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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 1 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 2 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 3 What we need to know… 1. How many salmon and trout are out there? 2. How much do they eat? 3. How capable are the prey fish of meeting this demand? 4. What happens to salmon and trout feeding (and survival) when prey numbers are low? Lake Michigan Decision Analysis - 2012 4 Our approach 1. Analyze the past Data we used • Salmonine abundance • Stocking and harvest • Salmonine consumption • Growth and diet data • Prey fish survey data • Prey fish production • Supply vs demand 2. Forecast the future • Simulation model Lake Michigan Decision Analysis - 2012 5 What does the past tell us? Lake Michigan Decision Analysis - 2012 6 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 7 How many salmon and trout are out there? Salmonine abundance Lake Michigan Decision Analysis - 2012 Age-3 Chinook numbers 8 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 9 How much do they eat? Consumption by predator type Consumption by all species of salmon and trout 100 80 60 1 KT = 2.2 million lbs 40 Consumption by prey type 20 0 1965 0.80 0.60 1975 1985 1995 2005 Lake Michigan Decision Analysis - 2012 Proportion Consumption (KT) 120 Large alewife Small alewife Smelt and others 0.40 0.20 0.00 1965 1975 1985 1995 2005 10 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 11 How capable are the prey fish of meeting this demand? Index of total predation mortality on alewife Predation Mortality Index 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 1960 1970 1980 Lake Michigan Decision Analysis - 2012 1990 2000 2010 12 How capable are the prey fish of meeting this demand? Age-0 recruits (109) 60 50 Stock and recruitment 40 30 20 10 0 0 10 20 30 Age 2+ numbers (109) 40 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 14 What happens to salmon and trout feeding when prey numbers are low? 23 Chinook age-3 ration Alewife age-3 abundance Ration (kg) 20 22 15 21 10 20 5 0 1960 Alewife abundance index 25 19 1970 1980 Lake Michigan Decision Analysis - 2012 1990 2000 2010 15 What can we say about the future? Lake Michigan Decision Analysis - 2012 16 Policy simulation model Accounts for uncertainties: key uncertainties concern prey recruitment (supply) and predator feeding (demand) What we Know Management Decisions LMDA Prediction of Outcome y x2 x 5 ƒ( x ) n ˆ 2 Lake Michigan Decision Analysis - 2012 x ˆ 2 i i 1 n 1 17 Multiple possible futures Age-0 recruits (109) 60 Variability in actual recruitment in a particular year from the “average” recruitment 50 40 Alternative relationships that are consistent with the data 30 20 10 0 0 10 20 Age 2+ numbers (109) 30 40 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 19 Generating results: First simulation Average biomass = 243 kT Alewife Biomass (kt) 350 300 250 200 150 100 50 0 0 5 10 15 20 25 Year Lake Michigan Decision Analysis - 2012 20 Generating results Number of Simulations 10 First simulation: average alewife biomass = 243 kt 8 6 4 2 0 < 100 100-500 > 500 Biomass (kt) Lake Michigan Decision Analysis - 2012 21 Generating results: Second simulation 140 Average biomass = 52 kT Alewife Biomass 120 100 80 60 40 20 0 0 5 10 15 20 25 Year Lake Michigan Decision Analysis - 2012 22 Generating results Number of Simulations 10 8 6 Second simulation: average alewife biomass = 52 kt 4 2 0 < 100 100-500 > 500 Biomass (kT) Lake Michigan Decision Analysis - 2012 23 Generating results Number of Simulations 10 8 … and so on (e.g., results after 15 simulations) 6 4 2 0 < 100 100-500 > 500 Biomass (kt) Lake Michigan Decision Analysis - 2012 24 An example result: Status quo policy Number of simulations 50 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 40 30 20 10 0 <100 100-500 >500 Biomass (KT) Lake Michigan Decision Analysis - 2012 25