Updated 2023-11-07 19:05:41
Lake Michigan -> 1.0 Salmonine (Salmon and Trout) -> Predator-Prey Ratio
Reporting Interval
2016 - 2021
Area
Lake Michigan
Meeting Target?
Meets
Indicator Trend
Downward trend
Confidence?
High
1.1.4 Lake Michigan Predator-Prey Ratio near the target reference point of 0.05
The Lake Michigan Predator-Prey Ratio (PPR) is a ratio of estimated total lake-wide biomass (i.e., weight) of Chinook salmon (≥ age 1 year) divided by the estimated total lake-wide biomass of alewives (≥ age 1 year) and is an indicator of predator-prey balance. Among Lake Michigan salmon and trout species, Chinook salmon exert the greatest predation pressure on alewives, which are the primary prey fish in the lake. Therefore, a high PPR value indicates that there are too many predators (i.e., Chinook salmon) relative to their available prey (i.e., alewives). In contrast, a low PPR value indicates that there are few predators relative to their available prey.
Relatively high PPR values during 2010 – 2016 motivated fishery managers to reduce stocking in 2013 and 2016 to encourage better predator/prey balance. Since 2016, Chinook salmon biomass has trended downward while alewife biomass has trended upward (SWG 2022). The cumulative result of these opposing trends is relatively more prey biomass per predator (i.e., low PPR): the PPR declined from 0.076 to 0.023 during the 2016 – 2021 reporting period and is within or below the management target range of 0.05 – 0.10 (Figure 1). This pattern is suggestive of healthy predator-prey balance during the reporting period. However, relative to the late 1990s and early 2000s, alewife biomass remains low and improvements in predator-prey balance over this longer period were driven primarily by declines in age-1+ Chinook salmon biomass (SWG 2022; Warner et al. 2022) as opposed to robust increases in alewife abundance.
Figure 1. Lake Michigan Predator-Prey Ratio (PPR) for Chinook salmon (≥ age 1 year) and alewife (≥ age 1 year), 1967 – 2021. The target reference point is 0.05 and the upper threshold is 0.1.
Methodology
Biomass of Chinook salmon and alewife used to calculate the PPR are estimated from statistical catch-at-age analysis (SCAA; Tsehaye et al. 2014a; Tsehaye et al. 2014b) that incorporate lake-wide datasets from several surveys and agencies (see SWG 2022 for list of datasets). Generally, SCAA models estimate fish abundance based on numbers of fish harvested, age of fish harvested, recruitment information (i.e., numbers of fish produced naturally and numbers stocked), and other factors. This modelling process can be explained simply as a mathematical approach to provide the most likely answer to the question of how many fish must have been present to produce the observed harvest data.
Other Resources
- SWG (Salmonid Working Group). 2022. Summary of the predator/prey ratio analysis for Chinook salmon and alewife in Lake Michigan. Report to the Lake Michigan Committee. PPR Summary for Outreach_8-18-2022.pdf (glfc.org).
- Tsehaye, I., M. L. Jones, T. O. Brenden, J. R. Bence, and R. M. Claramunt. 2014a. Changes in the Salmonine community of Lake Michigan and their implications for predator–prey balance. Transactions of the American Fisheries Society 143:420-437.
- Tsehaye, I., M. L. Jones, J. R. Bence, T. O. Brenden, C. P. Madenjian, and D. M. Warner. 2014b. A multispecies statistical age-structured model to assess predator–prey balance: application to an intensively managed Lake Michigan pelagic fish community. Canadian Journal of Fisheries and Aquatic Sciences 71:1-18.
- Warner, D.M., R. Tingley, C.P. Madenjian, B. Turschak, D. Hanson. In Review. Status of Pelagic and Benthic Prey Fish Populations in Lake Michigan, 2021. Report to the Lake Michigan Committee.
Contributing Author(s)
- Lake Michigan Technical Committee Salmonid Working Group - Predator-Prey Ratio Modeling Sub Team