Cisco Provide Linkage between Nearshore and Offshore HabitatsOct. 1st 2014
The nearshore region of large lakes consists of shallow water habitat where many organisms are available as a food source to fish. Fish diets change throughout the year as abundances of prey change. Macroinvertebrates, including zooplankton, are a main food source from spring to autumn when they are abundant. In the winter, their numbers are reduced, and fish living in the nearshore must find other prey. In this study, researchers investigated if the eggs of cisco, a highly mobile forage fish, provided a significant prey source to the nearshore fish community of Lake Superior during the winter. Cisco spawn in the autumn and their eggs do not hatch until spring. Lake Superior contains healthy populations of cisco as well as other native fishes. Dr. Jason Stockwell, Director of the Rubenstein Ecosystem Science Lab at the University of Vermont, was lead investigator of this project prior to his move to Vermont. Cisco live in the offshore deep waters of Lake Superior during most of the year, only spending time nearshore while spawning. Cisco feed on zooplankton in the offshore pelagic region, and then transfer that energy to the nearshore habitat during their spawning migration. The eggs provide a food source for fish in the nearshore, benthic region. Any eggs that survive to adulthood will ultimately return to the offshore pelagic region and continue the same cycle. The study quantified whitefish diet and growth during the year, including winter. Whitefish not only maintained weight during winter months, they exhibited growth, likely due to the availability of energy rich cisco eggs that were identified as an important winter food source. Whole-lake food webs rely on the movement of energy between the two habitats. The habitat coupling in Lake Superior may be unique to the Great Lakes because cisco and other native fish populations are relatively intact. Other Great Lakes have many non-native fish that change the dynamics of these systems. Non-native fish, such as rainbow smelt and alewife, spawn in the spring. Their eggs are not available as a food source to nearshore fish when needed in the winter, breaking the energy movement between habitats. Lake Champlain has 50 native and 12 non-native fish, including alewife. Cisco populations are small and their co-existence with native rainbow smelt is unique. Lake whitefish is also a native species to Lake Champlain. The nearshore-offshore connection may not be strong in Lake Champlain because of small population size of cisco, and the establishment of alewife may further weaken this link. Future studies focusing on lakes that have native and non-native fish populations may help further the understanding of coupling between offshore and nearshore habitats and the ecosystem processes involved in complex food webs.