Anatomy of the collapse and onset of recovery in the North Atlantic groundfish community
The Atlantic northwest cod (Gadus morhua) stocks collapsed in the early 1990s, causing significant biological and socio-economic impacts. Despite a fisheries moratorium during the subsequent decades, the cod population has only recovered to a fraction of its previous biomass. The failure to identify the imminent collapse, and the underestimation of the recovery timescale, highlight the need for a better understanding of the health of the larger groundfish ecosystem. Our study focused on answering three questions: To what extent was the collapse limited to the cod? Is there evidence of community recovery post-collapse? Finally, what were the relative rates of collapse and recovery of biomass, composition, and spatial structure of the community?
We measured shifts in community dynamics in a long-term benthic-trawl survey of the groundfish community on the Newfoundland shelf. The survey collected a depth-stratified random sample of biomasses for 51 groundfish species from 1981 to 2013. For each year, we calculated several metrics: biomass, community synchrony, composition, and mean trophic level. For each metric, we measured how it changed prior to, during, and after the collapse, and whether it showed signs of recovery. We also grouped trawls into spatio-temporal clusters to determine how different assemblages have shifted over time.
We show that signs of collapse and recovery of cod are contained in the spatial and temporal trajectory of the broader groundfish community. Our results show three key facts regarding the dynamics of the collapse. First, the groundfish community as a whole was declining synchronously prior to the cod collapse. Second, following the collapse, the community underwent a rapid change in composition, trophic structure, and spatial structure, with a relative increase in small fish, and a large decline in the spatial extend of the cod-associated community. Third, community composition, overall biomass, and cod biomass show signs of beginning to recover to their previous state beginning roughly 20 years after the collapse. However, composition is recovering faster than biomass or spatial structure.
This community perspective provides rich insight into the collapse and ongoing recovery, beyond what can be gained by focusing on only a single species. Overall our results show that neither the collapse nor the potential recovery is a single-species phenomenon, nor are they limited to large-biomass stocks. Instead, the groundfish community shows signs of responding as a single dynamic system, with community composition, spatial structure, and biomass shifting over multiple time scales.