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Searching for a good stocking policy for Lake Michigan salmonines

Searching for a good stocking policy for Lake Michigan salmonines. Michael L. Jones and Iyob Tsehaye Quantitative Fisheries Center, Fisheries and Wildlife Michigan State University. Decision Analysis. Structured, formal method for comparing alternative management actions

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Searching for a good stocking policy for Lake Michigan salmonines

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  1. Searching for a good stocking policy for Lake Michigan salmonines Michael L. Jones and Iyob Tsehaye Quantitative Fisheries Center, Fisheries and Wildlife Michigan State University Lake Michigan Decision Analysis - 2012

  2. Decision Analysis Structured, formal method for comparing alternative management actions Main components: Specify objectives Identify management options Assess knowledge and account for uncertainties Use model to forecast possible outcomes • Consider the possible consequences of a decision, rather than just predicting the most likely consequence Lake Michigan Decision Analysis - 2012

  3. The Big Question How many salmon and trout should we stock into Lake Michigan each year? • more stocking leads to greater harvest, and thus benefits - unless… • too much stocking leads to poor feeding conditions and increased mortality, but • too little stocking may lead to negative effects of alewife on other species Lake Michigan Decision Analysis - 2012

  4. How many salmon and trout are out there? • How much do they eat? • How capable are the prey fish of meeting this demand? • What happens to salmon and trout feeding (and survival) when prey numbers are low? What we need to know… Lake Michigan Decision Analysis - 2012

  5. Our approach • Analyze the past • Salmonine abundance • Salmonine consumption • Prey fish production • Supply vs demand • Forecast the future • Simulation model • Data we used • Stocking and harvest • Growth and diet data • Prey fish survey data Lake Michigan Decision Analysis - 2012

  6. What does the past tell us? Lake Michigan Decision Analysis - 2012

  7. How many salmon and trout are out there? • Total salmonine numbers have remained relatively stable since 1990 • Reduced Chinook stocking has been offset by increased wild fish production • More recently, improved survival of older Chinook salmon has also offset reduced stocking Lake Michigan Decision Analysis - 2012

  8. Age-3 Chinook numbers How many salmon and trout are out there? Salmonine abundance Lake Michigan Decision Analysis - 2012

  9. How much do they eat? • Total consumption has remained fairly stable for last decade • Chinook salmon have accounted for more than half of total demand consistently since 1980 • Large alewife accounted for more than 40% of total prey consumed since 1980, except in the late 1980s when small alewife dominated Lake Michigan Decision Analysis - 2012

  10. Consumption by all species of salmon and trout How much do they eat? Consumption by predator type 1 KT = 2.2 million lbs Consumption by prey type Lake Michigan Decision Analysis - 2012

  11. How capable are the prey fish of meeting this demand? • Predation rates on alewife have ranged from 25%-45% per year from the mid-1980’s to present • Predation mortality peaked in mid-1980’s and has approached peak levels again recently • Alewife (and rainbow smelt) recruitment is variable and not strongly related to adult abundance Lake Michigan Decision Analysis - 2012

  12. How capable are the prey fish of meeting this demand? Index of total predation mortality on alewife Lake Michigan Decision Analysis - 2012

  13. How capable are the prey fish of meeting this demand? Stock and recruitment

  14. What happens to salmon and trout feeding when prey numbers are low? • Chinook salmon consumption has declined when alewife abundance declined • Similar, but weaker pattern for lake trout Lake Michigan Decision Analysis - 2012

  15. What happens to salmon and trout feeding when prey numbers are low? Lake Michigan Decision Analysis - 2012

  16. What can we say about the future? Lake Michigan Decision Analysis - 2012

  17. Policy simulation model Accounts for uncertainties: key uncertainties concern prey recruitment (supply) and predator feeding (demand) Management Decisions LMDA Prediction of Outcome Lake Michigan Decision Analysis - 2012 What we Know

  18. Multiple possible futures Alternative relationships that are consistent with the data

  19. Model results • The model forecasts possible future changes in fish populations and harvest, given a stocking policy • There are many possible futures, so we need to look at the range of possible (likely) outcomes • This range tells us what we think is most likely, but also what might happen • Mainly we’re interested in how likely it is that bad things will happen • Here’s how it works… Lake Michigan Decision Analysis - 2012

  20. Generating results: First simulation Average biomass = 243 kT Lake Michigan Decision Analysis - 2012

  21. Generating results First simulation: average alewife biomass = 243 kt Lake Michigan Decision Analysis - 2012

  22. Generating results: Second simulation Average biomass = 52 kT Lake Michigan Decision Analysis - 2012

  23. Generating results Second simulation: average alewife biomass = 52 kt Lake Michigan Decision Analysis - 2012

  24. Generating results … and so on (e.g., results after 15 simulations) Lake Michigan Decision Analysis - 2012

  25. An example result: Status quo policy In 26 of 100 cases alewife biomass was less than 100 kt: BAD In 45 of 100 cases alewife biomass was between 100 and 500 kt: OK Lake Michigan Decision Analysis - 2012

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