The Challenges of Stock Assessment

Equipment used in Stock Assessment (Photo Credit – Pacific Biological Station)

Equipment used in Stock Assessment (Photo Credit – Pacific Biological Station)

The Science of Counting Fish

Transcript and video

One of the most critical functions carried out by Fisheries and Oceans Canada is assessing the state of fish stocks within its jurisdiction. It is critical both for conservation reasons, and for the continuance of viable commercial fisheries. By any measure, it is no simple job. Just ask Dr. Laura Brown, Manager of the Marine Ecosystems and Aquaculture Division of DFO Science Branch, at the Pacific Biological Station (PBS) in Nanaimo BC. In describing stock assessment complexities, she credits one of her PBS science colleagues, Mr. Rob Kronlund, with the following metaphor:

Suppose you have a bank account but you don't know exactly how much money is deposited.  This imprecise amount represents stock biomass.  You do know the account managers have taken out about an average of $75 per year over the last 30 years. That represents your catch. However somebody else is putting an unspecified amount of money back into the account and that represents stock immigration. In addition there are other withdrawals from time to time – those withdrawals represent stock emigration.

To make the matter more complex, no one is telling you the interest rate, which corresponds to new fish being born, or recruitment. And no one can tell you precisely the current banking fees. Those fees represent natural mortality.

The bank manager can tell you that the account has more money this year than last, but not as much as 10 years ago, which is analogous to the type of information often available from research surveys that reflect fluctuations in the fish stock over time, but not the absolute amount of fish.

Now - here are the two big questions.  How much money do you currently have in your bank account?  And how much money may be taken out each year if you want to keep making withdrawals for the next 30 years without depleting the account?

It is the job of DFO scientists to provide account advice and information to fisheries managers, who would be the bank managers in our metaphor.  It is these managers who, in consultation with stakeholders, make decisions about stock allocation, quotas, closures, and generally all of the various actions required to help maintain our marine natural resources.  To provide the best advice possible, the starting point is coming up with accurate stock assessment data.

Dr. Brown, "For traditional stock assessments we rely on a number of sources of information.  They include catch data, and these could be from commercial or recreational fisheries.  We also rely on observers that are on commercial vessels as well as on monitoring of fishery landings.  When the catches are landed we have people from the department, or contracted, who record the landed fish and talk to the commercial fishers.  We can also go to the processing plants to obtain biological data.  In addition, our own DFO surveys provide critical fishery-independent information.  These surveys are conducted with traditional fishing gear (e.g., hooks, trawls, traps), or with alternative fish and habitat measurement tools such as Remotely Operated Vehicles and acoustic sounders."

Research surveys are labour intensive and complex endeavours.  Stock assessment is not just about catching and counting fish. Counting the catch (retained and released) only tells you what was removed from the stock by the fishery.  There are a whole host of other variables to consider such as where the fish were caught, at what depth, their size and gender - as well as their age.

Based on these considerations and a multitude of other factors, the next step in the process is to construct a mathematical model of the fish stock that helps to sort out sometimes conflicting information and captures the degree of uncertainty implicit in assessment work.  To refine their techniques even further fisheries biologists are now using, for some fisheries,  an additional approach called Management Strategy Evaluation or MSE.  The goal of the MSE technique is to explicitly consider how the data, assessment model, and catch rules can best be combined to meet fishery objectives while taking into account the uncertainty about the status and productivity of the actual fish stock.  It changes the focus from trying to determine a "best stock assessment model" to identifying a "best procedure" for achieving desirable outcomes for a fishery and avoiding undesirable outcomes for the stock.  It is a way of thinking about how to deliberately design the management strategy to achieve an acceptable trade-off between conservation and harvests that can be sustained over the long-term, the nature of which may change over time.

In traditional stock assessment scientists try to come up with the best model they can - and they aim for an accurate representation of what is going on because they have to make a forecast about the abundance of the fish stock next year or over the next several years.  But that prediction is totally dependent on that model being correct - which is a real challenge since nature has a way of throwing so many variables into the assessment equation.

Mr. Kronlund:  "The real purpose of stock assessment and statistical modeling of fish stocks is trying to improve decision-making.  Now that Canada has adopted an ecosystem (as opposed to a single species) approach to fisheries management, the challenges are considerable.  Objectives are more complex now because they involve a multitude of interlocking ecosystem considerations.  The more objectives that are on the table the more likely you are going to run into a conflict of objectives.  So there's a series of trade-offs that have to be made.  This is where the MSE approach can be so helpful.  It involves trying to find the best strategy for harvesting fish that works adequately across a range of possible future conditions and therefore helps to reduce risk created by an inaccurate forecast.  And so the product we are providing to decision makers is not a recommended harvest; rather a recommended procedure for using stock and fishery monitoring data, assessment models, and harvest rules to calculate what the catch should be each year."

Scientists working on stock assessment now have to understand and consider all the policy issues at play, translate broad goals into measurable objectives, and then make the necessary calculations that would suggest acceptable harvest strategies within an ecological framework.  As experience with the approach accrues, it is anticipated that MSE will prove to be an invaluable tool in DFO’s stock assessment arsenal, since worldwide it is now regarded as state-of-the-art methodology.

However, MSE is a resource- and time-intensive process.  Among other things it requires a cooperative multi-stakeholder environment where everyone can work together on a process rather than have scientists take a few months to do an assessment, write a document and then advise the managers as to the state of the stock and what the catch could be.  It is much more involved and intricate because many objectives must be considered on behalf of the people of Canada: conservation objectives and the goal of viable and sustainable fisheries.

Mr. Kronlund. "So far MSE is proving to be a very good approach to being fully compliant with all the components of DFO’s Sustainable Fisheries Framework, and in particular, its policies on decision making incorporating the precautionary approach to fisheries management."

The increasing sophistication of traditional stock assessment methodology and surveying techniques, along with the adoption of approaches such as MSE, provides a more refined systematic and repeatable approach to finding good management procedures.  And that gets us just that much closer to being able to confidently answer the two questions posed by our banking metaphor.  How much do we have and how much can we spend?

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