New Model Predicts Where Red King Crab Bycatch Is Likely to Occur in Bering Sea Flatfish Trawl Fisheries

Three juvenile Bristol Bay red king crab. Credit: NOAA Fisheries.

NOAA Fisheries scientists developed new modelsto predict red king crab bycatch occurrence and abundance in Bristol Bay flatfish trawl fisheries. They found a general northward shift in occurrence and abundance of different ages of male and female red king crab in Bristol Bay.

"By demonstrating that it's possible to predict where and when bycatch occurs, this model is an important first step towards building predictive tools for fishing fleets and fisheries managers to reduce bycatch," said Emily Ryznar, lead author and fisheries biologist at the Alaska Fisheries Science Center.

Bristol Bay red king crab. Credit: NOAA Fisheries.

Model Provides Insights on What's Happening in the Non-Summer Months in Bristol Bay

We have assessed the abundance of Bristol Bay red king crab annually since 1975 with a standardized fisheries-independent summertime bottom trawl survey. Data collected over nearly 50 years provide a wealth of information for understanding summer red king crab distribution. However, fisheries-independent information on crab distribution in other seasons is lacking.

Surveying fish and crab stocks year-round is often difficult due to cost and weather conditions outside the summer months, especially in high-latitude ecosystems. In situations when the seasonal timing of surveys limits the availability of data, fisheries-dependent data can help fill in some gaps.

Fisheries dependent data can come from onboard observers, industry-reported logbooks, or fish landings. It may provide the best available information for understanding the biology of species caught in directed fisheries or as bycatch. These data also can identify the location and timing of bycatch hotspots.

Scientists used these data to build new models about the distribution of red king crab bycatch in groundfish fisheries during fall/winter/spring seasons. The goal is to provide managers and fishermen with information so they can evaluate the efficacy of static closure areas to limit bycatch and provide protections to red king crab.

To evaluate whether the closures are still effective requires:

1. An understanding of where bycatch is occurring
2. Information on how this has changed since closed areas were established

Crab pots in Dutch Harbor. Credit: NOAA Fisheries.

Study Findings

Summer survey data and fishery-dependent data from the trawl fisheries were generally much better predictors of bycatch distribution than environmental variables such as sea ice coverage, which drives bottom temperature.

Depth was also an important variable for predicting the bycatch occurrence for mature and immature females. It potentially indicated the depth fidelity of females compared to seasonally migrating males.

Centers of distribution for red king crab caught in the summer bottom trawl survey, and as bycatch in flatfish fisheries, were correlated with the model. Its output shows a general shift from southwestern Bristol Bay through time to be north of or slightly below 57°N after 2016.

As a result, crab in Bristol Bay are more likely to occur above or near the northern boundary of the Red King Crab Savings Area (bounded by 164˚-162˚W and 57˚-56.2˚N). This area is closed to flatfish trawling year round and was established in the 1990s based on red king crab distribution from the summer bottom trawl survey. This places them in Bycatch Limitation Zone 1 (south of 58˚N and east of 165˚W), which is closed to flatfish trawling when bycatch limits are reached.

Bristol Bay red king crab. Credit: NOAA Fisheries.

Northward movement and spatial expansion towards central Bristol Bay through time for females and males suggests there may be value in reassessing existing static closure areas. For reference, the proportion of observed red king crab bycatch occurring in zones entirely open to flatfish trawling year round has increased from roughly 5 percent (1997-2013) to 30 percent in the last 10 years.

The model may also prove useful for understanding seasonal movements of Bristol Bay red king crab. These movements are poorly documented but thought to cover hundreds of kilometers and future distribution shifts with warming ocean temperatures.

"Our study contributes timely insight regarding the distribution and drivers of Bristol Bay red king crab bycatch. This is a great example of the kind of research that we are conducting to give stakeholders and fisheries managers better information for responding to the current crisis in Bering Sea crab fisheries," said Mike Litzow, Kodiak Laboratory Director and Alaska Fisheries Science Center Shellfish Assessment Program Manager.