Science Advisory Report 2016/057

Ecological Risk Assessment of Grass Carp (Ctenopharyngodon idella) for the Great Lakes Basin

Summary

• Grass Carp (Ctenopharyngodon idella) is an herbivorous, freshwater fish that was first introduced in the United States in the early 1960s for use in biological control of aquatic vegetation. It has since escaped and dispersed through the Mississippi River basin towards the Great Lakes. To characterize the risk of Grass Carp to the Great Lakes basin, a binational ecological risk assessment of Grass Carp was conducted.
• This risk assessment covered both triploid (sterile) and diploid (fertile) Grass Carp and assessed the likelihood of arrival, survival, establishment, and spread, and the magnitude of the ecological consequences within 5, 10, 20 and 50 years from 2014 (i.e., the baseline year) to the connected Great Lakes basin (defined as the Great Lakes basin and its tributaries to the first impassable barrier); risk was assessed based on current climate conditions and at the individual lake scale but does not address a finer geographical scale (e.g., bay or sub-region).
• For triploid Grass Carp, the probability of occurrence (likelihood of arrival, survival, and spread) was assessed, and for diploid Grass Carp the probability of introduction (likelihood of arrival, survival, establishment and spread) was assessed

Arrival:

• Grass Carp (both triploid and diploid) have arrived from outside the Great Lakes basin to two lake basins: lakes Michigan and Erie, but the pathways are not clear.
• The most likely point of direct arrival for triploid and diploid Grass Carp into the Great Lakes basin is through the Chicago-Area Waterway System (CAWS) to Lake Michigan.
• The most likely vector of arrival for triploid and diploid Grass Carp to Lake Erie is human-mediated release.
• Likelihood of arrival to Lake Ontario is low at 5 years for both triploid (human-mediated release) and diploid (physical connections) Grass Carp, and increases to moderate at 10 (triploid) and 50 (diploid) years. Likelihood of arrival to lakes Superior and Huron by 50 years is considered to be very unlikely to low for both triploid and diploid Grass Carp.
• Regulations and their effective enforcement are important factors that may affect the likelihood of arrival.

Survival:

• Based on thermal tolerance, food availability, predation, pathogens and diseases, adult and juvenile (both triploid and diploid) Grass Carp will survive in the Great Lakes; there are no known factors that would preclude survival.
• Survival at northern latitudes of Lake Superior is less certain based on some climate-based models.

Establishment:

• Triploid Grass Carp are not expected to establish because they are sterile (failed triploids are considered as diploids).
• Evidence exists for the conditions to support establishment of diploid Grass Carp, such as, but not limited to, existence of spawning habitat, potential for positive population growth, and overwinter survival of early life stages.
• Establishment requires relatively few diploid individuals if older age classes are introduced. Population growth is most sensitive to the survivorship of juveniles.
• Likelihood of establishment by 5 years is high for Lake Erie due to evidence of recruitment in Lake Erie.
• For lakes Michigan, Huron, Erie and Ontario, the likelihood of establishment is very likely by 10 years.
• In contrast, the likelihood of establishment in Lake Superior remains low at 50 years given the low probability of overwinter survival and inability to mature based on current climate, which will limit establishment.

Spread:

• No known impediments to spread exist among the lakes.
• Spread to other Great Lakes in the basin is a concern based on the arrival of Grass Carp in lakes Erie and Michigan.
• Expect significant lake-to-lake movement within 10 years (Lake Michigan to Lake Huron); movement will be influenced by habitat and food availability, especially across lakes Michigan, Huron, and Erie.
• Possible movement expected between lakes Huron and Superior and from Erie to Huron, but less likely between Ontario and Erie (Welland Canal).

Ecological Consequences:

• Consumption of aquatic vegetation by Grass Carp (both ploidies) may lead to consequences to elements of the biotic community (high potential consequence was predicted for 33 of 136 fishes assessed; and for 18 of 47 bird species assessed) and abiotic environment.
• The ecological consequences for triploid Grass Carp for all lakes were ranked as negligible at the lake-scale for all time periods because of inability to establish.
• It is important to note that effects may be greater within localized wetlands if Grass Carp (regardless of ploidy) aggregate in these areas.
• Ecological consequences depend on the predicted density of Grass Carp to occur in each lake.
• Lakes Michigan, Huron, Erie, and Ontario are most likely to experience increasing ecological consequences within 20–50 years.

Overall Risk:

• Under current conditions, the overall risk for triploid Grass Carp ranges from low (lakes Superior, Huron and Ontario) to medium (lakes Michigan and Erie) for all years.
• Under current conditions, the overall risk for diploid Grass Carp is low for all lakes within 5 years, but increases to high for Lake Ontario and extreme for lakes Michigan, Huron and Erie, at 50 years.
• If the rate of arrival increases, the onset and magnitude of risk will increase.
• Lakes Michigan and Erie are at greater risk relative to the other lakes.

Overarching Conclusions:

• Grass Carp has arrived to the Great Lakes basin (lakes Michigan and Erie) and the invasion process has begun.
• There is an expected time lag associated with the full ecological consequences of an established population of Grass Carp in the Great Lakes basin.
• Immediate preventative actions would be most effective, especially in conjunction with control activities where Grass Carp have arrived, to reduce the probability of establishment and delay or reduce subsequent ecological consequences.

This Science Advisory Report is from the June 1-3, 2015 peer review of the Binational Ecological Risk Assessment for Grass Carp in the Great Lakes Basin. Additional publications from this meeting will be posted on the DFO Science Advisory Schedule as they become available.

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