The Theory of Stock Assessment

Atlantic cod captured Photo: Fisheries and Oceans Canada

Atlantic cod captured Photo: Fisheries and Oceans Canada

If you had to sum up in a few words what the process of fish stock assessment is all about, most science literature on the subject describes it as "turning data into advice." One scientist who lives by such a dictum is Dr. Noel Cadigan, a researcher with Fisheries and Oceans Canada (DFO), working out of St. John's, Newfoundland. The focus of his entire career has been stock assessment - trying to figure out as precisely as possible how many of which species of fish are in eastern Canadian waters. It is critical work, the results of which inform DFO about the health of the marine ecosystem in general, and more specifically, the status of specific commercial fish stocks from one fishing season to the next, and what are sustainable harvests.

He can't do this in isolation of course because fish tend to ignore national boundaries! That is why scientists from around the world pool their knowledge. In the case of the North Atlantic, they coordinate their efforts primarily under two organizations known as the International Council for the Exploration of the Sea, or ICES for short, and the Northwest Atlantic Fisheries Organization (NAFO). ICES acts as a meeting point for a community of more than 1,600 marine scientists from 20 countries. NAFO has 12 member countries from North America, Europe, Asia and the Caribbean. Working together gives them a much more balanced picture of what is really happening in the sea.

One of the challenges for those in the stock assessment business is the quantity and quality of data they have. Dr. Cadigan: "You just can't change a data deficiency situation quickly. It's not something that you say, 'Okay, we're going to assess the stock. Let's go out and collect the data on it.' You can't do that in one year. Usually, you need a time series of monitoring data that you collect annually. You track over time what has been happening to the stock."

The data comes from a variety of sources including landings at ports, from fishers, and research vessel surveys. Universities can be involved in some specialized monitoring, such as acoustic surveys. As Dr. Cadigan notes, "There's data being collected virtually every day of the year somewhere; including by Fisheries and Oceans Canada of course, whose annual monitoring surveys are primarily for stock assessment purposes."

Fish are examined for crucial information such as age, length, and breeding condition. Once enough records are collected, records that number in the thousands, mathematical and statistical models are used to convert the data into fish population estimates.

A fundamental principle of stock assessment advice is that there should be enough fish left in a stock - after fishing and deaths from natural causes - to spawn healthy new generations in future years. In Canada there are two important benchmarks that come from stock assessments: the biomass limit and the fishing mortality limit. The biomass is the total weight of fish in a fish stock, and fishing mortality is related to the fraction of the stock removed by fisheries. The biomass limit is the lowest level to which a stock should be allowed to fall. Below this level, the stock is so small that the numbers of young fish the adults are capable of spawning is likely to be seriously reduced. This means that the future of the stock is in jeopardy, and in a worst-case scenario, it may never recover to its former levels. The fishing mortality limit is the maximum acceptable removal rate for the stock. It should be less than the level of fishing that results in the greatest long-term sustainable yield.

At Fisheries and Oceans Canada, the science advice gets wrapped up with fisheries management advice in reports that eventually land on the Minister's desk for final decisions. Dr. Cadigan: "With stock assessment we could be asked to provide advice on a whole range of fundamental and common issues about trends in stock populations. We are often asked to evaluate the impact of some proposed quota options. If, in the next fishing year, we were to take X amount, X thousand pounds or whatever, what would be the impact on the stock? We would provide those evaluations for a range of quota options. We on the science side don't set quotas, but we do give fisheries managers our best understanding of the status of a stock and how a particular quota might impact it."

Asked how much of assessment is initiated on the science side, and how much is in response for specific information, Cadigan responds, "It varies. Mostly we get requests from fisheries managers, but we also respond to requests for advice from other groups. There will be some stocks that maybe no one has asked for advice on, but science decides to do the assessment anyway. Here on the East Coast we do different stocks at different times of the year. But usually we're doing those same stocks year after year. Of course, cod are always on our radar, as are American Plaice, Greenland Halibut, and Yellowtail Flounder, remembering of course that some of our stocks of interest go out beyond the 200-mile limit. And then too the shellfish stocks are really important - shrimp, lobster and Snow Crab as well as pelagics such as herring in the Maritimes regions of Nova Scotia and New Brunswick."

At times researchers are also asked to look at by-catch species. These are species caught incidentally to the primary target species, ones not regarded as commercially viable or valuable and which don't usually undergo much monitoring. However, some of these fall under the Species At Risk Act therefore the stock assessors must provide advice on these as well.

There are two aspects to stock assessment work. The biologists who go out to sea and do the sampling, and those who take the resulting data and try to turn it into advice. It is the second aspect that Dr. Cadigan is principally involved in. About the data he works with he notes, "It is not exactly the information that's recorded on the boats. There's data cleaning that goes on, but once it is archived in the database, I do the extraction and go with it from there."

From a training perspective he points out that there is really no definitive book on the subject. There are some that talk about the theoretical basis of modeling, say, marine populations. But a big component of stock assessment is actually analyzing and understanding data; this statistical element tends to be not well described in the population modelling texts. "To me", says Cadigan, "if you were to talk about the theory of stock assessment, you have to talk about the theory of fish dynamics, and the theory of statistics. You need to understand uncertainty theories and how it applies to data collection and fish stocks. I have a PhD in statistics - my thesis topic was about how to analyze fishery surveys. I mean, I've been learning for 20 years, and I'm still learning. The field is enormous. I remember doing statistics in university and taking all of these courses and thinking, 'I'll never use that again. Why do I need to know this?' And yet within five years at Fisheries and Oceans Canada, I think I had used all of it, or 90 percent of it at least."

It's a good thing those classes were taking though, since the science of stock assessment is one of the fundamental components of the Fisheries and Oceans Canada science program. Sound decision making on harvest levels is critically tied to accurate stock assessments. It requires years of experience, a dogged determination to do it year in and year out, and a love for all things statistical. All of which makes Dr. Noel Cadigan the right guy in the right job.
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