Cod vs. Crab: It’s Not Just Cod Abundance—Size Influences Predation on Crab

Pacific cod in Alaska waters. Credit: NOAA Fisheries.

A new studyby NOAA Alaska Fisheries Science Center biologists illuminates predator-prey dynamics between Pacific cod and commercially important snow and Tanner crabs. Scientists examined diet data collected during surveys of the eastern Bering Sea together with spatial models. They discovered the single most important factor related to total crab consumption by Pacific cod is the overall abundance of the Pacific cod population.

However, the study also revealed a critical size-based difference: The total quantity of crab eaten by small cod is related to their physical overlap with crab populations. For large cod, year-to-year variation in overlap had a smaller relative influence on the total quantity of crab eaten. These findings can help improve projections of cod predation on crab and contribute to more accurate crab population models.

Map of the Bering Sea showing the spatial extent of the annual eastern and northern Bering Sea trawl survey conducted by NOAA's Alaska Fisheries Science Center. Credit: NOAA Fisheries On the left, figures show time-averaged maps of predicted densities of Pacific cod, snow crab, and tanner crab in the study area. The right image illustrates the total consumption rates of crab by Pacific cod. Credit: NOAA Fisheries

Pacific Cod Predation on Crab

Snow and Tanner crab support major Alaska fisheries, yet their populations fluctuate dramatically from year to year. In 2018--2019, snow crab experienced a population collapsein the Bering Sea, leading to a 2-year closure of the fishery beginning in October 2022. The value of that fishery went from $219 million to $0 during this period. Scientists attribute the recent snow crab collapseto an ecological shift related to an extreme marine heatwave. Recently, more attention has been directed toward understanding why populations routinely fluctuate-and how to best manage these fisheries.

The insides of this adult Pacific cod's stomach show that it ate at least two crabs. Crabs are a common prey item for Pacific cod. Credit: NOAA Fisheries/Erin Fedewa

The role of predation by Pacific cod on snow and Tanner crab remains unclear and changes in its abundance could have cascading impacts on crab fisheries. Understanding what drives crab consumption is a critical first step toward managing these interconnected resources.

The Size Divide: Why Small Cod Are Different

This study modeled the distribution of cod consumption rates of crab across the survey area. It was the first to separate consumption patterns by small cod and large cod. For both size classes, total cod biomass was the most important predictor of total crab consumption, but a size-based distinction was uncovered:

Small Cod (The Overlap Factor)

For small cod (30-60 centimeters), the physical overlap between cod and crab populations was the second most important predictor of total crab consumption. This suggests that the small cod's feeding success is tied in part to simply encountering the crab-likely because their higher hunger levels mean they eat every crab they find.

Large Cod (The Appetite Factor)

For large cod (longer than 61 centimeters), overlap was unrelated to total crab consumption. When they do overlap, large cod do not always feed on crab. Their consumption was more a function of their total biomass. When the overall biomass of large cod is high in an area, the crab consumption is higher. Overlap with crab may matter less because large cod feed more heavily on alternative prey, which means they may be more picky about where and when they eat crab.

This finding suggests that projections of cod-crab overlap will be useful to predict small cod predation on crab populations, but less so for large cod.

A scientist holds up two small snow crabs that were found inside the stomach of an adult Pacific cod. Credit: NOAA Fisheries/Erin Fedewa

The Cold Pool Barrier: Less of a Factor Than Expected

Scientists have long considered the "cold pool" (an area of cold water near the seafloor) as a natural refuge for snow crab, separating them from predators like cod. However, the study downplays its overall importance: The cold pool extent was found to be a relatively small predictor of snow crab consumption by small cod. It accounted for only around 3 percent of the year-to-year variation. While the cold pool matters, the abundance of cod will remain the primary concern for crab predation mortality.