Temperature and fisheries drive a decline of cisco in Mille Lacs Lake, Minnesota: Learning from the past to shape the future

Background/Question/Methods

Cisco, Coregonus artedi, is a coregonid forage fish widely distributed in central and northern Minnesota. Since the 1980s, the large 537km² Mille Lacs Lake, Minnesota, has experienced a decline in its cisco population. As cisco is a cold water stenotherm, the population decline has been attributed to an increase in the lake temperature. However, there has also been a fishery for this species over the last 20 years. To investigate the influence of temperature and the fishery on the decline, a surplus production model (SPM) with a temperature driver was formulated: three versions enabled temperature and fishery effects to be partitioned.  Catch per unit effort (CPUE) of standardized sampling gill nets from 1985 to 2007 were used as an index of fish abundance for the lake population; model parameters were estimated by fitting model-predicted CPUE to observed CPUE. 

Results/Conclusions

Results indicate that temperature explained about 40% of the change in cisco abundance. Temperature influenced carrying capacity, fish production and maximum sustainable yield such that cisco were overfished in warmer years. We conclude that the causes of cisco decline were a combination of temperature and fishing pressure - especially the maintenance of average fish catch levels in years when cisco were likely stressed by higher temperature.