of the visual characteristics among deep water marine and freshwater fishes . The clear, offshore waters of Lake Superior allow greater light penetration compared to other freshwater systems, and offer an opportunity to examine the visual sensitivity of deep water fishes in a freshwater system. The goal of the current study was to characterize previously unmeasured visual sensitivity of deep water fishes in Lake Superior and to determine the potential role of vision in mediating predator-prey interactions. Electroretinography was performed on three species of deep water fish found in Lake Superior to determine dark adapted spectral sensitivity and to compare each visual system to the prevailing light environment. The fishes’ visual sensitivity was combined with estimates of the transmission of light in Lake Superior to model the depths at which vision may mediate predator-prey interactions.
Full study, including methods, results and discussion, published under open-access license in PLOS ONE.