The freshwater mysid Mysis diluviana (Audzijonyte & Väinölä, 2005) (Mysida: Mysidae) consumes detritus in the presence of Daphnia (Cladocera: Daphniidae)

Freshwater mysids of the Mysis relicta group are omnivorous macroinvertebrates that form an important link between fishes and lower trophic levels in many north temperate to Arctic lakes, where they exhibit diel vertical migration (DVM) to exploit subsurface food-rich layers at night. Benthic food resources have been assumed to be less important for mysid diets than pelagic zooplankton. Studies have nevertheless indicated that mysids consume benthic sedimented detritus, calling this assumption into question. We conducted a food-choice experiment to evaluate the feeding preferences of Mysis diluviana (Audzijonyte & Väinölä, 2005) by presenting field-caught specimens in individual foraging arenas with multiple choices of food. Experimental food treatments included a preferred pelagic prey (Daphnia), a presumed less desirable benthic resource (detritus), and a combination of both. We hypothesized that M. diluviana 1) prefers Daphnia over detritus and consumes only Daphnia in combination treatments, and 2) would not consume detritus except when detritus was the only food source available. Contrary to our hypothesis, M. diluviana readily consumed detritus in the presence of Daphnia. Mysis diluviana unexpectedly consumed more individuals of Daphnia in the presence rather than in the absence of detritus. Our results demonstrate that mysids take advantage of benthic food resources even in the presence of a presumably preferred zooplankton prey, calling to question the long-held assumption that benthic resources are unimportant when considering the trophic role of freshwater mysids of the M. relicta group.

Cisco diversity in a historical drainage of glacial Lake Algonquin

Cisco (Coregonus artedi (sensu lato) Lesueur, 1818) forms matching in appearance to Blackfin Cisco from the Laurentian Great Lakes occur in four lakes in Algonquin Park, Ontario, Canada, a historical drainage of glacial Lake Algonquin (precursor of lakes Michigan and Huron). Their occurrence may represent colonization from glacial Lake Algonquin drainage patterns 13 000 calibrated years BP or independent evolution within each lake. Gill-raker numbers, temperature at capture depth during lake stratification, and hurdle models of habitat distribution are summarized. Blackfin (nigripinnis-like) in the four lakes had higher gill-raker numbers than artedi-like cisco captured in nearby lakes or within the same lake. Two lakes have a bimodal gill-raker distribution that indicate co-occurrence of two forms. Blackfin occupied the hypolimnion with a peak depth distribution at 20–25 m. Maximum depth for blackfin was 35–40 m. The presence of the opossum shrimp (Mysis diluviana Audzijonyte and Väinölä, 2005) appears necessary for the occurrence of cisco diversity in lakes but not sufficient in all cases. The presence of two forms of cisco in at least two lakes points to the possibility of the colonization hypothesis or the ecological speciation hypothesis as accounting for this phenomenon. Genetic analysis is needed to determine which of these hypotheses best accounts for the occurrence of blackfin in Algonquin Park.

Changes in the condition, early growth, and trophic position of lake trout (Salvelinus namaycush) in response to an experimental aquaculture operation

We analyzed changes in the condition, early growth, and resource use of lake trout (Salvelinus namaycush) from a Boreal Shield lake in response to an experimental aquaculture operation. Annual small-scale commercial production of rainbow trout (Oncorhynchus mykiss) resulted in increased lake trout body condition during aquaculture operations, which was positively related to minnow catch per unit effort (CPUE). Incremental increases in growth led to larger lake trout following aquaculture. While the littoral energy assimilated by lake trout did not change linearly from 2002 to 2009, we observed a shift towards littoral energy use in the last year of aquaculture and for 2 years after the experiment, coincident with low densities of Mysis diluviana. Lake trout trophic position declined from 2002 to 2009, suggesting increased foraging on secondary versus tertiary consumers. Minnow and slimy sculpin (Cottus cognatus) CPUE increased by 5.2- and 5.5-fold, respectively, and Mysis diluviana densities decreased by 93%. Changes observed in the experimental lake were much greater than those observed in a nearby reference lake. Our results suggest that moderate nutrient enrichment from aquaculture may benefit lake trout in oligotrophic Boreal Shield lakes.