WHAT’S UP WITH 3K CRAB – PART II

Let’s delve into the issues with trawl survey results a bit more.

When trawl and trap surveys are trending together; it increases confidence that the change they are tracking is a real change in crab biomass.

For five of the seven years for which we have both trawl and trap data, the two indices are within 10-20% of each other (see chart below). While the difference between trawl and trap was higher in 2020 (when survey completion was impacted by COVID), the percent difference was still well below the difference between the two indices in 2024. In 2024, the trap exploitable biomass index was more than double that from the trawl survey.

When the survey indices differ, there is less confidence in results because at least one of the surveys must not be tracking eth status of the stock and/or is impacted by factors in addition to stock biomass.

Why would the two indices diverge?

We don’t know, but catch rates, whether in a survey or in a fishery, are a result of the interaction between the survey or fishing gear, fishing practices (e.g., soak time), the environment and fish behaviour. We expect snow crab may be less likely to be caught in trawls that have groundfish predators. It’s possible that the prevalence of groundfish predators causes crab in bury into the mud, making them unavailable to the trawl survey. It’s also possible that a large amount of groundfish in the trawl affects the buoyancy of the trawl, compromising bottom contact and therefore the ability of the trawl to catch crab. Another possibility is that poor weather and rough sea condition affect how well the trawl fishes.

The weather in the fall of 2024 was horrendous, with one person who was aboard the survey vessel having said 2024 was the roughest sea conditions they had ever seen during a survey. Given that the impact of sea state on performance of a trawl is well documented, this could be a possible cause for divergence of the trawl index from the trap index (Effects of sea-state on the physical performance of a survey bottom trawl – ScienceDirect).


Is this just a 3K problem?

No. The deviation between the trawl and trap indices is most pronounced in 3K at 54%, but we also saw the highest deviation between the trap and trawl indices in 2024 in 3LNO (39%) and 2HJ (31%) for the 2018-2024 period.


So, what do we want?

In the coming days, our new Minister of Fisheries, Oceans and the Canadian Coast Guard will be making a decision on crab quotas. The Department has suggested that in order to be compliant with the Precautionary Approach, 3K has to take a 49% quota cut. When considering a cut of that magnitude, it is incumbent upon the Minister to ensure that such a cut is indeed necessary to conserve the stock and evaluate the evidence that is brought forward. In the case of 3K, the data from the fishery (i.e., the catch per unit effort) and the trap survey align to indicate a relatively stable resource. The trawl survey is the outlier, as it was in other assessment divisions. We have outlined above several possible reasons for that difference.

Whatever the cause of lower catches in the trawl survey, its results with respect to snow crab are questionable – and importantly are inconsistent with other indicators – therefore we are calling on Minister Thompson to do the right thing: use the results of the trap survey and fishery CPUE when making her decision on the Total Allowable Catch.