Spatial and ontogenetic patterns of Pacific cod (Gadus macrocephalus Tilesius) predation on octopus in the eastern Bering Sea

Abstract

In 2012, the North Pacific Fishery Management Council adopted a consumption-based stock assessment method to determine catch limits for the non-target, multi-species octopus complex in the Bering Sea-Aleutian Islands (BSAI) fishery management area. The method uses Pacific cod (Gadus macrocephalus) diet data as a basis for estimating octopus complex natural mortality and minimum biomass. To enhance understanding of the predator-prey interaction between Pacific cod and octopus, we examined patterns of octopus consumption by Pacific cod using long-term stomach contents data from the eastern Bering Sea continental shelf, a large, ecologically unique subarea of the BSAI. Generalized additive modeling of octopus presence/absence in Pacific cod diets revealed distinct spatial, ontogenetic and seasonal consumption patterns. Prey octopus frequency of occurrence (FO) generally increased with bottom depth, latitude and Pacific cod fork length, and FO in the southern BSAI was lower during winter and spring than during summer. Prey octopus FO patterns may reflect overall consumption patterns and likely indicate long-term distribution patterns of small-sized (<1 kg) octopus on the EBS shelf, although we could not visually distinguish between octopus species using prey remains. Multi-species beak length-to-body mass regressions developed from three octopus species allowed reasonable estimation of prey octopus mass, and we found Pacific cod fork length was positively correlated with prey octopus mass, suggesting predator-prey interactions are sensitive to predator and prey size composition. Pacific cod consumed octopus with estimated masses ranging from 0.000017 kg to 4.62 kg, while octopus taken during concurrent bottom trawl surveys range from 0.05 kg to greater than 25 kg. Based on our findings, we expect the consumption-based stock assessment underestimates BSAI octopus complex biomass because it cannot account for larger octopus, such as the 10–20 kg Enteroctopus dofleini which dominate incidental take in BSAI Pacific cod pot fishery.