Habitat coupling in a large lake system: delivery of an energy subsidy by an offshore planktivore to the nearshore zone of Lake Superior

1. We hypothesised that the autumn spawning migration of Lake Superior cisco (Coregonus artedi)
provides a resource subsidy, in the form of energy-rich cisco eggs, from the offshore pelagic to the
nearshore benthic community over winter, when alternate prey production is likely to be low.

2. We tested this hypothesis using fish and macroinvertebrate surveys, fish population demographics,
diet and stable isotope analyses, and bioenergetics modelling.

3. The benthic, congeneric lake whitefish (C. clupeaformis) was a clear beneficiary of cisco spawning.
Cisco eggs represented 16% of lake whitefish annual consumption in terms of biomass, but 34% of
energy (because of their high energy density: >10 kJ g wet mass1
). Stable isotope analyses were
consistent with these results and suggest that other nearshore fish species may also rely on cisco
eggs.

4. The lipid content of lake whitefish liver almost doubled from 26 to 49% between November and
March, while that of muscle increased from 14 to 26% over the same period, suggesting lake white-
fish were building, rather than depleting, lipid reserves during winter.

5. In the other Laurentian Great Lakes, where cisco populations remain very low and rehabilitation
efforts are underway, the offshore-to-nearshore ecological link apparent in Lake Superior has been
replaced by non-native planktivorous species. These non-native species spawn in spring have smaller
eggs and shorter incubation periods. The rehabilitation of cisco in these systems should reinstate the
onshore subsidy as it has in Lake Superior.