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Breathing life into ICES stocks

Latest instalment of WKLIFE workshop helping to advance methods for stocks for which knowledge is limited.
Published: 4 November 2016

​​​​​​​​​The data-limited puzzle

Providing fishing opportunity advice on 260 individual fish and shellfish stocks relies on the pooling and processing of a vast quantity data. While for a number of these stocks it's possible to piece together a well-rounded picture underpinned by full quantitative assessments based on models and an estimate of stock size, over 60 percent of them lack sufficient data and knowledge to be able to carry out such an evaluation.

All ICES stocks are sorted into six categories depending on the amount and type information available to scientists for running an assessment of a stock's state stock and level of exploitation. Numbers one and two denote stocks like herring or hake for which the available data and information allow an analytical assessment and provision of stock size information. Categories three to six meanwhile are graded as information-limited, where only various pieces of the jigsaw are available. For categories five and six, this means only commercial landings and by-catch figures and for category four, just catch data.

For category three stocks, meanwhile, without a quantitative assessment and with perhaps a lower confidence in the data to hand, a reliable indicator for stock size has to be found in the absence of an absolute estimate. These can take the form of abundance indices from research surveys, which show patterns in the creatures' life-history traits such as biomass, recruitment, and mortality. From rays and skates to eastern Baltic cod, such indices provide a proxy for the size of category three stocks through which a catch value can be advised.

Because knowledge of stocks in category three through to six is incomplete, advice is based on the precautionary approach (conservation limits on fish caught and biomass as well as buffers accounting for uncertainties) rather than on the maximum sustainable yield (MSY) as it is for category one and two stocks. 

Data-limited comes to LIFE

Back in 2011, ICES embarked upon its data-limited journey, the process of defining a way of issuing advice along these lines. A year later and 68 data-limited stocks were added to the advice repertoire, helping to ensure that flounder, brill, and pollack fisheries among others could be conducted sustainably. It was in 2012 that the WKLIFE workshop was first convened, bringing together experts to examine how alternative methods could be adopted for the numerous species on which information and knowledge were either lacking or of limited quality. There was plenty of information for these, but not the sort that would allow for the traditional analytical assessment and thus a picture of the stock size. Other biological details – such as growth, maturity, and size (life history parameters) – however, were often available instead, which were used in new and creative ways. Because of this, ICES has moved away from the term 'data-limited' to describe these stocks, instead using the category system.

Although initially intended to run for three years, WKLIFE has continued, becoming a yearly fixture on the ICES calendar. It has carried on developing new means of gauging the stocks as well as setting advice rules (ARs) and evaluating management strategies – this to the backdrop of ICES overall goal of cementing an approach that would see category three and four stocks move from a precautionary framework to an MSY-based one.

Ascertaining uncertainty

WKLIFE's sixth meeting early this October was the latest chapter in the story. Chaired by Manuela Azevedo and Carl O'Brien, the group's main tasks included creating rules for providing precautionary advice for the stocks in a way consistent with the MSY approach, reflecting on advice rules already in place. This effort included investigating the effectiveness of the 20 percent precautionary buffer for category three stocks, a level to safeguard against true mortality or biomass slipping into the danger zone, something ICES has never been able to do before.

"We arbitrarily assumed a 20 percent buffer but in fact our simulations show that it could be increased to 25 percent," explained O'Brien. "The buffer came in because when ICES started to implement some of the methods, the advice would have potentially been inconsistent with the past, because you're looking at stocks where you don't really know exploitation rates or the stock status. If you just applied the uncertainty cap alone, you could end up giving advice that was higher than the previous trends based advice and the European Commission's arbitrary annual quota cuts of up to 30 percent, so the buffer was to guard against that. Twenty percent seemed a sensible starting point, but based on our simulations six years later, 25 percent is actually safer in terms of risk."

Indeed, the uncertainty cap applied to survey index data was another issue for WKLIFE. Survey indices, as often used with these stocks, can be quite noisy. If signals go up a lot it doesn't follow that the stock is really abundant and vice-versa if going down; instead, it could just mean the survey indices are merely tracking noise in the data. A 20 percent cap either way built into the advice takes this into account. Based on their simulations, the 20 percent value for categories three to six stocks was judged by the group to be appropriate.

 Reflecting on rules

O'Brien also outlined another success from the workshop, the decision to propose reinforcing the so-called two-over-three rule. For category three stocks, this means taking the average of the last two years' survey indices relative to the three preceding years. Increasingly, however, a three-over-five rule has since been favoured by many ICES stock assessment scientists.

"The three-over-five rule has been preferred for skates, rays, and elasmobranchs, which have really long generation times, and so this rule has been better because it picks longer time-series. Here it shows the mean time for recovery longer and you end up with instability in catches and also in the development of the stock, so it's a riskier strategy. If you use two-over-three, which was the original intention of the approach, you got a lower probability of the spawning stock biomass being below the critical half of Bmsy – so in terms of risk it's actually better."

Another issue addressed was examining how suitable it is using an advice rule of F (fishing mortality) = M (natural mortality) as an MSY proxy for stocks with very limited information, such as some of the shellfish stocks.

"Is fishing at natural mortality precautionary or not, or should ICES be advising to fish at a different lower level?" asked O'Brien. "What our work shows is that a reference point of ¾ natural mortality actually reduces the risk of falling below the biomass threshold and reduces catch variability. So you get more stability for the fishery in terms of catch rate and a lower risk of being outside safe biological limits."

Moving forward with stock status

Another important cog in the process was the WKPROXY process. Beginning in 2015, this saw ICES earlier this year provide the European Commission with a framework for classifying the status of category three and four stocks relative to MSY. Here each stock was ranked according to two proxy indicators for exploitation and biomass, and was colour coded either green to signify a desirable status or red to signify an undesirable status, depending on levels of the two. As O'Brien explained, WKLIFE was also key in moving this forward.

"What we managed to do this year was, having identified status with respect to desirable or undesirable, to try and embed that in a form of advice rule or management strategy evaluation. So we evaluated something called DLMtool, a data-limited methods toolkit. The tool allows you to set up a hypothetical population that mimics the stock you are interested in and then try and apply different types of fishing to it – simulate the effects of fishing – and see how the population evolves. Do you end up with a stock collapsing, rebuilding, or becoming stable? You then choose the strategy that gives you the best risk profiles or the best biomass trajectories."

"So that's a big change. I think all we really managed to say with the MSY proxies was whether a stock is in a good or bad state without saying what to do about it. Now, we're in a position to start giving some guidance on what you should be doing and identify fishing opportunities for next year and beyond." 

 

WKLIFE will report to ICES Advisory Committee (ACOM) with their recommendations for future advice.

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Several Norway lobster (nephrops) stocks are listed as category 3 and 4 stocks in various functional units

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Breathing life into ICES stocks

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