searching for a good stocking policy for lake michigan salmonines michael l. jones and iyob tsehaye...

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

1Lake Michigan Decision Analysis - 2012

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


The Big QuestionHow 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


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?


Our Our approachapproach

1. Analyze the past

• Salmonine abundance

• Salmonine consumption

• Prey fish production

• Supply vs demand

2. Forecast the future

• Simulation model

Data we used

• Stocking and harvest

• Growth and diet data

• Prey fish survey data


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


How many salmon and trout are out there?

Age-3 Chinook numbers Salmonine abundance


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


How much do they eat?

Consumption by all species of salmon and trout

1 KT = 2.2 million lbs

Consumption by predator type

Consumption by prey type


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


Index of total predation mortality on alewife




Stock and recruitment

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


What happens to salmon and trout feeding when prey numbers are low?


What can we say about the future?


Policy simulation model

Accounts for uncertainties: key uncertainties concern prey recruitment (supply) and predator feeding (demand)


What weKnow

Management Decisions

Prediction of Outcome

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LMDA

Multiple possible futuresMultiple possible futures

Alternative relationships that are consistent with the data

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) outcomesThis 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…

Model results


Generating results:First simulation

Average biomass = 243 kT


First simulation:average alewife

biomass = 243 kt

Generating results


Generating results:Second simulation

Average biomass = 52 kT


Second simulation:average alewife biomass = 52 kt

Generating results


… and so on (e.g., results after 15

simulations)

Generating results


An example result: Status quo policy

In 45 of 100 cases alewife biomass

was between 100 and 500 kt: OK

In 26 of 100 cases alewife biomass was less than 100 kt: BAD


searching for a good stocking policy for lake michigan salmonines michael l. jones and iyob tsehaye...

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