Meet our experts:

Panel members are:

Drs. Jim Bence and Travis Brenden, Quantitative Fisheries Center at Michigan State University;

http://www.fw.msu.edu/~bence/  

http://qfc.fw.msu.edu/travis/

Dr. Paul Venturelli, University of Minnesota;

http://individual.utoronto.ca/venturelli/VenturelliCV.pdf

Dr. Nigel Lester, Ontario Ministry of Natural Resources and the University of Toronto; and

http://www.harkness.ca/collaborators/omnr/nigel-lester/

Dr. Lars Rudstam, Cornell University and Oneida Lake Field Station.

http://www.glrrin.info/lakeontario/people/?person=rudstam

Since the Ontario Ministry of Natural Resources has been making such good progress in the management of lakes in this province, you can expect to see some of this kind of influence on Mille Lacs. From the study,

 Strategies for Managing Walleye in Ontario 

Diagnostics and Sampling Standards 

Biological Reference Points 

Productivity of walleye lakes depends on the availability of suitable habitat, nutrient 

levels, and climate. Maximum sustainable yield (MSY, kg.ha-1.year-1) can be estimated 

by a formula provided by Lester et al. (2002): 

MSY = 0.97 PTOHA TDS0.52 G1.30

where PTOHA is an index of habitat suitability, TDS (Total Dissolved Solids in mg L-1) is an 

index of nutrients, and G (Growing Degree Days x 103

) is a climatic index. This index of 

habitat suitability is a complex function of water clarity, lake bathymetry, and thermocline 

depth (see Lester et al. 2002 for details). This formula implies that relatively large, 

shallow lakes in Ontario with intermediate Secchi depth (e.g., 1-3 m), high TDS, and 

high GDD tend to offer the highest walleye yields. It predicts the highest walleye yields 

in Ontario inland lakes are approximately 3 kg.ha-1. These high yields are found in only 

a few southern lakes. 

Indications of Population Health 

When MSY is viewed as a threshold, a desired state of the population can be defined by 

critical levels of stock biomass and fishing mortality. These criteria depend on the 

equilibrium (i.e., long term sustainable) relationship between biomass (B) and fishing 

mortality (F) in which the equilibrium biomass of a stock decreases as fishing mortality 

increases. Yield (= FB) therefore has a dome shaped relationship with fishing rate. The 

peak of the yield curve (MSY) supplies a reference that identifies critical values of 

biomass (BMSY) and fishing mortality (MMSY). These biological reference points can be 

used to classify a fishery into one of four stages of development.

MSY-based definition of a desired state of a walleye population. In (a) the equilibrium 

relationship between exploitable fish biomass (В) and fishing mortality (F) is portrayed. In (b) the 

resulting equilibrium relationship between fish yield and fishing mortality is shown. The peak yield 

(MSY) in (b) occurs when F = Fmsy and В = Вmsy. These criteria (dotted lines in (a)) identify four 

stages of development (i.e., 4 quadrants). 

Stage 1 (healthy) : low fishing mortality and high biomass. This state is 

expected during the early stages of fishery development. 

Stage 2 (overexploited - early) : high fishing mortality and high biomass. This 

state is expected only during the early stages of overexploitation because stable 

combinations of fishing mortality rate and biomass do not exist in this quadrant.

Stage 3 (overexploited - late) : high fishing mortality and low biomass. This 

state indicates that the waterbody is being overexploited and that the expected 

decline in fish biomass has occurred. 

Stage 4 (degraded, recovering) : low fishing mortality and low biomass. This 

state indicates that the stock was probably overexploited in the past and is 

expected in the natural course of fishery development because anglers are likely 

to shift their effort to other lakes once catch rates on one lake decline due to the 

reduction in biomass. Stable (i.e., equilibrium) combinations of abundance and 

mortality are not expected in this quadrant. If fishing mortality rate remains low, a 

gradual transition to stage 1 should occur. This recovery process may be 

inhibited by changes in the fish community resulting from heavy exploitation of 

one species (Walters and Kitchell 2001). 

Stage 1 is the desired state because it implies a fishery has not been exploited beyond 

MSY and is thus consistent with the principle of sustainability. Maintenance of fisheries 

in this state should represent a goal for management. More stringent criteria (i.e., 

harvest controls), aimed at sustaining higher fish abundance, may be preferred for social 

or economic reasons. For example, the management objective might be to provide high 

fishing quality. In this case, MSY criteria would not be appropriate. To offer higher 

fishing quality, fish abundance must be sustained closer to the unexploited level. Thus, 

Stage 1 would be defined by a higher fish abundance criterion and lower fishing mortality 

criterion. This direction of change is acceptable; indeed, evaluation of its potential 

benefits is encouraged. On the other hand, less stringent criteria allowing fisheries to 

operate beyond MSY are not acceptable because they may compromise the 

sustainability objective. 

Sampling Standards and Protocols 

Index netting surveys are intended to provide a relatively quick and inexpensive means 

of assessing the health and relative abundance of the walleye population in a waterbody. 

Standardized techniques are required to ensure that information can be compared with 

similar data for other lakes and geographic regions. If the catchability and size 

selectivity of the index fishing method is known, the absolute abundance of a stock and 

its mortality rate can be estimated from catch data. These estimates can then be 

compared to reference values determined for a particular stock. Standard netting 

techniques described below are being calibrated through a variety of methods so that 

diagnostic methods described above can be applied.

Two netting techniques: fall walleye index netting (FWIN) (Morgan 2000) and end of 

spring trap netting (ESTN) (Skinner and Ball 2004), have been developed and field 

tested as new provincial sampling protocols for walleye. Both techniques involve a 

stratified sampling design, standardized gear specifications, depth criteria, and timing 

restrictions